U.S. patent application number 16/503841 was filed with the patent office on 2019-10-31 for negative electrode material for non-aqueous electrolyte secondary battery and method of producing negative electrode active mate.
This patent application is currently assigned to SHIN-ETSU CHEMICAL CO., LTD.. The applicant listed for this patent is SHIN-ETSU CHEMICAL CO., LTD.. Invention is credited to Takakazu HIROSE, Hiromichi KAMO, Hiroki YOSHIKAWA.
Application Number | 20190334167 16/503841 |
Document ID | / |
Family ID | 53542509 |
Filed Date | 2019-10-31 |
United States Patent
Application |
20190334167 |
Kind Code |
A1 |
KAMO; Hiromichi ; et
al. |
October 31, 2019 |
NEGATIVE ELECTRODE MATERIAL FOR NON-AQUEOUS ELECTROLYTE SECONDARY
BATTERY AND METHOD OF PRODUCING NEGATIVE ELECTRODE ACTIVE MATERIAL
PARTICLES
Abstract
A negative electrode material for a non-aqueous electrolyte
secondary battery contains negative electrode active material
particles containing a silicon compound expressed by SiO.sub.x,
where 0.5.ltoreq.x.ltoreq.1.6, and a coating layer composed of an
organic polymer coating the silicon compound, the silicon compound
containing a lithium compound on its surface or inside. As a
result, a negative electrode material for a non-aqueous electrolyte
secondary battery can increase the battery capacity and improve the
cycle performance and battery initial efficiency.
Inventors: |
KAMO; Hiromichi; (Takasaki,
JP) ; HIROSE; Takakazu; (Annaka, JP) ;
YOSHIKAWA; Hiroki; (Takasaki, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SHIN-ETSU CHEMICAL CO., LTD. |
Tokyo |
|
JP |
|
|
Assignee: |
SHIN-ETSU CHEMICAL CO.,
LTD.
Tokyo
JP
|
Family ID: |
53542509 |
Appl. No.: |
16/503841 |
Filed: |
July 5, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15107022 |
Jun 21, 2016 |
10396351 |
|
|
PCT/JP2014/005730 |
Nov 14, 2014 |
|
|
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16503841 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/366 20130101;
H01M 4/604 20130101; H01M 10/0525 20130101; H01M 2004/027 20130101;
H01M 4/386 20130101; H01M 4/624 20130101; H01M 4/483 20130101; H01M
4/587 20130101; H01M 4/364 20130101; H01M 4/622 20130101; H01M
4/485 20130101 |
International
Class: |
H01M 4/36 20060101
H01M004/36; H01M 4/62 20060101 H01M004/62; H01M 4/485 20060101
H01M004/485; H01M 4/587 20060101 H01M004/587; H01M 4/38 20060101
H01M004/38; H01M 4/48 20060101 H01M004/48; H01M 4/60 20060101
H01M004/60 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 16, 2014 |
JP |
2014-006201 |
Feb 21, 2014 |
JP |
2014-031273 |
Claims
1. A negative electrode material for a non-aqueous electrolyte
secondary battery, comprising negative electrode active material
particles comprising a silicon compound expressed by SiO.sub.x,
where 0.5.ltoreq.x.ltoreq.1.6, and a coating layer composed of an
organic polymer coating the silicon compound, wherein the silicon
compound contains Li.sub.2SiO.sub.3 inside.
2. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 1, wherein the organic polymer
contains at least one of a fluoroethylene group, a carboxyl group,
a hydroxyl group, and a carbonate group within its molecule.
3. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 1, wherein the organic polymer
comprises at least one polymer selected from the group consisting
of polyvinyl alcohol, polycarbonate, polyvinylidene fluoride, and
polyvinylidene tetrafluoride.
4. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 1, wherein a content of the
organic polymer is 0.01 mass % or more and less than 5 mass % with
respect to the whole negative electrode active material
particles.
5. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 1, wherein at least a part of
the organic polymer has an island-shaped structure.
6. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 1, wherein the silicon
compound further contains at least one lithium compound on its
surface, and the lithium compound is at least one selected from the
group consisting of Li.sub.2CO.sub.3, Li.sub.2O, and LiF.
7. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 1, wherein the silicon
compound further contains at least one lithium compound selected
from the group consisting of Li.sub.4SiO.sub.4 and
Li.sub.6Si.sub.2O.sub.7 inside.
8. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 1, wherein the silicon
compound is at least partially coated with a carbon compound.
9. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 8, wherein a coverage of the
carbon compound is 15 mass % or less with respect to a total of the
silicon compound and the carbon compound.
10. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 8, wherein the silicon
compound satisfies 0.05.ltoreq.A/B.ltoreq.3.00 when subjected to
X-ray photoelectron spectroscopy (XPS) on its surface layer where A
is a peak area of a peak when a C1s binding energy is about
287.5.+-.1.0 eV, and B is a peak area of a peak when the C1s
binding energy is about 284.0.+-.1.0 eV.
11. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 8, wherein the silicon
compound satisfies 0.10.ltoreq.A/B.ltoreq.1.00 when subjected to
X-ray photoelectron spectroscopy (XPS) on its surface layer where A
is a peak area of a peak when a C1s binding energy is about
287.5.+-.1.0 eV, and B is a peak area of a peak when the C1s
binding energy is about 284.0.+-.1.0 eV.
12. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 1, wherein the coating layer
composed of the organic polymer contains a particulate carbon-based
material having a median size smaller than that of the silicon
compound.
13. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 1, wherein the silicon
compound exhibits a diffraction peak having a half width (2.theta.)
of 1.2.degree. or more, the diffraction peak being attributable to
an (111) crystal face and obtained by X-ray diffraction, and a
crystallite size attributable to the crystal face is 7.5 nm or
less.
14. The negative electrode material for a non-aqueous electrolyte
secondary battery according to claim 1, wherein the silicon
compound containing Li.sub.2SiO.sub.3 inside is produced by a step
including an electrochemical manner.
15. A negative electrode for a non-aqueous electrolyte secondary
battery, comprising a negative electrode material according to
claim 1.
16. The negative electrode for a non-aqueous electrolyte secondary
battery according to claim 15, further comprising a carbon-based
negative electrode material.
17. The negative electrode for a non-aqueous electrolyte secondary
battery according to claim 16, wherein a median size Y of the
silicon compound and a median size X of the carbon-based negative
electrode material satisfy X/Y.gtoreq.1.
18. A non-aqueous electrolyte secondary battery comprising a
negative electrode according to claim 15.
19. A method of producing negative electrode active material
particles contained in a negative electrode material for a
non-aqueous electrolyte secondary battery, comprising the steps of:
producing a silicon compound expressed by SiO.sub.x where
0.55.ltoreq.x.ltoreq.1.6; inserting lithium into the silicon
compound and thereby forming a lithium compound that includes
Li.sub.2SiO.sub.3 at least inside the silicon compound to modify
the silicon compound; and coating the silicon compound with a
coating layer composed of an organic polymer to produce the
negative electrode active material particles.
20. The method of producing negative electrode active material
particles according to claim 19, wherein the step of modifying the
silicon compound is performed in an electrochemical manner.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a Continuation of application Ser. No. 15/107,022
filed Jun. 21, 2016, which in turn is allowed and is a national
stage entry of PCT/JP2014/005730 filed Nov. 14, 2014, which claims
priority to the following applications: Japanese Patent Application
No. 2014-006201 filed Jan. 16, 2014, and Japanese Patent
Application No. 2014-031273 filed Feb. 21, 2014. The disclosure of
each of the prior application is hereby incorporated by reference
herein in its entirety.
TECHNICAL FIELD
[0002] The present invention relates to a negative electrode
material for a non-aqueous electrolyte secondary battery and a
method of producing negative electrode active material particles.
The present invention also relates to a negative electrode for a
non-aqueous electrolyte secondary battery using the negative
electrode material and a non-aqueous electrolyte secondary battery
using the negative electrode.
BACKGROUND ART
[0003] In recent years, small electronic devices, represented by
mobile terminals, have been widely used and urgently required to
reduce the size and weight and to increase the life. Such
requirement has advanced the development of particularly small,
lightweight secondary batteries with higher energy density. These
secondary batteries are considered to find application not only for
small electronic devices but for large electronic devices such as,
typically, automobiles as well as power storage systems such as,
typically, houses.
[0004] Among those, lithium-ion secondary batteries are easy to
reduce the size and increase the capacity and have higher energy
density than those of lead or nickel-cadmium batteries, receiving
considerable attention.
[0005] The lithium-ion secondary battery has positive and negative
electrodes, a separator, and an electrolyte. The negative electrode
includes a negative electrode active material related to charging
and discharging reactions.
[0006] A negative electrode active material, which is usually made
of a carbon material, is required to further improve the battery
capacity for recent market requirement. Use of silicon as a
negative electrode active material is considered to improve the
battery capacity, for silicon has a logical capacity (4199 mAh/g)
ten times larger than does graphite (372 mAh/g). Such a material is
thus expected to significantly improve the battery capacity. The
development of silicon materials for use as negative electrode
active materials includes not only silicon as a simple but also
alloy thereof and a compound thereof such as typically oxides. The
consideration of active material shapes for carbon materials ranges
from a standard application type to an integrated type in which the
materials are directly accumulated on a current collector.
[0007] Use of silicon as a main material of a negative electrode
active material, however, expands or shrinks a negative electrode
active material when charging or discharging, thereby making the
negative electrode active material particle easy to break
particularly near its surface layer. In addition, this active
material particle produces ionic substances in its interior and is
thus easy to break. The breakage of the surface layer of the
negative electrode active material creates a new surface,
increasing a reaction area of the active material. The new surface
then causes the decomposition reaction of an electrolyte and is
coated with a decomposition product of the electrolyte, thereby
consuming the electrolyte. This makes the cycle performance easy to
reduce.
[0008] Various materials and configurations of a negative electrode
for a lithium-ion secondary battery mainly using a silicon material
have been considered to improve the initial battery efficiency and
the cycle performance.
