U.S. patent application number 11/166173 was filed with the patent office on 2006-01-05 for silicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode material.
This patent application is currently assigned to Shin-Etsu Chemical Co., Ltd.. Invention is credited to Mikio Aramata, Hirofumi Fukuoka, Satoru Miyawaki, Kazuma Momii, Kouichi Urano.
Application Number | 20060003227 11/166173 |
Document ID | / |
Family ID | 35514346 |
Filed Date | 2006-01-05 |
United States Patent
Application |
20060003227 |
Kind Code |
A1 |
Aramata; Mikio ; et
al. |
January 5, 2006 |
Silicon composite, making method, and non-aqueous electrolyte
secondary cell negative electrode material
Abstract
A silicon composite comprises silicon particles whose surface is
at least partially coated with a silicon carbide layer. It is
prepared by subjecting a silicon powder to thermal CVD with an
organic hydrocarbon gas and/or vapor at 900-1,400.degree. C., and
heating the powder for removing an excess free carbon layer from
the surface through oxidative decomposition.
Inventors: |
Aramata; Mikio; (Usui-gun,
JP) ; Miyawaki; Satoru; (Usui-gun, JP) ;
Fukuoka; Hirofumi; (Usui-gun, JP) ; Momii;
Kazuma; (Usui-gun, JP) ; Urano; Kouichi;
(Annaka-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Shin-Etsu Chemical Co.,
Ltd.
Tokyo
JP
|
Family ID: |
35514346 |
Appl. No.: |
11/166173 |
Filed: |
June 27, 2005 |
Current U.S.
Class: |
429/218.1 ;
252/182.1; 428/404 |
Current CPC
Class: |
H01M 10/44 20130101;
H01M 2004/027 20130101; H01M 4/139 20130101; H01M 4/386 20130101;
Y02E 60/10 20130101; H01M 4/134 20130101; Y10T 428/2993 20150115;
H01M 10/052 20130101; H01M 4/13 20130101; H01M 4/38 20130101; H01M
4/0421 20130101; Y02P 70/50 20151101; H01M 4/366 20130101; H01M
4/58 20130101 |
Class at
Publication: |
429/218.1 ;
252/182.1; 428/404 |
International
Class: |
H01M 4/58 20060101
H01M004/58; B32B 5/16 20060101 B32B005/16 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 1, 2004 |
JP |
2004-195586 |
Claims
1. A silicon composite comprising silicon particles whose surface
is at least partially coated with a silicon carbide layer.
2. The silicon composite of claim 1 wherein the silicon particles
have an average particle size of 50 nm to 50 .mu.m and the silicon
particle surface is at least partially fused to silicon
carbide.
3. The silicon composite of claim 1 wherein a diffraction line
attributable to silicon is observed when analyzed by x-ray
diffractometry, and which contains free carbon in an amount of up
to 5% by weight.
4. The silicon composite of claim 1 which contains 5 to 90% by
weight of zero-valent silicon capable of generating hydrogen gas
when reacted with an alkali hydroxide solution.
5. The silicon composite of claim 1 wherein after the silicon
composite is treated with a mixture of hydrofluoric acid and an
oxidizing agent and heat dried, silicon carbide is left as the
evaporation residue.
6. A method for preparing the silicon composite of claim 1,
comprising the steps of subjecting a silicon powder to
thermochemical vapor deposition treatment with an organic
hydrocarbon gas and/or vapor at 900.degree. C. to 1,400.degree. C.,
and heating the powder for removing a surface excess free carbon
layer through oxidation.
7. A negative electrode material for a non-aqueous electrolyte
secondary cell, comprising the silicon composite of claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This non-provisional application claims priority under 35
U.S.C. .sctn.119(a) on patent application No. 2004-195586 filed in
Japan on Jul. 1, 2004, the entire contents of which are hereby
incorporated by reference.
TECHNICAL FIELD
[0002] This invention relates to a silicon composite powder having
a capacity controlled to compensate for the drawback of silicon
which is believed useful as lithium ion secondary cell negative
electrode active material; a method for preparing the same; and a
non-aqueous electrolyte secondary cell negative electrode material
comprising the powder.
BACKGROUND ART
[0003] With the recent remarkable development of potable electronic
equipment, communications equipment and the like, a strong demand
for high energy density secondary batteries exists from the
standpoints of economy and size and weight reductions. One prior
art method for increasing the capacity of secondary batteries is to
use oxides as the negative electrode material, for example, oxides
of V, Si, B, Zr, Sn or the like or complex oxides thereof (see JP-A
5-174818 and JP-A 6-060867 corresponding to U.S. Pat. No.
5,478,671), metal oxides quenched from the melt (JP-A 10-294112),
silicon oxide (Japanese Patent No. 2,997,741 corresponding to U.S.
Pat. No. 5,395,711), and Si.sub.2N.sub.2O and Ge.sub.2N.sub.2O
(JP-A 11-102705 corresponding to U.S. Pat. No. 6,066,414).
