U.S. patent application number 11/028735 was filed with the patent office on 2005-09-22 for negative electrode for nonaqueous secondary battery, process of producing the negative electrode, and nonaqueous secondary battery.
Invention is credited to Dobashi, Makoto, Honda, Hitohiko, Matsushima, Tomoyoshi, Modeki, Akihiro, Musha, Shinichi, Sakaguchi, Yoshiki, Taguchi, Takeo, Taniguchi, Kazuko, Yasuda, Kiyotaka.
Application Number | 20050208379 11/028735 |
Document ID | / |
Family ID | 32475875 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050208379 |
Kind Code |
A1 |
Musha, Shinichi ; et
al. |
September 22, 2005 |
Negative electrode for nonaqueous secondary battery, process of
producing the negative electrode, and nonaqueous secondary
battery
Abstract
Disclosed is a negative electrode for a nonaqueous secondary
battery comprised of a current collector and an active material
structure containing an electro-conductive material having low
capability of forming a lithium compound on at least one side of
the current collector, the active material structure containing 5
to 80% by weight of active material particles containing a material
having high capability of forming a lithium compound. The active
material structure preferably has an active material layer
containing the active material particles and a surface coating
layer formed on the active material layer.
Inventors: |
Musha, Shinichi; (Ageo-Shi,
JP) ; Honda, Hitohiko; (Ageo-Shi, JP) ;
Sakaguchi, Yoshiki; (Ageo-Shi, JP) ; Yasuda,
Kiyotaka; (Ageo-Shi, JP) ; Modeki, Akihiro;
(Ageo-Shi, JP) ; Matsushima, Tomoyoshi; (Ageo-Shi,
JP) ; Taguchi, Takeo; (Ageo-Shi, JP) ;
Taniguchi, Kazuko; (Ageo-Shi, JP) ; Dobashi,
Makoto; (Ageo-Shi, JP) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET
2ND FLOOR
ARLINGTON
VA
22202
US
|
Family ID: |
32475875 |
Appl. No.: |
11/028735 |
Filed: |
January 5, 2005 |
Current U.S.
Class: |
429/231.95 ;
204/192.15; 205/300; 205/316; 429/232 |
Current CPC
Class: |
H01M 4/366 20130101;
H01M 4/387 20130101; H01M 4/1395 20130101; Y02E 60/10 20130101;
H01M 2004/027 20130101; H01M 4/134 20130101; H01M 4/624 20130101;
H01M 4/405 20130101; H01M 4/38 20130101; H01M 10/052 20130101; H01M
4/386 20130101 |
Class at
Publication: |
429/231.95 ;
429/232; 205/300; 205/316; 204/192.15 |
International
Class: |
H01M 004/58; H01M
004/62; C23C 014/34; C25D 003/30; C25D 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2002 |
JP |
2002-348990 |
Feb 7, 2003 |
JP |
2003-31636 |
Apr 9, 2003 |
JP |
2003-105797 |
Jul 23, 2003 |
JP |
2003-278615 |
Aug 8, 2003 |
JP |
2003-290726 |
Oct 21, 2003 |
JP |
2003-360938 |
Claims
1. A negative electrode for a nonaqueous secondary battery
comprising a current collector and an active material structure
containing an electro-conductive material having low capability of
forming a lithium compound on at least one side of the current
collector, the active material structure containing 5 to 80% by
weight of active material particles containing a material having
high capability of forming a lithium compound.
2. The negative electrode for a nonaqueous secondary battery
according to claim 1, wherein the active material structure has an
active material layer containing the active material particles and
a surface coating layer located on the active material layer.
3. The negative electrode for a nonaqueous secondary battery
according to claim 1, wherein the material having high capability
of forming a lithium compound is tin or silicon.
4. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the active material layer contains
0.1 to 20% by weight of an electro-conductive carbon material.
5. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the material constituting the surface
coating layer enters the active material layer or reaches the
current collector.
6. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the material constituting the surface
coating layer penetrates throughout the active material layer.
7. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the surface coating layer has a large
number of micropores extending in the thickness direction of the
surface coating layer and allowing a nonaqueous electrolyte to pass
therethrough.
8. The negative electrode for a nonaqueous secondary battery
according to claim 3, wherein the active material particles are
particles of single silicon or single tin.
9. The negative electrode for a nonaqueous secondary battery
according to claim 3, wherein the active material particles are
mixed particles comprising at least silicon or tin and carbon, the
mixed particles containing 10 to 90% by weight of silicon or tin
and 10 to 90% by weight of carbon.