[0009] More specifically, a vapor deposition method is used to
accumulate silicon and amorphous silicon dioxide simultaneously so
that better cycle performance and greater safety are achieved (See
Patent Document 1, for example). Moreover, a carbon material, an
electronic conduction material, is disposed on the surface of
silicon oxide particles so that a higher battery capacity and
greater safety are achieved (See Patent Document 2, for example).
Moreover, an active material including silicon and oxygen is
produced to form an active material layer having a higher ratio of
oxygen near a current collector so that improved cycle performance
and higher input-output performance are achieved (See Patent
Document 3, for example). Moreover, silicon active material is
formed so as to contain oxygen with an average content of 40 at %
or less and with a higher oxygen content near a current collector
so that improved cycle performance is achieved (See Patent Document
4, for example)
[0010] Moreover, a nano-complex including Si-phase, SiO.sub.2,
M.sub.yO metal oxide is used to improve the first charge and
discharge efficiency (See Patent Document 5, for example).
Moreover, a lithium-containing material is added to a negative
electrode, and pre-doping that decomposes lithium and moves the
lithium to a positive electrode at a higher negative-electrode
potential is performed so that the first charge and discharge
efficiency is improved (See Patent Document 6, for example).
[0011] Moreover, SiO.sub.x (0.85.ltoreq.x.ltoreq.1.5) having a
particle size ranging from 1 .mu.m to 50 .mu.m and a carbon
material are mixed and calcined at a high temperature so that
improved cycle performance is achieved (See Patent Document 7, for
example). Moreover, a mole ratio of oxygen to silicon in a negative
electrode active material is adjusted in the range from 0.1 to 0.2
so as to hold a difference between the maximum and the minimum of
the oxygen-to-silicon mole ratio near the interface between the
active material and a current collector at 0.4 or less, so that
improved cycle performance is achieved (See Patent Document 8, for
example). Moreover, a metal oxide containing lithium is used to
improve the battery load characteristic (See Patent Document 9, for
example). Moreover, a hydrophobic layer such as a silane compound
is formed in the surface layer of a silicon material so that
improved cycle performance is achieved (See Patent Document 10, for
example).
[0012] Moreover, a silicon oxide is used and coated with graphite
to give conductivity so that improved cycle performance is achieved
(See Patent Document 11, for example). Patent Document 11 describes
that a shift value of the graphite coating, which is obtained from
a Raman spectrum, has broad peaks at 1330 cm.sup.-1 and 1580
cm.sup.-1 and a ratio I.sub.1330/I.sub.1580 of its intensity shows
1.5<I.sub.1330/I.sub.1580<3.
[0013] Moreover, a particle having an Si-microcrystal phase
dispersing in a silicon dioxide is used to achieve a higher battery
capacity and improved cycle performance (See Patent Document 12,
for example). Finally, a silicon oxide having a silicon-to-oxygen
atomicity ratio of 1:y (0<y<2) is used to improve overcharge
and overdischarge performance (See Patent Document 13, for
example).
CITATION LIST
Patent Literature
[0014] Patent Document 1: Japanese Patent Application Publication
No. 2001-185127
[0015] Patent Document 2: Japanese Patent Application Publication
No. 2002-042806
[0016] Patent Document 3: Japanese Patent Application Publication
No. 2006-164954
[0017] Patent Document 4: Japanese Patent Application Publication
No. 2006-114454
[0018] Patent Document 5: Japanese Patent Application Publication
No. 2009-070825
[0019] Patent Document 6: Japanese Patent Application Publication
No. 2013-513206
[0020] Patent Document 7: Japanese Patent Application Publication
No. 2008-282819
[0021] Patent Document 8: Japanese Patent Application Publication
No. 2008-251369
[0022] Patent Document 9: Japanese Patent Application Publication
No. 2008-177346
[0023] Patent Document 10: Japanese Patent Application Publication
No. 2007-234255
[0024] Patent Document 11: Japanese Patent Application Publication
No. 2009-212074
[0025] Patent Document 12: Japanese Patent Application Publication
No. 2009-205950
[0026] Patent Document 13: Japanese Patent No. 2997741
SUMMARY OF INVENTION
Technical Problem
[0027] As described previously, small mobile devices, represented
by electronic devices, have been developed to improve their
performance and increase their functions. Non-aqueous electrolyte
secondary batteries, especially lithium-ion secondary batteries,
which are used as main sources of the devices, have been required
to increase the battery capacity. The development of non-aqueous
electrolyte secondary batteries including negative electrodes
mainly using silicon materials have been desired to solve this
problem. The non-aqueous electrolyte secondary batteries using
silicon materials need the same cycle performance as non-aqueous
electrolyte secondary batteries using carbon materials.
[0028] The present invention was accomplished in view of the above
problems, and an object thereof is to provide a negative electrode
material for a non-aqueous electrolyte secondary battery that can
increase the battery capacity and improve the cycle performance and
battery initial efficiency. Another object of the present invention
is to provide a negative electrode for a non-aqueous electrolyte
secondary battery using the negative electrode material and a
non-aqueous electrolyte secondary battery using the negative
electrode. Further object of the present invention is to provide a
method of producing negative electrode active material particles
usable in the negative electrode material.
Solution to Problem
[0029] To achieve this object, the present invention provides a
negative electrode material for a non-aqueous electrolyte secondary
battery, comprising negative electrode active material particles
comprising a silicon compound expressed by SiO.sub.x, where
0.5.ltoreq.x.ltoreq.1.6, and a coating layer composed of an organic
polymer coating the silicon compound, the silicon compound
containing a lithium compound on its surface or inside.
[0030] The negative electrode material having such negative
electrode active material particles, in which a SiO.sub.2 component
part to be destabilized with insertion and extraction of lithium
ion is previously modified into a lithium compound, can reduce
irreversible capacity generated at charging. In addition, the
coating layer composed of the organic polymer coating the silicon
compound provides excellent resistance to an organic solvent and an
aqueous solvent. This allows the negative electrode material to
have excellent resistance to an organic solvent and an aqueous
solvent as well as excellent capacity retention rate and first
efficiency. Furthermore, the negative electrode material mainly
composed of the silicon compound can increase battery capacity.
[0031] The organic polymer preferably contains at least one of a
fluoroethylene group, a carboxyl group, a hydroxyl group, and a
carbonate group within its molecule. Moreover, the organic polymer
preferably comprises at least one of polyacrylic acid and an alkali
metal salt thereof, carboxymethyl cellulose and an alkali metal
salt thereof, polyvinyl alcohol, polycarbonate, polyvinylidene
fluoride, and polyvinylidene tetrafluoride.
[0032] The coating layer using such an organic polymer can more
effectively impart excellent resistance to an aqueous solvent to
the negative electrode material.
[0033] A content of the organic polymer is preferably 0.01 mass %
or more and less than 5 mass % with respect to the whole negative
electrode active material particles.
[0034] Such a content of the organic polymer enables resistance to
an aqueous solvent to be more surely provided while keeping
electrical conductivity and ionic conductivity high.
[0035] At least a part of the organic polymer preferably has an
island-shaped structure.
[0036] The island-shaped structure of the organic polymer can
minimize the reduction of electrical conductivity and ionic
conductivity of the negative electrode active material particles
due to coating with the organic polymer and enables the coated
particles to have resistance to an aqueous solvent.
[0037] The silicon compound preferably contains at least one
lithium compound selected from Li.sub.2CO.sub.3, Li.sub.2O, and LiF
on its surface. Moreover, the silicon compound preferably contains
at least one lithium compound selected from Li.sub.4SiO.sub.4,
Li.sub.6Si.sub.2O.sub.7, and Li.sub.2SiO.sub.3 inside.
[0038] When the silicon compound contains such compounds on its
surface or inside, the effect of the present invention can be more
effectively exhibited.
[0039] The silicon compound is preferably at least partially coated
with a carbon compound. In this case, a coverage of the carbon
compound is preferably 15 mass % or less with respect to a total of
the silicon compound and the carbon compound.
[0040] At least partially coating the silicon compound with a
carbon compound yields an effect of improving conductivity.
Moreover, the above range of coverage can ensure adequate
capacity.
[0041] The silicon compound preferably satisfies
0.05.ltoreq.A/B.ltoreq.3.00, more preferably
0.10.ltoreq.A/B.ltoreq.1.00, when subjected to X-ray photoelectron
spectroscopy (XPS) on its surface layer where A is a peak area of a
peak when a C1s binding energy is about 287.5.+-.1.0 eV, and B is a
peak area of a peak when the C1s binding energy is about
284.0.+-.1.0 eV.
[0042] When the above relationship exists between the peak area A
for a binding energy mainly attributable to carboxyl group of about
287.5.+-.1.0 eV and the peak area B for a binding energy
attributable to single carbon of 284.0.+-.1.0 eV, a carboxyl group
on the surface layer of the silicon compound improves adhesion
between the organic polymer and a binder in a negative electrode,
resulting in improved battery performances.
[0043] The coating layer composed of the organic polymer preferably
contains a particulate carbon-based material having a median size
smaller than that of the silicon compound.
[0044] The coating layer containing the carbon-based material
having a median size smaller than that of the silicon compound can
improve electrical conductivity between particles of the silicon
compound.
[0045] The silicon compound preferably exhibits a diffraction peak
having a half width (2.theta.) of 1.2.degree. or more, the
diffraction peak being attributable to an (111) crystal face and
obtained by X-ray diffraction, and a crystallite size attributable
to the crystal face is preferably 7.5 nm or less.
[0046] The silicon compound which exhibits the above half width and
crystallite size has low crystallinity. Use of the silicon compound
having low crystallinity, which has a few Si crystal, enables
improvement in the battery performances.
[0047] The silicon compound containing the lithium compound is
preferably produced by a step including an electrochemical
manner.
[0048] When the silicon compound containing the lithium compound is
produced by the step including an electrochemical manner, a stable
lithium compound can be obtained.
[0049] Furthermore, the present invention provides a negative
electrode for a non-aqueous electrolyte secondary battery,
comprising any one of the above negative electrode material.