Conventional methods of imparting conductivity to the negative
electrode material include mechanical alloying of SiO with
graphite, followed by carbonization (see JP-A 2000-243396
corresponding to U.S. Pat. No. 6,638,662), coating of silicon
particles with a carbon layer by chemical vapor deposition (JP-A
2000-215887 corresponding to U.S. Pat. No. 6,383,686), and coating
of silicon oxide particles with a carbon layer by chemical vapor
deposition (JP-A 2002-42806). None of these patents relates to the
method of alleviating a substantial volume change of a silicon
negative electrode during charge/discharge cycles which is an
outstanding problem characteristic of the silicon negative
electrode nor the method of reducing the current collection
associated with the volume change. It remains an important task to
establish such techniques.
[0004] The prior art methods using silicon as such or making its
surface conductive for improving the cycle performance of negative
electrode material are successful in increasing the
charge/discharge capacity and energy density, but are not
necessarily satisfactory because of failure to fully meet the
characteristics required in the market, particularly the cycle
performance of importance in mobile phone and other applications.
There is a desire for further improvement in cycle performance.
[0005] In particular, Japanese Patent No. 2,997,741 uses silicon
oxide as the negative electrode material in a lithium ion secondary
cell to provide an electrode with a high capacity. As long as the
present inventors have confirmed, there is left a room for further
improvement as demonstrated by a still high irreversible capacity
on the first charge/discharge cycle and cycle performance below the
practical level. With respect to the technique of imparting
conductivity to the negative electrode material, JP-A 2000-243396
suffers from the problem that solid-to-solid fusion fails to form a
uniform carbon coating, resulting in insufficient conductivity. In
the method of JP-A 2000-215887 which can form a uniform carbon
coating, the negative electrode material based on silicon undergoes
excessive expansion and contraction upon adsorption and desorption
of lithium ions, meaning impractical operation, and loses cycle
performance. Thus, the charge/discharge quantity must be limited.
In JP-A 2002-42806, despite a discernible improvement of cycle
performance, due to precipitation of silicon crystallites,
insufficient structure of the carbon coating and insufficient
fusion of the carbon coating to the substrate, the capacity
gradually lowers as charge/discharge cycles are repeated, and
suddenly drops after a certain number of charge/discharge cycles.
This approach is thus insufficient for use in secondary cells.
SUMMARY OF THE INVENTION
[0006] An object of the present invention is to provide a silicon
composite which maintains the high initial efficiency inherent to
silicon, has excellent cycle performance, and has alleviated a
substantial volume change during charge/discharge cycles so that it
is effective as active material for lithium ion secondary cell
negative electrodes; a method for preparing the same; and a
non-aqueous electrolyte secondary cell negative electrode material
comprising the silicon composite.
[0007] The inventor has discovered a silicon composite which
maintains the high initial efficiency inherent to silicon, has
excellent cycle performance, and has alleviated a substantial
volume change during charge/discharge cycles and which is thus
effective as the active material for lithium ion secondary cell
negative electrodes.
[0008] The development of an electrode material having an increased
charge/discharge capacity is very important and many engineers have
been engaged in the research and development thereof. Under the
circumstances, silicon and amorphous silicon oxides represented by
the general formula SiO.sub.x wherein 1.0.ltoreq.x<1.6 are of
great interest as the negative electrode active material for
lithium ion secondary cells because their capacity is large. Only
few of them have been used in practice because of their
shortcomings including substantial degradation upon repeated
charge/discharge cycles, that is, poor cycle performance and in
particular, low initial efficiency.
[0009] Making investigations from such a standpoint with the target
of improving cycle performance and initial efficiency, the inventor
found that CVD treatment of silicon oxide powder led to a
substantial improvement in performance as compared with the prior
art. However, this approach starting with silicon oxide left the
problem of low initial efficiency due to the presence of oxygen
atoms. The problem could, of course, be solved by some means, for
example, by the addition of phenyl lithium which is known as a
method for compensating for the low initial efficiency. These
solutions, however, invited side issues that the cell manufacture
process becomes complex and unnecessary materials are left within
the cell.
[0010] In contrast, a silicon powder characterized by the absence
of oxygen is expected to have a far greater charge/discharge
capacity than the silicon oxide. On the other hand, the silicon
powder undergoes a substantial volume change while occluding and
releasing a large amount of lithium, which can cause separation
between silicon and binder, breakage of silicon particles, and even
separation between the electrode film and the current collector.
This leads to such problems as a failure of current collection and
cycle degradation. Among approaches contemplated to solve these
problems, electrically controlling the capacity is effective as a
method of alleviating the volume change by expansion and
contraction of silicon, but impractical. Under the circumstances
where a charge/discharge capacity as large as that of silicon is
not necessary, there is a need for a silicon base material having
the advantages of a less volume change and good adhesion to binders
or the like, notwithstanding a lower energy density than
silicon.
