U.S. patent application number 12/394736 was filed with the patent office on 2009-06-25 for method of producing lithium ion-storing/releasing material, lithium ion-storing/releasing material, and electrode structure and energy storage device using the material.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Toshiaki Aiba, Norishige Kakegawa, Akio Kashiwazaki, Soichiro Kawakami, Takashi Noma, Kaoru Ojima, Mikio Shimada, Rie Ueno.
Application Number | 20090162750 12/394736 |
Document ID | / |
Family ID | 40429017 |
Filed Date | 2009-06-25 |
United States Patent
Application |
20090162750 |
Kind Code |
A1 |
Kawakami; Soichiro ; et
al. |
June 25, 2009 |
METHOD OF PRODUCING LITHIUM ION-STORING/RELEASING MATERIAL, LITHIUM
ION-STORING/RELEASING MATERIAL, AND ELECTRODE STRUCTURE AND ENERGY
STORAGE DEVICE USING THE MATERIAL
Abstract
A method of producing a material capable of electrochemically
storing and releasing a large amount of lithium ions is provided.
The material is used as an electrode material for a negative
electrode, and includes silicon or tin primary particles composed
of crystal particles each having a specific diameter and an
amorphous surface layer formed of at least a metal oxide, having a
specific thickness. Gibbs free energy when the metal oxide is
produced by oxidation of a metal is smaller than Gibbs free energy
when silicon or tin is oxidized, and the metal oxide has higher
thermodynamic stability than silicon oxide or tin oxide. The method
of producing the electrode material includes reacting silicon or
tin with a metal oxide, reacting a silicon oxide or a tin oxide
with a metal, or reacting a silicon compound or a tin compound with
a metal compound to react with each other.
Inventors: |
Kawakami; Soichiro;
(Machida-shi, JP) ; Kakegawa; Norishige;
(Chofu-shi, JP) ; Kashiwazaki; Akio;
(Yokohama-shi, JP) ; Aiba; Toshiaki;
(Fujisawa-shi, JP) ; Ueno; Rie; (Hadano-shi,
JP) ; Shimada; Mikio; (Kawasaki-shi, JP) ;
Ojima; Kaoru; (Kawasaki-shi, JP) ; Noma; Takashi;
(Hadano-shi, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
40429017 |
Appl. No.: |
12/394736 |
Filed: |
February 27, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2008/066506 |
Sep 8, 2008 |
|
|
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12394736 |
|
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Current U.S.
Class: |
429/218.1 ;
252/182.1 |
Current CPC
Class: |
H01G 11/24 20130101;
H01M 4/58 20130101; H01M 4/62 20130101; H01G 11/50 20130101; H01M
4/48 20130101; H01G 11/46 20130101; Y02E 60/10 20130101; H01M 4/386
20130101; H01M 10/0525 20130101; Y10T 428/2927 20150115; H01M 4/134
20130101; Y02E 60/13 20130101; Y10T 428/259 20150115; H01M 4/387
20130101; H01M 4/366 20130101 |
Class at
Publication: |
429/218.1 ;
252/182.1 |
International
Class: |
H01M 4/58 20060101
H01M004/58; H01M 4/88 20060101 H01M004/88 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 6, 2007 |
JP |
2007-232090 |
Dec 12, 2007 |
JP |
2007-321373 |
Claims
1. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions, the electrode material comprising silicon or tin
primary particles composed of crystal particles each having a
diameter of 5 nm to 200 nm and an amorphous surface layer having a
thickness of 1 nm to 10 nm, wherein the amorphous surface layer of
each of the primary particles is formed of at least a metal oxide;
Gibbs free energy when the metal oxide is produced by oxidation of
a metal is smaller than Gibbs free energy when silicon or tin is
oxidized; and the metal oxide has higher thermodynamic stability
than silicon oxide or tin oxide.
2. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions according to claim 1, wherein a metal element accounts
for 0.3 atomic % or more of the metal oxide in the primary
particles.
3. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions according to claim 1, wherein the silicon or tin
primary particles are silicon particles.
4. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions according to claim 3, wherein a crystallite size
calculated from a half width of an Si(111) peak in an X-ray
diffraction chart of the silicon particles and Sherrer's equation
falls within a range of 20 to 60 nm.
5. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions according to claim 3, wherein a molar ratio of silicon
oxide to Si calculated from an X-ray photoelectron spectroscopy
(XPS) measurement spectrum of the silicon particles is 0.05 to 7.0,
and a ratio of an oxygen element to an Si element of the silicon
particles measured with an energy dispersive X-ray spectrometer
(EDX) of a scanning transmission electron microscope (STEM) is 0.05
to 0.8.
6. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions according to claim 3, wherein the crystal particles of
the silicon primary particles are formed into a network structure
with fibrous (filamentous) substances, and a surface of each of the
fibrous (filamentous) materials is coated with at least an
oxide.
7. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions according to claim 6, wherein the fibrous
(filamentous) substances constituting the network structure each
have a diameter in a range of 5 nm to 70 nm and a length in a range
of 100 nm to 2 .mu.m.
8. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions according to claim 6, wherein the fibrous
(filamentous) substances each have a core-shell structure, the core
portion comprises a silicon crystal, and the shell portion
comprises an amorphous silicon oxide or an amorphous metal oxide,
provided that Gibbs free energy when the metal oxide is produced by
oxidation of a metal is smaller than Gibbs free energy when silicon
or tin is oxidized, and the metal oxide has higher thermodynamic
stability than silicon oxide or tin oxide.
9. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions according to claim 6, wherein the fibrous
(filamentous) substances comprise amorphous silicon oxide.
10. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions according to claim 6, wherein the fibrous
(filamentous) substances comprise crystalline aluminum
oxynitride.
11. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions according to claim 1, wherein a metal element of the
metal oxide comprises one or more types of metals selected from Li,
Be, Mg, Al, Ca, Zr, Ba, Th, La, Ce, Nd, Sm, Eu, Dy, and Er.
12. An electrode material for a negative electrode of an energy
storage device capable of electrochemically storing and releasing
lithium ions according to claim 1, wherein a metal element of the
metal oxide comprises one or more types of metals selected from Li,
Mg, Al, Ca, Zr, Ba, La, Ce, and Nd.
13. A method of producing the electrode material for a negative
electrode of an energy storage device capable of electrochemically
storing and releasing lithium ions according to claim 1, comprising
any one of the following steps: (i) reacting silicon or tin with a
metal oxide; (ii) reacting a silicon oxide or a tin oxide with a
metal; and (iii) reacting a silicon compound or a tin compound with
a metal compound, provided that Gibbs free energy when a metal
oxide is produced by oxidation of a metal element of which the
metal oxide or the metal is smaller than Gibbs free energy when
silicon or tin is oxidized, and the metal oxide has higher
thermodynamic stability than silicon oxide or tin oxide.
14. A method of producing an electrode material for a negative
electrode of an energy storage device capable of electrochemically
storing and releasing lithium ions according to claim 13, wherein
the reaction of silicon or tin with the metal oxide in the step (i)
is (A) a reaction performed by introducing at least silicon or tin
and the metal oxide in a powder state into thermal plasma obtained
by turning an inert gas or a hydrogen gas into plasma, or (B) a
sintering reaction in spark plasma instantaneously generated by a
spark discharge phenomenon caused by subjecting at least silicon or
tin and the metal oxide to mechanical alloying treatment, pressing
a powder obtained by the mechanical alloying treatment, and
applying a pulsed current to gaps between the pressed powder
particles at a low voltage under reduced pressure.
15. A method of producing an electrode material for a negative
electrode of an energy storage device capable of electrochemically
storing and releasing lithium ions according to claim 13, wherein
the reaction of the silicon oxide or the tin oxide with the metal
in the step (ii) is at least one of (C) a reaction performed by
introducing the metal, and the silicon oxide or the tin oxide, or
silicon containing the silicon oxide or tin containing the tin
oxide in a powder state into thermal plasma, (D) a sintering
reaction in spark plasma instantaneously generated by a spark
discharge phenomenon caused by subjecting the metal, and the
silicon oxide or the tin oxide, or silicon containing the silicon
oxide or tin containing the tin oxide in a powder state to
mechanical alloying treatment, pressing a powder obtained by the
mechanical alloying treatment, and applying a pulsed current to
gaps between the pressed powder particles at a low voltage under
reduced pressure, (E) a heating reaction in an inert gas or a
hydrogen gas or under reduced pressure for a powder obtained by
subjecting the metal, and the silicon oxide or the tin oxide, or
silicon containing the silicon oxide or tin containing the tin
oxide in a powder state to mechanical alloying treatment, and (F) a
heating reaction for a composite layer formed on a substrate by
vapor deposition of the metal, and the silicon oxide or the tin
oxide, or silicon containing the silicon oxide or tin containing
the tin oxide.
16. A method of producing an electrode material for a negative
electrode of an energy storage device capable of electrochemically
storing and releasing lithium ions according to claim 13, wherein
in the reaction of the silicon compound or the tin compound with
the metal compound in the step (iii), the silicon compound is a
compound selected from silane, disilane, dichlorosilane,
trichlorosilane, tetrachlorosilane, tetramethoxysilane,
tetraethoxysilane, and tetrabutoxysilane, the tin compound is a
compound selected from tin tetrachloride, tetraethoxytin,
tetrapropoxytin, and tetrabutoxytin, the metal compound is a
compound selected from trichloroaluminum, trimethoxyaluminum,
triethoxyaluminum, tripropoxyaluminum, tributoxyaluminum, and
aluminum isoperoxide, and the compounds are heated in an inert gas
atmosphere or hydrogen gas atmosphere so that a reaction
temperature reaches 400 to 1,300.degree. C.
17. A method of producing an electrode material for a negative
electrode of an energy storage device capable of electrochemically
storing and releasing lithium ions according to claim 15, wherein
the heating in the reaction (E) or (F) is performed by radiation
with laser light or infrared light.
18. A method of producing an electrode material for a negative
electrode of an energy storage device capable of electrochemically
storing and releasing lithium ions according to claim 16, wherein
the heating is performed by radiation with laser light or infrared
light.
19. A method of producing an electrode material for a negative
electrode of an energy storage device capable of electrochemically
storing and releasing lithium ions according to claim 16, wherein
the atmosphere is an atmosphere under reduced pressure.
20. An electrode structure for a negative electrode of an energy
storage device, comprising: a current collector; and an electrode
material layer (main active material layer) formed of a main active
material as a powder material capable of storing and releasing
lithium ions by an electrochemical reaction, wherein the main
active material is the electrode material according to claim 1.
21. An electrode structure for a negative electrode of an energy
storage device according to claim 20, wherein the electrode
material layer is formed of the main active material and a
binder.
22. An electrode structure for a negative electrode of an energy
storage device according to claim 20, wherein the electrode
material layer is formed of the main active material, a conductive
auxiliary material, and a binder.
23. An electrode structure for a negative electrode of an energy
storage device according to claim 20, wherein the electrode
material layer has density in a range of 0.5 g/cm.sup.3 or more and
3.5 g/cm.sup.3 or less.
24. An energy storage device comprising: a negative electrode using
the electrode structure according to claim 20; a lithium ion
conductor; and a positive electrode, wherein the energy storage
device utilizes an oxidation reaction of lithium and a reduction
reaction of lithium ions.
25. An energy storage device according to claim 24, wherein the
positive electrode is formed of at least a powder material which
includes particles formed of a transition metal compound selected
from a transition metal oxide, a transition metal phosphate
compound, a lithium-transition metal oxide, and a
lithium-transition metal phosphate compound, and is turned into a
composite with a metal oxide having an amorphous phase.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of International
Application No. PCT/JP2008/066506, filed Sep. 8, 2008, which claims
the benefit of Japanese Patent Applications No. 2007-232090, filed
Sep. 6, 2007, and No. 2007-321373, filed Dec. 12, 2007.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method of producing a
lithium ion-storing/releasing material mainly composed of a metal
which alloys with lithium by an electrochemical reaction such as
silicon or tin and a metal oxide, an electrode structure formed of
the material, and an energy storage device having the electrode
structure.
[0004] 2. Description of the Related Arts
[0005] It has been pointed out that global warming may occur owing
to a greenhouse effect because the amount of CO.sub.2 gas in the
air has been recently increasing. In addition, air pollution due
to, for example, CO.sub.2, NO.sub.x, and a hydrocarbon exhausted
from automobiles is of serious concern. Also in view of a run-up in
crude oil prices, expectations have been placed from the viewpoint
of environmental protection on hybrid vehicles and electric
vehicles in each of which an electric motor to be actuated with
electricity stored in an energy storage device and an engine are
combined. Accordingly, the development of an energy storage device
such as a capacitor or a secondary battery which has both high
power density and high energy density has been desired in order
that the performance of a hybrid vehicle or electric vehicle may be
improved and a cost for the production of the hybrid vehicle or
electric vehicle may be reduced.
[0006] Further, the functions of portable instruments such as a
portable phone, a book type personal computer, a video camera, a
digital camera, and a personal digital assistant (PDA) have become
more and more sophisticated. The development of an energy storage
device such as a secondary battery which not only has a small size,
a light weight and a large capacity, but also can be charged
quickly, has been desired in order that the device may be able to
find use in applications including power sources for actuating the
instruments.
[0007] Representative examples of the above energy storage device
include the so-called "lithium ion batteries". Each of the
batteries is of a rocking chair type in which lithium ions are
released by a charging reaction from between the layers of a
lithium intercalation compound and lithium ions are inserted in
between the layers of a carbon material typified by graphite used
as a negative electrode, having a laminated structure including
six-membered network planes. The batteries have been in widespread
use as power sources for a large number of portable instruments
because of their high cell voltages and their high energy
densities. In addition, investigation has been conducted on whether
each of the batteries can be used as a power source for a hybrid
vehicle.
[0008] However, each of the "lithium ion batteries" can
theoretically intercalate only a maximum of one lithium atom per
six carbon atoms because its negative electrode is formed of the
carbon material. Accordingly, it is difficult to additionally
increase the capacity of each of the batteries, and a new electrode
material for an increase in capacity has been desired. Although the
above "lithium ion batteries" have been expected to serve as power
sources for hybrid vehicles and electric vehicles because of their
high energy densities, each of the batteries involves the following
problem: each of the batteries has so large an internal resistance
as to be incapable of discharging a sufficient electrical quantity,
that is, each of the batteries has so small a power density as to
be unqualified for quick discharging. In view of the foregoing, the
development of an energy storage device having a high power density
and a high energy density has been demanded.
[0009] The inventors of the present invention have proposed Patent
Document 1, Patent Document 2, Patent Document 3, Patent Document
4, Patent Document 5, Patent Document 6, Patent Document 7, and
Patent Document 8 each concerning a negative electrode for a
lithium secondary battery formed of a silicon or tin element for
additional increases in capacities of lithium secondary batteries
including the "lithium ion batteries".
[0010] Patent Document 1 proposes a lithium secondary battery using
a negative electrode obtained by forming, on a current collector
made of a metal material which does not alloy with lithium, an
electrode layer formed of a metal which alloys with lithium such as
silicon or tin and a metal which does not alloy with lithium such
as nickel or copper.
[0011] Patent Document 2 proposes a negative electrode formed of an
alloy powder made of an element such as nickel or copper and an
element such as tin. Patent Document 3 proposes a lithium secondary
battery using a negative electrode in which an electrode material
layer contains 35 wt % or more of particles having an average
particle diameter of 0.5 to 60 .mu.m, formed of silicon or tin, and
has a porosity of 0.10 to 0.86 and a density of 1.00 to 6.56
g/cm.sup.3.
[0012] Patent Document 4 proposes a lithium secondary battery using
a negative electrode having silicon or tin having an amorphous
phase. Patent Document 5 and Patent Document 6 each propose an
active material lithium secondary battery having an amorphous phase
obtained by turning a material mainly formed of a metal and inert
to a material except Li into a composite with a positive electrode
active material or negative electrode active material. Patent
Document 7 proposes a lithium secondary battery using a negative
electrode formed of amorphous tin-transition metal alloy particles
having non-stoichiometric composition. Patent Document 8 proposes a
lithium secondary battery using a negative electrode formed of
amorphous silicon-transition metal alloy particles having
non-stoichiometric composition.
[0013] A lithium secondary battery using the above amorphous alloy
in its negative electrode can not only realize a large capacity but
also reduce the expansion of the volume of the alloy at the time of
charging. Although an approach referred to as mechanical alloying
involving applying mechanical energy is an effective production
method involving reducing the size of the crystallite of the above
alloy to improve the amorphous property of the alloy, it cannot
uniformize the composition of the alloy in a microscopic range, and
cannot avoid the production of silicon oxide or tin oxide because
of the following reason: a material for the alloy is turned into a
fine powder to have an increased surface area, so there is no
choice but to remove the material by slow oxidation. In the above
alloy, lithium reacts with silicon oxide or tin oxide at the time
of charging to change into an inert lithium compound such as
lithium oxide which cannot release lithium reversibly, and the
inert lithium compound is responsible for a reduction in charge and
discharge efficiency of the battery. Further, alloy particles
coated with the above inert lithium compound produced by a charging
reaction each have increased electrical resistance because the
compound is an insulator. In addition, when each of the alloy
particles is coated with the compound non-uniformly, the intensity
of an electric field applied to the particles at the time of
charging become non-uniform, so alloying with lithium also becomes
non-uniform, and local expansion of the volume of the alloy occurs.
Moreover, it cannot be said that a reaction for alloying with
lithium by storage of lithium in the silicon alloy or tin alloy
lattice occurs uniformly because the alloy produced by mechanical
alloying originally has non-uniform alloy composition. Accordingly,
the volume expansion is still present, and an increase in internal
resistance of the battery caused by the repetition of charging and
discharging cannot be completely suppressed. In addition, it can
never be said that the rate at which lithium is turned into an
alloy at the time of charging is high, so, in quick charging, at
least one of the decomposition of an electrolyte solution and the
precipitation of metal lithium onto the surface of the negative
electrode may occur with a certain possibility depending on the
design of the structure of the battery. In view of the foregoing,
the development of an energy storage device has been desired which
maintains a large capacity, has a high power density, and can be
charged quickly.
[0014] An electric double layer capacitor which: uses active carbon
having a large specific surface area in each of its negative
electrode and positive electrode; and stores electricity in its
electric double layer has been expected to find use in applications
including power sources for hybrid vehicles because the capacitor
can be charged quickly, and has a large capacity. The electric
double layer capacitor has the following major advantages: the
capacitor has a long lifetime, specifically, the number of repeated
uses of the capacitor is about 10 to 100 times as many as that of
the "lithium ion battery", and the capacitor has a power density
about five times as high as that of the battery. However, the above
electric double layer capacitor has not yet been adopted as a power
source for a movable body owing to the following disadvantages: the
capacitor has low energy densities, specifically, the capacitor has
a weight energy density about one tenth to one half as high as that
of each of the "lithium ion battery", and has a volume energy
density about one fiftieth to one twentieth as high as that of the
battery. In view of the foregoing, the development of an energy
storage device having increased energy densities while taking
advantage of good characteristics of the above electric double
layer capacitor, in other words, maintaining the following
advantages has been desired: the device can be charged quickly, can
be repeatedly used a large number of times, and has a high power
density. A proposal concerning the use of a carbon material capable
of storing and releasing lithium ions and anions at the time of
charging and discharging in an electrode, and a proposal concerning
a hybrid type capacitor using a metal oxide material capable of
storing and releasing lithium ions at the time of charging and
discharging in an electrode have been made in order that the
shortcomings of the above-mentioned electric double layer capacitor
may be alleviated. For example, Patent Documents 9 to 21 and
Non-patent Document 1 have been proposed.
[0015] Patent Document 9 proposes a battery (an energy storage
device) using a polyacene-based material capable of being doped
with an ion electrochemically in at least one of its negative
electrode and positive electrode. Patent Document 10 proposes a
capacitor using a polyacene-based material in each of its positive
and negative electrodes and using a quaternary ammonium salt as an
electrolyte. Patent Document 11 proposes a battery (an energy
storage device) using a polyacene-based material carrying lithium
in advance in its negative electrode.
[0016] Patent Document 12 proposes a capacitor using a carbon
material which has been caused to absorb lithium in its negative
electrode and active carbon in its positive electrode. Patent
Document 13 proposes a capacitor using an electrode made of a
carbon material containing a metal or metal compound and having
micropores in each of its positive and negative electrodes. Patent
Document 14 proposes an electric double layer capacitor using, in
each of its negative and positive electrodes, an electrode formed
of non-porous carbon having graphite-like microcrystalline carbon
in which electrolyte ions are intercalated between layers together
with a solvent.
[0017] Patent Document 15 proposes an energy storage device in
which a composite porous material obtained by adhering a carbon
material to the surface of active carbon is used in its negative
electrode and active carbon is used in its positive electrode.
Patent Document 16 proposes an electric double layer capacitor
formed of an electrode member obtained by electrochemically
activating a carbon member and having pores larger than electrolyte
ions.
[0018] A proposal in which a metal oxide is used as an electrode
material has also been made. Patent Document 17 proposes an
electrochemical capacitor using an electrode formed of a lithium
vanadium oxide and a conductive agent as its negative electrode and
an electrode formed of active carbon as its positive electrode.
Patent Document 18 proposes an electric double layer capacitor
using a porous conductive ceramic having a mesoporous structure in
an electrode. Patent Document 19 proposes an electric double layer
capacitor using an electrode obtained by coating the surface of a
porous material with a conductive ceramic.