[0050] The negative electrode using the inventive negative
electrode material enables a non-aqueous electrolyte secondary
battery using this negative electrode to have improved cycle
performance and first charge and discharge efficiency.
[0051] The negative electrode for a non-aqueous electrolyte
secondary battery preferably further comprises a carbon-based
negative electrode material. In this case, a median size Y of the
silicon compound and a median size X of the carbon-based negative
electrode material preferably satisfy X/Y.gtoreq.1.
[0052] When the negative electrode contains both the carbon-based
negative electrode material and the negative electrode material
which includes the silicon compound containing the lithium compound
and the coating layer, the negative electrode can be prevented from
breaking due to change of its volume. In particular, this effect is
effectively exhibited when the carbon-based negative electrode
material is equal to or larger than the silicon compound.
[0053] Furthermore, the present invention provides a non-aqueous
electrolyte secondary battery comprising any one of the above
negative electrode.
[0054] The non-aqueous electrolyte secondary battery using the
inventive negative electrode can improve the cycle performance and
first charge and discharge efficiency.
[0055] Furthermore, the present invention provides a method of
producing negative electrode active material particles contained in
a negative electrode material for a non-aqueous electrolyte
secondary battery, comprising the steps of: producing a silicon
compound expressed by SiO.sub.x where 0.55.ltoreq.x.ltoreq.1.6;
inserting lithium into the silicon compound and thereby forming a
lithium compound on and/or inside the silicon compound to modify
the silicon compound; and coating the silicon compound with a
coating layer composed of an organic polymer to produce the
negative electrode active material particles.
[0056] The method of producing negative electrode active material
particles including such steps allows the SiO.sub.2 component part
contained in the inventive negative electrode material for a
non-aqueous electrolyte secondary battery to be previously modified
into a lithium compound, whereby negative electrode active material
particles having the coating layer composed of the organic polymer
can be manufactured.
[0057] The step of modifying the silicon compound is preferably
performed in an electrochemical manner.
[0058] Modifying the silicon compound in the electrochemical manner
can yield a stable lithium compound.
Advantageous Effects of Invention
[0059] The inventive negative electrode material for a non-aqueous
electrolyte secondary battery, in which a SiO.sub.2 component part
to be destabilized with insertion and extraction of lithium ion is
previously modified into a lithium compound, can reduce
irreversible capacity generated at charging. In addition, the
coating layer composed of the organic polymer coating the silicon
compound provides excellent resistance to an organic solvent and an
aqueous solvent. This allows the negative electrode material to
have excellent resistance to an organic solvent and an aqueous
solvent as well as excellent capacity retention rate and first
efficiency.
[0060] A negative electrode for a non-aqueous electrolyte secondary
battery using the inventive negative electrode material and a
non-aqueous electrolyte secondary battery using this negative
electrode can improve cycle performance and first charge and
discharge efficiency. In addition, electronic devices, machine
tools, electric vehicles, and power storage systems, etc., using
the inventive secondary battery also can achieve the same
effect.
[0061] Furthermore, the inventive method of producing negative
electrode active material particles can produce negative electrode
active material particles to be contained such a negative electrode
material for a non-aqueous electrolyte secondary battery.
BRIEF DESCRIPTION OF DRAWINGS
[0062] FIG. 1 is a schematic cross-sectional view of a
configuration of a negative electrode for a non-aqueous electrolyte
secondary battery according to an embodiment of the invention;
[0063] FIG. 2 is a schematic view of a bulk modification apparatus
that can used in the inventive method of producing negative
electrode active material particles; and
[0064] FIG. 3 is an exploded view of a laminate film type of
secondary battery according to an embodiment of the invention.
DESCRIPTION OF EMBODIMENTS
[0065] As described previously, use of a negative electrode mainly
made of a silicon material, for use in a non-aqueous electrolyte
secondary battery, has been considered to increase the capacity of
the non-aqueous electrolyte secondary battery.
[0066] The non-aqueous electrolyte secondary battery using a
silicon material is required to have the same cycle performance as
a non-aqueous electrolyte secondary battery using a carbon
material; however, no one has yet proposed a negative electrode
material for this type of battery having the same cycle stability
as a non-aqueous electrolyte secondary battery using a carbon
material.
[0067] In view of this, the inventors diligently conducted study on
a negative electrode active material that allows a non-aqueous
secondary battery using a negative electrode made of this material
to have good cycle performance, bringing the present invention to
completion.
[0068] The inventive negative electrode material for a non-aqueous
electrolyte secondary battery contains negative electrode active
material particles. The negative electrode active material
particles contain a silicon compound expressed by SiO.sub.x, where
0.55.ltoreq.x.ltoreq.1.6, and the silicon compound contains a
lithium compound on its surface or inside. The negative electrode
active material particles further contain a coating layer composed
of an organic polymer coating the silicon compound.
<Negative Electrode for Non-Aqueous Electrolyte Secondary
Battery>
[0069] A negative electrode for a non-aqueous electrolyte secondary
battery using the inventive negative electrode active material will
be described. FIG. 1 shows a cross-sectional view of a
configuration of a negative electrode for a non-aqueous electrolyte
secondary battery (also simply referred to as a "negative
electrode" below) according to an embodiment of the invention.
[Configuration of Negative Electrode]
[0070] As shown in FIG. 1, the negative electrode 10 has a negative
electrode active material layer 12 on a negative electrode current
collector 11. The negative electrode active material layer 12 may
be disposed on one side or both sides of the negative electrode
current collector 11. The negative electrode current collector 11
is not necessarily needed in a negative electrode using the
inventive negative electrode active material.
[Negative Electrode Current Collector]
[0071] The negative electrode current collector 11 is made of a
highly conductive and mechanically strong material. Examples of the
conductive material used for the negative electrode current
collector 11 include copper (Cu) and nickel (Ni). Such conductive
materials preferably have inability to form an intermetallic
compound with lithium (Li).
[0072] The negative electrode current collector 11 preferably
contains carbon (C) and sulfur (S) besides the main element since
these elements improve the physical strength of the current
collector. In particular, when the active material layer contains a
material expandable at charging, the current collector including
the above elements can inhibit deformation of the electrodes and
the current collector itself. Each content of the above elements is
preferably, but not particularly limited to, 100 ppm or less. This
content enables the deformation to be effectively inhibited.
[0073] The surface of the negative electrode current collector 11
may or may not be roughed. Examples of the negative electrode
current collector roughened include a metallic foil subjected to an
electrolyzing process, an embossing process, or a chemical etching
process. Examples of the negative electrode current collector that
is not roughened include a rolled metallic foil.
[Negative Electrode Active Material Layer]
[0074] The negative electrode active material layer 12 contains a
particulate negative electrode material (hereinafter, referred to
as negative electrode active material particles) that can occlude
and emit lithium ions and may further contain other materials such
as a negative-electrode binder and a conductive additive depending
on battery design. The inventive negative electrode material for a
non-aqueous electrolyte secondary battery can be used for the
negative electrode active material layer 12.
[0075] The negative electrode active material particles used in the
inventive negative electrode material each contain a silicon
compound that can occlude and emit lithium ions and a lithium
compound on the surface or in the interior of the silicon compound.
The silicon compound is coated with the coating layer of the
organic polymer. This structure is observed by photography of
transmission electron microscope-energy dispersive X-ray
spectroscopy (TEM-EDX). Occluding and emitting lithium ions may be
done in at least a part of the coating layer of the organic
polymer.
[0076] The negative electrode active material particles used in the
inventive negative electrode material is a silicon oxide containing
the silicon compound (SiO.sub.x, where 0.55.ltoreq.x.ltoreq.1.6); a
preferable composition of the silicon compound is that x is close
to 1. The reason is that this composition enables high cycle
performance. The present invention does not necessarily intend a
silicon material composition of 100% but permits a silicon material
containing a minute amount of impurities.
[0077] Such negative electrode active material particles can be
obtained by selectively modifying a part of the SiO.sub.2 component
formed inside the silicon compound into a lithium compound.
Examples of the lithium compound inside the silicon compound
include Li.sub.4SiO.sub.4, Li.sub.6Si.sub.2O.sub.7, and
Li.sub.2SiO.sub.3, which especially have good performance.
[0078] The lithium compound can be quantified by NMR (Nuclear
Magnetic Resonance) and XPS (X-ray Photoelectron Spectroscopy). XPS
and NMR measurements can be performed, for example, under the
following conditions.
XPS
[0079] Apparatus: an X-ray photoelectron spectroscopy apparatus
[0080] X-ray Source: a monochromatic Al-K.alpha. ray [0081] X-ray
Spot Diameter: 100 .mu.m [0082] Ar-ion Sputtering Gun Conditions:
0.5 kV, 2 mm.times.2 mm
.sup.29Si-MAS-NMR
[0082] [0083] Apparatus: a 700-NMR spectroscope made by Bruker
Corp. [0084] Probe: a 4-mm-HR-MAS rotor, 50 .mu.L [0085] Sample
Rotation Speed: 10 kHz [0086] Temperature of Measurement
Environment: 25.degree. C.
[0087] The formation of the selective compound, i.e., the
modification of the silicon compound is preferably carried out in
the electrochemical manner.
[0088] Such modification (bulk modification) to produce the
negative electrode active material particles can reduce or prevent
the lithium compound formation in an Si region, whereby a substance
stable in the air, water slurry, or solvent slurry can be obtained.
Moreover, electrochemical modification can produce a more stable
substance than thermal modification (thermal doping method), in
which the compound is randomly modified.
[0089] Li.sub.4SiO.sub.4, Li.sub.6Si.sub.2O.sub.7, and
Li.sub.2SiO.sub.3 can improve the performance when at least one of
them is formed within a bulk of the silicon compound, and the
combination of two or more of them can further improve the
performance.
[0090] In addition, the silicon compound preferably contains at
least one lithium compound selected from Li.sub.2CO.sub.3,
Li.sub.2O, and LiF on its surface.