[0011] Making extensive investigations from this standpoint on a
material which undergoes a less volume change upon occlusion and
release of lithium even during full charge/discharge operation and
has highly adhesive surfaces, the inventor has found that the above
problems of lithium ion secondary cell negative electrode active
material are overcome by coating silicon particles or
micro-particles with an inert robust material, that is, silicon
carbide. The resulting material has an initial efficiency
comparable to or surpassing the existing carbonaceous materials and
an extremely greater charge/discharge capacity than the
carbonaceous materials and achieves drastic improvements in cyclic
charge/discharge operation and efficiency thereof.
[0012] In one aspect, the present invention provides a silicon
composite comprising silicon particles whose surface is at least
partially coated with a silicon carbide layer.
[0013] In preferred embodiments, the silicon particles have an
average particle size of 50 nm to 50 .mu.m; the silicon particle
surface is at least partially fused to silicon carbide; a
diffraction line attributable to silicon is observed when the
silicon composite is analyzed by x-ray diffractometry; the silicon
composite contains free carbon in an amount of up to 5% by weight;
the silicon composite contains 5 to 90% by weight of zero-valent
silicon capable of generating hydrogen gas when reacted with an
alkali hydroxide solution; and after the silicon composite is
treated with a mixture of hydrofluoric acid and an oxidizing agent
and heat dried, silicon carbide is left as the evaporation
residue.
[0014] In another aspect, the present invention provides a method
for preparing the silicon composite defined above, comprising the
steps of subjecting a silicon powder to thermochemical vapor
deposition treatment with an organic hydrocarbon gas and/or vapor
at 900.degree. C. to 1,400.degree. C., and heating the powder for
removing a surface excess free carbon layer through oxidation.
[0015] Also contemplated herein is a negative electrode material
for a non-aqueous electrolyte secondary cell, comprising the
silicon composite defined above.
[0016] The silicon composite of the present invention maintains the
high initial efficiency inherent to silicon, has excellent cycle
performance, and alleviates a substantial volume change during
charge/discharge cycles so that it is effective as the active
material for lithium ion secondary cell negative electrodes. When
the silicon composite is used as the active material for a lithium
ion secondary cell negative electrode, the resulting lithium ion
secondary cell negative electrode material is adherent to a binder,
has a high initial efficiency, alleviates a volume change during
charge/discharge cycles, and is improved in repeated cyclic
operation and efficiency thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 diagrammatically illustrates in cross section the
structure of a silicon composite particle of the invention.
[0018] FIG. 2 is a chart of x-ray diffraction (Cu--K.alpha.) on a
silicon composite obtained by starting with a silicon powder,
conducting thermal CVD using methane gas, and conducting oxidative
decomposition for removing carbon.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] For use as the lithium ion secondary cell negative electrode
active material a siliceous material is expected promising because
of its charge/discharge capacity which is several times greater
than that of the current mainstream graphite-derived materials, but
is prevented from practical use by the degradation of performance
with repeated charge/discharge operation and a substantial volume
change during charge/discharge operation. The present invention
relates to a silicon composite which maintains the high initial
efficiency, improves the cycle performance, and alleviates the
volume change of the siliceous material. The silicon composite is
arrived at by effecting chemical vapor deposition on silicon
micro-crystals for coating their surfaces with carbon and forming a
silicon carbide layer at the interface, and then removing the
carbon layer on the surfaces through oxidation.
[0020] More particularly, the silicon powder used herein is
metallic silicon of the industrial grade, polycrystalline silicon
for semiconductor or semiconductor single-crystal powder which has
been previously pulverized to a predetermined particle size, or a
microparticulate silicon powder which has been prepared by inert
gas quenching, gas phase precipitation or the like. A hydrocarbon
compound gas and/or vapor is introduced, after which thermochemical
vapor deposition treatment is effected on the silicon powder at
900.degree. C. to 1,400.degree. C., thereby forming a carbon layer
on the silicon surface and at the same time forming a silicon
carbide layer at the interface between the silicon and the carbon
layer. Subsequent heating in an oxidizing atmosphere causes
oxidative decomposition of the carbon layer whereby the carbon
layer is removed, leaving the silicon particles whose surface is at
least partially coated with silicon carbide. In this way, silicon
carbide-surface-coated silicon particles, referred to herein as
silicon composite particles, are readily prepared. In the silicon
composite particles, it is preferred that at least part of the
silicon surface be fused to silicon carbide. The outermost coating
may be of an electrically conductive material which is not
decomposed or altered in the cell although silicon carbide is most
preferred from the adhesion standpoint as well.
[0021] The silicon composite of the invention should preferably
meet the following conditions.
[0022] i) By varying the temperature and time of chemical vapor
deposition treatment, the thickness and proportion of the silicon
carbide layer can be controlled. With this technique, therefore,
the quantity of zero-valent silicon serving as a lithium occluding
mass, that represents a charge/discharge capacity, can be
controlled, and the volume change associated with charge/discharge
cycles is eventually alleviated.
[0023] ii) In x-ray diffractometry (Cu--K.alpha.) using copper as a
counter cathode, a diffraction pattern attributable to silicon, for
example, a diffraction peak centering approximately at
2.theta.=28.4.degree. and attributable to Si(111) is
observable.