10. The negative electrode for a nonaqueous secondary battery
according to claim 3, wherein the active material particles are
mixed particles comprising silicon or tin and a metal, the mixed
particles containing 30% to 99.9% by weight of silicon or tin and
0.1 to 70% by weight of at least one element selected from the
group consisting of Cu, Ag, Li, Ni, Co, Fe, Cr, Zn, B, Al, Ge, Sn
(except for a case where the active material particles contain
tin), Si (except for a case where the active material particles
contain silicon), In, V, Ti, Y, Zr, Nb, Ta, W, La, Ce, Pr, Pd, and
Nd.
11. The negative electrode for a nonaqueous secondary battery
according to claim 3, wherein the active material particles are
silicon compound particles or tin compound particles, the silicon
compound particles or the tin compound particles containing 30% to
99.9% by weight of silicon or tin and 0.1 to 70% by weight of at
least one element selected from the group consisting of Cu, Ag, Li,
Ni, Co, Fe, Cr, Zn, B, Al, Ge, Sn (except for a case where the
active material particles contain tin), Si (except for a case where
the active material particles contain silicon), In, V, Ti, Y, Zr,
Nb, Ta, W, La, Ce, Pr, Pd, and Nd.
12. The negative electrode for a nonaqueous secondary battery
according to claim 3, wherein the active material particles are
mixed particles comprising silicon compound particles or tin
compound particles and metal particles, the mixed particles
containing 30% to 99.9% by weight of silicon compound particles or
tin compound particles and 0.1 to 70% by weight of particles of at
least one element selected from the group consisting of Cu, Ag, Li,
Ni, Co, Fe, Cr, Zn, B, Al, Ge, Sn (except for a case where the
active material particles contain tin), Si (except for a case where
the active material particles contain silicon), In, V, Ti, Y, Zr,
Nb, Ta, W, La, Ce, Pr, Pd, and Nd, and the compound particles
containing 30% to 99.9% by weight of silicon or tin and 0.1 to 70%
by weight of at least one element selected from the group
consisting of Cu, Ag, Li, Ni, Co, Fe, Cr, Zn, B, Al, Ge, Sn (except
for a case where the active material particles contain tin), Si
(except for a case where the active material particles contain
silicon), In, V, Ti, Y, Zr, Nb, Ta, W, La, Ce, Pr, Pd, and Nd.
13. The negative electrode for a nonaqueous secondary battery
according to claim 3, wherein the active material particles are
single silicon or single tin particles coated with a metal, the
metal being at least one element selected from the group consisting
of Cu, Ag, Ni, Co, Fe, Cr, Zn, B, Al, Ge, Sn (except for a case
where the active material particles contain tin), Si (except for a
case where the active material particles contain silicon), In, V,
Ti, Y, Zr, Nb, Ta, W, La, Ce, Pr, Pd, and Nd, and the active
material particles containing 30% to 99.9% by weight of silicon or
tin and 0.1 to 70% by weight of the metal.
14. The negative electrode for a nonaqueous secondary battery
according to claim 1, wherein the active material particles have a
maximum particle size of 50 .mu.m or smaller.
15. The negative electrode for a nonaqueous secondary battery
according to claim 1, wherein the active material particles contain
silicon and have an average particle size of 0.1 to 10 .mu.m in
terms of D.sub.50 and an oxygen concentration of less than 2.5% by
weight, the outermost surface of the active material particles
having a silicon concentration of higher than half of an oxygen
concentration of the outermost surface.
16. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the surface coating layer contains at
least one element selected from the group consisting of Cu, Ag, Ni,
Co, Cr, Fe, and In.
17. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the surface coating layer is formed
by electroplating.
18. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the surface coating layer is formed
by sputtering, chemical vapor deposition or physical vapor
deposition.
19. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the surface coating layer is formed
by rolling an electro-conductive foil.
20. The negative electrode for a nonaqueous secondary battery
according to claim 19, wherein the electro-conductive foil is a
metal foil or an electro-conductive plastic foil.
21. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the active material layer is formed
by applying a slurry containing the active material particles to a
surface of the current collector.
22. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the surface coating layer has a
thickness of 0.3 to 50 .mu.m, and the active material layer has a
thickness of 1 to 100 .mu.m.
23. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the surface coating layer has a
thickness of 0.3 to 50 .mu.m, and the active material structure has
a thickness of 2 to 100.
24. The negative electrode for a nonaqueous secondary battery
according to claim 2, wherein the surface coating layer has a
thickness of 0.3 to 50 .mu.m, and the electrode has a total
thickness of 2 to 200 .mu.m.
25. The negative electrode for a nonaqueous secondary battery
according to claim 1, wherein the current collector has a large
number of micropores of 0.01 to 200 .mu.m in diameter at a density
of 5 to 10000 pores per cm.sup.2 and has a thickness of 1 to 100
.mu.m.