[0091] Above all, formation of a fluorine compound such as LiF on
the outermost surface of the silicon compound dramatically improves
the powder storage property. In particular, this compound is
preferably formed with a coverage of 30% or more. The most
preferable material is LiF, and the most preferable forming method
is an electrochemical method, though not particularly limited
thereto.
[0092] As previously described, in the inventive negative electrode
material, the negative electrode active material particles contain
the coating layer composed of the organic polymer coating the
surface of the silicon compound. Above all, the organic polymer
containing at least one of a fluoroethylene group, a carboxyl
group, a hydroxyl group, and a carbonate ester group within its
skeleton exhibits particularly good performance. The method of
forming the coating layer may include liquid immersion and drying,
mixing and melting of the organic polymer and the silicon compound,
and fusing with a ball mill.
[0093] The organic polymer used for the coating layer preferably
includes at least one of polyacrylic acid and an alkali metal salt
thereof, carboxymethyl cellulose and an alkali metal salt thereof,
polyvinyl alcohol, polycarbonate, polyvinylidene fluoride, and
polyvinylidene tetrafluoride.
[0094] The particle of the silicon compound having the coating
layer composed of the organic polymer can improve resistance to an
organic solvent and an aqueous solvent even while containing the
lithium compound obtained by modifying a part of the SiO.sub.2
component.
[0095] The content of the organic polymer constituting the coating
layer of the silicon compound particle (the content of the organic
polymer with respect to the whole negative electrode active
material particles) is preferably 0.01 mass % or more and less than
5 mass %, more preferably 0.05 mass % or more and less than 3 mass
%. When the content of the organic polymer is 0.01 mass % or more,
the effect of the coating layer can be more surely exhibited,
resulting in improved resistance to an organic solvent and an
aqueous solvent. When the content is less than 5 mass %, the
surface of the negative electrode active material particles can
keep good electrical conductivity and ionic conductivity, and thus
battery performances can be prevented from deteriorating.
[0096] The coating amount of the coating layer can be calculated,
for example, by comparing the weight of the negative electrode
active material particles before and after the formation of the
coating layer or by elemental analysis.
[0097] In this case, the organic polymer constituting the coating
layer preferably at least partially forms an island-structure. This
structure can minimize the reduction of electrical conductivity and
ionic conductivity of the negative electrode active material
particles due to coating with the organic polymer and enables the
coated particles to have resistance to an aqueous solution.
[0098] The silicon compound is preferably at least partially coated
with a carbon compound. In this case, the coverage of the carbon
compound is preferably 15 mass % or less with respect to the total
of the silicon compound and the carbon compound. This allows
improvement of electrical conductivity. Although the carbon coating
exceeding 15 mass % does not deteriorate the battery performance,
it reduces battery capacity. Therefore, the coverage is preferably
15 mass % or less. The coating method with the carbon compound is
preferably, but not particularly limited to, sugar carbonization or
pyrolysis of hydrocarbon gas, for these methods can improve the
coverage.
[0099] The silicon compound preferably satisfies
0.05.ltoreq.A/B.ltoreq.3.00, more preferably
0.10.ltoreq.A/B.ltoreq.1.00, when subjected to XPS on its surface
layer where A is a peak area of a peak when a C1s orbital binding
energy is about 287.5.+-.1.0 eV, and B is a peak area of a peak
when the C1s orbital binding energy is about 284.0.+-.1.0 eV. The
reason is that when the above relationship exists between the peak
for a binding energy mainly attributable to carboxyl group of about
287.5.+-.1.0 eV and the peak for a binding energy attributable to
single carbon of 284.0.+-.1.0 eV, a carboxyl group on the surface
layer of the silicon compound improves adhesion between the organic
polymer and a binder in a negative electrode, resulting in improved
battery performances. A/B of 3.00 or less prevents a carboxyl group
and the like from excessively increasing on the surface, thus
prevents the carbon coating from being covered, and consequently
prevents electrical and ionic conductivities from decreasing. A/B
of 0.05 or more allows a carboxyl group and the like to be on the
surface layer of the silicon compound in sufficient amount to
exhibit the effect.
[0100] The coating layer composed of the organic polymer preferably
contains a particulate carbon-based material having a median size
smaller than that of the silicon compound. In other words,
particles of a carbon compound having a median size smaller than
that of the silicon compound are preferably around the silicon
compound. This allows improvement of electrical conductivity
between particles of the silicon compound. This carbon compound can
be around the negative electrode active material particles by, for
example, physically mixing with negative electrode active material
particles.
[0101] A lower crystallinity of the silicon compound contained in
the inventive negative electrode material is better. More
specifically, the silicon compound preferably exhibits a
diffraction peak having a half width (2.theta.) of 1.20 or more
that is attributable to an Si(111) crystal face and obtained when
X-ray diffraction is performed on the silicon compound, and a
crystallite size of 7.5 nm or less that is attributable to the
crystal face. Use of the silicon compound with low crystallinity,
which has a few Si crystal, can improve the battery performances
and allows production of a stable lithium compound.
[0102] The median size of the silicon compound preferably ranges
from 0.5 .mu.m to 20 .mu.m, but not particularly limited thereto.
This range makes it easy to occlude and emit lithium ions and
inhibits the breakage of the particles at charging and discharging.
A median size of 0.5 .mu.m or more then prevents the silicon
compound surface from increasing and can thus reduce the battery
irreversible capacity; a median size of 20 .mu.m or less inhibits
the breakage of the particles and the creation of a new surface, so
that this range is preferable.
[0103] The negative-electrode binder may be, for example, one or
more of a polymer material and a synthetic rubber. Examples of the
polymer material include polyvinylidene fluoride, polyimide,
polyamideimide, aramid, polyacrylic acid, lithium polyacrylate, and
carboxymethyl cellulose. Examples of the synthetic rubber include
styrene-butadiene rubber, a fluorinated rubber, and an
ethylene-propylene-diene.
[0104] Examples of the negative-electrode conductive additive
include carbon materials such as carbon black, acetylene black,
graphite, ketjen black, carbon nanotube, carbon nanofiber, and the
combination thereof.
[0105] The negative electrode active material layer 12 of FIG. 1
may be produced with a mixture of the inventive negative electrode
material including the negative electrode active material particles
(silicon-based negative electrode material) and a carbon material
(carbon-based negative electrode material). In this manner, the
negative electrode active material layer 12 can reduce its
electrical resistance and a stress due to its expansion at
charging. Examples of the carbon-based negative electrode material
include pyrolytic carbons, cokes, glassy carbon fiber, a fired
organic polymeric compound, and carbon black.
[0106] The negative electrode active material layer 12 may be
formed by, for example, an application method. The application
method is to mix the negative electrode active material particles
and the binders, in addition to the conductive additive and the
carbon material as needed, and disperse the resultant mixture into
an organic solvent or water to apply the resultant to a
subject.
[Method of Producing Negative Electrode]
[0107] The method of producing the inventive negative electrode
will be now described.
[0108] First, the inventive method of producing negative electrode
active material particles contained in a negative electrode
material for a non-aqueous electrolyte secondary battery will be
described. The method begins with a production of a silicon
compound expressed by SiO.sub.x where 0.55.ltoreq.x.ltoreq.1.6. The
silicon compound is then modified by inserting lithium into the
silicon compound and thereby forming a lithium compound either or
both of on the silicon compound and inside the silicon compound.
The silicon compound is then coated with a coating layer composed
of an organic polymer.
[0109] More specifically, the negative electrode active material
particles can be produced by, for example, the following
procedure.
[0110] A raw material capable of generating a silicon oxide gas is
first heated under an inert gas atmosphere or a reduced pressure at
a temperature ranging from 900.degree. C. to 1600.degree. C. to
produce the silicon oxide gas. The raw material is a mixture of
metallic silicon powder and silicon dioxide powder. The mole ratio
of the mixture preferably satisfies the relationship of
0.8<metallic silicon powder/silicon dioxide powder<1.3, in
consideration of the existence of oxygen on the metallic silicon
powder surface and a minute amount of oxygen in a reactor. The
Si-crystallites in the particles are controlled by adjustment of an
arrangement range and a vaporization temperature, or heat treatment
after the production. The produced gas is deposited on an
adsorption plate. The temperature in the reactor is decreased to
100.degree. C. or less and then a deposit is taken out. The deposit
is pulverized with a ball mill or a jet mill to form powder.
[0111] The obtained powder material may be coated with a carbon
coating, but this step is not essential.
[0112] Thermal CVD is desirably used to coat the obtained powder
material with the carbon layer. This thermal CVD is to fill a
furnace in which the silicon oxide powder is placed with a
hydrocarbon gas and heat the interior of the furnace. The pyrolysis
temperature is preferably, but not particularly limited to,
1200.degree. C. or less, more preferably 950.degree. C. or less.
This temperature range enables the inhibition of an unintended
disproportionation of the active material particles. The
hydrocarbon gas preferably has a composition of C.sub.nH.sub.m
where 3.gtoreq.n, though not particularly limited thereto, for this
composition enables reduction in production cost and improvement in
physical properties of a pyrolysis product.
[0113] The bulk modification is preferably performed by inserting
and extracting lithium in the electrochemical manner. Although
apparatus structure is not particularly limited, bulk modification
can be performed with, for example, a bulk modification apparatus
20 shown in FIG. 2. The bulk modification apparatus 20 shown in
FIG. 2 includes a bath 27 filled with an organic solvent 23, a
positive electrode 21 (lithium source) provided within the bath 27
and connected to one terminal of a power source 26, a powder
storage container 25 provided within the bath 27 and connected to
the other terminal of the power source 26, and a separator 24
provided between the positive electrode 21 and the powder storage
container 25. In the powder storage container 25, silicon compound
powder 22 is stored.
[0114] The modified silicon compound powder 22 is immersed in a
solution dissolving an organic polymer and then dried to form a
coating layer composed of the organic polymer.
[0115] In the bulk modification treatment, when a fluorine compound
is formed on the surface, the fluorine compound is preferably
formed by changing voltage and temperature conditions. This yields
a dense film. In particular, when fluorinated lithium is formed, it
is preferable to keep the temperature 45.degree. C. or higher
during insertion and extraction of lithium.