[0024] iii) The quantity of zero-valent silicon capable of
occluding and releasing lithium ions when used in a lithium ion
secondary cell negative electrode can be measured in terms of the
quantity of hydrogen generated upon reaction with an alkali
hydroxide according to ISO DIS 9286, the method of measuring free
silicon in silicon carbide fine powder. As calculated from the
quantity of hydrogen generated, zero-valent silicon is contained in
an amount of 5 to 90% by weight, more preferably 20 to 80% by
weight, and even more preferably 30 to 60% by weight of the silicon
composite.
[0025] iv) When the silicon composite is dissolved in hydrofluoric
acid containing an oxidizing agent such as hydrogen peroxide,
silicon carbide settles down without being dissolved. When this
solution is evaporated to dryness, a green tinted residue is left.
The silicon composite has the structure shown by a schematic image
of FIG. 1.
[0026] v) When used as a lithium ion secondary cell negative
electrode active material, the silicon composite has an initial
efficiency equal to or greater than that of the current
graphite-derived materials and undergoes a reduced volume change
and minimized cycle degradation as compared with silicon
itself.
[0027] In the silicon composite powder of the invention, the
quantity of free carbon and the quantity of carbon in silicon
carbide form are determined from the quantity of free carbon and a
difference between the total carbon quantity and the free carbon
quantity as measured according to JIS R-6124. The quantity of
silicon carbide coated is determined by multiplying the quantity of
carbon in silicon carbide form by 40/12=3.33.
[0028] Preference is given to a less quantity of free carbon
relative to the silicon composite powder (i.e., silicon composite
powder surface coated with silicon carbide, prepared by thermal CVD
and subsequent oxidative decomposition). Specifically, the quantity
of free carbon is preferably up to 5% by weight (5 to 0% by
weight), more preferably up to 3% by weight (3 to 0% by weight) and
even more preferably up to 2% by weight (2 to 0% by weight) of the
silicon composite. The quantity of silicon carbide coated is
preferably 10 to. 95% by weight, more preferably 20 to 80% by
weight, and even more preferably 40 to 70% by weight of the silicon
composite. If the quantity of silicon carbide coated is less than
10 wt % of the silicon composite, the silicon composite may have
insufficient cycle performance when incorporated in a lithium ion
secondary cell. If the lo quantity of silicon carbide coated is
more than 95 wt % of the silicon composite, the inactive silicon
carbide coating has an increased thickness, which may inhibit
migration of lithium ions and reduce the negative electrode
capacity.
[0029] Next, the method for preparing the electrically conductive
silicon composite according to the invention is described.
[0030] The method starts with a silicon powder preferably having an
average particle size of 50 nm to 50 .mu.m which is obtained by
mechanically pulverizing metallic silicon of the industrial grade,
polycrystalline silicon for semiconductor, or semiconductor
silicon, or a microparticulate silicon powder preferably having an
average particle size of 50 nm to 10 .mu.m, more preferably 100 nm
to 5 .mu.m, which is obtained by quenching silicon vapor with an
inert gas such as argon. In a fluidizing gas atmosphere containing
at least an organic matter gas and/or vapor, thermal chemical vapor
deposition (CVD) treatment is effected on the silicon powder at a
temperature of preferably 900.degree. C. to 1,400.degree. C., more
preferably 900.degree. C. to 1,300.degree. C. The resulting
carbon/silicon carbide coated silicon powder is heat treated in an
oxidizing atmosphere, typically air, at a temperature of preferably
600 to 1,400.degree. C., more preferably 600 to 900.degree. C., and
even more preferably 650 to 800.degree. C., whereby the surface
layer of free carbon is oxidatively decomposed away. There are
obtained silicon composite particles in which at least part of SiC
is fused to or merged with Si.
[0031] The silicon particles whose surface will be at least
partially coated with silicon carbide should preferably have an
average particle size of 50 nm to 50 .mu.m, more preferably 100 nm
to 20 .mu.m, even more preferably 200 nm to 10 .mu.m, and most
preferably 500 nm to 5 .mu.m. Silicon particles with an average
particle size of less than 50 nm may be difficult to handle whereas
silicon particles with an average particle size of more than 50
.mu.m may penetrate through the negative electrode film.
[0032] At a CVD temperature below 900.degree. C., the formation of
a silicon carbide coating proceeds at a slow rate, takes a long
time, and becomes inefficient. Inversely, at a CVD temperature
above 1,400.degree. C., silicon is melted, deviating from a uniform
particle form. With respect to the oxidative decomposition of the
free carbon layer, a temperature below 600.degree. C. may induce
insufficient oxidative decomposition. The temperature and time for
CVD are determined as appropriate relative to the thickness of the
carbon layer. During the CVD treatment, particles can agglomerate
together. In such a case, the agglomerates are preferably
disintegrated on a ball mill or the like in order to facilitate the
subsequent oxidative decomposition. In some cases, the CVD
treatment may be repeated similarly.