[0019] Patent Document 20 proposes a capacitor using a carbon fine
powder coated with a metal oxide, metal nitride, or metal carbide
as an electrode material. Patent Document 21 proposes a lithium
non-aqueous electrolyte energy storage device which: uses a
composite porous material obtained by adhering a carbonaceous
material to the surface of active carbon in its negative electrode
and an amorphous metal oxide containing at least one of Mn and V in
its positive electrode; and contains a lithium salt as an
electrolyte. Patent Document 22 proposes an electrode for an
electrochemical element containing an octatitanate nanosheet
represented by H.sub.2Ti.sub.8O.sub.17nH.sub.2O (n=0 to 2.0) and a
carbon material. Patent Document 23 proposes a rechargeable energy
battery system using a material which reversibly intercalates a
cation of, for electrochemical insertion/example, an alkali metal
such as Li.sub.4Ti.sub.5O.sub.12 in its negative electrode and a
material which reversibly adsorbs an anion in its positive
electrode. In addition, Non-patent Document 1 reports a nonaqueous
battery cell produced from a negative electrode formed of
Li.sub.4Ti.sub.5O.sub.12 and a positive electrode formed of active
carbon.
[0020] However, each of the above proposed energy storage devices
such as a capacitor has an energy density not more than one tenth
as high as that of the lithium secondary battery (including the
lithium ion battery), so an additional increase in energy density
of the device has been desired.
[0021] In addition, Patent Document 24, Patent Document 25,
Non-patent Document 2, and Non-patent Document 3 each propose a
secondary battery using carbon composite particles in which SiO is
heated to cause a disproportionation reaction and Si crystals
nanometers in size are dispersed in SiO.sub.2 as a negative
electrode material and having good charging and discharging cycle
properties. However, the above electrode using silicon dispersed in
a silicon oxide involves the following problem: the amount of Li
that cannot be desorbed in an extraction reaction for Li
(irreversible amount) is large.
[0022] Patent Document 26 proposes that a silicon compound such as
silicon from which a metal has been removed, or a material obtained
by adhering a ceramic to the silicon compound be used as a negative
electrode material. Si--SiO.sub.2 is obtained by mixing and heating
silicon and colloidal silica, and Si--Al.sub.2O.sub.3 is obtained
by mixing and heating silicon and alumina sol.
[0023] Patent Document 27 proposes, as a negative electrode
material for a nonaqueous electrolyte solution secondary battery, a
material formed of composite particles obtained by coating the
whole surfaces, or part of the surfaces, of inorganic particles
(Si, Sn, or Zn) capable of absorbing and desorbing lithium ions
with a ceramic (oxide, nitride, or carbide of a material selected
from Si, Ti, Al, and Zr). The above composite particles have an
average particle diameter of 1 .mu.m to 50 .mu.m, and are prepared
by: adding and mixing the inorganic particles into sol as a source
of the ceramic; drying the mixture; and subjecting the dried
product to heat treatment.
[0024] Each of Patent Document 26 and Patent Document 27 described
above involves the following problem: the oxidation of silicon is
promoted in the step of adhering the ceramic to the surfaces, so
the content of silicon oxide to be produced increases, and the
amount of Li that reacts with silicon oxide so as to be incapable
of being desorbed in an initial electrochemical
insertion/extraction reaction for Li (irreversible amount) is
large.
[0025] Non-patent Document 4 announces the following: when a
repetitive experiment on electrochemical insertion/extraction of Li
is performed by using an electrode, which is obtained by forming a
silicon nanowire on a stainless substrate with gold Au as a
catalyst, as a working electrode and metal lithium as a counter
electrode, coulomb efficiency for the first insertion is 73%,
efficiency for each of the second and subsequent insertions is 90%,
and a reduction in amount of Li to be inserted/desorbed during a
period from the second insertion to the tenth insertion is small.
The coulombic efficiency of the extraction of Li for the first
insertion of Li is low probably because silicon oxide is formed at
the time of forming the silicon nanowire.
[0026] Patent Document 28 proposes a nonaqueous electrolyte
secondary battery using fibrous silicon the surface of which is
coated with a carbon material as a negative electrode material.
However, the document discloses neither a method of obtaining
fibrous silicon nor a method of preparing fibrous silicon.
Moreover, the document does not disclose any specific method of
coating the surface with the carbon material.
[0027] Patent Document 29 proposes a negative electrode active
material formed of: metal core particles each having a carbon-based
coating layer on its surface and each containing a metal capable of
forming an alloy with lithium (Si, Sn, Al, Ge, Pb, Bi, Sb, and
alloys of them); and metal nanowires formed integrally to the metal
core particles. The document discloses that the active material is
obtained by the following procedure: a metal particle powder, a
polymer material, and a pore-forming substance are mixed and baked
so that the polymer material carbonizes to provide the carbon-based
coating layer, and the metal nanowires grow from metal particles
each contacting the carbon-based coating layer, whereby the active
material is obtained. However, the document does not disclose any
analysis for the shape and material of each of the metal nanowires.
In addition, the document does not disclose any large-capacity
negative electrode material having a charged and discharged
capacity in excess of 900 mAh/g.
[0028] On the other hand, a method of producing a whisker-, wire-,
or needle-like nanosilicon is proposed as described below.
[0029] Patent Document 30 proposes a production method in which a
metal serving as a catalyst (Au, Cu, Pt, Pd, Ni, Gd, or Mg) is
heated to melt under reduced pressure in an atmosphere containing
an oxygen element as an oxidation source for silicon, and a silicon
gas molecule is brought into contact with the molten metal so that
a whisker-like chain is formed in which silicon crystal nanospheres
each coated with an SiO.sub.2 oxide film are arrayed by the network
of the SiO.sub.2 oxide film.
[0030] Non-patent Document 4 announces that the whisker of
crystalline silicon is formed by: mounting a gold small particle on
a silicon wafer; heating the resultant to 950.degree. C.; and
introducing a mixed gas of hydrogen and silane tetrachloride into
the heated product.
[0031] Non-patent Document 5 announces that an Si Powder mixed with
0.5% of Fe is irradiated with excimer laser light in a quartz tube
in an Ar gas flow at 500 Torr and 50 sccm so that nanowires each
using crystalline silicon in its core and amorphous silicon oxide
in its surface layer, and each having a diameter of 3 to 43 nm and
a length of 2 to 3 .mu.m are formed on the inner wall of the quartz
tube.
[0032] Patent Document 31 proposes a method of growing a silicon
nanoneedle involving: providing a thin film of a metal (gold,
silver, or copper) that forms an alloy droplet with silicon on a
silicon substrate; and heating the resultant to 1,200.degree. C. or
higher in the presence of sulfur in a vacuum inclusion closed
vessel to produce silicon in a vapor phase.
[0033] Patent Document 32 proposes a production method involving
evaporating silicon or a silicon/germanium alloy at a temperature
equal to or lower than the melting point of silicon or the alloy,
specifically a temperature in excess of 1,300.degree. C. and
1,400.degree. C. or lower in a stream of a carrier gas (an argon
gas, a hydrogen gas, or a mixed gas of them) to grow nanowires of
silicon or the silicon/germanium alloy in the temperature range of
900.degree. C. or higher and 1,300.degree. C. or lower. The
document discloses that the produced nanowires each have a diameter
of 50 nm to 100 nm and a length of several millimeters.
[0034] Patent Document 33 proposes a production method involving:
evaporating sintered body of a silicon powder in a stream of an
inert gas; and forming a silicon nanowire on a substrate placed at
a position where a temperature gradient of 10.degree. C./cm or more
is formed in the range of 1,200.degree. C. to 900.degree. C. on a
downstream side of the stream of the inert gas.
[0035] Patent Document 34 proposes a method of producing silicon
nanowires by the heat decomposition of a polysilane gas (such as a
disilane gas) with a metal that forms a low-melting eutectic alloy
with silicon (gold, silver, iron, or nickel) as a catalyst under
reduced pressure. The formation of silicon nanowires each having a
diameter of about 50 nm and each having a length of up to 4 .mu.m
has been initiated.
[0036] However, any method of producing such nanoscale silicon as
described above involves problems in that a large amount of the
nanoscale silicon cannot be produced at a low cost, and the content
of silicon oxide inevitably increases.
[0037] Therefore, it has been desired to develop a negative
electrode material capable of providing an energy storage device
having high energy density close to the energy density of a lithium
secondary battery, showing high initial charge and discharge
efficiency, and capable of being repeatedly used a large number of
times; an electrode using the negative electrode material; and an
energy storage device adopting the electrode has been desired. It
has been desired to develop also a method by which a large amount
of the negative electrode material can be produced at a low cost.
[0038] Patent Document 1: U.S. Pat. No. 6,051,340 [0039] Patent
Document 2: U.S. Pat. No. 5,795,679 [0040] Patent Document 3: U.S.
Pat. No. 6,432,585 [0041] Patent Document 4: Japanese Patent
Application Laid-Open No. H11-283627 [0042] Patent Document 5: U.S.
Pat. No. 6,517,974 [0043] Patent Document 6: U.S. Pat. No.
6,569,568 [0044] Patent Document 7: Japanese Patent Application
Laid-Open No. 2000-311681 [0045] Patent Document 8: International
Publication WO2000/17949 [0046] Patent Document 9: Japanese Patent
Application Laid-Open No. S60-170163 [0047] Patent Document 10:
Japanese Patent Application Laid-Open No. H02-181365 [0048] Patent
Document 11: Japanese Patent Application Laid-Open No. H04-034870
[0049] Patent Document 12: Japanese Patent Application Laid-Open
No. H08-107048 [0050] Patent Document 13: Japanese Patent
Application Laid-Open No. 2000-340470 [0051] Patent Document 14:
Japanese Patent Application Laid-Open No. 2002-25867 [0052] Patent
Document 15: Japanese Patent Application Laid-Open No. 2004-079321
[0053] Patent Document 16: Japanese Patent Application Laid-Open
No. 2005-086113 [0054] Patent Document 17: Japanese Patent
Application Laid-Open No. 2000-268881 [0055] Patent Document 18:
Japanese Patent Application Laid-Open No. 2003-109873 [0056] Patent
Document 19: Japanese Patent Application Laid-Open No. 2003-224037
[0057] Patent Document 20: Japanese Patent Application Laid-Open
No. 2004-103669 [0058] Patent Document 21: Japanese Patent
Application Laid-Open No. 2004-178828 [0059] Patent Document 22:
Japanese Patent Application Laid-Open No. 2005-108595 [0060] Patent
Document 23: U.S. Pat. No. 6,252,762 [0061] Patent Document 24:
Japanese Patent Application Laid-Open No. 2007-42393 [0062] Patent
Document 25: Japanese Patent Application Laid-Open No. 2007-59213
[0063] Patent Document 26: Japanese Patent Application Laid-Open
No. 2000-36323 [0064] Patent Document 27: Japanese Patent
Application Laid-Open No. 2004-335334 [0065] Patent Document 28:
Japanese Patent Application Laid-Open No. 2003-168426 [0066] Patent
Document 29: Japanese Patent Application Laid-Open No. 2007-115687
[0067] Patent Document 30: Japanese Patent Application Laid-Open
No. 2001-48699 [0068] Patent Document 31: Japanese Patent
Application Laid-Open No. 2003-246700 [0069] Patent Document 32:
Japanese Patent Application Laid-Open No. 2004-296750 [0070] Patent
Document 33: Japanese Patent Application Laid-Open No. 2005-112701
[0071] Patent Document 34: Japanese Patent Application Laid-Open
No. 2006-117475 [0072] Non-Patent Document 1: Journal of the
Electrochemical Society, 148 A930-A939 (2001) [0073] Non-Patent
Document 2: Journal of the Electrochemical Society, 153 A425-A430
(2006) [0074] Non-Patent Document 3: Journal of Power Sources, 170
456-459 (2007) [0075] Non-Patent Document 3: Nature Nanotechnology
3, 31-35 (2008) [0076] Non-Patent Document 4: Applied Physics
Letters 4, 89-90 (1998) [0077] Non-Patent Document 5: Applied
Physics Letters 72, 1835-1 837 (1998)
SUMMARY OF THE INVENTION
[0078] In view of the foregoing, an object of the present invention
is to provide a method of producing a material, in particular, a
silicon material or tin material capable of electrochemically
storing and releasing a large amount of lithium ions and having a
high ratio of the amount of lithium ions to be released to the
initial amount of lithium ions to be stored.
[0079] Another object of the present invention is to provide a
material prepared by the above production method and capable of
electrochemically storing or releasing a large amount of lithium
ions, an electrode structure formed of the material, and an energy
storage device provided with the electrode structure and having
such a characteristic that a reduction in capacity is small even
when charging and discharging are repeatedly performed, in
conjunction with high power density and a high energy density.
[0080] It should be noted that the term "energy storage device"
refers to a capacitor, a secondary battery, a device obtained by
combining a capacitor and a secondary battery, and a device
obtained by imparting a power-generating function to any one of
them.
[0081] Si and Sn capable of electrochemically absorbing and
releasing a large amount of Li has conventionally involved such
problems that they show a large volume expansion at the time of the
absorption of Li, and are turned into a fine powder by volume
expansion or shrinkage due to the absorption or release of Li to
increase the resistance of an electrode, and a reduction in
performance of a cell occurs owing to the repetition of charging
and discharging. The following attempts have been made to solve the
above problems: the particles of Si or Sn are reduced in size in
advance, or are made amorphous so that crystallites are
additionally reduced in size. The problem referred to as the
reduction in performance of the cell due to the repetition of
charging and discharging has been alleviated by those approaches,
but the extent to which the problem is alleviated is still
insufficient. The reason for the insufficient extent is that a
ratio of the amount of Li to be released to the amount of Li to be
inserted for the first charging and discharging is low. The low
ratio is due to the following fact: Si or Sn has an increased
specific surface area and is oxidized at the stage of reducing the
size of the particles of Si or Sn, so the amount of the oxide of Si
or Sn to be produced increases. For example, in the case of Si, the
production of SiO.sub.2 leads to the occurrence of an irreversible
reaction for the production of lithium oxide by an electrochemical
insertion reaction for Li.
SiO.sub.2+Li Li.sub.2O+Si
[0082] As a result of various experiments, the inventors of the
present invention have found the following: the oxide of Si or Sn
in particles formed of Si, Sn, or an alloy of any one of such
elements is allowed to react with a metal so that a metal oxide
stabler than the oxide of Si or Sn is produced, and the particles
formed of Si, Sn, or an alloy of any one of such elements are
coated with the metal oxide stabler than the oxide of Si or Sn,
whereby the content of the oxide of Si or Sn can be reduced, and
hence the oxidation of the particles formed of Si, Sn, or an alloy
of any one of such elements can be suppressed, and the coulombic
efficiency of a cell for the first charging and discharging can be
improved. In addition, the inventors have found that an electrode
produced by using an active material having a network structure
made up from Si particles with fibrous (filamentous) substances
each having a diameter of a nanometer to submicron size shows an
additional improvement in durability.
[0083] A method of producing a lithium ion-storing/releasing
material according to the first invention for solving the above
problems is a method of producing an electrode material for a
negative electrode of an energy storage device capable of
electrochemically storing and releasing lithium ions, the method
being characterized by including any one of the following steps:
(i) reacting silicon or tin with a metal oxide; (ii) reacting a
silicon oxide or a tin oxide with a metal; and (iii) reacting a
silicon compound or a tin compound with a metal compound.
[0084] It should be noted that Gibbs free energy at the time of
producing a metal oxide by oxidation of a metal element included in
the metal oxide or the metal is smaller than Gibbs free energy at
the time of oxidizing silicon or tin, and the metal oxide has
higher thermodynamic stability than silicon oxide or tin oxide.
[0085] In addition, the reaction of silicon or tin with a metal
oxide in the above (i) is one of (A) a reaction performed by
introducing at least silicon or tin and the metal oxide in a powder
state into thermal plasma obtained by turning an inert gas or a
hydrogen gas into plasma, and (B) a sintering reaction in spark
plasma instantaneously generated by a spark discharge phenomenon
caused by subjecting at least silicon or tin and the metal oxide to
mechanical alloying treatment, pressing a powder obtained by the
mechanical alloying treatment, and applying a pulsed current to
gaps between the pressed powder particles at a low voltage under
reduced pressure.
[0086] The reaction of silicon oxide or tin oxide with a metal in
the above (ii) is at least one of (C) a reaction performed by
introducing the metal, and the silicon oxide or the tin oxide, or
silicon containing the silicon oxide or tin containing the tin
oxide in a powder state into thermal plasma, (D) a sintering
reaction in spark plasma instantaneously generated by a spark
discharge phenomenon caused by subjecting the metal, and the
silicon oxide or the tin oxide, or silicon containing the silicon
oxide or tin containing the tin oxide in a powder state to
mechanical alloying treatment, pressing a powder obtained by the
mechanical alloying treatment (including mechanically milling
treatment), and applying a pulsed large current to gaps between the
pressed powder particles at a low voltage under reduced pressure,
(E) a heating reaction in an inert gas or a hydrogen gas or under
reduced pressure for a powder obtained by subjecting the metal, and
the silicon oxide or the tin oxide, or silicon containing the
silicon oxide or tin containing the tin oxide in a powder state to
mechanical alloying treatment, and (F) a heating reaction for a
composite layer formed on a substrate by vapor deposition of the
metal, and the silicon oxide or the tin oxide, or silicon
containing the silicon oxide or tin containing the tin oxide. The
metal element of the metal oxide or of the metal is preferably one
or more types of metal elements selected from Li, Be, Mg, Al, Ca,
Zr, Ba, Th, La, Ce, Nd, Sm, Eu, Dy, and Er.
[0087] In the reaction of a silicon compound or a tin compound with
a metal compound in the above (iii), the silicon compound is a
compound selected from silane, disilane, dichlorosilane,
trichlorosilane, tetrachlorosilane, tetramethoxysilane,
tetraethoxysilane and tetrabutoxysilane, the tin compound is a
compound selected from tin tetrachloride, tetraethoxytin,
tetrapropoxytin and tetrabutoxytin, the metal compound is a
compound selected from trichloroaluminum, trimethoxyaluminum,
triethoxyaluminum, tripropoxyaluminum, tributoxyaluminum, and
aluminum isoperoxide, and the compounds are heated in an inert gas
atmosphere or hydrogen gas atmosphere so that a reaction
temperature reaches 400 to 1,300.degree. C.
[0088] The heating in the above (E) or (F) or in the above (iii)
may be performed by irradiation with laser light or infrared light.
The atmosphere in the above (iii) is more preferably an atmosphere
under reduced pressure.
[0089] An electrode material for a negative electrode of a lithium
ion-storing/releasing energy storage device according to the second
invention for solving the above problems is a material capable of
electrochemically storing and releasing lithium ions, including
silicon or tin primary particles composed of crystal particles each
having a diameter of 5 nm to 200 nm and an amorphous surface layer
having a thickness of 1 nm to 10 nm, wherein the amorphous surface
layer of each of the primary particles is formed of at least a
metal oxide; Gibbs free energy when the metal oxide is produced by
oxidation of a metal is smaller than Gibbs free energy when silicon
or tin is oxidized; and the metal oxide has higher thermodynamic
stability than silicon oxide or tin oxide.
[0090] Further, it is preferred that the primary particles of the
above electrode material are silicon particles, the crystal
particles of silicon are formed into a network structure with
fibrous (filamentous) substances, and the surface of each of the
fibrous (filamentous) materials is coated with at least an oxide.
In addition, the fibrous (filamentous) substances making up the
networking structure each preferably have a diameter in a range of
5 nm to 70 nm and a length in a range of 100 nm to 2 .mu.m. It is
also preferred that the fibrous (filamentous) substances each have
a core-shell structure, the core portion includes a silicon
crystal, and the shell portion includes an amorphous silicon oxide
or an amorphous metal oxide. It should be noted that Gibbs free
energy when the metal oxide is produced by oxidation of a metal is
smaller than Gibbs free energy when silicon or tin is oxidized, and
the metal oxide has higher thermodynamic stability than silicon
oxide or tin oxide.
[0091] An electrode structure according to the third invention for
solving the above problems includes a current collector and an
electrode material layer formed of an active material including a
material capable of storing and releasing lithium ions by an
electrochemical reaction, wherein the active material is the above
lithium ion-storing/releasing material.
[0092] An energy storage device according to the fourth invention
for solving the above problems includes a negative electrode using
the above electrode structure, a lithium ion conductor, and a
positive electrode including a positive electrode active material
layer and a current collector, wherein the energy storage device
utilizes an oxidation reaction of lithium and a reduction reaction
of lithium ions.
[0093] According to the method of producing a lithium
ion-storing/releasing material of the present invention, a material
capable of electrochemically storing and releasing a large amount
of lithium ions can be produced without using any complicated
production step.
[0094] In addition, the energy storage device of the present
invention utilizing the electrochemical oxidation-reduction
reaction of lithium ions using the electrode structure of the
present invention can provide high initial charge and discharge
efficiency, high power density and high energy density, and the
long cycle life of the device for charging and discharging can be
ensured. In addition, the energy storage device thus obtained can
be charged quickly.
[0095] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0096] FIGS. 1A and 1B are each a schematic sectional view of an
example of a powder material of the present invention.
[0097] FIGS. 2C and 2D are each a schematic sectional view of an
example of the powder material of the present invention.
[0098] FIGS. 3A and 3B are each a schematic sectional view of an
example of an electrode structure of the present invention.
[0099] FIG. 4 is a conceptual sectional view of an energy storage
device of the present invention.
[0100] FIG. 5 is a schematic cell sectional view of a monolayer,
flat (coin) type energy storage device.
[0101] FIG. 6 is a schematic cell sectional view of a spiral,
cylindrical energy storage device.
[0102] FIG. 7 is an image as a result of observation with an
electron microscope in Example TP1.
[0103] FIG. 8 is an image as a result of observation with an
electron microscope in Example TP2.
[0104] FIG. 9 is an image as a result of observation with an
electron microscope in Example TP3.
[0105] FIG. 10 is an image as a result of observation with an
electron microscope in Example TP4.
[0106] FIG. 11 is an image as a result of observation with an
electron microscope in Example TP5.
[0107] FIGS. 12A and 12B are each an image as a result of
observation with an electron microscope in Example TP6.
[0108] FIG. 13 is an image as a result of observation with an
electron microscope in Example TP7.