[0116] As described previously, the modified particles thus
obtained may contain no carbon layer. However, when more uniform
control is required in the bulk modification treatment, voltage
distribution needs to be reduced, and thus the carbon layer is
desirably contained.
[0117] Examples of the organic solvent 23 in the bath 27 include
ethylene carbonate, propylene carbonate, dimethyl carbonate,
diethyl carbonate, ethylmethyl carbonate, fluoromethylmethyl
carbonate, and difluoromethylmethyl carbonate. Examples of
electrolyte salt contained in the organic solvent 23 include
lithium hexafluorophosphate (LiPF.sub.6) and lithium
tetrafluoroborate (LiBF.sub.4).
[0118] The positive electrode 21 may be a lithium foil or a
Li-containing compound. Examples of the Li-containing compound
include lithium carbonate and lithium oxide.
[0119] The negative electrode active material particles, the
negative-electrode binder, and other materials such as conductive
additives are then mixed, and the obtained negative electrode
mixture is mixed with an organic solvent, water or the like to form
slurry.
[0120] The mixture slurry is then applied to the surface of the
negative-electrode current collector and dried to form a negative
electrode active material layer 12 shown in FIG. 1. At this time,
heating press may be performed, if necessary.
[0121] This negative electrode, in which the SiO.sub.2 component in
the bulk is modified into a stable lithium compound, can improve
the battery initial efficiency and stability of the active material
with cycle performance. Higher effect can be achieved by forming
lithium silicate in the bulk.
[0122] In addition, the coating layer composed of the organic
polymer coating the silicon compound improves resistance to an
aqueous solvent.
[0123] In addition, coating the silicon compound with a carbon
compound makes the compound condition in the bulk more uniform, and
a fluorine compound on the surface layer improves stability of the
active material, resulting in more effective negative
electrode.
[0124] A negative-electrode current collector containing carbon and
sulfur in an amount of 90 ppm or less is more effective.
<2. Lithium-Ion Secondary Battery>
[0125] As an embodiment of the non-aqueous electrolyte secondary
battery using the above negative electrode, lithium-ion secondary
battery will be now described.
[Configuration of Laminate Film Secondary Battery]
[0126] The laminate film secondary battery 30 shown in FIG. 3
includes a wound electrode body 31 interposed between sheet-shaped
outer parts 35. The wound electrode body are formed by winding a
positive electrode, a negative electrode, and a separator disposed
between these electrodes. The electrode body may also be composed
of a laminated part of the positive and negative electrodes, and a
separator disposed between these electrodes. The electrode bodies
of both types have a positive-electrode lead 32 attached to the
positive electrode and a negative-electrode lead 33 attached to the
negative electrode. The outermost circumference of the electrode
bodies is protected by a protecting tape.
[0127] The positive-electrode lead and the negative-electrode lead,
for example, extends from the interior of the outer parts 35 toward
the exterior in one direction. The positive-electrode lead 32 is
made of, for example, a conductive material such as aluminum; the
negative-electrode lead 33 is made of, for example, a conductive
material such as nickel or copper.
[0128] An example of the outer part 35 is a laminate film composed
of a fusion-bond layer, a metallic layer, and a surface protecting
layer stacked in this order. Two laminate films are fusion-bonded
or stuck with an adhesive at the outer edge of their fusion-bond
layers such that each fusion-bond layer faces the electrode body
31. The fusion-bond layer may be, for example, a film such as a
polyethylene or polypropylene film; the metallic layer aluminum
foil; the protecting layer nylon.
[0129] The space between the outer parts 35 and the positive and
negative electrodes is filled with close adhesion films 34 to
prevent air from entering therein. Exemplary materials of the close
adhesion films include polyethylene, polypropylene, and
polyolefin.
[Positive Electrode]
[0130] The positive electrode has a positive electrode active
material layer disposed on one side or both sides of a
positive-electrode current collector as in the negative electrode
10, for example, shown in FIG. 1.
[0131] The positive-electrode current collector is made of, for
example, a conductive material such as aluminum.
[0132] The positive electrode active material layer contains a
material that can occlude and emit lithium ions or the combination
thereof, and may contain a binder, a conductive additive, a
dispersing agent, or other materials according to design. The same
detailed description as described for the negative-electrode
binders and negative-electrode conductive additive, for example, is
then given for this binder and this conductive additive.
[0133] The positive electrode material is preferably a compound
containing lithium. Examples of this compound include a complex
oxide composed of lithium and transition metal elements, and a
phosphoric acid compound containing lithium and transition metal
elements. Among them, a compound containing at least one of nickel,
iron, manganese, and cobalt is preferable for the positive
electrode material. The chemical formula of this compound is
expressed by, for example, Li.sub.xM.sub.1O.sub.2 or
Li.sub.yM.sub.2PO.sub.4, where M.sub.1 and M.sub.2 represent at
least one kind of transition metal elements, and x and y represent
a value varied depending on a charging or discharging status of a
battery, which typically satisfy 0.055.ltoreq.x.ltoreq.1.10 and
0.055.ltoreq.y.ltoreq.1.10.
[0134] Examples of the complex oxide composed of lithium and
transition metal elements include a lithium cobalt complex oxide
(Li.sub.xCoO.sub.2), a lithium nickel complex oxide
(Li.sub.xNiO.sub.2). Examples of the phosphoric acid compound
containing lithium and transition metal elements include a lithium
iron phosphoric acid compound (LiFePO.sub.4), a lithium iron
manganese phosphoric acid compound
(LiFe.sub.1-uMn.sub.uPO.sub.4(u<1)). Use of these positive
electrode materials enables a higher battery capacity and excellent
cycle performance.
[Negative Electrode]
[0135] The negative electrode is configured as in the above
negative electrode 10 for a lithium-ion secondary battery shown in
FIG. 1, and has the negative electrode active material layer 12,
for example, on both faces of the current collector 11. The
negative electrode preferably has a negative-electrode charge
capacity larger than a battery charge capacity (electrical
capacitance) provided by the positive electrode active material,
for this negative electrode itself can inhibit the precipitation of
lithium metal.
[0136] The positive electrode active material layer is formed
partially on both faces of the positive-electrode current
collector. The same is true of the negative electrode active
material layer. Such a negative electrode may have, for example, an
area at which the positive electrode active material layer is not
present on the surface of the positive-electrode current collector
that the negative electrode active material layer faces. This area
permits stable battery design.
[0137] The above area at which the positive and negative electrode
active material layers do not face one another, a non-facing area,
is hardly affected by charging and discharging. The status of the
negative electrode active material layer is consequently maintained
since its formation. This enables repeatable investigation of the
composition of negative electrode active material with high
precision without being affected by charging and discharging.
[Separator]
[0138] The separator separates the positive electrode and the
negative electrode, prevents short circuit current due to contact
of these electrodes, and passes lithium ions therethrough. This
separator may be made of, for example, a porous film of synthetic
resin or ceramics, or two or more stacked porous films. Examples of
the synthetic resin include polytetrafluoroethylene, polypropylene,
and polyethylene.
[Electrolyte]
[0139] A part of the active material layers or the separator is
impregnated with a liquid electrolyte (an electrolyte solution).
The electrolyte is composed of electrolyte salt dissolved in a
solvent and may contain other materials such as additives.
[0140] The solvent may be, for example, a non-aqueous solvent.
Examples of the non-aqueous solvent include ethylene carbonate,
propylene carbonate, butylene carbonate, dimethyl carbonate,
diethyl carbonate, ethylmethyl carbonate, carbonic acid
propylmethyl ester, 1,2-dimethoxyethane, and tetrahydrofuran. Among
these, ethylene carbonate, propylene carbonate, dimethyl carbonate,
diethyl carbonate, or ethylmethyl carbonate, or the combination
thereof is preferable. Such solvent enables better performances.
The combination of a viscous solvent, such as ethylene carbonate or
propylene carbonate, and a non-viscous solvent, such as dimethyl
carbonate, diethyl carbonate or ethylmethyl carbonate allows much
better performances, for such a solvent improves the dissociation
of electrolyte salt and ionic mobility.
[0141] For an alloyed electrode, the solvent preferably contains a
halogenated chain carbonic acid ester, or a halogenated cyclic
carbonic acid ester. Such a solvent enables the negative electrode
active material to be coated with a stable coating at discharging
and particularly charging. The halogenated chain carbonic acid
ester is a chain carbonic acid ester including halogen, in which at
least one hydrogen atom is replaced by a halogen atom. The
halogenated cyclic carbonic acid ester is a cyclic carbonic acid
ester including halogen, in which at least one hydrogen atom is
replaced by a halogen atom.
[0142] The halogen is preferably, but not limited to, fluorine, for
fluorine enables the formation of better coating than other
halogens do. A larger number of halogens is better, for a more
stable coating can be obtained which reduces a decomposition
reaction of an electrolyte.
[0143] Examples of the halogenated chain carbonic acid ester
include carbonic acid fluoromethylmethyl ester, and carbonic acid
methyl(difluoromethyl) ester. Examples of the halogenated cyclic
carbonic acid ester include 4-fluoro-1,3-dioxolan-2-one or
4,5-difluoro-1,3-dioxolane-2-one.
[0144] The solvent preferably contains an unsaturated carbon bond
cyclic carbonate as an additive, for this enables the formation of
a stable coating on an electrode at charging and discharging and
the inhibition of a decomposition reaction of an electrolyte.
Examples of the unsaturated carbon bond cyclic carbonate include
vinylene carbonate and vinyl ethylene carbonate.
[0145] In addition, the solvent preferably contains sultone (cyclic
sulfonic acid ester) as an additive, for this enables improvement
in chemical stability of a battery. Examples of the sultone include
propane sultone and propene sultone.
[0146] In addition, the solvent preferably contains acid anhydride,
for this enables improvement in chemical stability of a battery.
The acid anhydride may be, for example, propane disulfonic acid
anhydride.