[0033] The organic materials used as the raw material for
generating the organic matter gas and/or vapor include those which
are pyrolyzed at the heat treatment temperature, particularly in a
non-oxidizing atmosphere, to form carbon or graphite, for example,
hydrocarbons such as methane, ethane, ethylene, acetylene, propane,
butane, butene, pentane, isobutane and hexane, alone or in
admixture, and mono- to tri-cyclic aromatic hydrocarbons such as
benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane,
naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene,
coumarone, pyridine, anthracene, and phenanthrene, alone or in
admixture. Also useful are gas light oil, creosote oil, anthracene
oil and naphtha-cracked tar oil resulting from tar distillation
process, alone or in admixture.
[0034] The heat treatment of silicon powder and the organic matter
gas and/or vapor is not particularly limited as long as a reactor
system including a heating mechanism is employed in a non-oxidizing
atmosphere. The treatment may be either continuous or batchwise.
Depending on a particular purpose, a choice may be made among a
fluidized bed reactor, rotary furnace, vertical moving bed reactor,
tunnel furnace, batch furnace, and rotary kiln.
[0035] The fluidizing gas used herein may be the above-described
organic matter gas alone or a mixture of the organic matter gas and
a non-oxidizing gas such as Ar, He, H.sub.2 and N.sub.2. Preferably
the linear velocity u (m/sec) of the fluidizing gas is controlled
such that the ratio of the linear velocity u to the minimum
fluidization velocity u.sub.mf may be in the range of 1.5 to 5
(i.e., 1.5.ltoreq.u/u.sub.mf.ltoreq.5), whereby a conductive
coating is formed more efficiently. A u/u.sub.mf ratio below 1.5
may lead to insufficient fluidization, introducing variations in
the conductive coating. Inversely, a u/u.sub.mf ratio in excess of
5 may allow for secondary agglomeration of particles, failing to
form a uniform conductive coating.
[0036] It is noted that the minimum fluidization velocity varies
with the size of particles, treating temperature, treating
atmosphere and the like. The minimum fluidization velocity is
defined as the linear velocity of the fluidizing gas at which a
pressure loss across the powder becomes equal to W/A wherein W is
the weight of the powder and A is the cross-sectional area of the
fluidized layer, when the linear velocity of the fluidizing gas is
slowly increased the minimum fluidization velocity u.sub.mf is
typically in the range of about 0.1 to 30 cm/sec, preferably about
0.5 to 10 cm/sec. The average particle size of silicon particles
imparting u.sub.mf in this range is typically 0.5 to 50 .mu.m, and
preferably 1 to 30 .mu.m. With an average particle size of less
than 0.5 .mu.m, secondary agglomeration may take place, inhibiting
effective treatment of surfaces of individual particles. Particles
with an average particle size of more than 50 .mu.m may be
difficult to uniformly coat on the surface of a current collector
in a lithium ion secondary cell.
[0037] The thus obtained silicon composite powder of the invention
has an average particle size of typically 0.08 to 52 .mu.m,
preferably 0.1 to 50 .mu.m, more preferably 0.5 to 40 .mu.m, and
most preferably 1 to 20 .mu.m. Too small an average particle size
corresponds to too large a surface area, which may lead to too low
a negative electrode film density. Too large an average particle
size has the risk of penetrating through the negative electrode
film. It is noted that throughout the specification, the average
particle size is determined as a weight average diameter D.sub.50
(particle diameter at 50% by weight cumulative, or median diameter)
upon measurement of particle size distribution by laser light
diffractometry.
[0038] The silicon composite powder obtained by the invention may
be used as an active material for non-aqueous electrolyte secondary
cell negative electrodes. Due to many advantages including a high
capacity as compared with the existing graphite and the like, a
high initial efficiency as compared with silicon oxide or silicon
oxide-derived materials, a controlled volume change upon
charge/discharge cycles as compared with silicon, and a good
adhesion between particles and a binder, the silicon composite
powder may be used to construct a non-aqueous electrolyte secondary
cell, especially lithium ion secondary cell, having improved cycle
performance.
[0039] When a negative electrode is prepared using the silicon
composite powder, a conductive agent such as graphite may be added
to the silicon composite powder. The type of conductive agent is
not particularly limited as long as it is an electron conductive
material which does not undergo decomposition or alteration in the
cell associated therewith. Illustrative conductive agents include
metals in powder or fiber form such as Al, Ti, Fe, Ni, Cu, Zn, Ag,
Sn and Si, natural graphite, synthetic graphite, various coke
powders, meso-phase carbon, vapor phase grown carbon fibers, pitch
base carbon fibers, PAN base carbon fibers, and graphite obtained
by firing various resins.
[0040] In the embodiment wherein the conductive agent is added, the
amount of the conductive agent is preferably 1 to 60% by weight,
more preferably 10 to 50% by weight, and most preferably 20 to 50%
by weight of a mixture of the silicon composite powder and the
conductive agent. A mixture with less than 1% of the conductive
agent may fail to withstand expansion and contraction on
charge/discharge cycles, whereas a mixture with more than 60% of
the conductive agent may have a reduced charge/discharge capacity.