[0109] FIG. 14 is an image as a result of observation with an
electron microscope in Example MAT1.
[0110] FIG. 15 is an image as a result of the observation of a bead
mill-pulverized silicon powder used as an active material for the
electrode of Comparative Example EA1 with a scanning electron
microscope (SEM).
[0111] FIGS. 16A, 16B, 16C, 16D, 16E, 16F, 16G, 16H and 16I are
various X-ray diffraction charts of Examples TP1 to TP7,
Comparative Example TP1, and Example TP8.
[0112] FIG. 17 shows Si2p spectra according to XPS.
[0113] FIG. 18 shows a discharge curve of a cell of Example EA6 and
a discharge curve of a cell obtained by combining a graphite
electrode and a positive electrode.
[0114] FIG. 19A shows a Ragone plot of a cell obtained by combining
the electrode of Example EA6 and a positive electrode per weight
and FIG. 19B shows a Ragone plot of the cell per volume.
DESCRIPTION OF REFERENCE NUMERALS
[0115] 100 silicon or tin crystal particle [0116] 101 coating layer
formed of an amorphous oxide of a metal [0117] 102 surface layer
formed of silicon and an amorphous oxide of a metal or formed of
tin and an amorphous oxide of a metal [0118] 103 surface layer
formed of an amorphous oxide of silicon or tin [0119] 200 silicon
or tin crystal particle [0120] 201 amorphous oxide layer as a
surface layer [0121] 202, 205 fibrous (filamentous) substance
[0122] 203 primary particle in the present invention [0123] 204
secondary particle as an aggregate of primary particles [0124] 300
current collector [0125] 301 silicon or tin crystal particle [0126]
302 metal oxide [0127] 303 material powder particle containing
silicon or tin crystal particles [0128] 304 conductive auxiliary
material [0129] 305 binder [0130] 306 electrode material layer
(active material layer) [0131] 307 electrode structure [0132] 401,
501, 603 negative electrode [0133] 402, 503, 606 positive electrode
[0134] 403, 502, 607 ion conductor [0135] 404 negative electrode
terminal [0136] 405 positive electrode terminal [0137] 406 battery
case (cell housing) [0138] 504 negative electrode cap [0139] 505
positive electrode can [0140] 506, 610 gasket [0141] 601 negative
electrode current collector [0142] 602 negative electrode active
material layer [0143] 604 positive electrode current collector
[0144] 605 positive electrode active material layer [0145] 608
negative electrode can (negative electrode terminal) [0146] 611
insulating plate [0147] 612 negative electrode lead [0148] 613
positive electrode lead [0149] 614 safety valve
DESCRIPTION OF THE EMBODIMENTS
[0150] Hereinafter, the present invention will be described in
detail.
[0151] A method of producing a lithium ion-storing/releasing
material according to the present invention is a method of
producing an electrode material for the negative electrode of an
energy storage device capable of electrochemically storing and
releasing lithium ions, the method being characterized by including
any one of the following steps: (i) causing silicon or tin and a
metal oxide to react with each other; (ii) causing a silicon oxide
or a tin oxide and a metal to react with each other; and (iii)
causing a silicon compound or a tin compound and a metal compound
to react with each other. It should be noted that the silicon oxide
or the tin oxide in the above (ii) may be silicon oxide as an
impurity in silicon or a surface natural oxide film on silicon, or
tin oxide as an impurity in tin or a surface natural oxide film on
tin.
[0152] Further, the reaction between silicon or tin and the metal
oxide in the above (i) is one of (A) a reaction performed by
introducing at least silicon or tin and the metal oxide each in a
powder state into thermal plasma obtained by turning an inert gas
or a hydrogen gas into plasma, and (B) a sintering reaction in
spark plasma instantaneously generated by a spark discharge
phenomenon caused by subjecting at least silicon or tin and the
metal oxide to mechanical alloying treatment, pressing a powder
obtained by the mechanical alloying treatment, and applying a
pulsed current into gaps between pressed powder particles at a low
voltage under reduced pressure.
[0153] The reaction between the silicon oxide or the tin oxide and
the metal in the above (ii) is at least one of (C) a reaction
performed by introducing the metal, and the silicon oxide or the
tin oxide, or silicon containing the silicon oxide or tin
containing the tin oxide each in a powder state into thermal
plasma, (D) a sintering reaction with spark plasma instantaneously
generated by a spark discharge phenomenon caused by subjecting the
metal, and the silicon oxide or the tin oxide, or silicon
containing the silicon oxide or tin containing the tin oxide each
in a powder state to mechanical alloying treatment, pressing a
powder obtained by the mechanical alloying treatment, and applying
a pulsed current into gaps between pressed powder particles at a
low voltage under reduced pressure, (E) a heating reaction in an
inert gas or a hydrogen gas or under reduced pressure for a powder
obtained by subjecting the metal, and the silicon oxide or the tin
oxide, or silicon containing the silicon oxide or tin containing
the tin oxide each in a powder state to mechanical alloying
treatment, and (F) a heating reaction for a composite layer formed
on a substrate by vapor deposition of the metal, and the silicon
oxide or the tin oxide, or silicon containing the silicon oxide or
tin containing the tin oxide.
[0154] The metal element of the metal oxide or of the metal is
preferably at least one type of metal element selected from Li, Be,
Mg, Al, Ca, Zr, Ba, Th, La, Ce, Nd, Sm, Eu, Dy, and Er. Further,
the metal element is more preferably Li, Mg, Al, Ca, Zr, La, or Nd,
or is most preferably Al or Zr because any such metal element is
available at a low cost, and can be easily handled.
[0155] In the reaction between the silicon compound or the tin
compound and the metal compound in the above (iii), the silicon
compound is a compound selected from silane, disilane,
dichlorosilane, trichlorosilane, tetrachlorosilane,
tetramethoxysilane, tetraethoxysilane, and tetrabutoxysilane, the
tin compound is a compound selected from tin tetrachloride,
tetraethoxytin, tetrapropoxytin, and tetrabutoxytin, the metal
compound is a compound selected from trichloroaluminum,
trimethoxyaluminum, triethoxyaluminum, tripropoxyaluminum,
tributoxyaluminum, and aluminum isoperoxide, and the compounds are
heated in an inert gas atmosphere or hydrogen gas atmosphere so
that a reaction temperature reaches 400 to 1,300.degree. C.
[0156] The heating in the above (E) or (F) or in the above (iii)
may be performed by irradiation with laser light or infrared light.
The atmosphere in the above (iii) is more preferably an atmosphere
under reduced pressure. The above laser light has a wavelength of
preferably 532 nm or less because the above materials to be
deposited can easily absorb the energy of the light. At least one
type of gas selected from an argon gas, a helium gas and a nitrogen
gas can be used as the above inert gas.
[0157] The thermal plasma in the above (A) and (C) is generated by
an approach selected from arc discharge, high-frequency discharge,
microwave discharge, and laser light irradiation.
[0158] A method for the vapor deposition in the above (F) is
preferably at least one type of method selected from the group
consisting of electron-beam deposition, laser ablation, sputtering,
cluster ion beam deposition, chemical vapor deposition (CVD),
plasma CVD, and resistance heating deposition. In the above (F),
the materials are preferably deposited onto a conductor layer.
[0159] The temperature at which the heating in the above (E) or (F)
is performed is preferably equal to or higher than the melting
temperature of silicon, tin, or the metal. The heated product is
more preferably cooled quickly, and a rate for the above cooling is
preferably 10.sup.3 K/sec or more, or more preferably 10.sup.4
K/sec or more.
[0160] When silicon or tin and the metal oxide are caused to react
with each other in the above (i), a ratio of the metal oxide as a
raw material to silicon or tin as another raw material falls within
the range of preferably 1 to 30 wt %, or more preferably 3 to 15 wt
%.
[0161] When the silicon oxide or the tin oxide and the metal are
caused to react with each other in the above (ii), a ratio of the
metal as a raw material to the silicon oxide or the tin oxide as
another raw material is preferably equal to or higher than a ratio
in which the total amount of oxygen constituting the silicon oxide
or the tin oxide and the metal react with each other to form a
metal oxide.
[0162] When the silicon compound or the tin compound and the metal
compound are caused to react with each other in the above (iii), a
ratio of the metal compound as a raw material to the silicon
compound or the tin compound as another raw material falls within
the range of preferably 1 to 30 atomic %, or more preferably 3 to
15 atomic %.
[0163] The ratio between the raw materials is determined from a
range in which, while the amount of lithium ions to be
electrochemically stored and released is prevented from
significantly decreasing, the silicon or tin particles are each
coated with the stable metal oxide so that the oxidation of silicon
or tin can be suppressed.
[0164] In the above (A) and (C), a slow oxidation step is
preferably performed before a reaction product is exposed to the
air. To be specific, the above slow oxidation is performed by
exposing the above reaction product to an inert gas atmosphere
containing a trace amount of oxygen.
[0165] In the above (B) and (D), a fine powder is preferably
obtained by pulverization after the sintering.
[0166] Another method of producing a lithium ion-storing/releasing
material including the step of causing a silicon oxide or a tin
oxide and a metal to react with each other in the above (ii) is,
for example, a method involving: mixing an oxide-containing silicon
and a metal or an oxide-containing tin and a metal; melting the
mixture to form a molten metal; quenching the molten metal by an
atomization method, a gun method, a single-roll method, or a
twin-roll method to provide a powder material of a lithium
ion-storing/releasing material in a powder or ribbon state; finely
pulverizing the material; and slowly oxidizing the finely
pulverized products. As the above raw material metal, a metal is
used in which Gibbs free energy at the time of producing a metal
oxide by oxidation is smaller than Gibbs free energy at the time of
oxidizing silicon or tin and the metal oxide has higher
thermodynamic stability than silicon oxide or tin oxide.
[0167] Thermal Plasma Method:
[0168] The thermal plasma method is a method in which raw materials
are introduced into generated thermal plasma to allow the materials
to react with each other in the high-temperature plasma.
[0169] The thermal plasma can be generated by, for example, any one
of the following methods: (1) a gas is inductively subjected to
heating by electric discharge by utilizing a radiofrequency
electromagnetic field; (2) a gas is irradiated with a microwave so
as to be subjected to heating by electric discharge; and (3) arc
discharge is performed between electrodes. The discharge by
radiofrequency (RF) in the above method (1) is non-polar discharge,
and has such an advantage that an electrode substance is not
included as an impurity into plasma. An inductively coupled plasma
torch based on a radiofrequency is basically inductively coupled
discharge where a gas-introducing portion is provided at one end of
a water-cooled torch made of an insulating material such as a
quartz tube, and a gas in the torch is brought into a plasma state
with an induction coil outside the torch. A high-temperature region
of 10,000 K or higher is generated inside the induction coil. A raw
material introduced into the plasma is instantaneously brought into
an atom or ion state in the above high-temperature region. For
example, when the raw material is a metal oxide, a metal element
and an oxygen element each dissociate into an atom or ion state,
and react during cooling to solidify.
[0170] Spark Plasma Sintering
[0171] A spark plasma sintering (SPS) process is involved in the
effective application of high energy of spark plasma
instantaneously generated by a spark discharge phenomenon caused by
applying a large pulsed current into gaps between pressed powder
particles at a low voltage (high-temperature plasma: a
high-temperature field of several thousands to ten thousand degrees
centigrade is instantaneously generated between the particles) to,
for example, thermal diffusion or electric field diffusion.
Sintering is completed within a time period as short as about 5 to
20 minutes including a temperature-increase time and a retention
time in a temperature region about 200 to 500.degree. C. lower than
that of a conventional method in a range from a low temperature to
an ultra-high-temperature of 2,000.degree. C. or higher.
Vaporization and a melting phenomenon occur locally on the surface
of each particle, and a constricted portion called a neck is formed
at a portion where particles come in contact with each other so
that the particles are brought into a welded state. The neck formed
between adjacent particles gradually develops, and expands a
diffused portion while undergoing plastic deformation, whereby a
high-density sintered body having a density of 99% or more can be
finally formed. Since the temperature of each of the particles can
be quickly increased by the self-heating of only the surface of
each particle, the grain growth of a starting material can be
suppressed, and a dense sintered body can be obtained within a
short time period. Of course, a porous body can be produced. In
addition, a powder having an amorphous structure or nanocrystal
texture can be made into a bulk without being processed because a
texture in pressed powder can be prevented from changing.
[0172] A lithium ion-storing/releasing material obtained by the
production method is preferably as follows: the material is formed
of at least silicon or tin primary particles formed of crystal
particles having a diameter of 5 nm to 200 nm and an amorphous
surface layer having a thickness of 1 nm to 10 nm, and the
amorphous surface layer of each of the primary particles is formed
of at least a metal oxide. It should be noted that Gibbs free
energy when the metal oxide is produced by the oxidation of a metal
is smaller than Gibbs free energy when silicon or tin is oxidized,
and the metal oxide has higher thermodynamic stability than silicon
oxide or tin oxide.
[0173] A composite powder of silicon or tin and the metal oxide may
be formed of fine particles each having a core-shell structure in
which the periphery of a silicon or tin crystal particle having a
diameter of 5 nm to 200 nm is coated with the amorphous metal oxide
of the amorphous surface layer having a thickness of 1 nm to 10
nm.
[0174] A metal element accounts for preferably 0.3 atomic % or
more, or more preferably 0.3 atomic % or more and 30 atomic % or
less of the metal oxide of the primary particles.
[0175] The lithium ion-storing/releasing material is preferably
composed of silicon particles.
[0176] A crystallite size calculated from a half width of a Si(111)
peak in an X-ray diffraction chart of the silicon particles and
Sherrer's equation falls within a range of 20 to 60 nm.
[0177] A molar ratio of silicon oxide to Si calculated from an
X-ray photoelectron spectroscopy (XPS) measurement spectrum is
preferably 0.05 to 7.0. In addition, a ratio of an oxygen element
to an Si element measured with an energy dispersive X-ray
spectrometer (EDX) of a scanning transmission electron microscope
(STEM) is preferably 0.05 to 0.8.
[0178] It is preferred that the crystal particles of the silicon
primary particles are formed into a network structure with fibrous
(filamentous) substances, and the surface of each of the fibrous
(filamentous) materials is coated with at least an oxide. The
fibrous (filamentous) substances constituting the network structure
preferably have a diameter in a range of 5 nm to 70 nm and a length
in a range of 100 nm to 2 .mu.m. It is preferred that the fibrous
(filamentous) substances each have a core-shell structure, the core
portion is composed of a silicon crystal, and the shell portion is
composed of an amorphous silicon oxide or an amorphous metal oxide.
In addition, Gibbs free energy when the metal oxide is produced by
oxidation of a metal is smaller than Gibbs free energy when silicon
or tin is oxidized, and the metal oxide has higher thermodynamic
stability than silicon oxide or tin oxide.
[0179] The fibrous (filamentous) substances may each be amorphous
silicon oxide or aluminum oxynitride.
[0180] The metal element of the metal oxide is preferably at least
one type of metal selected from Li, Be, Mg, Al, Ca, Zr, Ba, Th, La,
Ce, Nd, Sm, Eu, Dy, and Er, or more preferably at least one type of
metal selected from Li, Mg, Al, Ca, Zr, Ba, La, Ce, and Nd. Of the
above metal elements, Al or Zr is the most preferable element in
consideration of the fact that any such element is available at a
low cost, and can be easily handled.
[0181] The content of the oxide of a metal except silicon and tin
in the lithium ion-storing/releasing material falls within the
range of preferably 1 wt % or more to 50 wt % or less, or more
preferably 10 wt % or more to 35 wt % or less.
[0182] The content of silicon or tin in the lithium
ion-storing/releasing material falls within the range of preferably
30 wt % or more and 98 wt % or less, or more preferably 50 wt % or
more and 90 wt % or less.
[0183] In the lithium ion-storing/releasing material, silicon or
tin and a metal except tin preferably form a eutectic crystal.
[0184] According to the Ellingham diagrams of oxides in which the
abscissa indicates temperature T and the axis of ordinate indicates
change .DELTA.G.sup.0 in standard Gibbs energy of oxide formation
reaction for 1 mol of oxygen, for example, Cr.sub.2O.sub.3, MnO and
V.sub.2O.sub.3 each have higher thermodynamic stability than
SnO.sub.2 because a line for "Sn+O.sub.2.fwdarw.SnO.sub.2" is
positioned above (.DELTA.G.sup.0 is larger) each of a line for
"4/3Cr+O.sub.2.fwdarw.2/3Cr.sub.2O.sub.3", a line for
"2Mn+O.sub.2.fwdarw.2MnO", and a line for
"3/4V+O.sub.2.fwdarw.2/3V.sub.2O.sub.3". In addition, the line for
"Si+O.sub.2.fwdarw.SiO.sub.2" is positioned below the line for
"3/4V+O.sub.2.fwdarw.2/3V.sub.2O.sub.3" In addition, a line for
"Ti+O.sub.2.fwdarw.TiO.sub.2" is positioned below, but considerably
close to, the line for "Si+O.sub.2.fwdarw.SiO.sub.2". Since a line
for "4/3Al+O.sub.2.fwdarw.2/3Al.sub.2O.sub.3", a line for
"2Mg+O.sub.2.fwdarw.2MgO" and a line for "2Ca+O.sub.2.fwdarw.2CaO"
are each positioned distantly below (.DELTA.G.sup.0 is smaller) the
line for "Si+O.sub.2.fwdarw.SiO.sub.2", SiO.sub.2 is stabler than
SnO.sub.2, and furthermore, TiO.sub.2, Al.sub.2O.sub.3, MgO and CaO
are stabler than SiO.sub.2. Similarly, the Ellingham diagrams of
oxides show that Nb.sub.2O.sub.3 and Ta.sub.2O.sub.3 are each
stabler than SnO.sub.2, and B.sub.2O.sub.3, CeO.sub.2, BaO,
ZrO.sub.2, SrO, ThO.sub.2, BeO and La.sub.2O.sub.3 are also stabler
than SiO.sub.2. Exemplary metal oxides stabler than tin monoxide,
tin dioxide, or silicon dioxide are shown below using inequality
signs.
[0185] SnO<MoO.sub.2, WO.sub.2, WO.sub.3
[0186] SnO.sub.2<MoO.sub.2, WO.sub.2, V.sub.2O.sub.3
[0187] SiO.sub.2<Al.sub.2O.sub.3, ZrO.sub.2, Li.sub.2O, MgO,
CaO, BaO, CeO.sub.2, La.sub.2O.sub.3 Nd.sub.2O.sub.3,
Sm.sub.2O.sub.3, ThO.sub.2, Tm.sub.2O.sub.3, Yb.sub.2O.sub.3,
Dy.sub.2O.sub.3, Er.sub.2O.sub.3, Eu.sub.2O.sub.3,
Eu.sub.3O.sub.4
[0188] SnO<SnO.sub.2<SiO.sub.2
[0189] In this connection, known examples of numerical values for
the standard Gibbs energies of formation .DELTA.G.sup.0.sub.f's per
mole of representative metal oxides at a temperature of 298.15 K
are as follows: -251.9 kJ/mol for SnO, -515.8 kJ/mol for SnO.sub.2,
-1,582.3 kJ/mol for Al.sub.2O.sub.3, -888.8 kJ/mol for TiO.sub.2,
-569.3 kJ/mol for MgO, -561.2 kJ/mol for Li.sub.2O, -1,042.8 kJ/mol
for ZrO.sub.2, -1,816.6 kJ/mol for Y.sub.2O.sub.3, -1,789.0 kJ/mol
for La.sub.2O.sub.3, and -1,720.8 kJ/mol for Nd.sub.2O.sub.3 (as
the absolute values of the negative values of the above standard
Gibbs energy of formation are larger, the metal oxides become
stabler).
[0190] For example, when the metal is titanium, zirconium or
aluminum, a reaction with silicon oxide or tin oxide proceeds as
shown below in consideration of the Ellingham diagrams.
SiO.sub.2+Zr.fwdarw.Si+ZrO.sub.2
3SiO.sub.2+4Al.fwdarw.3Si+2Al.sub.2O.sub.3
3SiO+2Al.fwdarw.3Si+Al.sub.2O.sub.3
SnO.sub.2+Ti.fwdarw.Sn+TiO.sub.2
3SnO.sub.2+4Al.fwdarw.3Sn+2Al.sub.2O.sub.3
2SnO+Ti.fwdarw.2Sn+TiO.sub.2
3SnO+2Al.fwdarw.3Sn+Al.sub.2O.sub.3
[0191] Since the weight ratio of silicon or tin in a metal oxide in
which silicon or tin crystals obtained only by the above reduction
reaction of silicon oxide or tin oxide are dispersed, is not large,
the amount of lithium ions to be electrochemically stored and
released cannot be increased so much when the metal oxide is used
as an active material for an electrode of an energy storage device.
In order to increase the amount of lithium ions to be
electrochemically stored and released, it is necessary to increase
the above ratio of silicon or tin. To this end, in the above step,
it is preferable that the above reduction reaction of silicon oxide
or tin oxide is brought about by adding extra silicon or tin in
addition to silicon oxide or tin oxide and the metal, and it is
more preferable that silicon oxide included in silicon or tin oxide
included in tin are allowed to react with the metal. In this case,
it is also desirable that a metal species that forms a eutectic
crystal with silicon or tin is selected as the metal, and the
amount of the metal remaining without being oxidized by the above
reduction reaction of silicon oxide or tin oxide is such a
composition amount that the metal forms a eutectic crystal with
silicon or tin.
[0192] In addition, silicon and a metal, or tin and a metal are
mixed, the mixture is melted to form a molten metal, and the molten
metal is quenched by an atomization method, a gun method, a
single-roll method, or a twin-roll method, whereby a powder
material of a lithium ion-storing/releasing material in a powder or
ribbon state is obtained. Next, the powder material obtained by the
above method can be additionally finely pulverized with a
pulverizing apparatus. For example, a ball mill such as a planetary
ball mill, a vibrating ball mill, a conical mill or a tube mill, a
media mill of, for example, an attritor type, sand grinder type,
annealer mill type or tower mill type, or an apparatus for
pulverizing slurry in which a raw material is dispersed by causing
the slurry to collide at a high pressure can be used as the above
pulverizing apparatus. The metal in the finely pulverized particles
formed of silicon and the metal or of tin and the metal is
preferentially oxidized, whereby silicon or tin particles coated
with the metal oxide are obtained.