26. The negative electrode for a nonaqueous secondary battery
according to claim 1, wherein the current collector is formed of
punching metal or expanded metal, each having a large number of
openings each having an opening area of 0.0001 to 4 mm.sup.2 or
metal foam.
27. The negative electrode for a nonaqueous secondary battery
according to claim 1, wherein the current collector is formed of
electrolytic metal foil.
28. A process of producing the negative electrode for a nonaqueous
secondary battery of claim 4, which comprises applying a slurry
comprising the active material particles, the electro-conductive
carbon material, a binder, and a diluting solvent to a surface of
the current collector, drying the coating to form the active
material layer, and electroplating the active material layer with
the electro-conductive material having low capability of forming a
lithium compound to form the surface coating layer.
29. A process of producing the negative electrode for a nonaqueous
secondary battery of claim 4, which comprises applying a slurry
comprising the active material particles, the electro-conductive
carbon material, a binder, and a diluting solvent to a surface of
the current collector, drying the coating to form the active
material layer, and depositing the electro-conductive material
having low capability of forming a lithium compound on the active
material layer by sputtering, chemical vapor deposition or physical
vapor deposition to form the surface coating layer.
30. A process of producing the negative electrode for a nonaqueous
secondary battery of claim 25, which comprises forming a coat of a
material different from the material making up the current
collector on a carrier foil to a thickness of 0.001 to 1 .mu.m,
electroplating the carrier foil having the coat with the material
making up the current collector to form the current collector,
applying a slurry comprising the active material particles, the
electro-conductive carbon material, a binder, and a diluting
solvent to a surface of the current collector, drying the coating
to form the active material layer, electroplating the active
material layer with the electro-conductive material having low
capability of forming a lithium compound to form the surface
coating layer, and separating the current collector from the
carrier foil.
31. A nonaqueous secondary battery having the negative electrode
for a nonaqueous secondary battery according to claim 1.
Description
TECHNICAL FIELD
[0001] This invention relates to a negative electrode for
nonaqueous secondary batteries. More particularly, it relates to a
negative electrode capable of intercalating and deintercalating a
large amount of lithium and providing a nonaqueous secondary
battery with high energy density and improved cycle life. The
present invention also relates to a nonaqueous secondary battery
using the negative electrode.
BACKGROUND ART
[0002] Secondary batteries now used in mobile phones and notebook
computers are mostly lithium ion secondary batteries, due to a
higher energy density than other secondary batteries. With the
latest tendency of mobile phones and personal computers toward
multifunctionality, power consumption of these devices has shown a
remarkable increase. Therefore, the demands for higher capacity
secondary batteries have been increasing. As long as the present
electrode active materials are used, it would be difficult to meet
the increasing demands in the near future.
[0003] Lithium ion secondary batteries generally use graphite as a
negative electrode active material. Now Sn alloys and Si alloys
which offer 5 to 10 times the capacity potential of graphite are
being actively developed. For instance, it has been proposed to
produce Sn--Cu-based alloy flakes by mechanical alloying, roll
casting or gas atomization (see J. Electrochem. Soc., 148 (5),
A471-A481 (2001)). Production of Ni--Si-based alloys and
Co--Si-based alloys by gas atomization etc. is also proposed (see
JP-A-2001-297757). While these alloys have high capacity, they have
not yet been put to practical use on account of the problems of
large irreversible capacity and short cycle life.
[0004] There is an attempt to use copper foil, which is used as a
current collector, electroplated with tin, as a negative electrode
(see JP-A-2001-68094). On the other hand, though silicon has higher
capacity potential than tin, there is no report on the development
of silicon-containing plated copper foil for use in lithium ion
secondary batteries because silicon is an element incapable of
electroplating.
[0005] The aforesaid Si alloys and Sn alloys and, in addition, Al
alloys are negative electrode active materials exhibiting high
charge and discharge capacities. However, they have the drawback
that they incur large changes in volume with alternate repetition
of charging and discharging and, as a result, undergo cracking and
pulverizing and finally fall off the current collector. To address
this problem, techniques for preparing a negative electrode, of
which the active material is prevented from falling off, have been
proposed, in which a mixture of a negative electrode active
material containing Si or an Si alloy and an electro-conductive
metal powder is applied to a conductive metal foil, followed by
sintering in a non-oxidative atmosphere (see JP-A-11-339777,
JP-A-2000-12089, JP-A-2001-254261, and JP-A-2002-260637). It has
also been proposed to prevent fall-off of a negative electrode
active material by forming a thin film of Si on a current collector
with good adhesion by plasma-enhanced CVD or sputtering (see
JP-A-2000-18499). Moreover, extensive studies have been devoted to
development of various Sn- or Si-based intermetallic compounds (see
JP-A-10-312804, JP-A-2001-243946, and JP-2001-307723). Even with
these techniques, however, it is still impossible to perfectly
prevent fall-off of the negative electrode active material from the
current collector as a result of cracking and pulverizing of the
active material, accompanying charge and discharge of a secondary
battery.