The total amount of carbon in said mixture is preferably 25 to 90%
by weight, more preferably 30 to 50% by weight. A mixture with less
than 25% by weight of carbon may fail to withstand expansion and
contraction on charge/discharge cycles, whereas a mixture with more
than 90% of carbon may have a reduced charge/discharge
capacity.
[0041] When the negative electrode is prepared using the silicon
composite powder, an organic polymer binder may be added to the
silicon composite powder. Examples of organic polymer binder
include polymers such as polyethylene, polypropylene, polyethylene
terephthalate, aromatic polyamides, aromatic polyimides, cellulose,
poly(vinylidene fluoride), polytetrafluoroethylene, and
copolymerized fluoro-polymers including tetrafluoroethylene;
rubbery polymers such as styrene-butadiene rubber, isoprene rubber,
butadiene rubber and ethylene-propylene rubber; flexible polymers
such as ethylene-vinyl acetate copolymers and
propylene-alpha-olefin copolymers; and ion conductive polymers such
as organic polymers (e.g., polyethylene oxide, polypropylene oxide,
polyepichlorohydrin, polyphosphazene, polyvinylidene fluoride,
polyacrylonitrile), combined with lithium salts or alkali metal
salts primarily containing lithium.
[0042] With respect to the mixing proportion of the organic polymer
binder and the silicon composite powder, it is preferred to use 0.1
to 30 parts by weight, more preferably 0.5 to 20 parts by weight,
and even more preferably 1 to 15 parts by weight of the organic
polymer binder per 100 parts by weight of the silicon composite
powder. Outside the range, too small an amount of the binder may
allow silicon composite particles to separate off whereas too large
an amount may lead to a reduced percent void and/or a thicker
insulating film, prohibiting lithium ions from migration.
[0043] In addition to the organic polymer binder, a viscosity
adjusting agent may be added, for example, carboxymethyl cellulose,
sodium polyacrylate and other acrylic polymers.
[0044] The lithium ion secondary cell-forming negative electrode
material (or non-aqueous electrolyte secondary cell-forming
negative electrode material) of the invention can be shaped into a
lithium ion secondary cell-forming negative electrode by the
following exemplary procedure. For example the silicon composite
powder, conductive agent, organic polymer binder and other
additives are admixed with a solvent suitable for dissolving or
dispersing the binder, such as N-methylpyrrolidone or water to form
a paste mix, which is applied in a sheet form to a current
collector. The current collector may be copper foil, nickel foil or
any other materials which are typically used as the negative
electrode current collector. The method of shaping the mix into a
sheet is not particularly limited and any of well-known methods may
be used.
[0045] Using the lithium ion secondary cell-forming negative
electrode thus obtained, a lithium ion secondary cell can be
fabricated. The lithium ion secondary cell thus constructed is
characterized by the use of the silicon composite as the negative
electrode active material while the materials of the positive
electrode, electrolyte, and separator and the cell design are not
critical. For example, the positive electrode active material used
herein may be selected from transition metal oxides such as
LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4, V.sub.2O.sub.6,
MnO.sub.2, TiS.sub.2 and MoS.sub.2 and chalcogen compounds. The
electrolytes used herein may be lithium salts such as lithium
perchlorate in non-aqueous solution form. Examples of the
non-aqueous solvent include propylene carbonate, ethylene
carbonate, dimethoxyethane, .gamma.-butyrolactone and
2-methyltetrahydrofuran, alone or in admixture. Use may also be
made of other various non-aqueous electrolytes and solid
electrolytes.
EXAMPLE
[0046] Examples of the invention are given below by way of
illustration and not by way of limitation. In Examples, all parts
and percents are by weight unless otherwise stated. The average
particle size is determined as a cumulative weight average diameter
D.sub.50 (or median diameter) upon measurement of particle size
distribution by laser light diffractometry.
Example 1
[0047] To illustrate the structure of the silicon composite of the
invention, a silicon composite was prepared using a milled powder
of industrial metallic silicon.
[0048] A metallic silicon mass of industrial grade was crushed on a
crusher, and milled on a ball mill and then a bead mill using
hexane as a dispersing medium until fine particles having a
predetermined average particle size (about 4 .mu.m) were obtained.
A vertical reactor was charged with the silicon fine powder, and
thermal CVD was conducted in a stream of a methane-argon mixture at
1,150.degree. C. for an average residence time of about 4 hours.
The black mass thus obtained was heated in air at 800.degree. C.
for one hour for removing free carbon from the surface. After the
oxidizing treatment, the mass was disintegrated on an automated
mortar into a fine powder having an average particle size of about
4 .mu.m.
[0049] The silicon composite powder was analyzed by x-ray
diffractometry (Cu--K.alpha.). FIG. 2 illustrates an x-ray
diffraction pattern, which proves the presence of a diffraction
line near 2.theta.=28.40 attributable to Si(111) of crystalline
silicon (diamond structure). It is thus seen that fine silicon
crystals were left. Also the quantity of active silicon or
zero-valent silicon was measured in terms of the quantity of
hydrogen generated upon reaction with a sodium hydroxide solution
according to ISO DIS 9286, the method of measuring free silicon in
silicon carbide fine powder. The quantity of active silicon in the
silicon composite powder was found to be about 62%, confirming the
presence of zero-valent silicon having a lithium occluding
ability.