[0193] The material capable of storing and releasing lithium ions
may be turned into a composite with carbon, and a weight ratio of
the carbon element to be used in the composite to the material is
preferably 0.05 or more and 1.0 or less.
[0194] An electrode structure according to the present invention
includes a current collector and an electrode material layer formed
of an active material as a material capable of storing and
releasing lithium ions by an electrochemical reaction, wherein the
active material is the above lithium ion-storing/releasing
material.
[0195] To be specific, the electrode structure is constituted of a
current collector and an electrode material layer formed of an
active material as a material capable of storing and releasing
lithium ions by an electrochemical reaction, and is characterized
in that the material capable of storing and releasing lithium ions
includes a composite material including silicon or tin primary
particles composed of crystal particles each having a diameter of 5
nm to 200 nm and an amorphous surface layer having a thickness of 1
nm to 10 nm, and the amorphous surface layer of each of the primary
particles is formed of at least a metal oxide. In addition, Gibbs
free energy when the metal of the metal oxide is oxidized is
smaller than Gibbs free energy when silicon or tin is oxidized.
Further, the metal may form a eutectic crystal with silicon or tin
as well as the metal oxide.
[0196] In particular, the electrode structure of the present
invention is characterized in that the material capable of storing
and releasing lithium ions is formed of silicon having a
crystallite size of 20 to 60 nm.
[0197] In addition, the electrode structure is characterized in
that the crystal particles of the silicon primary particles are
formed into a network structure with fibrous (filamentous)
substances, and the surface of each of the fibrous (filamentous)
substances is coated with at least an oxide.
[0198] The electrode material layer is preferably formed of the
active material and a binder. The active material is formed of the
above material capable of storing and releasing lithium ions.
[0199] The electrode material layer is preferably formed of the
active material, a conductive auxiliary material, and a binder.
[0200] The density of the electrode material layer is preferably in
a range of 0.5 g/cm.sup.3 or more and 3.5 g/cm.sup.3 or less.
[0201] The content of silicon or tin in the lithium
ion-storing/releasing material constituting the electrode structure
preferably falls within the range of 30 wt % or more to 98 wt % or
less of the material.
[0202] The content of the oxide of a metal except silicon and tin
(metal oxide) in the lithium ion-storing/releasing material falls
within the range of preferably 1 wt % or more and 50 wt % or less,
or more preferably 10 wt % or more and 35 wt % or less.
[0203] The content of silicon or tin in the lithium
ion-storing/releasing material preferably falls within the range of
50 wt % or more and 90 wt % or less.
[0204] An energy storage device according to the present invention
includes a negative electrode using the above electrode structure,
a lithium ion conductor, and a positive electrode including a
positive electrode substance layer and a current collector, wherein
the energy storage device utilizes an oxidation reaction of lithium
and a reduction reaction of lithium ions.
[0205] The content of silicon or tin in the lithium
ion-storing/releasing material constituting the negative electrode
preferably falls within the range of 30 wt % or more and 98 wt % or
less of the material.
[0206] A metal oxide content in the lithium ion-storing/releasing
material constituting the negative electrode falls within the range
of preferably 1 wt % or more and 50 wt % or less, or more
preferably 10 wt % or more and 35 wt % or less.
[0207] The positive electrode is preferably formed of at least a
powder material which includes particles formed of a transition
metal compound selected from a transition metal oxide, a transition
metal phosphate compound, a lithium-transition metal oxide and a
lithium-transition metal phosphate compound, and is turned into a
composite with at least one of particles each having an amorphous
phase and an oxide containing a metal oxide semimetal.
[0208] The positive electrode active material is composed of a
transition metal compound selected from a transition metal oxide, a
transition metal phosphate compound, a lithium-transition metal
oxide and a lithium-transition metal phosphate compound, or a
carbon material. Further, the above positive electrode active
material is more preferably turned into a composite with an oxide
or composite oxide having an amorphous phase and primarily composed
of an element selected from Mo, W, Nb, Ta, V, B, Ti, Ce, Al, Ba,
Zr, Sr, Th, Mg, Be, La, Ca and Y. Further, it is preferred that the
content of the oxide or composite oxide to be used in the composite
is 1 wt % or more and 20 wt % or less of the above positive
electrode active material turned into the composite, and the
contribution ratio of the oxide or composite oxide to the charge
and discharge electrical quantity is 20% or less.
[0209] The positive electrode active material is preferably turned
into a composite also with a carbon material having a specific
surface area in the range of 10 to 3,000 m.sup.2/g.
[0210] The carbon material is preferably selected from active
carbon, mesoporous carbon, carbon fiber and carbon nanotube.
[0211] The positive electrode active material turned into the
composite preferably has a crystallite size of 100 nm or less.
[0212] A method of producing the positive electrode active material
turned into the composite is, for example, a method in which a
metal oxide material to be turned into a composite with the
selected active material is mixed and the mixture is milled using a
mill such as a vibrating mill or an attritor to produce a composite
(mechanical alloying).
[0213] The inventors of the present invention have reached the
present invention by detailed investigation into an alloy-based
negative electrode for a lithium secondary battery. In the case of
a lithium secondary battery using as a negative electrode an
electrode obtained by forming, on a current collector made of metal
foil, an electrode material layer (active material layer) formed of
a powder made of a metal or alloy of silicon or tin and a binder,
the internal resistance of the battery increases owing to the
repetition of charging and discharging, with the result that a
reduction in performance of the battery occurs. In particular, when
a charging current density is increased, the influence of the
increase is large, so the reduction in performance is large. The
inventors of the present invention have assumed, by the observation
and analysis of the active material of the above negative
electrode, that the following is responsible for the above
reduction in performance. At the time of charging, electric field
intensity on the particles each formed of the metal or alloy of
silicon or tin is non-uniform, and the precipitation of lithium is
more liable to occur at a portion where electric field intensity is
large than at a portion where an electric field intensity is small,
and the diffusion of lithium occurs non-uniformly in an alloy
lattice, so non-uniform expansion and collapse of the alloy
particles seem to occur. Accordingly, the precipitation of a
lithium metal in direct contact with an electrolyte solution must
be suppressed by uniformizing the electric field intensity on the
alloy particles. The addition of, for example, a vinyl monomer
capable of forming a coating film capable of functioning as a solid
electrolyte interface (SEI) or passivating film to the electrolyte
solution upon charging and discharging is effective for the
suppression to some extent. However, the rate at which lithium
diffuses in the alloy is not high, so the non-uniform expansion and
collapse of the alloy particles become remarkable at high charging
current density, and the addition serves also as a cause for the
occurrence of an irreversible side reaction. As a result, a
reduction in discharge capacity and a reduction in the cycle life
of the repetition of charging and discharging are accelerated.
[0214] In consideration of the above assumed cause, the inventors
of the present invention have found a method of suppressing the
non-uniform precipitation or diffusion of lithium at the time of
charging to metal particles each formed of silicon or tin or alloy
particles each formed of any such element, with the metal particles
or the alloy particles being capable of electrochemically storing
lithium.
[0215] The inventors of the present invention had as the above
method the idea that the diffusion of lithium ions at the time of
charging and discharging can be rendered more uniform and faster by
making as fine as possible the primary particles of the metal
particles formed of silicon or tin, or the alloy particles formed
of any such element. However, when the primary particles of the
metal particles formed of silicon or tin, or the alloy particles
formed of any such element is made as fine as possible, a problem
is raised in that the particles have an increased specific surface
area, and become easily oxidized, with the result that the
production of a large amount of silicon oxide or tin oxide becomes
inevitable. Silicon oxide or tin oxide thus produced reacts with
lithium to form irreversible lithium oxide, so the initial charge
and discharge coulombic efficiency of a lithium secondary battery
formed of a negative electrode using fine silicon or tin metal
particles containing a large amount of silicon oxide or tin oxide
described above as an active material is reduced, and a subsequent
electrical quantity which can be charged into or discharged from
the battery is reduced.
[0216] To solve the above problem, the inventors of the present
invention had the idea that the fine silicon or tin metal particles
are coated with a metal oxide having higher thermodynamic stability
than silicon oxide or tin oxide in order that the oxidation of the
particles may be suppressed.
[0217] A material capable of maintaining the performance of silicon
or tin metal particles which store a large amount of lithium ions
by a charging reaction and realizing such performance that the
diffusion of lithium ions at the time of charging and discharging
is more uniform and faster is obtained by coating fine silicon or
tin crystal particles as an active material with a
thermodynamically stable metal oxide, and is produced by the
above-mentioned production method in the present invention such as
the oxidation-reduction reaction between silicon oxide or tin oxide
and a metal or a reaction between silicon or tin and the stable
metal oxide.
[0218] The silicon or tin metal particles are separated into finer
regions by the metal oxide. Even when primary particles aggregate
to form secondary particles, the crystal particles remain isolated
by the metal oxide.
[0219] In addition, when an energy storage device is produced by
using the electrode of the present invention formed of a powder
material obtained by reducing the sizes of silicon or tin metal
particles, and lithium is electrochemically stored in the electrode
by a charging reaction, lithium can easily diffuse uniformly to the
above fine regions, and the specific surface area increases, so
substantial current density is reduced, and the device can be
charged more quickly. In addition, reserved lithium can be released
uniformly at a higher rate by a discharging reaction. In addition,
the content of silicon oxide or tin oxide can be reduced, so the
initial charge and discharge coulombic efficiency is improved. As a
result, a large-capacity energy storage device such as a secondary
battery can be provided.
[0220] In addition, the above metal oxide (including the composite
oxide) should be stable at temperature at the time of forming the
composite, and the silicon or tin metal particles storing lithium
should not be oxidized by depriving oxides of metals other than
silicon and tin of oxygen. The thermodynamically stable metal oxide
(oxide containing a semimetal) can be selected from the Ellingham
diagrams of oxides representing the stability of oxides as a
function of temperature. First, it is desirable that the oxides are
thermodynamically more stable than tin oxide, that is, the oxides
have larger absolute values of negative values of Gibbs free energy
in the oxidation reaction of the metals than Gibbs free energy in
the oxidation reaction of tin. To be specific, oxides or composite
oxides mainly composed of metal elements selected from Li, Be, B,
Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Zn, Ga, Y, Zr, Nb, Mo, Ba, Hf, Ta,
W, Th, La, Ce, Nd, Sm, Eu, Dy and Er are preferable as the above
oxides. Further, it is desirable that the oxides are
thermodynamically more stable than silicon oxide, that is, the
oxides have larger absolute values of negative values of Gibbs free
energy in the oxidation reaction of the metals than Gibbs free
energy in the oxidation reaction of silicon. As the above oxides of
the specific metals, oxides or composite oxides mainly composed of
metal elements selected from Li, Mg, Al, Zr, Th, La, Ce, Nd, Sm,
Eu, Dy, and Er are more preferable.
[0221] In general, the surfaces of particles formed of a silicon or
tin metal or of an alloy of one of these metals to be handled in
the air are coated with an oxide film. In an energy storage device
using an electrode formed of those particles and utilizing the
oxidation-reduction reaction of lithium ions, the conduction of
electrons between the particles and the conduction of lithium ions
between the particles are suppressed to low levels by the oxide
coating film.
[0222] In addition, at the time of charging, oxygen atoms of
silicon oxide or tin oxide in the coating film reacts with lithium
to form lithium oxide which cannot reversibly release any Li ion by
a discharging reaction, so the performance of discharge may be
suppressed.
[0223] However, silicon or tin crystal particles produced by the
production method of the present invention are coated with an oxide
of a metal other than silicon and tin at the time of the
production, so the particles are not easily oxidized, silicon oxide
or tin oxide is difficult to newly form, and the change of the
particles over time at the time of storage is small. In addition,
the silicon or tin crystal particles are produced in minute states
at the time of the production. As a result, an energy storage
device using an electrode formed of the above composite material
obtained by the present invention has the following
characteristics: an electrochemical reaction in a discharging
reaction occurs uniformly at a higher rate, the device can be
quickly charged and discharged, the quantity of electricity
comparable to the electrical quantity charged into the device can
be discharged with high efficiency, and the device has high energy
density and high power density.
[0224] It is also preferred that the oxide of a metal to be turned
into a composite with the above positive electrode active material
is an oxide or composite oxide mainly formed of a metal element
selected from Mo, W, Nb, Ta, V, B, Ti, Ce, Al, Ba, Zr, Sr, Th, Mg,
Be, La, Ca and Y. The weight ratio of the oxide containing a metal
oxide semimetal to be turned into a composite with the positive
electrode active material falls within the range of preferably 0.01
to 0.2, or more preferably 0.02 to 0.1 with respect to 1 of the
positive electrode active material. Additionally, specific examples
of the oxide containing a metal oxide semimetal to be turned into a
composite include WO.sub.2, TiO.sub.2, MOC.sub.2, Nb.sub.2O.sub.5,
MoO.sub.3, WO.sub.3, Li.sub.4Ti.sub.5O.sub.12,
Li.sub.2Nb.sub.2O.sub.5, LiNbO.sub.3, LiWO.sub.2, LiMoO.sub.2,
LiTi.sub.2O.sub.4, Li.sub.2Ti.sub.2O.sub.4,
H.sub.2Ti.sub.12O.sub.25, Na.sub.2Ti.sub.12O.sub.252, VO.sub.2,
V.sub.6O.sub.13, Al.sub.2O.sub.3, Al.sub.2O.sub.3Na.sub.2O, MgO,
ZrO.sub.2, and La.sub.2O.sub.3. The ratio between the elements of
those oxides is not necessarily needed to be a stoichiometric
ratio.
[0225] Hereinafter, embodiments of the present invention will be
described with reference to FIGS. 1A to 6.
[0226] (Lithium Ion-Storing/Releasing Material)
[0227] FIG. 1A is a conceptual view showing the structures of two
types of primary particles capable of electrochemically reserving
and releasing Li of the present invention. FIG. 1B is a conceptual
view showing the structures of two types of secondary particles
formed by aggregation of the primary particles shown in FIG. 1A. In
FIG. 1A, reference numeral 100 represents a silicon or tin crystal
particle; 101, a coating layer formed of an amorphous oxide of a
metal; 102, a surface layer formed of silicon and an amorphous
oxide of a metal or formed of tin and an amorphous oxide of a
metal; and 103, a surface layer formed of an amorphous oxide of
silicon or tin. The whole or part of the surface of each of the
primary particles capable of electrochemically storing and
releasing Li in the present invention is coated with 101, 102, or
101 and 102. FIG. 1B is a conceptual view showing the constitutions
of two types of secondary particles formed by aggregation of the
primary particles shown in FIG. 1A. The particles capable of
electrochemically storing and releasing Li of the present invention
may be formed of either the two types of primary particles shown in
FIG. 1A or the two types of secondary particles shown in FIG. 1B
alone, or may be formed of a mixture of them.
[0228] FIGS. 2C and 2D are conceptual structural views when the
primary particles capable of electrochemically storing and
releasing Li in the present invention are connected with fibrous
substances to form a network structure. FIG. 2C is a conceptual
structural view when the primary particles in the present invention
of one type shown in FIG. 1A are connected with fibrous substances
to form a network structure. FIG. 2D is a conceptual structural
view when parts of the secondary particles formed by aggregation of
the primary particles in the present invention and other parts of
the secondary particles are connected with fibrous substances to
form a network structure. In FIGS. 2C and 2D, reference numeral 200
represents a silicon or tin crystal particle; 201, an amorphous
oxide layer as a surface layer; 202 and 205, fibrous substances;
203, a primary particle in the present invention; and 204, a
secondary particle as an aggregate of the primary particles. It
should be noted that the particle, which appears to have a surface
entirely coated with the amorphous oxide in FIG. 2C, may be partly
coated.
[0229] It is preferable in terms of the easiness of production and
the performance of the electrode that the above silicon or tin
crystal particles 100 and 200 have a diameter of 5 nm to 200 nm,
and the amorphous oxide layers 102, 103, and 201 have a thickness
of 1 nm to 10 nm. The crystallite size of each of the above silicon
or tin crystal particles preferably falls within the range of 20 to
60 nm in terms of the performance of the electrode. When the above
particle size or the above crystallite size is excessively large,
upon electrochemical insertion/extraction of lithium into/from the
formed electrode, a local reaction is apt to occur, and is
responsible for a reduction in lifetime of the electrode. When the
crystallite size is excessively small, the resistance of the
electrode increases.
[0230] In addition, a metal element accounts for preferably 0.3
atomic % or more, or more preferably 0.3 atomic % or more and 30
atomic % or less of the metal oxide in the primary particles shown
in FIG. 1A and the secondary particles shown in FIG. 1B in order
that the electrode may provided with the performance of favorably
storing and releasing lithium ions.
[0231] When the crystal particles of the primary particles are
formed of silicon, a molar ratio of silicon oxide to Si of the
secondary particles by XPS as a surface analysis approach is
preferably 0.05 to 7.0, and a ratio of an oxygen element to an Si
element by analysis with the EDX of an STEM is preferably 0.05 to
0.8 in order that the electrode may be provided with the
performance of favorably storing and releasing lithium ions.
[0232] Further, the fibrous substances 202 and 205 shown in FIGS.
2C and 2D preferably have a diameter in the range of 5 nm to 70 nm
and a length in the range of 100 nm to 2 .mu.m in terms of the
easiness of production and an increase in mechanical strength of
the electrode. A material of the fibrous substances is preferably
amorphous silicon oxide or crystalline aluminum oxynitride. In
addition, the fibrous substances are preferably selected from
materials having a core-shell structure in which the core portion
is crystalline silicon and the shell portion is an amorphous
silicon oxide, an amorphous metal oxide or a composite oxide of a
metal and silicon. It is preferred that Gibbs free energy when a
metal oxide is produced by oxidation of the metal element of the
metal oxide or composite oxide is smaller than Gibbs free energy
when silicon or tin is oxidized, and the metal oxide have higher
thermodynamic stability than silicon oxide or tin oxide. To be
specific, the metal element is preferably at least one type of
metal selected from Li, Be, Mg, Al, Ca, Zr, Ba, Th, La, Ce, Nd, Sm,
Eu, Dy and Er, or more preferably at least one type of metal
selected from Li, Mg, Al, Ca, Zr, Ba, La, Ce and Nd; the metal
element is most preferably Al or Zr because any such element is
available at a low cost, is stable in the air, and can be easily
handled.
[0233] The content of the oxide of a metal other than silicon and
tin in the lithium ion-storing/releasing material falls within the
range of preferably 1 wt % or more and 50 wt % or less, or more
preferably 10 wt % or more and 35 wt % or less.
[0234] The content of silicon or tin in the lithium
ion-storing/releasing material falls within the range of preferably
30 wt % or more and 98 wt % or less, or more preferably 50 wt % or
more and 90 wt % or less.
[0235] The above powder material is used as an active material for
the electrode of an electrochemical device, in particular, an
energy storage device. The above powder material can be used as an
electrode material for an electrode for other electrolysis or an
electrode for electrochemical synthesis. In addition, the above
powder material can be used also as a photocatalyst for decomposing
water or organic matter by photoirradiation or a material for a
photovoltaic device.
[0236] (Electrode Structure)
[0237] FIGS. 3A and 3B are each a schematic view showing the
sectional structure of an electrode structure formed of the lithium
ion-storing/releasing material produced by the method of the
present invention.
[0238] FIG. 3A shows a state in which the layer of the lithium
ion-storing/releasing material is formed by vapor deposition on a
current collector in the reaction (F) in the production method of
the present invention. In FIGS. 3A and 3B, reference numeral 300
denotes the current collector; 301, a silicon or tin crystal
particle; 302, a metal oxide; 306, an electrode material layer
(active material layer); and 307, an electrode structure. The metal
oxide 302 shown in FIG. 3A may contain an unoxidized metal, the
electrode material layer 306 may contain a metal crystal (not
shown), and the composition of the crystal is preferably the
eutectic composition of silicon or tin. In FIG. 3A, the surface
layer of the silicon or tin crystal particle 301 is coated with an
amorphous oxide.
[0239] An electrode structure shown in FIG. 3B is obtained by
forming, on the current collector, the electrode material layer
(active material layer) from the powder of the lithium
ion-storing/releasing material shown in FIG. 1A or 1B, or FIG. 2C
or 2D produced by the method of the present invention. In FIG. 3B,
reference numeral 300 denotes the current collector; 303, a
material powder particle containing silicon or tin crystal
particles; 304, a conductive auxiliary material; 305, a binder;
306, the electrode material layer (active material layer); and 307,
the electrode structure. The electrode structure shown in FIG. 3B
is produced by: mixing the material powder particles 303 containing
silicon or tin crystal particles, the conductive auxiliary material
304, and the binder 305; appropriately adding, for example, a
solvent for the binder to the mixture to prepare slurry; applying
the prepared slurry onto the current collector 300 with an
application apparatus; drying the resultant; and appropriately
adjusting the thickness and density of the electrode material layer
with an apparatus such as a roll press after the drying.
[0240] The electrode material layer shown in FIG. 3B preferably has
density in the range of 0.5 g/cm.sup.3 or more to 3.5 g/cm.sup.3 or
less in order that an energy storage device having high energy
density and high power density can be obtained.