[0147] The electrolyte salt may contain, for example, at least one
light metal salt such as lithium salt. Examples of the lithium salt
include lithium hexafluorophosphate (LiPF.sub.6), and lithium
tetrafluoroborate (LiBF.sub.4).
[0148] The content of the electrolyte salt in the solvent is
preferably in the range from 0.5 mol/kg to 2.5 mol/kg. This content
enables high ionic conductivity.
[Manufacture of Laminate Film Secondary Battery]
[0149] Firstly, a positive electrode is produced with the above
positive-electrode material as follows. A positive-electrode
mixture is created by mixing the positive-electrode material with
as necessary the binder, the conductive additive, and other
materials, and dispersed in an organic solvent to form slurry of
the positive-electrode mixture. This slurry is then applied to a
positive-electrode current collector with a coating apparatus such
as a die coater having a knife roll or a die head, and dried by hot
air to obtain a positive electrode active material layer. The
positive electrode active material layer is finally compressed
with, for example, a roll press. The compression may be performed
under heating. The heating may be repeated multiple times.
[0150] Secondly, a negative electrode active material layer is
formed on a negative-electrode current collector to produce a
negative electrode through the same procedure as in the above
production of the negative electrode 10 for a lithium-ion secondary
battery.
[0151] When the positive electrode and the negative electrode are
produced, the active material layers are formed on both faces of
the positive- and negative-electrode current collector. In both the
electrodes, the length of these active material layers formed on
the faces may differ from one another (See FIG. 1).
[0152] Finally, the following steps are carried out in the order
described. An electrolyte is prepared. With ultrasonic welding, the
positive-electrode lead 32 is attached to the positive-electrode
current collector and the negative-electrode lead 33 is attached to
the negative-electrode current collector. The positive and negative
electrodes and the separator interposed therebetween are stacked or
wound to produce the electrode body 31 and a protecting tape is
stuck to the outermost circumference of the body. The electrode
body is flattened. The film-shaped outer part 35 is folded in half
to interpose the electrode body therebetween. The outer edge of the
half parts is stuck to one another by heat sealing such that one of
the four sides is opened to house the electrode body. The close
adhesion films are inserted between the outer part and the
positive- and negative-electrode leads. The prepared electrolyte is
introduced from the open side in a prescribed amount to perform the
impregnation of the electrolyte under a vacuum. The open side is
stuck by vacuum heat sealing.
[0153] In this manner, the laminate film secondary battery 30 can
be produced.
EXAMPLE
[0154] Hereinafter, the present invention will be more specifically
described with reference to examples and comparative examples, but
the present invention is not limited to these examples.
Example 1-1
[0155] The laminate film secondary battery 30 shown in FIG. 3 was
produced by the following procedure.
[0156] The procedure began with the production of a positive
electrode. Positive electrode active materials of 95 mass parts of
LiCoO.sub.2 (lithium cobalt complex oxide), 2.5 mass parts of
positive-electrode conductive additive, and 2.5 mass parts of
positive-electrode binders (polyvinylidene fluoride, PVDF) were
mixed to produce a positive-electrode mixture. The
positive-electrode mixture was dispersed in an organic solvent
(N-methyl-2-pyrrolidone, NMP) to form paste slurry. The slurry was
applied to both surfaces of a positive-electrode current collector
with a coating apparatus having a die head and dried with a drying
apparatus of hot-air type. The positive-electrode current collector
had a thickness of 15 .mu.m. The resultant was finally compressed
with a roll press.
[0157] Next, a negative electrode was produced. For the production
of a negative electrode active material, a mixed raw material of
metallic silicon and silicon dioxide was placed in a reactor and
evaporated under a vacuum of 10 Pa to deposit the evaporated
material on an adsorption plate. The deposit was sufficiently
cooled and then taken out to pulverize the deposit with a ball
mill. After adjusting the particle size of the obtained powder, the
powder was coated with a carbon layer by thermal CVD, as needed.
The produced powder was bulk-modified by the electrochemical method
in a mixed solvent having a
propylene-carbonate-to-ethylene-carbonate volume ratio of 1:1,
including 1.3 mol/kg of electrolyte salt. The obtained material was
immersed in a NMP solution containing polyvinylidene fluoride and
then dried. The powder was thereby coated with a coating layer
composed of the organic polymer of polyvinylidene fluoride. The
negative electrode active material particles thus obtained, a
negative-electrode binder, a first conductive additive, and a
second conductive additive were mixed at a dry-weight ratio of
80:8:10:2. The mixture was diluted with water to form paste slurry
of a negative-electrode mixture. The water was used as a solvent
for the negative-electrode binder of polyacrylic acid. The
negative-electrode mixture slurry was then applied to both surfaces
of a negative-electrode current collector with a coating apparatus
and dried. The negative-electrode current collector used was an
electrolytic copper foil, having a thickness of 15 .mu.m. The
resultant negative-electrode current collector was fired under a
vacuum at 70.degree. C. for 5 hour.
[0158] A solvent was produced by mixing 4-fluoro-1,3-dioxolan-2-one
(FEC), ethylene carbonate (EC), and dimethyl carbonate (DMC) and an
electrolyte salt (lithium hexafluorophosphate, LiPF.sub.6) was
dissolved therein to produce an electrolyte. The composite of the
solvent was FEC:EC:DMC=10:20:70 in term of the accumulation amount.
The content of the electrolyte salt in the solvent was 1.2
mol/kg.
[0159] The secondary battery was assembled by the following
procedure. An aluminum lead was first ultrasonic-welded to one end
of the positive-electrode current collector. A nickel lead was
welded to one end of the negative-electrode current collector. The
positive electrode, a separator, the negative electrode, a
separator were then stacked in this order and wound in a
longitudinal direction to obtain a wound electrode body. The end of
the wounded part was fixed by a PET protecting tape. The separators
were a 12-.mu.m laminate film composed of a porous polyethylene
film interposed between porous polypropylene films. The electrode
body was interposed between outer parts and the outer
circumferences except one side were heat-sealed to house the
electrode body therein. The outer parts were an aluminum laminate
film composed of a nylon film, aluminum foil, and a polypropylene
film stacked. The prepared electrolyte was poured from an open side
to perform the impregnation of the electrolyte under a vacuum. The
open side was stuck by heat sealing.
Examples 1-2 to 1-7
[0160] A secondary battery was produced as in example 1-1 except
for changing the type of the organic polymer constituting the
coating layer coating the powder surface of the silicon compound in
the negative electrode material. As the organic polymer for the
coating layer, polyacrylic acid, sodium polyacrylate, polypropylene
carbonate, sodium carboxymethyl cellulose, polypropylene carbonate,
and polyethylene were used in examples 1-2 to 1-7,
respectively.
Comparative Example 1-1
[0161] A secondary battery was produced as in example 1-1 except
that the coating layer coating the powder surface of the silicon
compound in the negative electrode material was not formed.
[0162] The silicon compounds in examples 1-1 to 1-7 and comparative
example 1-1 had the following physical properties: the silicon
compounds had a median size D.sub.50 of 5 .mu.m; the half width
(2.theta.) of the diffraction peak attributable to an Si(111)
crystal face and obtainable by X-ray diffraction was 1.220; the
crystallite size attributable to the (111) crystal face was 7.21
nm; the silicon compounds were SiO.sub.x where x=0.9; the silicon
compound contained LiF, Li.sub.2CO.sub.3, and Li.sub.2O on its
surface; the active material contained Li.sub.4SiO.sub.4,
Li.sub.6Si.sub.2O.sub.7, and Li.sub.2SiO.sub.3 inside.
[0163] In examples 1-1 to 1-7 and comparative example 1-1, the
carbon coverage was 5.0 mass %. In examples 1-1 to 1-7 and
comparative example 1-1, the organic polymer content was 1% with
respect to the whole negative electrode material particles.
[0164] The cycle performance and the first charge and discharge
efficiency of the secondary batteries in examples 1-1 to 1-7 and
comparative example 1-1 were investigated. The result is given in
Table 1.
[0165] The cycle performance was investigated in the following
manner: First, two cycles of charging and discharging were
performed at 25.degree. C. to stabilize the battery and the
discharge capacity in the second cycle was measured. Next, the
cycle of charging and discharging was repeated until the total
number of cycles reached 100 cycles and the discharge capacity was
measured every cycle. Finally, a capacity retention rate % was
calculated by dividing the discharge capacity in the 100-th cycle
by the discharge capacity in the second cycle and multiply the
resultant by 100 to express as a percent. The cycle conditions were
as follows: The secondary batteries were charged with a constant
current of 2.5 mA/cm.sup.2 until the voltage reached 4.2V. After
this voltage reached 4.2V, the charging was continued while the
current density became 0.25 mA/cm.sup.2 at 4.2V. The batteries were
then discharged with a constant current density of 2.5 mA/cm.sup.2
until the voltage reached 2.5V.
[0166] The first charge and discharge efficiency was calculated by
the following expression:
Initial Efficiency(%)=(First Discharge Capacity/First Charge
Capacity).times.100
[0167] The atmosphere temperate was the same as the cycle
performance was investigated. The charging and discharging
conditions were 0.2 times the conditions of the investigation of
the cycle performance.
TABLE-US-00001 TABLE 1 Capacity Initial retention efficiency
Organic polymer rate (%) (%) Comparative -- 64.3 70.1 example 1-1
Example 1-1 polyvinylidene fluoride 84.2 75.0 Example 1-2
polyacrylic acid 84.4 74.3 Example 1-3 sodium polyacrylate 84.0
76.0 Example 1-4 polypropylene carbonate 83.7 75.0 Example 1-5
sodium carboxymethyl 83.6 75.0 cellulose Example 1-6 polypropylene
carbonate 70.2 73.0 Example 1-7 polyethylene 67.9 69.0
[0168] As shown in Table 1, when the type of the organic polymer
coating the silicon compound particle was changed, the battery
performances were changed depending on the polymer skeleton. The
polymer containing at least one of a fluoroethylene group, a
carboxyl group, a hydroxyl group, and a carbonate group within its
skeleton particularly improved the capacity retention rate and the
first efficiency.