[0050] Further, an aliquot (about 2 g) of the silicon composite
powder was placed on a platinum dish, to which a hydrofluoric
acid/hydrogen peroxide mixture was added for dissolving the powder.
The presence of a precipitate was observed. The liquid mixture was
evaporated to dryness by heating in a draft, with a greenish gray
residue characteristic of silicon carbide being observed.
[0051] The quantities of free carbon and total carbon were measured
to be 1.5% and 12.3%, respectively, according to JIS R-6124, the
method of chemical analysis on silicon carbide abrasives. The
difference (10.8%) between them is attributed to carbon in silicon
carbide form. It is thus seen that 10.8.times.3.33=36.0% is present
as silicon carbide.
[0052] The silicon composite powder has a structure in which the
surface of a silicon particle is coated with silicon carbide as
shown in FIG. 1
Example 2
[0053] A metallic silicon mass of industrial grade (low Al silicon
available from SIMCOA Operations Pty. Ltd., Australia, Al 0.4%, Fe
0.21%, etc.) was crushed on a jaw crusher and atomized on a jet
mill, obtaining fine silicon particles having an average particle
size of about 4 .mu.m. The silicon fine powder was fed to a rotary
kiln reactor, and thermal CVD was conducted in a stream of a
methane-argon mixture at 1,200.degree. C. for an average residence
time of about 2 hours. The black mass thus obtained was
disintegrated on an automated mortar. The powder was fed to the
rotary kiln reactor again, and thermal CVD was conducted in a
stream of a methane-argon mixture at 1,200.degree. C. for an
average residence time of about 2 hours. The black mass thus
obtained was similarly disintegrated on the automated mortar again.
There was obtained a black powder having a total carbon content of
47% and a free carbon content of 36%. The black powder was then
placed in an alumina bowl, and heated in air at 800.degree. C. for
one hour for oxidizing and removing free carbon on the surface,
yielding a blackish gray fine powder.
[0054] The fine powder had a total carbon content of 12.1%, a free
carbon content of 1.3%, an oxygen content of 1.5%, and an average
particle size of 4.0 .mu.m. On x-ray analysis, a diffraction line
attributable to silicon was found. Upon treatment with a
hydrofluoric acid/hydrogen peroxide mixture, a precipitate in the
solution was observed, and after evaporation to dryness, a grayish
green residue was observed. These data confirmed the structure in
which the surface of a silicon particle is coated with silicon
carbide.
[0055] From the analytical data, the powder was found to have a
composition of 62.7% silicon and 36.0% silicon carbide.
Cell Test
[0056] The evaluation of silicon composite powder as the negative
electrode active material for a lithium ion secondary cell was
carried out by the following procedure which was common to all
Examples and Comparative Examples. A mixture was first obtained by
adding synthetic graphite (average particle diameter D.sub.50=5
.mu.m) to the silicon composite in such amounts that the total of
carbon in synthetic graphite and carbon deposited on the silicon
composite was 40%. To the mixture were added 3.0 pbw of
carboxymethyl cellulose (ammonium salt) as a water-soluble
thickener, 6.0 pbw of a styrene-butadiene copolymer latex (solids
50%) as a binder using water as a dispersing medium, and 50 pbw of
water. The mixture was agitated to form a slurry. The slurry was
coated onto a copper foil of 20 .mu.m gage and dried at 120.degree.
C. for one hour. Using a roller press, the coated foil was shaped
under pressure into an electrode sheet, of which 2 cm.sup.2 discs
were punched out as the negative electrode.
[0057] To evaluate the charge/discharge performance of the negative
electrode, a test lithium ion secondary cell was constructed using
a lithium foil as the counter electrode. The electrolyte solution
used was a non-aqueous electrolyte solution of lithium phosphorus
hexafluoride in a 1/1 (by volume) mixture of ethylene carbonate and
1,2-dimethoxyethane in a concentration of 1 mol/liter. The
separator used was a microporous polyethylene film of 30 .mu.m
thick.
[0058] The lithium ion secondary cell thus constructed was allowed
to stand overnight at room temperature. Using a secondary cell
charge/discharge tester (Nagano K.K.), a charge/discharge test was
carried out on the cell. Charging was conducted with a constant
current flow of 3 mA until the voltage of the test cell reached 0
V, and after reaching 0 V, continued with a reduced current flow so
that the cell voltage was kept at 0 V, and terminated when the
current flow decreased below 100 .mu.A. Discharging was conducted
with a constant current flow of 3 mA and terminated when the cell
voltage rose above 2.0 V, from which a discharge capacity was
determined.
[0059] The initial charging capacity and efficiency of this lithium
ion secondary cell were determined. By repeating the above
operations, the charge/discharge test on the lithium ion secondary
cell was carried out 50 cycles. The discharging capacity on the
50th cycle was measured, from which a percent cycle retention
(after 50 cycles) was computed. The test results are shown in Table
1.