[0241] (Energy Storage Device)
[0242] FIG. 4 is a view showing the basic constitution of an energy
storage device utilizing the reduction-oxidation reaction of
lithium ions. In the energy storage device shown in FIG. 4,
reference numeral 401 represents a negative electrode; 403, a
lithium ion conductor; 402, a positive electrode; 404, a negative
electrode terminal; 405, a positive electrode terminal; and 406, a
battery case (housing). When the above electrode structure shown in
FIG. 3A or 3B is used in the negative electrode 401, in
consideration of an electrochemical reaction at the time of
charging and discharging with reference to FIGS. 1A and 1B, 2C and
2D, and 3A, in the case where the device is charged by connecting
the negative electrode terminal 401 and the positive electrode
terminal 405 to an external power source, silicon or tin in which
lithium is stored has been turned into fine particles in the
material layer of the negative electrode 401 in the present
invention upon inserting lithium ions into the active material for
the negative electrode to prompt a reduction reaction at the time
of charging. Since the particles have a large specific surface
area, substantial current density at the time of the charging is
reduced, and the electrochemical reaction is performed moderately
and uniformly. In addition, the initial charge and discharge
efficiency of the device is high because the content of silicon
oxide or tin oxide which reacts with lithium to inactivate lithium
is reduced to the uppermost. As a result of the foregoing, the
expansion of the volume of the negative electrode 401 due to the
insertion of lithium is suppressed, so the lifetime of the
electrode is lengthened. In addition, the suppression means that a
larger current can be flowed into the electrode than that in the
case of an electrode in which silicon or tin has not been turned
into fine particles. Accordingly, the energy storage device of the
present invention can be more quickly charged. When the device is
discharged by connecting the negative electrode terminal 401 and
the positive electrode terminal 405 to an external load,
discharging current density per silicon or tin particle is reduced
by turning silicon or tin into fine particles, so the energy
storage device of the present invention can be discharged at a
larger current per electrode, and provide high power.
[0243] (Regarding Storage Characteristic of Electrode Material)
[0244] Although fine particles formed of silicon or tin alone react
with oxygen and moisture in the air and are easily oxidized, the
oxidation is suppressed, and the particles can be easily handled in
the air when the periphery of each of the particles is coated with
a metal oxide as in the material powders 100 and 200 shown in FIGS.
1A and 1B and FIGS. 2C and 2D, and the electrode material layer 306
shown in FIG. 3A. In addition, even when the particles are stored
for a long time period, the particles show smaller chemical
changes, and are stable, so the particles can show stable
performance when used as an electrode material for an energy
storage device.
[0245] (Evaluation of Lithium Ion-Storing/Releasing Material for
Crystallite Size)
[0246] In the present invention, the crystallite size of a particle
can be determined by means of a transmission electron microscope
image or selected area electron diffraction, or from the half width
of a peak in an X-ray diffraction curve and a diffraction angle by
using Scherrer's equation described below.
Lc=0.94.lamda./(.beta. cos .theta.) (Scherrer's equation)
[0247] Lc: crystallite size
[0248] .lamda.: wavelength of an X-ray beam
[0249] .beta.: half width (radian) of a peak
[0250] .theta.: Bragg angle of a diffraction beam
[0251] The half width of diffraction intensity with respect to
2.theta. of the main peak in the X-ray diffraction chart of the
silicon or tin metal region of the lithium ion-storing/releasing
material of the present invention is preferably 0.10 or more, or
more preferably 0.2.degree. or more. The crystallite size of
silicon or tin of the lithium ion-storing/releasing material of the
present invention calculated by using Scherrer's equation described
above preferably falls within the range of 20 nm or more and 60 nm
or less. In this case, an amorphous region may be included. An
insertion/extraction reaction for lithium may occur from a crystal
grain boundary. Since silicon or tin crystals have a large number
of grain boundaries when being made microcrystalline or amorphous,
so that an insertion/extraction reaction for lithium is uniformly
performed, thereby increasing battery capacity and improving charge
and discharge efficiency. When the crystals are made
microcrystalline or amorphous, the structure of the crystal loses
its long-range order, so the crystalline structure shows an
increased degree of freedom and a reduced change at the time of the
insertion of lithium. As a result, the expansion of the structure
at the time of the insertion of lithium is also reduced.
[0252] The primary particles constituting the composite particles
as the powder material in the present invention have an average
particle diameter in the range of preferably 1 nm to 500 nm, or
more preferably 5 nm to 200 nm.
[0253] (Energy Storage Device)
[0254] The energy storage device according to the present invention
includes a negative electrode using the above-mentioned powder
material of the present invention as an active material, an ion
conductor (electrolyte), and a positive electrode, and utilizes the
oxidation reaction of lithium and the reduction reaction of lithium
ions. An electrode structure formed of a lithium
ion-storing/releasing material including at least primary particles
composed of silicon or tin crystal particles and the oxide of a
metal other than silicon and tin is used in the negative electrode.
Examples of the energy storage device according to the present
invention include a secondary battery and a capacitor.
[0255] (Negative Electrode 401)
[0256] The negative electrode to be used in the energy storage
device of the present invention includes a current collector and an
electrode material layer (active material layer) provided on the
current collector. The above electrode material layer is formed of
the above-mentioned active material using the powder material in
the present invention. An electrode structure adopting a schematic
sectional structure shown in FIG. 3B or 3A is used as the electrode
structure for a negative electrode in the present invention.
[0257] The electrode structure shown in FIG. 3B to be used in the
negative electrode is produced through the following procedure.
[0258] A conductive auxiliary material powder and a binder are
mixed into the powder material in the present invention. A solvent
for the binder is appropriately added to the mixture, and the
mixture is kneaded, whereby slurry is prepared. In this case, a
foaming agent such as azodicarbonamide or
P,P'-oxybisbenzenesulfenyl dihydrazide which produces a nitrogen
gas at the time of heating may be added in order that voids of the
electrode layer may be actively formed.
[0259] The slurry is applied onto the current collector, whereby
the electrode material layer (active material layer) is formed. The
layer is dried, whereby the electrode structure is formed. Further,
as required, the resultant is dried in the range of 100 to
300.degree. C. under reduced pressure, and the thickness and
density of the electrode material layer are adjusted with a
pressing machine.
[0260] The electrode structure obtained in the above (2) is
appropriately cut in accordance with the housing of the energy
storage device so that an electrode shape is adjusted. Then, as
required, an electrode tab for current extraction is welded to the
resultant, whereby the negative electrode is produced.
[0261] For example, a coater application method or a screen
printing method is applicable as the above application method.
Alternatively, the electrode material layer can be formed by
press-forming the above powder material as an active material, the
conductive auxiliary material, and the binder on the current
collector without adding any solvent. It should be noted that the
electrode material layer for the negative electrode of the energy
storage device of the present invention has density in the range of
preferably 0.5 to 3.5 g/cm.sup.3, or more preferably 0.9 to 2.5
g/cm.sup.3. When the density of the electrode material layer is
excessively large, the expansion of the layer at the time of
inserting lithium becomes large, so the peeling of the layer from
the current collector occurs. In addition, when the density of the
electrode material layer is excessively small, the resistance of
the electrode increases, so a reduction in charge and discharge
efficiency and a voltage drop at the time of the discharging of the
battery become remarkable.
[0262] When the electrode material slurry to which the foaming
agent has been added is used, the foaming agent is decomposed after
the application or the drying in an inert atmosphere or under
reduced pressure to foam so that voids are formed in the electrode
layer. As a result, voids are formed in the resin as the binder in
the electrode layer so that the resin is brought into a sponge
state. The density of the electrode layer formed into a network
structure with the above sponge-like binder is adjusted by
pressing. However, in association with the expansion of the active
material particles in the electrode layer at the time of charging,
the pressed sponge is elongated, that is, the resin as the binder
having a network structure is elongated, and the expanded particles
are fitted into voids formed by elongation. As a result, even when
the expansion and shrinkage are repeated owing to the repetition of
the charging and discharging, the resin used as the binder in the
electrode layer is inhibited from being broken due to fatigue,
whereby the lifetime of the electrode is lengthened.
[0263] [Conductive Auxiliary Material for Negative Electrode]
[0264] For example, amorphous carbon such as acetylene black or
ketjen black, a carbon material such as graphite structure carbon,
carbon nanofiber, or carbon nanotube, nickel, copper, silver,
titanium, platinum, cobalt, iron, or chromium can be used as the
conductive auxiliary material of the electrode material layer
(active material layer). The above carbon material is more
preferable because the material can retain an electrolyte solution,
and has a large specific surface area. A shape selected from, for
example, a spherical shape, a flaky shape, a filamentous shape, a
fibrous shape, a spike shape, and a needle shape can be preferably
adopted as the shape of the above conductive auxiliary material.
Further, the adoption of powders having two or more different types
of shapes can increase packing density at the time of forming the
electrode material layer to reduce the impedance of the electrode
structure. The average particle size of (the secondary particles
of) the above conductive auxiliary material is preferably 10 .mu.m
or less, or more preferably 5 .mu.m or less.
[0265] [Current Collector for Negative Electrode]
[0266] The current collector of the negative electrode in the
present invention serves to efficiently supply a current to be
consumed by an electrode reaction at the time of charging or to
collect a current to be generated at the time of discharging. In
particular, when the electrode structure is applied to the negative
electrode of the energy storage device, a material of which the
current collector is formed is desirably a material which has high
electric conductivity and is inert to the electrode reaction of the
energy storage device. A preferable material for the current
collector is, for example, at least one type of metal material
selected from copper, nickel, iron, stainless steel, titanium,
platinum, and aluminum; of these materials, copper is more
preferably used because copper is available at a low cost, and has
low electrical resistance. Aluminum foil having an increased
specific surface area can also be used. In addition, the current
collector is of a plate shape; the "plate shape" is not limited in
terms of its thickness as long as the shape can be put into
practical use, and a shape called "foil" having a thickness of
about 5 .mu.m to 100 .mu.m is also included in the category of the
shape. When copper foil is used in the above current collector,
copper foil appropriately containing, for example, Zr, Cr, Ni, or
Si and having high mechanical strength (high proof stress) is
particularly preferably used as the copper foil because the copper
foil is resistant to the repeated expansion and shrinkage of the
electrode layer at the time of charging and discharging.
Alternatively, for example, a plate member of a mesh, sponge or
fibrous shape, a plate punching metal, a plate metal having a
three-dimensional indented pattern formed on the front and rear
surfaces, or a plate expanded metal can be adopted. The above plate
or foil metal having a three-dimensional indented pattern formed on
the surface can be produced, for example, by applying pressure to a
metallic or ceramic roll having a microarray pattern or
line-and-space pattern on the surface to transfer the pattern onto
a plate or foil metal. In particular, an energy storage device
adopting a current collector on which a three-dimensional indented
pattern has been formed exerts the following effects: the reduction
of substantial current density per electrode area at the time of
charging and discharging, an improvement in adhesiveness to the
electrode layer, an increase in mechanical strength, improvements
in charging and discharging current characteristics, and an
increase in charging and discharging cycle lifetime.
[0267] [Binder for Negative Electrode]
[0268] A material for the binder in the active material layer of
the negative electrode is, for example, an organic polymer material
such as: a fluorine resin such as polytetrafluoroethylene or
polyvinylidene fluoride; polyamideimide; polyimide; a polyimide
precursor (polyamic acid before being turned into polyimide, or
polyamic acid which is incompletely turned into polyimide); a
styrene-butadiene rubber; a modified polyvinyl alcohol type resin
with its water absorbing property reduced; a polyacrylate type
resin; or a polyacrylate type resin-carboxymethylcellulose. When
the polyimide precursor (polyamic acid before being turned into
polyimide, or polyamic acid which is incompletely turned into
polyimide) is used, it is preferable that after the application of
the electrode layer, the precursor is subjected to a heat treatment
in the range of 150 to 300.degree. C. so that the extent to which
the precursor is turned into polyimide is enlarged.
[0269] In order that the negative electrode may exert such
performance that even after the repetition of charging and
discharging, the binding of the active material is maintained and a
larger electrical quantity is stored, the content of the above
binder in the electrode material layer is preferably 2 to 30 wt %,
or more preferably 5 to 20 wt %. When the component ratio of the
metal oxide (containing a semimetal oxide) in the active material
of the negative electrode is large, the expansion of the volume of
the negative electrode layer at the time of charging is small, so a
fluorine resin such as polytetrafluoroethylene or polyvinylidene
fluoride, or a polymer material such as a styrene-butadiene rubber
having the following characteristics may be used as the binder, in
which adhesive force is not high, the ratio at which the surface of
the active material is coated with the binder is small, and a large
surface area effective in a reaction is provided. When the
component ratio of a silicon or tin metal in the material layer
(active material layer) of the negative electrode is large, the
volume expansion at the time of charging is large, so a binder
having high adhesive force is preferable. In this case,
polyamideimide, polyimide, a polyimide precursor, a modified
polyvinyl alcohol type resin, polyvinylidene fluoride, or the like
is preferably used as the binder.
[0270] (Positive Electrode 402)
[0271] The positive electrode 402 serving as a counter electrode
for an energy storage device using the above-mentioned active
material of the present invention in its negative electrode is
roughly classified into the following three cases.
[0272] (1) In order that the energy density may be increased, a
crystalline lithium-transition metal oxide or a lithium-transition
metal phosphate compound having a relatively flat electric
potential at the time of discharging is used as an active material
for the positive electrode. Ni, Co, Mn, Fe, Cr, or the like is more
preferably mainly used as a transition metal element to be
incorporated into the above positive electrode active material.
[0273] (2) When the power density is intended to be increased as
compared with the case of the positive electrode in the above (1),
a transition metal oxide, a transition metal phosphate compound, a
lithium-transition metal oxide or a lithium-transition metal
phosphate compound, which is amorphous, is used as an active
material for the positive electrode. The above positive electrode
active material has a crystallite size of preferably 10 nm or more
and 100 nm or less, or more preferably 10 nm or more and 50 nm or
less. An element selected from Mn, Co, Ni, Fe, and Cr is suitably
used as a transition metal element serving as the main element of
the above positive electrode active material. The above positive
electrode active material can increase the power density as
compared with the case of the positive electrode in the above (1)
probably because the material has small crystal particles and a
large specific surface area, and is hence capable of utilizing not
only an intercalation reaction for lithium ions but also an
adsorption reaction of ions to the surface. The above positive
electrode active material is preferably turned into a composite
with an oxide or composite oxide mainly formed of an element
selected from Mo, W, Nb, Ta, V, B, Ti, Ce, Al, Ba, Zr, Sr, Th, Mg,
Be, La, Ca, and Y. As in the case of the negative electrode active
material, the crystal particles of the positive electrode active
material can be reduced in size by turning the material into a
composite with the above oxide, and the extent to which the
material is made amorphous is enlarged. In addition, the positive
electrode active material is preferably turned into a composite
with a carbon material such as amorphous carbon, carbon nanofiber
(carbon fibers each having a diameter of the order of nanometers),
carbon nanotube, or a graphite powder in order that the electronic
conductivity of the positive electrode active material may be
improved.
[0274] (3) When high power density is intended to be provided, a
carbon material which has a large specific surface area and/or is
porous such as active carbon, mesoporous carbon (carbon in which a
large number of pores in a meso region develop, in other words, a
carbon material having a large number of pores in a meso region),
carbon nanofiber (carbon fibers each having a diameter of the order
of nanometers), carbon nanotube, or graphite with its specific
surface area increased by pulverization or the like, or a metal
oxide (containing a semimetal oxide) having a large specific
surface area is used as an active material for the positive
electrode. In this case, at the time of the assembly of the cell of
the energy storage device, lithium must be stored in the negative
electrode in advance, or lithium must be stored in the positive
electrode in advance. A method of storing lithium is a method
involving bringing a lithium metal into contact with the negative
electrode or positive electrode to result in short-circuit so that
lithium is introduced, or a method involving introducing lithium in
the form of a lithium-metal oxide or lithium-semimetal oxide into
the active material.
[0275] In addition, the power density can be additionally increased
by making the above positive electrode active material porous.
Further, the material in the above (3) may be turned into a
composite. When the above positive electrode active material does
not contain lithium which can be deintercalated, lithium must be
stored by, for example, bringing metal lithium into contact with
the negative electrode or positive electrode in advance as in the
case of the above (3). In addition, the positive electrode active
material in each of the above (1), (2), and (3) can be turned into
a composite with a polymer capable of electrochemically storing
ions such as a conductive polymer.
[0276] (Positive Electrode Active Material)
[0277] An oxide or phosphate compound of a transition metal element
such as Co, Ni, Mn, Fe, or Cr which can be used in a lithium
secondary battery can be used as the crystalline lithium-transition
metal oxide or lithium-transition metal phosphate compound to be
used in the positive electrode active material in the above (1).
The above compound can be obtained by mixing a lithium salt or
lithium hydroxide and a salt of the transition metal at a
predetermined ratio (and, furthermore, adding phosphoric acid or
the like when the phosphate compound is prepared) and subjecting
the mixture to a reaction at a high temperature of 700.degree. C.
or higher. Alternatively, the fine powder of the above positive
electrode active material can be obtained by employing an approach
such as a sol-gel method.
[0278] A lithium-transition metal oxide, lithium-transition metal
phosphate compound, transition metal oxide, or transition metal
phosphate compound the transition metal element of which is Co, Ni,
Mn, Fe, Cr, V, or the like is used as the positive electrode active
material in the above (2), and the material preferably has an
amorphous phase having a small crystallite size. A transition metal
oxide or transition metal phosphate compound having the above
amorphous phase is obtained by making amorphous a
lithium-transition metal oxide, lithium-transition metal phosphate
compound, transition metal oxide, or transition metal phosphate
compound, which is crystalline, by mechanical milling using, for
example, a planetary ball mill, a vibrating mill, or an attritor. A
lithium-transition metal oxide, lithium-transition metal phosphate
compound, transition metal oxide, or transition metal phosphate
compound which is amorphous can be prepared also by directly mixing
raw materials with the above mill, subjecting the mixture to
mechanical alloying and appropriately subjecting the resultant to
heat treatment. Alternatively, such amorphous material can be
obtained by subjecting, for example, an oxide obtained by a
reaction in a sol-gel method from a solution of a salt, a complex,
and an alkoxide as raw materials to heat treatment in the
temperature range of preferably 100 to 700.degree. C., or more
preferably 150 to 550.degree. C. Heat treatment at a temperature in
excess of 700.degree. C. reduces the pore volume of the above
transition metal oxide to promote the crystallization of the oxide,
with the result that the specific surface area is reduced and
charge and discharge characteristics at high current density
deteriorate. The positive electrode active material has a
crystallite size of preferably 100 nm or less, or more preferably
50 nm or less. A positive electrode in which reactions for the
intercalation and deintercalation of lithium ions, and for the
adsorption and extraction of lithium ions are quicker, is produced
from a positive electrode active material having such a crystallite
size.
[0279] Examples of the carbon material which has a large specific
surface area and/or is porous, to be used as the positive electrode
active material in the above (3), include a carbon material
obtained by carbonizing an organic polymer under an inert gas
atmosphere and a carbon material obtained by forming pores in the
carbonized material by treatment with an alkali or the like.
Mesoporous carbon obtained by inserting an organic polymer material
into a mold made of, for example, an oxide in which pores are
aligned, produced in the presence of an amphiphilic surfactant to
carbonize the material and removing a metal oxide by etching, can
also be used as the positive electrode active material. The above
carbon material preferably has a specific surface area in the range
of 10 to 3,000 m.sup.2/g. A transition metal oxide having a large
specific surface area such as a manganese oxide as well as the
above carbon material can be used.
[0280] In addition, the positive electrode active material in the
present invention having high energy density and a certain degree
of power density is composed of an active material selected from a
lithium-transition metal oxide, a lithium-transition metal
phosphate compound, a transition metal oxide and a transition metal
phosphate compound whose transition metal elements are Co, Ni, Mn,
Fe, Cr, V, or the like. The active material is composed of
particles each having an amorphous phase, and is turned into a
composite with an oxide or composite oxide whose main component is
an element selected from Mo, W, Nb, Ta, V, B, Ti, Ce, Al, Ba, Zr,
Sr, Th, Mg, Be, La, Ca, and Y. The oxide or composite oxide added
for forming the composite accounts for preferably 1 wt % or more
and 20 wt % or less, or more preferably 2 wt % or more and 10 wt %
or less of the above entire positive electrode active material
turned into the composite. When the oxide or composite oxide for
forming the composite is incorporated to exceed the above weight
range, the battery capacity of the positive electrode is lowered.
The contribution of the above oxide or composite oxide to the
charge and discharge electrical quantity is desirably 20% or less.
Since the particle size of the above positive electrode active
material can be reduced by turning the positive electrode active
material into a composite as in the case of the negative electrode
material in the present invention, the ratio at which the positive
electrode active material is utilized in charging and discharging
increases, and an electrochemical reaction in charging and
discharging occurs in a more uniform and quick fashion. As a
result, both the energy density and power density increase. In
addition, the above oxide is desirably a lithium ion conductor such
as a composite oxide with lithium.
[0281] At the time of the above formation of the composite, it is
also preferable that the positive electrode material is further
turned into a composite with a carbon material such as amorphous
carbon, mesoporous carbon (carbon material having a large number of
pores in a meso region), carbon nanofiber (carbon fibers each
having a diameter of the order of nanometers), carbon nanotube, or
graphite with its specific surface area increased by pulverization
treatment or the like.
[0282] Further, a mixture of two or more types of materials
selected from the materials in the above (1), (2), and (3) may be
used as the above positive electrode active material.
[0283] Method of Producing Positive Electrode:
[0284] The positive electrode to be used in the energy storage
device of the present invention is produced by forming an electrode
material layer (layer of a positive electrode active material) on a
current collector. The electrode structure 307 having a schematic
sectional structure shown in FIG. 3B described for the negative
electrode and adopting the above-mentioned positive electrode
active material instead of the material powder particles 303 each
containing silicon or tin microcrystals is used in the positive
electrode of the present invention.
[0285] The electrode structure to be used in the positive electrode
is produced by the following procedure.
[0286] (1) A conductive auxiliary material powder and a binder are
mixed with the positive electrode active material. A solvent for
the binder is appropriately added to the mixture, and the mixture
is kneaded, whereby slurry is prepared.