[0169] In the following examples, polyvinylidene fluoride was used
as the organic polymer.
Examples 2-1 to 2-5 and Comparative Examples 2-1 and 2-2
[0170] A secondary battery was produced as in example 1-1 except
that oxygen amount in a bulk of the silicon compound was adjusted
when the negative electrode material was produced. The amount of
accumulated oxygen was adjusted by changing the temperature and the
ratio of raw materials to be vaporized. Table 2 shows the x-value
of the silicon compound expressed by SiO.sub.x in examples 2-1 to
2-5 and comparative examples 2-1 and 2-2.
[0171] The silicon compounds in examples 2-1 to 2-5 and comparative
examples 2-1 and 2-2 had the following physical properties: the
silicon compounds had a median size D.sub.50 of 5 .mu.m; the half
width (2.theta.) of the diffraction peak attributable to an Si(111)
crystal face and obtainable by X-ray diffraction was 1.220; the
crystallite size attributable to the (111) crystal face was 7.21
nm; the silicon compound contained LiF, Li.sub.2CO.sub.3, and
Li.sub.2O on its surface; the active material contained
Li.sub.4SiO.sub.4, Li.sub.6Si.sub.2O.sub.7, and Li.sub.2SiO.sub.3
inside.
[0172] In examples 2-1 to 2-5 and comparative examples 2-1 and 2-2,
the carbon coverage was 5.0 mass %, and the organic polymer
(polyvinylidene fluoride) content was 1% with respect to the whole
negative electrode material particles.
[0173] The cycle performance and the first charge and discharge
efficiency of the secondary batteries in examples 2-1 to 2-5 and
comparative examples 2-1 and 2-2 were investigated. The result is
given in Table 2.
TABLE-US-00002 TABLE 2 Capacity Initial retention efficiency X
value of SiOx rate (%) (%) Comparative 0.3 65.3 81.0 example 2-1
Example 2-1 0.5 80.2 77.1 Example 2-2 0.7 80.8 75.9 Example 2-3 0.9
84.1 75.6 Example 2-4 1.2 81.0 75.2 Example 2-5 1.6 81.1 74.3
Comparative 1.8 75.3 72.1 example 2-2
[0174] As shown in Table 2, when the oxygen amount was lack
(comparative example 2-1, x=0.3), the capacity retention rate
significantly degraded although the initial efficiency was
improved. When the oxygen amount was excess (comparative example
2-2, x=1.8), both the capacity retention rate and the initial
efficiency degraded due to reduction in conductivity. In the
following examples, SiO with an x-value of 0.9 was used.
Examples 3-1 and 3-2
[0175] A secondary battery was produced as in example 1-1 except
that, when the silicon compound was bulk-modified to produce a
lithium compound, voltage, current, and the method for inserting
and extracting lithium were changed to adjust the condition of the
compound produced in the silicon compound. The electrochemical
modification produces LiF, Li.sub.2CO.sub.3, and Li.sub.2O on the
surface, and Li.sub.2SiO.sub.3, Li.sub.6Si.sub.2O.sub.7, and
Li.sub.4SiO.sub.4 inside. Moreover, the lithium compound on the
surface can be removed by cleaning with water. In this way, in
example 3-1, LiF, Li.sub.2CO.sub.3, and Li.sub.2O were produced on
the surface and Li.sub.2SiO.sub.3, Li.sub.6Si.sub.2O.sub.7, and
Li.sub.4SiO.sub.4 were produced inside the silicon compound. In
example 3-2, the lithium compounds on the surface of the silicon
compound were removed while Li.sub.2SiO.sub.3,
Li.sub.6Si.sub.2O.sub.7, and Li.sub.4SiO.sub.4 remained inside.
[0176] The obtained lithium compounds could be observed by XPS. For
example, Li.sub.4SiO.sub.4 was detected by a binding energy of
about 532 eV and Li.sub.2SiO.sub.3 was detected by a binding energy
of about 530 eV. The obtained lithium compounds could also be
observed by .sup.29Si-MAS-NMR spectrum.
Comparative Example 3-1
[0177] A secondary battery was produced as in example 1-1 except
that the silicon compound was not bulk-modified.
[0178] The silicon compounds in examples 3-1 and 3-2 and
comparative example 3-1 had the following physical properties: the
silicon compounds had a median size D.sub.50 of 5 .mu.m; the half
width (2.theta.) of the diffraction peak attributable to an Si(111)
crystal face and obtainable by X-ray diffraction was 1.220; the
crystallite size attributable to the (111) crystal face was 7.21
nm.
[0179] In examples 3-1 and 3-2 and comparative example 3-1, the
carbon coverage was 5.0 mass %, and the organic polymer
(polyvinylidene fluoride) content was 1% with respect to the whole
negative electrode material particles.
[0180] The cycle performance and the first charge and discharge
efficiency of the secondary batteries in examples 3-1 and 3-2 and
comparative example 3-1 were investigated. The result is given in
Table 3.
TABLE-US-00003 TABLE 3 Compound Compound Capacity Initial contained
on contained inside retention efficiency surface layer active
material rate (%) (%) Comparative -- -- 68.1 67.1 example 3-1
Example 3-1 LiF, Li.sub.2CO.sub.3, Li.sub.2SiO.sub.3,
Li.sub.6Si.sub.2O.sub.7, 83.1 75.3 Li.sub.2O Li.sub.4SiO.sub.4
Example 3-2 -- Li.sub.2SiO.sub.3, Li.sub.6Si.sub.2O.sub.7, 84.2
68.3 Li.sub.4SiO.sub.4
[0181] As mentioned above, when the silicon compound is modified in
the electrochemical manner, LiF, Li.sub.2CO.sub.3, and Li.sub.2O
are produced on the surface, and Li.sub.2SiO.sub.3,
Li.sub.6Si.sub.2O.sub.7, and Li.sub.4SiO.sub.4 are produced inside.
Moreover, the lithium compound on the surface can be removed by
cleaning with water. Example 3-1, in which the best bulk condition
was achieved by these reaction, particularly improved the capacity
retention rate and the initial efficiency. In other words, the
silicon compound preferably contains Li.sub.2CO.sub.3, Li.sub.2O,
and LiF on its surface, and the active material preferably contains
Li.sub.4SiO.sub.4, Li.sub.6Si.sub.2O.sub.7, and Li.sub.2SiO.sub.3
inside.
Examples 4-1 to 4-3
[0182] A secondary battery was produced as in example 1-1 except
that, when the silicon compound particles were coated with the
coating layer of the organic polymer (polyvinylidene fluoride), the
amount of the organic polymer was adjusted. More specifically, the
negative electrode active material particles were immersed in a
polyvinylidene fluoride NMP solution and dried repeatedly until a
desired content was achieved. Table 4 shows specific content of
polyvinylidene fluoride.
Comparative Example 4-1
[0183] A secondary battery was produced as in example 1-1 except
that the silicon compound particles were not coated with the
coating layer of the organic polymer (polyvinylidene fluoride).
[0184] The silicon compounds in examples 4-1 to 4-3 and comparative
example 4-1 had the following physical properties: the silicon
compounds had a median size D.sub.50 of 5 .mu.m; the half width
(2.theta.) of the diffraction peak attributable to an Si(111)
crystal face and obtainable by X-ray diffraction was 1.220; the
crystallite size attributable to the (111) crystal face was 7.21
nm.
[0185] In examples 4-1 to 4-3 and comparative example 4-1, the
carbon coverage was 5.0 mass %.
[0186] The cycle performance and the first charge and discharge
efficiency of the secondary batteries in examples 4-1 to 4-3 and
comparative example 4-1 were investigated. The result is given in
Table 4.
TABLE-US-00004 TABLE 4 Content of polyvinylidene fluoride contained
in Capacity Initial negative electrode active retention efficiency
material particles (mass %) rate (%) (%) Comparative -- 64.3 70.1
example 4-1 Example 4-1 0.1 84.1 75.3 Example 4-2 3 80.5 74.4
Example 4-3 5 78.8 73.9
[0187] As shown in Table 4, the content of the organic polymer is
preferably 0.01 mass % or more and less than 5 mass %. When the
organic polymer was not contained (comparative example 4-1), the
battery performances degraded due to reduction in resistance to an
aqueous solvent. When the content of the organic polymer was less
than 5%, the negative electrode active material particles was not
decreased in electrical conductivity and ionic conductivity, and
thus the battery performances could be prevented from
deteriorating.
[0188] In the following examples, the content of the organic
polymer (polyvinylidene fluoride) was 1 mass %.
Examples 5-1 to 5-4
[0189] In examples 5-1 to 5-3, a secondary battery was produced as
in example 1-1 except for adjusting the carbon amount of the
coating layer coating the silicon compound (the particle of the
silicon oxide material) by the thermal CVD method. In example 5-4,
a secondary battery was produced as in example 1-1 except that the
carbon layer was not formed on the surface of the silicon
compound.
[0190] The silicon compounds in examples 5-1 to 5-4 had the
following physical properties: the silicon compounds had a median
size D.sub.50 of 5 .mu.m; the half width (2.theta.) of the
diffraction peak attributable to an Si(111) crystal face and
obtainable by X-ray diffraction was 1.220; the crystallite size
attributable to the (111) crystal face was 7.21 nm.
[0191] The cycle performance and the first charge and discharge
efficiency of the secondary batteries in examples 5-1 to 5-4 were
investigated. The result is given in Table 5.
TABLE-US-00005 TABLE 5 Content of carbon contained Capacity Initial
in negative electrode active retention efficiency material
particles (mass %) rate (%) (%) Example 5-1 5.2 84.1 75.1 Example
5-2 10.1 83.8 75.0 Example 5-3 15.2 83.2 74.8 Example 5-4 0 76.2
71.3
[0192] As shown in Table 5, formation of the carbon layer yielded
an effect of improving conductivity and thus improved the battery
performances. However, increasing the carbon content can reduce the
capacity, depending on battery design. Accordingly, the content is
preferably 15 mass % or less in view of the battery capacity and
performances.