Example 3
[0060] The metallic silicon powder with an average particle size of
about 4 .mu.m obtained through the pulverization in Example 2 was
milled on a bead mill using hexane as a dispersing medium to a
predetermined particle size (the target average particle size below
1 .mu.m). A milled silicon powder having an average particle size
of about 0.8 .mu.m was obtained. The resulting slurry was passed
through a filter for removing hexane, leaving a cake-like mass
containing hexane, which was placed in an alumina bowl. The bowl
was set in a lateral reactor, and thermal CVD was conducted in a
stream of a methane-argon mixture at 1,150.degree. C. for about 2
hours. The black mass thus obtained was disintegrated on an
automated mortar. The powder was placed in the alumina bowl and the
reactor again, and thermal CVD was conducted under the same
conditions. The black mass thus obtained was similarly
disintegrated on the automated mortar again. There was obtained a
black powder having a total carbon content of 59% and a free carbon
content of 41%. The black powder was then placed in an alumina
bowl, and heated in air at 800.degree. C. for one hour for
oxidizing and removing free carbon on the surface, yielding a
blackish gray fine powder.
[0061] The fine powder had a total carbon content of 19.3%, a free
carbon content of 1.8%, an oxygen content of 1.8%, and an average
particle size of about 1.1 .mu.m. On x-ray analysis, a diffraction
line attributable to silicon was found. Upon treatment with a
hydrofluoric acid/hydrogen peroxide mixture, a precipitate in the
solution was observed, and after evaporation to dryness, a grayish
green residue was observed. These data confirmed the structure in
which the surface of a silicon particle is coated with silicon
carbide.
[0062] From the analytical data, the powder was found to have a
composition of 39.9% silicon and 58.3% silicon carbide.
[0063] As in Example 2, this silicon composite powder was examined
by a charge/discharge test as a lithium ion secondary cell negative
electrode material. The results are also shown in Table 1.
Comparative Example 1
[0064] The silicon fine powder resulting from pulverization in
Example 2 was fed to a rotary kiln reactor, and thermal CVD was
conducted in a stream of a methane-argon mixture at 1,200.degree.
C. for an average residence time of about 2 hours. The black mass
thus obtained was disintegrated on an automated mortar. The powder
was fed to the rotary kiln reactor again, and thermal CVD was
conducted in a stream of a methane-argon mixture at 1,200.degree.
C. for an average residence time of about 2 hours. The black mass
thus obtained was similarly disintegrated on the automated mortar
again. There was obtained a black powder having a total carbon
content of 47.2% and a free carbon content of 36.3%. As in Example
2, this black powder was examined by a charge/discharge test as a
lithium ion secondary cell negative electrode material. The results
are also shown in Table 1.
Comparative Example 2
[0065] The silicon fine powder resulting from pulverization in
Example 2 was fed to a vertical tubular furnace (inner diameter
.about.50 mm), and thermal CVD was conducted in a stream of an
acetylene-argon mixture at 800.degree. C. for 3 hours. The
resulting black mass was a carbon-CVD-treated silicon powder, which
on analysis had a total carbon content of 41% and a free carbon
content of 40%. The black mass was disintegrated on an automated
mortar, after which free carbon was oxidized and removed as in
Example 2.
[0066] The fine powder had a total carbon content of 2.3%, a free
carbon content of 1.8%, an oxygen content of 1.3%, and an average
particle size of 4.1 .mu.m. On x-ray analysis, a diffraction line
attributable to silicon was found. Upon treatment with a
hydrofluoric acid/hydrogen peroxide mixture, little precipitate was
observed, and after evaporation to dryness, no residue was
observed.
[0067] From the analytical data, the powder was found to have a
composition of 96.5% silicon and 1.7% silicon carbide.
[0068] As in Example 2, this powder was examined by a
charge/discharge test as a lithium ion secondary cell negative
electrode material. The results are also shown in Table 1.
TABLE-US-00001 TABLE 1 Comparative Example Example Silicon
composite 2 3 1 2 Average particle size (.mu.m) 4.0 1.1 4.9 4.1
Total carbon content (wt %) 12.1 19.3 47.2 2.3 Free carbon content
(wt %) 1.3 1.8 36.3 1.8 Amount of silicon carbide coated 36.0 58.3
36.3 1.7 (wt %) Silicon content (wt %) 62.7 39.9 27.4 96.5 Test
results Initial charging capacity* (mAh/g) 1570 1210 1050 2630
Initial efficiency* (%) 93 90 89 89 Cycle retention after 50 cycles
(%) 93 90 89 25 *calculated based on the total weight of negative
electrode film including graphite
[0069] Japanese Patent Application No. 2004-195586 is incorporated
herein by reference.
[0070] Although some preferred embodiments have been described,
many modifications and variations may be made thereto in light of
the above teachings. It is therefore to be understood that the
invention may be practiced otherwise than as specifically described
without departing from the scope of the appended claims.
* * * * *