[0287] (2) The slurry is applied onto the current collector,
whereby the electrode material layer (active material layer) is
formed. The layer is dried, whereby the electrode structure is
formed. Further, as required, the resultant is dried in the range
of 100 to 300.degree. C. under reduced pressure, and the thickness
and density of the electrode material layer are adjusted with a
pressing machine.
[0288] (3) The electrode structure obtained in the above (2) is
appropriately cut in accordance with the housing of the energy
storage device so that an electrode shape is adjusted. Then, as
required, an electrode tab for current extraction is welded to the
resultant, whereby the positive electrode is produced.
[0289] For example, a coater application method or a screen
printing method is applicable as the above application method.
Alternatively, the electrode material layer can be formed by
press-forming the above positive electrode active material, the
conductive auxiliary material and the binder on the current
collector without adding any solvent. It should be noted that the
electrode material layer of the present invention has density in
the range of preferably 0.5 to 3.5 g/cm.sup.3, or more preferably
0.6 to 3.5 g/cm.sup.3. In the above density range of the electrode
layer, the density of the electrode layer is set to be low in the
case of an electrode for high power density, and the density of the
electrode layer is set to be high in the case of an electrode for
high energy density.
[0290] [Conductive Auxiliary Material for Positive Electrode]
[0291] The same material as the conductive auxiliary material for
the negative electrode can be used.
[0292] [Current Collector for Positive Electrode]
[0293] The current collector of the positive electrode of the
present invention also serves to efficiently supply a current to be
consumed by an electrode reaction at the time of charging or to
collect a current to be generated at the time of discharging as in
the case of the negative electrode. In particular, when the
electrode structure is applied to the positive electrode of a
secondary battery, a material of which the current collector is
formed is desirably a material which has a high electric
conductivity and is inert to a cell reaction. A preferable material
for the current collector is, for example, at least one type of
metal material selected from aluminum, nickel, iron, stainless
steel, titanium, and platinum; of these materials, aluminum is more
preferably used because aluminum is available at a low cost and has
low electrical resistance. In addition, the current collector is of
a plate shape; the "plate shape" is not limited in terms of its
thickness as long as the current collector can be put into
practical use, and a shape called "foil" having a thickness of
about 5 .mu.m to 100 .mu.m is also included in the category of the
shape. Alternatively, for example, a plate member of a mesh, sponge
or fibrous shape, a plate punching metal, a plate metal having a
three-dimensional indented pattern formed on the front and rear
surfaces, or a plate expanded metal can be adopted. The above plate
or foil metal having a three-dimensional indented pattern formed on
the surface can be produced, for example, by applying pressure to a
metallic or ceramic roll having a microarray pattern or
line-and-space pattern on the surface to transfer the pattern onto
a plate or foil metal. In particular, an energy storage device
adopting a current collector on which a three-dimensional indented
pattern has been formed exerts the following effects: the reduction
of a substantial current density per electrode area at the time of
charging and discharging, an improvement in adhesiveness to the
electrode layer, an increase in mechanical strength, improvements
in charging and discharging current characteristics, and an
increase in charging and discharging cycle lifetime.
[0294] [Binder for Positive Electrode]
[0295] The binder for the negative electrode can be used also as a
binder for the positive electrode. In order that a surface area
effective in the reaction of the active material may be enlarged, a
polymer material with which the surface of the active material is
difficult to coat, for example, a fluorine resin such as
polytetrafluoroethylene or polyvinylidene fluoride;
styrene-butadiene rubber; modified acrylic resin; polyimide; or
polyamideimide is preferably used as the binder. In order that the
positive electrode may exert such performance that even after
repetiting charging and discharging, the binding of the active
material is maintained, and a larger electrical quantity is stored,
the content of the above binder in the electrode material layer
(active material layer) of the positive electrode is preferably 1
to 20 wt %, or more preferably 2 to 10 wt %.
[0296] (Ion Conductor 403)
[0297] A lithium ion conductor such as a separator holding an
electrolyte solution (electrolyte solution prepared by dissolving
an electrolyte in a solvent), a solid electrolyte, a solidified
electrolyte obtained by causing an electrolyte solution to gel with
polymer gel or the like, a composite of polymer gel and a solid
electrolyte, or an ionic liquid can be used as the ion conductor of
the lithium secondary battery of the present invention.
[0298] The conductivity of the ion conductor to be used in the
secondary battery of the present invention at 25.degree. C. is
preferably 1.times.10.sup.-3 S/cm or more, or more preferably
5.times.10.sup.-3 S/cm or more.
[0299] Examples of the electrolyte include salts each formed of
lithium ions (Li.sup.+) and Lewis acid ions (BF.sub.4.sup.-,
PF.sub.6.sup.-, AsF.sub.6.sup.-, ClO.sub.4.sup.-,
CF.sub.3SO.sub.3.sup.-, or BPh.sub.4.sup.- (Ph: phenyl group)) and
a mixture of these salts, and ionic liquids. Each of the above
salts is desirably dehydrated and deoxygenated sufficiently by
heating under reduced pressure. Further, an electrolyte prepared by
dissolving any one of the above lithium salts in an ionic liquid
can also be used.
[0300] As a solvent of the electrolyte, the following can be used:
for example, acetonitrile, benzonitrile, propylene carbonate,
ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl
methyl carbonate, dimethyl formamide, tetrahydrofuran,
nitrobenzene, dichloroethane, diethoxyethane, 1,2-dimethoxyethane,
chlorobenzene, .gamma.-butyrolactone, dioxolane, sulfolane,
nitromethane, dimethyl sulfide, dimethyl sulfoxide, methyl formate,
3-methyl-2-oxydazolidinone, 2-methyltetrahydrofuran,
3-propylsydnone, sulfur dioxide, phosphoryl chloride, thionyl
chloride, sulfuryl chloride, or a mixture thereof. Further, an
ionic liquid can also be used.
[0301] The above solvent is desirably dehydrated with, for example,
active alumina, a molecular sieve, phosphorus pentoxide, or calcium
chloride; depending on the type of solvent, the solvent is
desirably distilled in the coexistence of an alkali metal in an
inert gas so that impurities are removed from the solvent and the
solvent is dehydrated. The electrolyte concentration of the
electrolyte solution prepared by dissolving the electrolyte in the
solvent preferably falls within the range of 0.5 to 3.0 mol/l in
order that the electrolyte solution may have high ionic
conductivity.
[0302] In addition, it is also preferable to add a vinyl monomer
which easily undergoes an electropolymerization reaction to the
above electrolyte solution in order that a reaction between the
electrode and the electrolyte solution may be suppressed. The
addition of the vinyl monomer to the electrolyte solution results
in the formation of a polymerized coating film capable of
functioning as a solid electrolyte interface (SEI) or passivating
film on the surface of the above active material for the electrode
in a charging reaction for the battery, whereby the charging and
discharging cycle lifetime can be lengthened. When the amount in
which the vinyl monomer is added to the electrolyte solution is
excessively small, the above effect does not appear. When the
amount is excessively large, the ionic conductivity of the
electrolyte solution is reduced, and the thickness of the
polymerized coating film to be formed at the time of charging
increases, so the resistance of the electrode increases.
Accordingly, the amount in which the vinyl monomer is added to the
electrolyte solution preferably falls within the range of 0.5 to 5
wt %.
[0303] Specific and preferable examples of the vinyl monomer
include styrene, 2-vinylnaphthalene, 2-vinylpyridine,
N-vinyl-2-pyrrolidone, divinyl ether, ethyl vinyl ether, vinyl
phenyl ether, methyl methacrylate, methyl acrylate, acrylonitrile,
and vinylene carbonate. More preferable examples include styrene,
2-vinylnaphthalene, 2-vinylpyridine, N-vinyl-2-pyrrolidone, divinyl
ether, ethyl vinyl ether, vinyl phenyl ether, and vinylene
carbonate. Those vinyl monomers are preferred when having an
aromatic group, because the affinity thereof with lithium ions is
high. Further, it is also preferred to use N-vinyl-2-pyrrolidone,
divinyl ether, ethyl vinyl ether, vinyl phenyl ether, vinylene
carbonate, or the like, which has high affinity with the solvent of
the electrolyte, in combination with the vinyl monomer having an
aromatic group.
[0304] A solid electrolyte or a solidified electrolyte is
preferably used in order that the electrolyte solution may be
prevented from being leaked. Examples of the solid electrolyte
include: glass such as an oxide formed of a lithium element, a
silicon element, an oxygen element, and a phosphorus element or a
sulfur element; and a polymer complex of an organic polymer having
an ether structure. The solidified electrolyte is preferably a
product obtained by causing the electrolyte solution to gel with a
gelling agent to solidify. A porous material having large liquid
absorption which absorbs the solvent of the electrolyte solution to
swell, such as a polymer or silica gel, is desirably used as the
gelling agent. Polyethylene oxide, polyvinyl alcohol,
polyacrylonitrile, polymethyl methacrylate, a vinylidene
fluoride-hexafluoropropylene copolymer, or the like is used as the
above polymer. Further, the above polymer more preferably has a
crosslinked structure.
[0305] The separator functioning also as a holding member for the
electrolyte solution as an ion conductor serves to prevent
short-circuit due to direct contact between the negative electrode
401 and the positive electrode 403 in the energy storage device.
The separator should have a large number of pores through which
lithium ions can move and be insoluble in, and stable against, the
electrolyte solution. Therefore, a film of a micropore structure or
non-woven fabric structure having fine pores and made of a material
such as glass, polyolefin such as polypropylene or polyethylene,
fluorine resin, cellulose, or polyimide is suitably used as the
separator. Alternatively, a metal oxide film having fine pores, or
a resin film turned into a composite with a metal oxide can be
used.
[0306] Assembly of the Energy Storage Device:
[0307] The above energy storage device is assembled by superposing
the ion conductor 403, the negative electrode 401 and the positive
electrode 402 so that the conductor is interposed between the
electrodes to form a group of electrodes, inserting the group of
electrodes into the battery case 406 under a dry air or dry inert
gas atmosphere with the dew point temperature sufficiently
controlled, connecting the respective electrodes and respective
electrode terminals after the insertion, and hermetically sealing
the battery case 406. When a product obtained by causing a
microporous polymer film to hold the electrolyte solution is used
as the ion conductor, the battery is assembled by interposing the
microporous polymer film as a separator for preventing
short-circuit between the negative electrode and the positive
electrode to form a group of electrodes, inserting the group of
electrodes into the battery case; connecting the respective
electrodes and the respective electrode terminals, injecting the
electrolyte solution into the battery case and hermetically sealing
the battery case.
[0308] As described above, the discharge voltage, battery capacity,
power density, and energy density of the energy storage device of
the present invention can be optimally designed in accordance with
applications by using the material capable of storing and releasing
lithium ions in the present invention in the negative electrode of
the device and appropriately selecting a material for the positive
electrode of the device from the above-mentioned types of
materials. As a result, an energy storage device having high power
density and high energy density can be obtained.
[0309] In an application in which the highest priority is given to
high energy density, and a certain level of high power density is
required, and besides, a certain degree of quick charging and
discharging are required, a crystalline lithium-transition metal
oxide or lithium-transition metal phosphate compound is adopted as
an active material for the positive electrode in the energy storage
device of the present invention. As a result, an energy storage
device having high energy density and a certain level of high power
density can be provided.
[0310] In an application in which both high energy density and high
power density are required, a transition metal oxide having an
amorphous phase is adopted as an active material for the positive
electrode in the energy storage device of the present invention, so
that an energy storage device having a certain level of high energy
density and a certain level of high power density can be
provided.
[0311] In an application in which the highest priority is given to
high power density and quick charging, and a certain level of high
energy density is required, a carbon material or metal oxide which
has a large specific surface area and/or is porous is adopted as an
active material for the positive electrode in the energy storage
device of the present invention, so that an energy storage device
having a high power density and a somewhat high energy density can
be provided. Further, a product obtained by turning the powder
material of the material capable of storing and releasing lithium
ions of the present invention into a composite with the carbon
material which has a large specific surface area and/or is porous
may be adopted for the negative electrode.
[0312] Therefore, in the case of an energy storage device using an
electrode structure formed of the powder material of the present
invention in its negative electrode, an energy storage device
having optimum energy density, optimum power density, and optimum
charge and discharge characteristics can be produced by selecting
the component ratio of the powder material and selecting a positive
electrode formed of an optimum positive electrode active material
species in accordance with desired characteristics.
[0313] [Shape and Structure of Cell]
[0314] The cell shape of the energy storage device of the present
invention is specifically, for example, a flat shape, a cylindrical
shape, a rectangular shape, or a sheet shape. In addition, the cell
structure is of, for example, a monolayer type, a multilayer type,
or a spiral type. Of those, a spiral, cylindrical cell has the
following characteristics: a separator is sandwiched between and
wound with a negative electrode and a positive electrode, so an
electrode area can be increased, and a large current can be flowed
at the time of charging and discharging. In addition, a rectangular
cell or sheet cell has the following characteristic: the storage
space of an instrument in which multiple cells are placed can be
effectively utilized.
[0315] Hereinafter, the shape and structure of a cell will be
described in more detail with reference to FIGS. 5 and 6. FIG. 5 is
a sectional view of a monolayer, flat (coin type) cell, and FIG. 6
is a sectional view of a spiral, cylindrical cell. An energy
storage device having such a shape as described above is basically
of the same constitution as shown in FIG. 4, and has a negative
electrode, a positive electrode, an ion conductor, a battery case
(cell housing), and an output terminal.
[0316] In FIGS. 5 and 6, reference numerals 501 and 603 each denote
a negative electrode; 503 and 606, a positive electrode; 504 and
608, a negative electrode terminal (a negative electrode cap or a
negative electrode can); 505 and 609, a positive electrode terminal
(a positive electrode can or a positive electrode cap); 502 and
607, an ion conductor; 506 and 610, a gasket; 601, a negative
electrode current collector; 604, a positive electrode current
collector; 611, an insulating plate; 612, a negative electrode
lead; 613, a positive electrode lead; and 614, a safety valve.
[0317] In the flat (coin) type cell as shown in FIG. 5, the
positive electrode 503 including a positive electrode material
layer and the negative electrode 501 including a negative electrode
material layer are superposed through, for example, the ion
conductor 502 formed of a separator holding at least an electrolyte
solution. Such a laminate is placed in the positive electrode can
505 as a positive electrode terminal from the positive electrode
side, and the negative electrode side is covered with the negative
electrode cap 504 as a negative electrode terminal. In addition,
the gasket 506 is placed at the other portion in the positive
electrode can.
[0318] In the spiral, cylindrical cell as shown in FIG. 6, the
positive electrode 606 having a positive electrode active material
layer 605 formed on the positive electrode current collector 604
and the negative electrode 603 having a negative electrode active
material layer 602 formed on the negative electrode current
collector 601 are opposite to each other through, for example, the
ion conductor 607 formed of a separator holding at least an
electrolyte solution, and the combination of the positive
electrode, the ion conductor, and the negative electrode is wound
up multiple times, whereby a laminate of a cylindrical structure is
formed.
[0319] The laminate of a cylindrical structure is placed in the
negative electrode can 608 as a negative electrode terminal. In
addition, the positive electrode cap 609 as a positive electrode
terminal is provided on the opening portion side of the negative
electrode can 608, and the gasket 610 is placed at the other
portion in the negative electrode can. The electrode laminate of a
cylindrical structure is separated from the side of the positive
electrode cap through the insulating plate 611. The positive
electrode 606 is connected to the positive electrode cap 609
through the positive electrode lead 613. In addition, the negative
electrode 603 is connected to the negative electrode can 608
through the negative electrode lead 612. The safety valve 614 for
adjusting the internal pressure of the cell is provided on the side
of the positive electrode cap. The above-mentioned electrode
structure of the present invention is used in the negative
electrode 603.
[0320] Hereinafter, an example of a method of assembling the energy
storage device shown in FIG. 5 or 6 will be described.
[0321] (1) The separator (502 or 607) is sandwiched between the
negative electrode (501 or 603) and the formed positive electrode
(503 or 606), and the resultant is incorporated into the positive
electrode can (505) or the negative electrode can (608).
[0322] (2) After the electrolyte solution has been injected, the
negative electrode cap (504) or the positive electrode cap (609)
and the gasket (506 or 610) are assembled.
[0323] (3) The product obtained in the above (2) is caulked,
whereby an energy storage device is completed.
[0324] It should be noted that the preparation of a material for
the above-mentioned energy storage device, and the assembly of the
cell are desirably performed in dry air or a dry inert gas from
which moisture has been sufficiently removed.
[0325] The members constituting such an energy storage device as
described above will be described.
[0326] (Gasket)
[0327] For example, a fluorine resin, a polyolefin resin, a
polyamide resin, a polysulfone resin, and various types of rubbers
can be used as a material for the gasket (506 or 610). A method
such as glass sealing, an adhesive, welding, or soldering as well
as "caulking" using the gasket as shown in FIG. 5 or 6 is employed
as a method of sealing the cell. In addition, various types of
organic resin materials or ceramics are used as a material for the
insulating plate (611) shown in FIG. 6.
[0328] (External Can)
[0329] The external can of a cell is composed of the positive
electrode can or negative electrode can (505 or 608) of the cell,
and the negative electrode cap or positive electrode cap (504 or
609) of the cell. Stainless steel is suitably used as a material
for the external can. An aluminum alloy, a titanium clad stainless
metal, a copper clad stainless metal, a nickel-plated steel plate,
or the like is also frequently used as another material for the
external can.
[0330] Stainless steel described above is preferable because the
positive electrode can (605) doubles as the battery case (cell
housing) and the terminal in FIG. 5, and the negative electrode can
(608) doubles as the battery case (cell housing) and the terminal
in FIG. 6; provided that when the positive electrode can or the
negative electrode can does not double as the battery case and the
terminal, in addition to stainless steel, a metal such as zinc, a
plastic such as polypropylene, a composite of a metal or glass
fiber and plastic, or a film obtained by superposing a plastic film
on metal foil made of aluminum or the like can be used as a
material for the battery case.
[0331] (Safety Valve)
[0332] The lithium secondary battery is provided with a safety
valve as a security measure when the internal pressure of the
battery increases. For example, rubber, a spring, a metal ball, or
rupture foil can be used as the safety valve.
EXAMPLES
[0333] Hereinafter, the present invention will be described in more
detail by way of examples.
[0334] [Preparation of an Electrode Material (Active Material) for
the Negative Electrode of the Energy Storage Device]
[0335] An example of a method of preparing a powder material to be
used as an active material for the negative electrode of the energy
storage device in the present invention will be given below.
[0336] Examples of procedure for preparation of powder material
[0337] Preparation by Thermal Plasma Method
[0338] Here, nanoparticles serving as a negative electrode material
were synthesized with a radiofrequency (RF) inductively coupled
thermal plasma-generating apparatus in accordance with the
following procedure. The above inductively coupled thermal
plasma-generating apparatus is constituted of a reactor connected
with a vacuum pump and a thermal plasma torch. The thermal plasma
torch is provided with a gas introducing portion for generating
plasma and a raw material powder-introducing portion at one end of
a water-cooled torch composed of a quartz double tube, and is
connected to the reactor. An induction coil for applying
radiofrequency is provided outside the torch. The pressure in the
reactor connected with the torch is reduced with the vacuum pump, a
gas for generating plasma such as an argon gas is flowed at a
predetermined flow rate from the gas-introducing portion, and a
radiofrequency of 3 to 40 MHz (4 MHz in general) is applied to the
induction coil, whereby plasma is inductively generated in the
torch by a magnetic field generated by a radiofrequency current.
Next, a raw material is introduced into the generated plasma to
react, thereby preparing silicon nanoparticles. When the raw
material is a solid powder material, the raw material is introduced
together with a carrier gas.
Example TP1
[0339] The above radiofrequency (RF) inductively coupled thermal
plasma-generating apparatus was used. First, the inside of the
reactor was evacuated to a vacuum with the vacuum pump, and an
argon gas and a hydrogen gas as gases for generating plasma were
flowed at flow rates of 200 liters/min and 10 liters/min,
respectively, and the total pressure of the gases was controlled to
50 kPa. A radiofrequency of 4 kHz was applied to the induction coil
with an electric power of 80 kW so that plasma was generated. Next,
a powder raw material obtained by mixing 97 parts by weight of a
silicon powder having an average particle diameter of 4 .mu.m and 3
parts by weight of zirconia having an average particle diameter of
1 .mu.m was fed into the thermal plasma at a feeding rate of about
500 g/h together with an argon gas flowed at a flow rate of 15
liters/min as a carrier gas. The raw material was subjected to a
reaction for a predetermined reaction time, whereby a fine powder
material was obtained. Then, the application of the radiofrequency
was stopped, the introduction of the gases for generating plasma
was stopped, and slow oxidation was carried out. After that,
nanoparticles were taken out. It should be noted that the slow
oxidation was performed by flowing an argon gas containing oxygen
as an impurity and having a purity of 999.99% into the reactor.
[0340] The material powder obtained by the above method was
irradiated with an ultrasonic wave in isopropyl alcohol so as to be
dispersed. The resultant droplets were dropped to a microgrid
obtained by coating a copper mesh with a carbon membrane having
pores, and were dried, whereby a sample for observation with an
electron microscope was produced. The shape of the above composite
powder was observed by using the sample for observation with a
scanning electron microscope (SEM), a scanning transmission
electron microscope (STEM), or a transmission electron microscope
(TEM), and the composite powder was analyzed for its composition by
using the sample with an energy dispersive characteristic X-ray
spectrometer (EDX).
[0341] FIG. 7 shows the bright field image and high-resolution
image of the resultant powder with a TEM. In the high-resolution
image of FIG. 7, lattice fringes of silicon were observed. In the
TEM image of FIG. 7, an amorphous phase having a thickness of
several nanometers to ten nanometers was observed on the surface of
crystalline silicon having a diameter of 10 nm to 80 nm.