Examples 6-1 to 6-4
[0193] In examples 6-1 to 6-4, a secondary battery was produced as
in example 1-1 except for adjusting the relationship A/B between
the peak area A for a C1s binding energy of about 287.5.+-.1.0 eV
and the peak area B for a C1s binding energy of about 284.0.+-.1.0
eV when the silicon compound was subjected to XPS on its surface
layer. The adjustment was performed by changing the treatment
temperature in the electrochemical modification method.
[0194] The silicon compounds in examples 6-1 to 6-4 had the
following physical properties: the silicon compounds had a median
size D.sub.50 of 5 .mu.m; the half width (2.theta.) of the
diffraction peak attributable to an Si(111) crystal face and
obtainable by X-ray diffraction was 1.220; the crystallite size
attributable to the (111) crystal face was 7.21 nm.
[0195] The cycle performance and the first charge and discharge
efficiency of the secondary batteries in examples 6-1 to 6-4 were
investigated. The result is given in Table 6.
TABLE-US-00006 TABLE 6 Capacity Initial retention efficiency A/B
rate (%) (%) Example 6-1 4.20 82.1 74.8 Example 6-2 1.62 83.8 75.0
Example 6-3 0.33 84.1 75.1 Example 6-4 0.02 81.8 74.3
[0196] As shown in Table 6, when A/B satisfied
0.05.ltoreq.A/B.ltoreq.3.00 (examples 6-2 and 6-3), the battery
performances were improved. Thus, A/B is preferably
0.05.ltoreq.A/B.ltoreq.3.00. A/B of 3.00 or less prevented a
carboxyl group and the like from excessively increasing on the
surface, thus prevented the carbon coating from being covered, and
consequently prevented electrical and ionic conductivities from
decreasing. A/B of 0.05 or more allowed a carboxyl group and the
like to be on the surface layer of the silicon compound in
sufficient amount to exhibit the effect.
Examples 7-1 to 7-5
[0197] A secondary battery was produced as in example 1-1 except
that a negative electrode containing a carbon-based material was
produced as follows. A silicon compound and a carbon-based material
whose median sizes have been adjusted were mixed with a shaker to
form an electrode. The negative electrode thus produced had a
structure in which the carbon-based material was around the silicon
compound. The median size of the silicon compound and the median
size and the content of the carbon-based material in examples 7-1
to 7-5 are as shown in Table 7.
[0198] The silicon compounds in examples 7-1 to 7-5 had the
following physical properties: the half width (2.theta.) of the
diffraction peak attributable to an Si(111) crystal face and
obtainable by X-ray diffraction was 1.220; the crystallite size
attributable to the (111) crystal face was 7.21 nm.
[0199] The cycle performance and the first charge and discharge
efficiency of the secondary batteries in examples 7-1 and 7-5 were
investigated. The result is given in Table 7.
TABLE-US-00007 TABLE 7 Median size of Content of Median size
carbon-based carbon-based Capacity Initial of silicon material
material retention efficiency compound (.mu.m) (.mu.m) (mass %)
rate (%) (%) Example 7-1 8 6 2 82.1 70.3 Example 7-2 4 6 2 82.4
71.8 Example 7-3 4 0.2 2 84.1 75.3 Example 7-4 4 0.2 5 83.9 73.5
Example 7-5 4 0.2 10 83.5 71.2
[0200] As shown in Table 7, when the carbon-based material whose
median size was smaller than that of the silicon compound was mixed
(examples 7-3 to 7-5), the carbon-based material has irreversible
capacity, the content thereof is preferably 5 mass % or less in
view of the battery capacity and performances.
Examples 8-1 to 8-8
[0201] A secondary battery was produced as in example 1-1 except
that the crystallinity of the silicon compound was changed. The
crystallinity can be changed by a heat treatment under a
non-atmospheric condition after insertion and extraction of
lithium. Table 8 shows the half width of the silicon compound in
examples 8-1 to 8-8. Although example 8-1 exhibited a crystallite
size of 1.542, this value was obtained by fitting with analysis
software because the peak value was not obtained. The silicon
compound in example 8-1 was substantially amorphous.
[0202] The silicon compounds in examples 8-1 to 8-8 had a median
size D.sub.50 of 5 .mu.m.
[0203] The cycle performance and the first charge and discharge
efficiency of the secondary batteries in examples 8-1 to 8-8 were
investigated. The result is given in Table 8.
TABLE-US-00008 TABLE 8 Capacity Initial Half width crystallite
retention efficiency 2.theta.(.degree.) size (nm) rate (%) (%)
Example 8-1 10.123 1.542 84.4 74.5 Example 8-2 5.01 3.29 84.3 74.6
Example 8-3 2.257 3.77 84.1 74.9 Example 8-4 1.845 4.62 83.6 75.2
Example 8-5 1.271 6.63 83.1 75.3 Example 8-6 1.025 8.55 82.0 75.3
Example 8-7 0.796 10.84 79.8 75.5 Example 8-8 0.756 11.42 76.5
75.7
[0204] As shown in Table 8, the capacity retention rate and the
initial efficiency changed in response to the variation in the
crystallinity of the silicon compound. In particular, a high
capacity retention rate and a high initial efficiency were obtained
by low crystallinity materials with a half width of 1.2.degree. or
more and a crystallite size of 7.5 nm or less, which is
attributable to an Si(111) crystal face. The best battery
performances were obtained when the silicon compound was
amorphous.
Examples 9-1 to 9-4
[0205] A secondary battery was produced as in example 1-1 except
that the median size of the silicon compound was adjusted. Table 9
shows the median size of the silicon compound in examples 9-1 to
9-4.
[0206] The silicon compounds in examples 9-1 to 9-4 exhibited a
half width (2.theta.) of the diffraction peak attributable to a
(111) crystal face and obtainable by X-ray diffraction of 1.220 and
a crystallite size attributable to the (111) crystal face of 7.21
nm.
[0207] The cycle performance and the first charge and discharge
efficiency of the secondary batteries in examples 9-1 to 9-4 were
investigated. The result is given in Table 9.
TABLE-US-00009 TABLE 9 Capacity Initial Median size retention
efficiency (.mu.m) rate (%) (%) Example 9-1 5.1 84.2 75.3 Example
9-2 1.3 82.3 74.8 Example 9-3 8.1 84.1 75.2 Example 9-4 12.3 83.5
75.3
[0208] As shown in Table 9, when the median size of the silicon
compound particle ranged from 0.5 .mu.m to 20 .mu.m, the capacity
retention rate and the initial efficiency were kept high.
Example 10-1
[0209] A secondary battery was produced by modifying the silicon
compound in the electrochemical manner as in example 1-1.
Example 10-2
[0210] A secondary battery was produced as in example 1-1 except
that the silicon compound was modified by the thermal doping method
with lithium.
Comparative Example 10-1
[0211] A secondary battery was produced as in example 1-1 except
that the silicon compound was not modified by introducing
lithium.
[0212] The silicon compounds in examples 10-1 and 10-2 and
comparative example 10-1 had the following physical properties: the
silicon compounds had a median size D.sub.50 of 5 .mu.m; the half
width (2.theta.) of the diffraction peak attributable to an Si(111)
crystal face and obtainable by X-ray diffraction was 1.220; the
crystallite size attributable to the (111) crystal face was 7.21
nm.
[0213] The cycle performance and the first charge and discharge
efficiency of the secondary batteries in examples 10-1 and 10-2 and
comparative example 10-1 were investigated. The result is given in
Table 10.
TABLE-US-00010 TABLE 10 Capacity Initial retention efficiency
Modification method rate (%) (%) Comparative -- 64.3 70.1 example
10-1 Example 10-1 Electrochemical method 86.5 77.6 Example 10-2
Thermal doping method 75 74
[0214] As shown in Table 10, the electrochemical method was
preferred as the modification method of the silicon compound. The
thermal doping method, in which a silicon material mixed with
lithium metal or a lithium compound was subjected to heat
treatment, could not modify the active material well.
Examples 11-1 to 11-8
[0215] A secondary battery was produced as in example 1-1 except
that a carbon-based negative electrode material was added to the
negative electrode, in addition to the negative electrode material
particles containing the silicon compound. Table 11 shows the
median size D.sub.50 (Y) of the silicon compound, the median size
D.sub.50 (X) of the carbon-based negative electrode material, and
the value of X/Y.
[0216] The silicon compounds in examples 11-1 to 11-8 had the
following physical properties: the half width (2.theta.) of the
diffraction peak attributable to an Si(111) crystal face and
obtainable by X-ray diffraction was 1.220; the crystallite size
attributable to the (111) crystal face was 7.21 nm.
[0217] The cycle performance and the first charge and discharge
efficiency of the secondary batteries in examples 11-1 to 11-8 were
investigated. The result is given in Table 11.
TABLE-US-00011 TABLE 11 D.sub.50 of carbon-based D.sub.50 of
negative silicon electrode Capacity Initial compound material
retention efficiency (.mu.m) (.mu.m) X/Y rate (%) (%) Example 11-1
4 20 5 88 86.3 Example 11-2 4 16 4 88.1 86.1 Example 11-3 4 12 3 88
85.8 Example 11-4 4 8 2 87.6 84.2 Example 11-5 4 4 1 87 83.6
Example 11-6 4 2 0.5 83.2 82.9 Example 11-7 8 4 0.5 81.5 85.3
Example 11-8 8 16 2 85.6 86.5
[0218] As shown in Table 11, the carbon material in the negative
electrode active material layer is preferably equal to or larger
than the silicon compound. When the silicon compound, which was
expandable and contractible, was equal to or smaller than the
carbon-based negative electrode material, breakage of the mixture
layer could be prevented. When the carbon-based negative electrode
material was larger than the silicon compound, the volume density
of the negative electrode at charging, the initial efficiency, and
thus the battery energy density were improved.
[0219] It is to be noted that the present invention is not limited
to the foregoing embodiment. The embodiment is just an
exemplification, and any examples that have substantially the same
feature and demonstrate the same functions and effects as those in
the technical concept described in claims of the present invention
are included in the technical scope of the present invention.
* * * * *