Example TP2
[0342] Nanoparticles were obtained by the same operations as in
Example TP1 described above except that a powder raw material
obtained by mixing 95 parts by weight of a silicon powder having an
average particle diameter of 4 .mu.m and 5 parts by weight of
zirconia having an average particle diameter of 1 .mu.m was used
instead of the powder raw material obtained by mixing 97 parts by
weight of a silicon powder having an average particle diameter of 4
.mu.m and 3 parts by weight of zirconia having an average particle
diameter of 1 .mu.m.
[0343] FIG. 8 shows the high-resolution image of the resultant
powder with a TEM.
[0344] In the high-resolution image with a TEM of FIG. 8, an
amorphous phase having a thickness of 0.5 nm to 5 nm was observed
on the surface of crystalline silicon having a diameter of 5 nm to
100 nm.
[0345] Comparison between the results of X-ray photoelectron
spectroscopy (XPS) of Examples TP1 and TP2 (see FIG. 17) showed
that the amount of silicon oxide of Example TP2 was small, and that
an increase in the amount of zirconia ZrO.sub.2 as a raw material
could suppress the oxidation of silicon.
[0346] Metal zirconium can also be used as a raw material for the
Zr element of each of Examples TP1 and TP2 described above, though
zirconia cheaper than the metal was used in the Examples.
Example TP3
[0347] Nanoparticles were obtained by the same operations as in
Example TP1 described above except that a powder raw material
obtained by mixing 97 parts by weight of a silicon powder having an
average particle diameter of 4 .mu.m and 3 parts by weight of
lanthanum oxide having an average particle diameter of 10 .mu.m was
used instead of the powder raw material obtained by mixing 97 parts
by weight of a silicon powder having an average particle diameter
of 4 .mu.m and 3 parts by weight of zirconia having an average
particle diameter of 1 .mu.m.
[0348] FIG. 9 shows the bright field image and high-resolution
image of the resultant powder with a TEM. In the high-resolution
image with a TEM of FIG. 9, an amorphous phase having a thickness
of 1 nm to 5 nm was observed on the surface of crystalline silicon
having a diameter of 10 nm to 50 nm.
Example TP4
[0349] Nanoparticles were obtained by the same operations as in
Example TP1 described above except that a powder raw material
obtained by mixing 97 parts by weight of a silicon powder having an
average particle diameter of 4 .mu.m and 3 parts by weight of
calcium oxide having an average particle diameter of 5 .mu.m was
used instead of the powder raw material obtained by mixing 97 parts
by weight of a silicon powder having an average particle diameter
of 4 .mu.m and 3 parts by weight of zirconia having an average
particle diameter of 1 .mu.m.
[0350] FIG. 10 shows the bright field image and high-resolution
image of the resultant powder with a TEM. In the high-resolution
image with a TEM of FIG. 10, an amorphous phase having a thickness
of several nanometers was observed on the surface of crystalline
silicon having a diameter of 10 nm to 50 nm.
Example TP5
[0351] Nanoparticles were obtained by the same operations as in
Example TP1 described above except that a powder raw material
obtained by mixing 97 parts by weight of a silicon powder having an
average particle diameter of 4 .mu.m and 3 parts by weight of
magnesia having an average particle diameter of 10 .mu.m was used
instead of the powder raw material obtained by mixing 97 parts by
weight of a silicon powder having an average particle diameter of 4
.mu.m and 3 parts by weight of zirconia having an average particle
diameter of 1 .mu.m.
[0352] FIG. 11 shows the bright field image and high-resolution
image of the resultant powder with a TEM. In the high-resolution
image with a TEM of FIG. 11, an amorphous phase having a thickness
of several nanometers was observed on the surface of crystalline
silicon having a diameter of 10 nm to 100 nm.
Example TP6
[0353] Nanoparticles were obtained by the same operations as in
Example TP1 described above except that a powder raw material
obtained by mixing 95 parts by weight of a silicon powder having an
average particle diameter of 4 .mu.m and 5 parts by weight of
alumina having an average particle diameter of 1 .mu.m was used
instead of the powder raw material obtained by mixing 97 parts by
weight of a silicon powder having an average particle diameter of 4
.mu.m and 3 parts by weight of zirconia having an average particle
diameter of 1 .mu.m.
[0354] FIG. 12A shows a secondary electron image of the resultant
powder with an SEM, and FIG. 12B shows the bright field image and
high-resolution image of the powder with a TEM at its upper stage
and lower stage, respectively. Fibrous substances each connecting a
silicon crystal particle and another silicon crystal particle were
observed. From analysis with the energy dispersive X-ray
spectrometer (EDX) of the TEM, it was inferred that most of the
fibrous substances in the resultant powder were each formed of
amorphous silicon oxide SiO.sub.2, but lattice fringes of silicon
were observed as shown in the lower stage of FIG. 12B, and at some
sites, a core portion was formed of a silicon crystal and a shell
portion outside the core portion was formed of an amorphous silicon
oxide. Probably, fibrous silicon was initially produced, and then
fibrous silicon having a large surface area was oxidized by slow
oxidation to form amorphous silicon oxide in consideration of: an
estimate that the amount of oxygen atoms in 5 parts by weight of
introduced alumina could not result in the production of such
amount of silicon oxide as described above; and the high-resolution
image with the TEM at the lower stage of FIG. 12B.
[0355] In the SEM image and TEM image of FIGS. 12A and 12B, an
amorphous phase having a thickness of several nanometers was
observed on the surface of crystalline silicon having a diameter of
20 nm to 50 nm. In addition, the amorphous fibrous substances each
had a diameter of 10 nm to 70 nm and a length of 100 nm to 2
.mu.m.
Example TP7
[0356] A powder was obtained by the same operations as in Example
TP1 described above except that a powder raw material obtained by
mixing 90 parts by weight of a silicon powder having an average
particle diameter of 4 .mu.m and 10 parts by weight of metal
aluminum having an average particle diameter of 1 .mu.m was used
instead of the powder raw material obtained by mixing 97 parts by
weight of a silicon powder having an average particle diameter of 4
.mu.m and 3 parts by weight of zirconia having an average particle
diameter of 1 .mu.m.
[0357] FIG. 13 shows the bright field image and high-resolution
image of the resultant powder with a TEM at its upper stage and
lower stage, respectively. Although fibrous portions were partly
observed with the TEM, most of the observed products were particles
each obtained by providing an amorphous phase having a thickness of
1 nm to 10 nm on the surface of crystalline silicon having a
diameter of 20 nm to 200 nm. In addition, analysis with the EDX of
the TEM showed that aluminum oxide was formed on the surface of
each of the particles.
Example TP8
[0358] Nanoparticles were obtained by the same operations as in
Example TP1 described above except that a powder raw material
obtained by mixing 95 parts by weight of silicon dioxide (Quartz)
powder having an average particle diameter of 4 .mu.m and 5 parts
by weight of metal aluminum having an average particle diameter of
1 .mu.m was used instead of the powder raw material obtained by
mixing 97 parts by weight of a silicon powder having an average
particle diameter of 4 .mu.m and 3 parts by weight of zirconia
having an average particle diameter of 1 .mu.m.
[0359] A peak in the X-ray diffraction chart of the resultant
powder showed that Si crystals were produced.
Comparative Example TP1
[0360] Particles were obtained by the same operations as in Example
TP1 described above except that a powder raw material obtained by
mixing 90 parts by weight of a silicon powder having an average
particle diameter of 4 .mu.m and 10 parts by weight of titanium
dioxide having an average particle diameter of 1 .mu.m was used
instead of the powder raw material obtained by mixing 97 parts by
weight of a silicon powder having an average particle diameter of 4
.mu.m and 3 parts by weight of zirconia having an average particle
diameter of 1 .mu.m.
[0361] A peak in the X-ray diffraction chart of the resultant
powder showed that SiO.sub.2 was produced, though Si remained.
SiO.sub.2 was considered to be produced by a reaction between
atomic oxygen produced by the decomposition reaction of TiO.sub.2
in high-temperature plasma and part of Si atoms probably because
Gibbs free energy in the oxidation reaction of Si was close to that
in the oxidation reaction of Ti.
[0362] It should be noted that although silicon and alumina were
used as raw materials in Example TP6 described above, and silicon
and aluminum were used as raw materials in Example TP7 described
above, silane tetrachloride, silane trichloride, silane dichloride,
silane monochloride, silane or disilane can be used instead of
silicon as one raw material, and aluminum chloride can be used
instead of alumina or aluminum as another raw material.
[0363] Although silicon is used as a raw material in each of
Examples TP1 to TP8 described above, tin nanoparticles can be
prepared by using metal tin instead of silicon as a raw material.
Of course, a tin compound can be used instead of metal tin.
[0364] B. Preparation by Heat Treatment
[0365] In addition, a specific method of obtaining silicon or tin
nanoparticles by heating a silicon compound (a silicon elementary
substance is also included in the category of the "silicon
compound") or a tin compound (a tin elementary substance is also
included in the category of the "tin compound"), and a metal
compound containing a metal elementary substance other than silicon
and tin is as described below; provided that Gibbs free energy at
the time of the production of the oxide of the metal is smaller
than Gibbs free energy when silicon or tin is oxidized, and the
metal oxide has higher thermodynamic stability than silicon oxide
or tin oxide.
[0366] One specific example is as follows: a fine powder of an
alloy formed of at least silicon or tin and the metal is subjected
to heat treatment under an inert gas or hydrogen gas atmosphere at
a temperature equal to or higher than the melting point of the
metal and lower than the melting point of silicon (1,412.degree.
C.). Since tin has a melting point as low as 232.degree. C., a
carbon powder of graphite or the like is mixed into the powder
before the above heat treatment is performed in order that tin may
be dispersed.
Example MAT1
[0367] 100 parts by weight of a silicon powder having an average
particle diameter of 0.2 .mu.m obtained by pulverizing metal
silicon with a bead mill and 15 parts by weight of an aluminum
powder having an average particle diameter of 3 .mu.m were
subjected to mechanical alloying treatment in a planetary ball mill
apparatus using a ball made of zirconia and a pot made of zirconia
at 800 rpm for 90 minutes, whereby a silicon-aluminum powder was
obtained. Next, the resultant powder was subjected to heat
treatment in a high-temperature reactor, which had been evacuated
to a vacuum, in a stream of an argon gas at atmospheric pressure
(0.1 MPa) and 900.degree. C. for 1 hour, whereby a silicon-aluminum
heat-treated powder was obtained. The surfaces of the silicon
powder as a raw material turned into fine particles by
pulverization with the bead mill are oxidized so that a silicon
oxide layer is formed on the surface, but are reduced by
aluminum.
[0368] FIG. 14 shows a secondary electron image of the resultant
powder with an SEM at its upper stage, and shows the bright field
image and high-resolution image of the powder with a TEM at its
middle stage and lower stage, respectively. Fibrous substances each
connecting a silicon crystal particle and another silicon crystal
particle were observed. Fibrous substances each connecting a
silicon nanoparticle and another silicon nanoparticle were
observed. Analysis with the EDX of the TEM showed that each of the
above fibrous substances was a crystal mainly formed of the
following elements: Al, N, and O. In addition, in X-ray
photoelectron spectroscopy (XPS), alumina and silicon oxide were
detected at the surface layer of the resultant powder. In view of
the foregoing, it is considered that fibrous (filamentous) aluminum
oxynitride connecting a silicon crystal particle and another
silicon crystal particle has grown by virtue of heat treatment
using as a catalyst the aluminum element of the Si--Al alloy powder
obtained by mechanical alloying treatment. In view of the decrease
in the amount of silicon oxide, aluminum is considered to have
reacted with the oxygen of silicon oxide. In addition, as for the
nitrogen element, it is considered that nitrogen in the air was
included to react with Al during the mechanical alloying.
Production Method Example MAT2
[0369] 100 parts by weight of a silicon powder having an average
particle diameter of 0.2 .mu.m obtained by pulverizing metal
silicon (having a purity of 99%) with a bead mill and 15 parts by
weight of an aluminum powder having an average particle diameter of
3 .mu.m are subjected to mechanical alloying treatment in a
planetary ball mill apparatus using a ball made of zirconia and a
pot made of zirconia at 800 rpm for 90 minutes, whereby a
silicon-aluminum alloy powder is obtained. Next, the resultant
powder is formed into pellets with a high-pressure press under
reduced pressure. The pellets are inserted into a reaction chamber
of a laser ablation apparatus, and the chamber is evacuated to a
vacuum. The pellets are heated to 450.degree. C. The atmosphere in
the chamber is replaced with an argon gas, and the argon gas is
flowed so that the pressure in the chamber is adjusted to 0.65 MPa.
Next, the silicon-aluminum alloy pellets are irradiated with an
excimer laser pulse, whereby a fine powder is obtained on the inner
wall of the reaction chamber. Next, the argon gas is flowed so that
the fine powder is slowly oxidized with impurity oxygen in the
argon gas. After that, the fine powder is taken out.
[0370] The amount of silicon oxide to be produced can be made
extremely small because aluminum reacts with oxygen more readily
than silicon.
[0371] The above pellets as the target of laser ablation is
produced by the mechanical alloying and the press molding; an ingot
produced by mixing and melting silicon and metal aluminum as raw
materials may be used. Although an example in which excimer laser
light is used has been described here, CO.sub.2 laser light or YAG
laser light can also be used as long as laser light to be used has
power sufficient for the evaporation of silicon.
Production Method Example MAT3
[0372] 100 parts by weight of a silicon powder having an average
particle diameter of 0.2 .mu.m obtained by pulverizing metal
silicon with a bead mill, 15 parts by weight of an aluminum powder
having an average particle diameter of 3 .mu.m, and 2 parts by
weight of a silicon dioxide powder having an average particle
diameter of 1 .mu.m are subjected to mechanical alloying treatment
in a planetary ball mill apparatus using a ball made of zirconia
and a pot made of zirconia at 800 rpm for 90 minutes, whereby a
silicon-aluminum-silicon dioxide composite powder is obtained.
Next, the resultant powder is placed into the graphite mold of a
spark plasma sintering apparatus and pressed, and the apparatus is
evacuated to a vacuum. After that, the pressed powder is heated to
600.degree. C. After that, a current pulse of 10 V and 200 A is
applied across the graphite mold at intervals of microseconds so
that spark plasma is generated across the powder to sinter the
powder. The resultant sintered pellets are pulverized, whereby a
silicon composite powder is obtained.
[0373] Another method example is as follows: a silicon compound and
a compound of the metal, or a tin compound and a compound of the
metal are subjected to a heat treatment at a temperature equal to
or higher than the boiling point of the above compound and lower
than the melting point of silicon (1,412.degree. C.).
[0374] Specific examples of the preparation of a silicon powder are
given below.
[0375] A silicon powder can be obtained also by the heat
decomposition reaction of a silicon compound.
Production Method Example TC1
[0376] A silicon-aluminum composite fine powder can be obtained by
the heat decomposition reaction of a mixture obtained by mixing 12
parts by weight of aluminum trichloride and 87 parts by weight of
silane tetrachloride, the mixture being introduced in advance with
a hydrogen gas as a carrier gas into a heat decomposition reactor
in which an argon gas set at 900.degree. C. is flowed. The fine
powder is slowly oxidized, whereby a silicon-aluminum oxide
composite fine powder is obtained.
[0377] Table 1 collectively shows the diameters of the primary
particles of the powder materials obtained in the above Examples,
determined from images as a result of observation with an electron
microscope.
TABLE-US-00001 TABLE 1 Observation with electron microscope
Diameter of Thickness of spherical amorphous Fibrous substance
Example Raw material primary particle surface layer Diameter Length
TP1 Si--3% ZrO.sub.2 10-80 nm 2-10 nm -- -- TP2 Si--5% ZrO.sub.2
5-100 nm 0.5-5 nm -- -- TP3 Si--3% La.sub.2O.sub.3 10-50 nm 1-5 nm
-- -- TP4 Si--3% CaO 10-50 nm 2-3 nm -- -- TP5 Si--3% MgO 10-100 nm
2-3 nm -- -- TP6 Si--5% Al.sub.2O.sub.3 20-50 nm 2-3 nm 10-70 nm
100 nm-2 .mu.m TP7 Si--10% Al 20-200 nm 1-10 nm -- -- MAT1
Pulverized 5-100 nm 0.5-10 nm 5-100 nm 100-500 nm Si--13% Al
[0378] Further, each of the above composite powders can be
evaluated for its crystallite size from the half width of a peak in
a chart obtained by the X-ray diffractometry of the powder and
Scherrer's equation described above.
[0379] FIGS. 16A to 16I show various X-ray diffraction charts of
the powders obtained in Examples TP1 to TP7, Comparative Example
TP1, and Example TP8 measured with a Cu tube. The axis of abscissa
indicates 2.theta. (.theta. represents the Bragg angle of a
diffraction beam), and the axis of ordinate indicates X-ray
intensity. FIGS. 16A to 16G show that Si crystals are produced in
each of Examples TP1 to TP7.
[0380] In the chart of the powder material obtained in Comparative
Example TP1 shown in FIG. 16H, the peak of SiO.sub.2 as well as the
peak of Si was observed around 2.theta.=26.6.degree., so it was
found that SiO.sub.2 was also produced by a reaction between Si and
TiO.sub.2 in thermal plasma. The production of SiO.sub.2 and the
production of TiO.sub.2 are considered to occur simultaneously
because the Gibbs free energy of the production of SiO.sub.2 and
that of the production of TiO.sub.2 in Ellingham plots are
substantially equal to each other. In a reaction in thermal plasma
caused by the addition of any other oxide stabler than SiO.sub.2,
almost no SiO.sub.2 to be detected by X-ray diffractometry is
produced. In the chart of the powder material obtained in Example
TP8 shown in FIG. 16I, the peak of Si was observed around
2.theta.=28.4.degree., so it was found that Si was produced by a
reaction between SiO.sub.2 and Al in thermal plasma.
[0381] In addition, Table 2 collectively shows crystallite sizes
each calculated from the half width of an Si(111) peak in the X-ray
diffraction chart of each of the powder materials obtained in the
above examples and Scherrer's equation.
TABLE-US-00002 TABLE 2 Peak Half Raw position width Half width
Crystallite Example material 2.theta. (.degree.) 2.theta.
(.degree.) (rad) size (nm) TP1 Si--3% ZrO.sub.2 28.39 0.35 0.006108
24.5 TP2 Si--5% ZrO.sub.2 28.43 0.34 0.005934 25.2 TP3 Si--3%
La.sub.2O.sub.3 28.43 0.34 0.005934 25.2 TP4 Si--3% CaO 28.44 0.31
0.00541 27.6 TP5 Si--3% MgO 28.44 0.33 0.002729 25.9 TP6 Si--5%
Al.sub.2O.sub.3 28.77 0.32 0.005585 26.8 TP7 Si--10% Al 28.47 0.34
0.005931 25.2 MAT1 Pulverized 28.47 0.19 0.003299 54.2 Si--13%
Al
[0382] Further, Table 3 collectively shows the results of the
analysis of the powder materials obtained in the above Examples
with the EDX of an STEM.
TABLE-US-00003 TABLE 3 Analysis with EDX of STEM Wt % Atomic % Raw
Metal Metal Molar ratio Example material Si O element Si O element
O/Si TP1 Si--3% ZrO.sub.2 72.23 26.6 1.16 60.55 39.14 0.3 0.64 TP2
Si--5% ZrO.sub.2 85.26 11.67 3.06 79.91 19.2 0.88 0.24 TP3 Si--3%
La.sub.2O.sub.3 88.05 10.41 1.52 82.56 17.14 0.28 0.20 TP4 Si--3%
CaO 82.06 16.12 1.80 73.50 25.36 1.13 0.34 TP5 Si--3% MgO 81.2
18.06 0.73 71.37 27.87 0.74 0.39 TP6 Si--5% Al.sub.2O3 68.22 29.67
2.09 55.69 42.52 1.78 0.76 TP7 Si--10% Al 77.61 4.18 18.20 74.70
7.06 18.23 0.09 MAT1 Pulverized 51.93 9.66 28.81 43.96 14.36 25.39
0.32 Si--13% Al
[0383] In addition, Table 4 collectively shows SiO.sub.x/Si ratios
based on Si2p peaks in the XPS of the powder materials obtained in
the above Examples. It should be noted that each SiO.sub.x/Si ratio
was calculated on the assumption that an Si2p peak was formed of
two peaks, i.e., the peak of metal Si and the peak of SiO.sub.x,
and SiO.sub.x and metal Si were uniformly distributed.
TABLE-US-00004 TABLE 4 SiO.sub.x/Si ratio based on Si2p peak in XPS
Example Raw material SiO.sub.x/Si TP1 Si--3% ZrO.sub.2 1.39 TP2
Si--5% ZrO.sub.2 0.37 TP3 Si--3% La.sub.2O.sub.3 0.42 TP4 Si--3%
CaO 0.64 TP5 Si--3% MgO 0.51 TP6 Si--5% Al.sub.2O.sub.3 7.14 TP7
Si--10% Al 0.28 MAT1 Pulverized Si--13% Al 1.16 (900.degree.
C.)
[0384] Further, FIG. 17 shows Si2p peaks in the XPS of the material
powders obtained in Examples TP1 to TP6 and powder silicon obtained
by pulverizing metal silicon with a bead mill. A peak at lower
binding energy is considered to be the peak of Si, and a peak at
higher binding energy is considered to be the peak of silicon oxide
SiO.sub.x mainly formed of SiO.sub.2. The results of the XPS are
considered to reflect information about the surfaces of the
measurement samples.
[0385] Comparison between the results of the X-ray photoelectron
spectroscopy (XPS) of Examples TP1 and TP2 (see FIG. 17 and Table
4) showed that the amount of silicon oxide in Example TP2 was
small, and that an increase in the amount of zirconia ZrO.sub.2 as
a raw material could suppress the oxidation of silicon. Since
zirconia ZrO.sub.2 as a raw material is decomposed into atoms or
ions in thermal plasma, the above increase is a de facto increase
in the amount of Zr.
[0386] In addition, an SiO.sub.x/Si ratio in any other measurement
sample obtained under the production conditions in Example TP7 was
as low as 0.06.
[0387] [Production of Electrode Structure for Negative Electrode of
Energy Storage Device]
Example EA1 to EA7
[0388] 100 parts by weight of each of the composite powders
prepared in Examples TP1 to TP4, Example TP6, Example TP7, and
Example MAT1, 70 parts by weight of artificial graphite having an
average particle diameter of 5 .mu.m, and 3 parts by weight of
acetylene black are mixed by means of a planetary ball mill
apparatus using a ball made of agate at 300 rpm for 20 minutes.
Next, 132 parts by weight of a solution of N-methyl-2-pyrrolidone
containing 15 wt % of polyamideimide and 130 parts by weight of
N-methyl-2-pyrrolidone are added to the resultant mixture, and the
whole is mixed by means of the planetary ball mill apparatus at 300
rpm for 10 minutes, whereby slurry for the formation of an
electrode active material layer is prepared. The resultant slurry
is applied onto copper foil having a thickness of 10 .mu.m with an
applicator, and is then dried at 110.degree. C. for 0.5 hour.
Further, the resultant is dried under reduced pressure at
200.degree. C., and its thickness and density are adjusted with a
roll press, whereby an electrode structure in which an electrode
active material layer having a thickness in the range of 20 to 40
.mu.m and density in the range of 0.9 to 1.9 g/cm.sup.3 has been
formed on a current collector made of the copper foil is
obtained.
[0389] It should be noted that electrode structures each produced
from any one of the respective composite powders prepared in
Examples TP1 to TP4, Example TP6, Example TP7, and Example MAT1 by
the above operations are defined as the electrodes of Examples EA1
to EA7.
Comparative Example EA1
[0390] 100 parts by weight of a silicon powder obtained by
pulverizing metal silicon with a bead mill in isopropyl alcohol and
having an average particle diameter of 0.2 .mu.m, 70 parts by
weight of artificial graphite, and 3 parts by weight of acetylene
black are mixed by means of a planetary ball mill apparatus using a
ball made of agate at 300 rpm for 20 minutes. It should be noted
that FIG. 15 is an image as a result of the observation of the bead
mill-pulverized silicon powder with a scanning electron microscope
(SEM). Next, 132 parts by weight of a solution of
N-methyl-2-pyrrolidone containing 15 wt % of polyamideimide and 130
parts by weight of N-methyl-2-pyrrolidone are added to the
resultant mixture, and the whole is mixed by mean of the planetary
ball mill apparatus at 300 rpm for 10 minutes, whereby slurry for
the formation of an electrode active material layer is prepared.
The resultant slurry is applied onto copper foil having a thickness
of 10 .mu.m with an applicator, and is then dried at 110.degree. C.
for 0.5 hour. Further, the resultant is dried under reduced
pressure at 200.degree. C., and its thickness and density are
adjusted with a roll press, whereby an electrode structure in which
an electrode active material layer having a thickness of 20 .mu.m
and a density of 1.3 g/cm.sup.3 has been formed on a current
collector made of the copper foil is obtained.
[0391] It should be noted that the electrode active material layer
can be formed on the copper foil by the following method as well:
after the viscosity of the slurry obtained by the above procedure
has been adjusted, a high voltage is applied between the copper
foil as a current collector and a nozzle of an electrospinning
apparatus.
[0392] [Evaluation of the Electrode Structure for the Negative
Electrode of The Energy Storage Device for Amount of Lithium to be
Electrochemically Inserted]
[0393] Each of the above electrode structures for the negative
electrodes of the energy storage devices was evaluated for the
amount of lithium to be electrochemically inserted by the following
procedure.
[0394] Each of the above electrode structures of Examples EA1 to
EA7 and Comparative Example EA1 is cut into a predetermined size,
and a lead made of a nickel ribbon is connected to the above
electrode structure by spot welding, whereby an electrode as a
working electrode is produced. A cell is produced by combining the
produced electrode and metal lithium as a counter electrode, and is
evaluated for the amount of lithium to be electrochemically
inserted.
[0395] The lithium electrode is produced by pressure-adhering metal
lithium foil having a thickness of 140 .mu.m to copper foil to
which a lead made of a nickel ribbon connected by spot welding and
whose one surface is roughened.
[0396] An evaluation cell is produced by the following procedure.
That is, a polyethylene film of a micropore structure having a
thickness of 17 .mu.m and a porosity of 40% as a separator is
sandwiched between an electrode produced from each of the above
electrode structures and the above lithium electrode under a dry
atmosphere having a dew point of -50.degree. C. or lower. A set of
the electrode (working electrode), the separator, and the lithium
electrode (counter electrode) is inserted into a battery case
obtained by forming an aluminum laminated film of a
polyethylene/aluminum foil/nylon structure into a pocket shape. An
electrolyte solution is dropped to the above battery case, and the
laminated film at the opening portion of the battery case is
thermally welded in a state in which the lead is taken out of the
battery case, whereby the evaluation cell is produced. A solution
obtained by dissolving lithium hexafluorophosphate (LiPF.sub.6) in
a solvent prepared by mixing ethylene carbonate from which moisture
has been sufficiently removed and diethyl carbonate from which
moisture has been sufficiently removed at a volume ratio of 3:7 so
that the solution has a concentration of 1 mol/l (M), is used as
the above electrolyte solution.
[0397] Evaluation for the amount of lithium to be electrochemically
inserted is performed by: discharging the above cell thus produced
with the lithium electrode of the cell as a negative electrode and
each produced working electrode as a positive electrode until the
voltage of the cell comes to be 0.01 V; and charging the cell until
the voltage of the cell comes to be 1.80 V. That is, the quantity
of discharged electricity is defined as an electrical quantity
utilized for the insertion of lithium, and the quantity of charged
electricity is defined as an electrical quantity utilized for the
release of lithium.
[0398] [Evaluation for Insertion/Extraction of Li of Electrode]
[0399] The cell was charged and discharged 50 times at a current
density of 0.48 mA/cm.sup.2, and an electrode formed of any one of
the various active materials was evaluated for insertion/extraction
of Li on the basis of the amount of Li to be inserted (electrical
quantity) for the first charging and discharging, the amount of Li
to be extracted (electrical quantity) for the first charging and
discharging, a ratio (%) of the amount of Li to be extracted to the
amount of Li to be inserted for the first charging and discharging,
a ratio of the amount of Li to be extracted (electrical quantity)
for the tenth charging and discharging to the amount of Li to be
released for the first charging and discharging, and a ratio of the
amount of Li to be extracted (electrical quantity) for the fiftieth
charging and discharging to the amount of Li to be extracted for
the tenth charging and discharging.
[0400] Table 4 collectively shows the evaluation results.
TABLE-US-00005 TABLE 5 Li insertion/extraction performance of
electrode Raw material for preparation 1st Li 1st Li Li of
electrode insertion extraction 1st Li extraction Li extraction
Example material mAh/g mAh/g extraction/insertion % 10th/1st
50th/10th Example EA1 Si--3% ZrO.sub.2 1,509 865 57.3 0.74 0.82
Example EA2 Si--5% ZrO.sub.2 1,829 1,457 79.1 0.49 0.22 Example EA3
Si--3% La.sub.2O.sub.3 1,895 1,520 80.2 0.79 0.47 Example EA4
Si--3% CaO 1,734 1,353 78.0 0.56 0.22 Example EA5 Si--5%
Al.sub.2O.sub.3 1,331 673 50.6 0.83 0.98 Example EA6 Si--10% Al
1,935 1,671 86.4 0.82 -- Example EA7 Pulverized 1,629 1,411 86.6 --
-- Si--13% Al Comparative Bead mill- 1,745 1,398 80.1 0.53 --
Example EA1 pulverized Si
[0401] The results of the evaluation of the above electrode for
electrochemical insertion/extraction of lithium, and the results
previously shown in Tables 3 and 4 show that an electrode using a
material having a small silicon oxide content as an active material
shows a high ratio of the amount of lithium to be released to the
amount of lithium to be inserted for the first charging and
discharging. The foregoing suggests that the present invention
allows the production of a material having a small silicon oxide
content, and an energy storage device adopting an electrode formed
of the material as its negative electrode shows improved initial
charge and discharge coulombic efficiency. Therefore, according to
the method of the present invention, silicon or tin fine particles
in which a silicon oxide or tin oxide content is reduced can be
obtained, and as a result, a material showing high lithium
extraction/insertion efficiency can be obtained.
[0402] In addition, the electrode of Example EA5 produced from the
material powder obtained in Example TP6 contains a large amount of
silicon oxide owing to slow oxidation, and shows low initial
lithium extraction/insertion efficiency. In this case, however, the
amount of lithium to be extracted shows a small change, and is
stable over a long time period (see "Li extraction 50th/10th" in
Table 5), so silicon particles formed into a network structure with
fibrous (filamentous) substances is considered to be effective in
lengthening the lifetime of the electrode. Further, there is a
possibility that silicon oxide content can be reduced, and hence
initial lithium extraction/insertion efficiency can be improved by
changing the production conditions of Example TP6 (such as an
increase in the raw material ratio of Al.sub.2O.sub.3 or the
addition of Al to the raw materials).
[0403] When making reference to Example EA6 as an example of the
above electrode structure of the present invention, since the
content of the active material produced in Example TP7 in the
electrode material layer is 51.9 wt %, the amount of Li to be
inserted for the first discharging in the active material and the
amount of Li to be extracted for the first charging in the active
material are 3,730 mAh/g and 3,221 mAh/g, respectively, which are
about ten times as much as the amount of lithium to be
inserted/extracted in graphite.
[0404] [Production of Electrode Structure for Positive Electrode of
the Energy Storage Device]
[0405] An example of a method of preparing the electrode structure
of the present invention to be used in the positive electrode
active material of the energy storage device will be given
below.
Example EC1
[0406] 100 parts by weight of a lithium nickel cobalt manganite
LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2 powder and 4 parts by
weight of acetylene black are mixed with a planetary ball mill
apparatus using a ball made of agate at 300 rpm for 10 minutes.
Further, 50 parts by weight of a solution of N-methyl-2-pyrrolidone
containing 10 wt % of polyvinylidene fluoride and 50 parts by
weight of N-methyl-2-pyrrolidone are added to the resultant
mixture, and the whole is mixed with the planetary ball mill
apparatus at 300 rpm for 10 minutes, whereby slurry for the
formation of an electrode active material layer is prepared.
[0407] The resultant slurry was applied onto aluminum foil having a
thickness of 14 .mu.m with a coater, and was then dried at
110.degree. C. for 1 hour. Further, the resultant was dried under
reduced pressure at 150.degree. C. Subsequently, the thickness of
the resultant was adjusted with a roll press, whereby an electrode
structure in which an electrode active material layer having a
thickness of 82 .mu.m and a density of 3.2 g/cm.sup.3 had been
formed on a current collector made of the aluminum foil was
obtained.
[0408] The resultant electrode structure was cut into a
predetermined size, and a lead made of an aluminum ribbon was
connected to the above electrode structure by ultrasonic welding,
whereby a LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2 electrode was
produced.
Example of Procedure for Production of the Energy Storage
Devices
[0409] Assembling was carried out totally under a dry atmosphere in
which moisture was controlled so that a dew point was -50.degree.
C. or lower.
[0410] Each of the electrode structures of Examples EA1 to EA7 and
Comparative Example EA1 is cut into a predetermined size, and a
lead made of a nickel ribbon is connected to the above electrode
structure by spot welding, whereby a negative electrode is
produced.
[0411] A separator was sandwiched between the negative electrode
prepared by the above operations and the positive electrode of
Example EC1, and the group of electrodes, i.e., the negative
electrode, the separator, and the positive electrode was inserted
into a cell case obtained by forming an aluminum laminated film of
a polyethylene/aluminum foil/nylon structure into a pocket shape.
An electrolyte solution was injected into the case, an electrode
lead was taken out, and the resultant was heat-sealed, whereby a
cell for evaluation with its positive electrode capacity limited
was produced. The outside of the above aluminum laminated film is
made of a nylon film, and the inside of the laminated film is made
of a polyethylene film.
[0412] In addition, for example, a polyethylene microporous film
having a thickness of 17 .mu.m was used as the above separator.
[0413] An electrolyte solution prepared by, for example, the
following procedure was used as the above electrolyte solution.
First, a solvent was prepared by mixing ethylene carbonate from
which moisture had been sufficiently removed and diethyl carbonate
from which moisture had been sufficiently removed at a volume ratio
of 3:7. Next, a lithium hexafluorophosphate (LiPF.sub.6) was
dissolved in the above solvent thus obtained so as to have a
concentration of 1 mol/l (M), whereby the electrolyte solution was
prepared.
[0414] [Charging and Discharging Test]
[0415] Each of the above energy storage devices was charged at a
constant current density of 0.48 mA/cm.sup.2 until its cell voltage
reached 4.2 V. After that, the device was charged at a constant
voltage of 4.2 V. After 10 minutes pause, the device was discharged
at a constant current density of 0.48 mA/cm.sup.2 until its cell
voltage was lowered to 2.7 V. After that, the device was caused to
pause for 10 minutes. Such charging and discharging was repeated
twice, and then the device was repeatedly charged and discharged at
a current density of 1.6 mA/cm.sup.2.
[0416] FIG. 18 shows the discharge characteristics of a cell using
the electrode of Example EA6 as a negative electrode and the
electrode of Example EC1 as a positive electrode for the first
charging and discharging.
[0417] [Method for Evaluation Test for Power Density and Energy
Density]
[0418] Each of the above energy storage devices was charged at a
constant current and a constant voltage. After that, the device was
discharged at predetermined power until its cell voltage was
lowered to a predetermined value, and the energy which had been
discharged was measured. The device was charged at a constant
current density of 1.6 mA/cm.sup.2 until its cell voltage reached
4.2 V. After that, the device was further charged at a constant
voltage of 4.2 V. After 5 minutes pause, the device was discharged
at constant power until its cell voltage was lowered to 2.7 V, and
then discharged energy was measured.
[0419] Each energy storage device was evaluated for its
power-energy characteristics with the so-called Ragon plots as a
view showing power density (W/kg) with respect to energy density
(Wh/kg) or specific power (W/L) with respect to specific energy
(Wh/L). The density was calculated by using the volume and weight
of each of the negative electrode, the separator, and the positive
electrode. The calculation was made on the assumption that the
respective electrode material layers were formed on both sides of
current collector foil.
[0420] FIGS. 19A and 19B show the Ragone plots of a cell using the
electrode of Example EA6 as a negative electrode and the electrode
of Example EC1 as a positive electrode with reference to those of a
capacitor using an active carbon electrode as each of its positive
electrode and negative electrode.
[0421] FIG. 19A shows power density (W/kg) with respect to energy
density (Wh/kg), and FIG. 19B shows specific power (W/L) with
respect to specific energy (Wh/L).
[0422] FIG. 19A showed that the cell (energy storage device) of the
present invention using the negative electrode of Example EA6 and
the positive electrode of Example EC1 had power density comparable
to that of the capacitor using an active carbon electrode as both
electrodes. In addition, FIG. 19B showed that the energy storage
device of the present invention using the negative electrode of
Example EA6 and the positive electrode of Example EC1 had specific
power higher than that of the capacitor using an active carbon
electrode as both electrodes. Although not shown here, it was found
that the cell of the present invention using the negative electrode
of Example EA6 and the positive electrode of Example EC1 had power
density and energy density higher than those of a cell using a
graphite electrode as a negative electrode and the electrode of
Example EC1 as a positive electrode.
[0423] A secondary battery and a capacitor using the above graphite
negative electrode can be produced by the following procedure.
[0424] (Secondary Battery Using Graphite Negative Electrode and
LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2 Positive Electrode)
[0425] Production of Graphite Electrode
[0426] 1 part by weight of carboxymethylcellulose and 1.5 parts by
weight of a styrene-butadiene rubber (in the form of a water
dispersion of the styrene-butadiene rubber) are added to 100 parts
by weight of an artificial graphite powder having an average
particle diameter of 20 .mu.m, and the whole is mixed with a
planetary ball mill apparatus at 300 rpm for 10 minutes, whereby
slurry for the formation of an electrode active material layer is
prepared.
[0427] The resultant slurry is applied onto copper foil having a
thickness of 10 .mu.m with a coater, and is then dried at
110.degree. C. for 1 hour. Further, the resultant is dried under
reduced pressure at 150.degree. C. Subsequently, the thickness of
the resultant is adjusted with a roll press, whereby an electrode
structure in which an electrode active material layer having a
thickness of 62 .mu.m and a density of 1.67 g/cm.sup.3 has been
formed on a current collector made of the copper foil is
obtained.
[0428] The resultant electrode structure is cut into a
predetermined size, and a lead made of a nickel ribbon is connected
to the above electrode structure by spot welding, whereby an
electrode as a negative electrode is obtained.
[0429] A cell (secondary battery) is assembled by using the
electrode of Example EC1 as a positive electrode in the same manner
as in "Example of procedure for production of the energy storage
device" described above.
[0430] An electrolyte solution obtained by dissolving a lithium
hexafluorophosphate (LiPF.sub.6) in a solvent prepared by mixing
ethylene carbonate and diethyl carbonate at a volume ratio of 3:7
so that the solution has a concentration of 1 mol/l (M) is
used.
[0431] The above secondary battery is charged and discharged in the
cell voltage range of 4.2 V to 2.7 V, and is then evaluated.
[0432] (Capacitor)
[0433] Production of Active Carbon Electrode
[0434] 1.7 parts by weight of carboxymethylcellulose and 2.7 parts
by weight of a styrene-butadiene rubber (in the form of a water
dispersion of the styrene-butadiene rubber) are added to 100 parts
by weight of active carbon having a specific surface area of 2,500
m.sup.2/g as measured by a BET method, and the whole is mixed with
a planetary ball mill apparatus at 300 rpm for 10 minutes, whereby
slurry for the formation of an electrode active material layer is
prepared.
[0435] The resultant slurry is applied onto aluminum foil having a
thickness of 28 .mu.m with a coater, and is then dried at
110.degree. C. for 1 hour. Further, the resultant is dried under
reduced pressure at 150.degree. C. Subsequently, the thickness of
the resultant is adjusted with a roll press, whereby an electrode
structure in which an electrode active material layer having a
thickness of 95 .mu.m and a density of 0.53 g/cm.sup.3 has been
formed on a current collector made of the aluminum foil is
obtained.
[0436] The resultant electrode structure is cut into a
predetermined size, and a lead made of an aluminum ribbon is
connected to the above electrode structure by ultrasonic welding,
whereby an active carbon electrode is produced.
[0437] A cell (capacitor) is assembled by using the above active
carbon electrode as its positive electrode and negative electrode
in the same manner as in "Example of procedure for production of
the energy storage device" described above.
[0438] An electrolyte solution is used which is obtained by
dissolving a tetraethylammonium tetrafluoroborate
((C.sub.2H.sub.5).sub.4NBF.sub.4) in propylene carbonate from which
moisture has been removed so that the solution has a concentration
of 1 mol/l (M).
[0439] The above capacitor is charged and discharged in the cell
voltage range of 2.7 V to 1.3 V, and is then evaluated.
[0440] (Energy Storage Device 1 of Another Structure)
[0441] A material layer capable of electrochemically storing and
releasing lithium ions is formed by the above-mentioned production
method (F) of the present invention.
[0442] First, a tungsten layer having a thickness of 100 nm is
formed as a collecting electrode on a silicon wafer substrate by
electron-beam deposition.
[0443] Next, an aluminum layer having a thickness of 50 nm, a
silicon layer having a thickness of 50 nm, an aluminum oxide layer
having a thickness of 50 nm, a silicon layer having a thickness of
50 nm, and an aluminum layer having a thickness of 50 nm are
sequentially formed on the above tungsten layer with a sputtering
apparatus.
[0444] After that, the resultant is subjected to treatment in a
heat treatment furnace under an argon gas at 1,000.degree. C. for 2
hours, whereby a negative electrode material layer is formed.
[0445] Subsequently, Li.sub.3.25Ge.sub.0.25P.sub.0.75S.sub.4 is
formed into a layer having a thickness of 50 nm by sputtering.
LiCoO.sub.2 is deposited onto the layer by laser ablation to form a
positive electrode material layer having a thickness of 300 nm.
Then, an aluminum layer having a smaller area than that of the
negative electrode current collector layer and a thickness of 100
nm is formed as a positive electrode current collector by using a
positive mask by electron-beam deposition.
[0446] Further, the resultant is subjected to heat treatment in a
vacuum at 200.degree. C., whereby an all-solid energy storage
device is produced.
[0447] Although a silicon wafer has been used as a substrate
material in the above operations, an insulating substrate such as a
quartz substrate can also be used. Although an electron beam has
been used as means for vapor deposition, means such as sputtering
can also be adopted.
[0448] As can be inferred from the evaluation of the device
produced by the above operations, an electrode formed of the
particles of a composite material capable of electrochemically
storing and releasing lithium ions produced by the production
method of the present invention shows high lithium
insertion/extraction efficiency, and an energy storage device
obtained by combining the electrode as a negative electrode and a
positive electrode formed of a transition metal compound such as a
transition metal lithium oxide has larger energy density and larger
power density than a conventional energy storage device.
[0449] In addition, an energy storage device having desired power
and desired energy can be designed by appropriately turning the
negative electrode of the energy storage device of the present
invention into a composite further with a material having a large
specific surface area such as active carbon or a material having
high conductivity such as graphite.
[0450] As described above, according to the present invention, an
energy storage device can be provided having high power density and
high energy density and being capable of being repeatedly used a
large number of times.
[0451] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0452] This application claims the benefit of Japanese Patent
Applications Nos. 2007-232090, filed Sep. 6, 2007, and No.
2007-321373, filed Dec. 12, 2007, which are hereby incorporated by
reference herein in their entirety.
* * * * *