[0006] JP-A-8-50922 proposes a negative electrode having a layer
containing a metallic element capable of forming an alloy with
lithium and a layer of a metallic element incapable of forming an
alloy with lithium. According to the disclosure, this layer
structure prevents the layer containing the lithium alloy-forming
metal element from cracking and pulverizing accompanying charge and
discharge of a battery. Judging from Examples of the publication,
however, since the thickness of the layer of the metallic element
incapable of forming a lithium alloy, which is the outermost layer,
is as extremely small as 50 nm, there is a possibility that the
outermost layer fails to sufficiently coat the underlying layer
containing the lithium alloy-forming metal element. If so, the
layer containing the metallic element capable of forming a lithium
alloy cannot be sufficiently prevented from falling off when
pulverized with repeated charging and discharging of the battery.
Conversely, if the layer of the metal element incapable of forming
a lithium alloy completely covers the layer containing the lithium
alloy-forming metal element, the former layer would inhibit an
electrolyte from passing through the latter layer, which will
interfere with sufficient electrode reaction. No proposal has ever
been made to satisfy these conflicting functions.
[0007] Besides the aforementioned, current collectors with
appropriate surface roughness and current collectors having
micropores that pierce the thickness are known to be used in
lithium ion secondary batteries. For example, JP-A-8-236120
proposes a current collector formed of a porous electrolytic metal
foil having pores winding across the thickness and making a
three-dimensional network. The porous electrolytic metal foil is
produced by a process including the steps of electrodepositing a
metal on the surface of a cathode drum to form an electrolytic foil
of the metal and separating the foil from the drum, wherein an
oxide film having a thickness of at least 14 nm is formed on the
surface of the cathode drum exposed after separation of the foil,
and electrolytic metal foil is deposited on the oxide film. The
porosity and pore size of the metal foil are dependent on the
thickness of the oxide film formed on the cathode drum. However,
since the oxide film comes off little by little, together with the
foil, it is difficult to control the porosity and pore size.
Additionally, since the pores have a relatively small diameter and
form a three-dimensional network, active material paste applied to
one side of the foil and that applied to the other side hardly come
into contact with each other. There seems to be a limit, therefore,
in improving the adhesion between the paste and the foil.
[0008] In order to solve the problems associated with the
above-described metal foil, Applicant previously proposed a porous
copper foil formed by electrodeposition such that copper grains,
having an average planar grain size of 1 to 50 .mu.m, are
two-dimensionally bonded to one another. The porous copper foil has
an optical transmittance of 0.01% or higher and a surface roughness
difference of 5 to 20 .mu.m, in terms of Rz, between the side in
contact with a cathode for foil formation and the opposite side
(see WO 00/15875). When the copper foil is used as a current
collector of a lithium ion secondary battery, the following
advantages are offered. (1) Since an electrolyte is able to pass
through the copper foil so easily, even a limited amount of an
electrolyte is permitted to uniformly penetrate into an active
material. (2) The copper foil hardly interferes with donation and
acceptance of Li ions and electrons during charge and discharge.
(3) Having proper surface roughness, the copper foil exhibits
excellent adhesion to an active material. According to the process
of making the porous copper foil, however, the electrolytic copper
foil deposited on a cathode drum and separated from the drum, is
subjected to various processing treatments, which make the copper
foil unstable. Therefore, the process cannot be seen as
satisfactory in ease of handling the foil and fit for large volume
production. Additionally, a nonaqueous secondary battery using a
negative electrode, prepared by applying a negative electrode
active material mixture to the porous copper foil (a current
collector), still has the problem that the negative electrode
active material tends to fall off accompanying intercalation and
deintercalation of lithium, resulting in reduction of cycle
characteristics.
DISCLOSURE OF THE INVENTION
[0009] Accordingly, an object of the present invention is to
provide a negative electrode for a nonaqueous secondary battery
that can solve the aforementioned various problems and a nonaqueous
secondary battery having the negative electrode.
[0010] The present invention provides a negative electrode for a
nonaqueous secondary battery made up of an active material
structure including an electro-conductive material having low
capability of forming a lithium compound on at least one side of a
current collector. The active material structure contains 5 to 80%
by weight of active material particles containing a material having
high capability of forming a lithium compound.
[0011] The present invention also provides a preferred process of
producing the negative electrode. The process comprises applying a
slurry comprising the active material particles, an
electro-conductive carbon material, a binder, and a diluting
solvent to a surface of the current collector, drying the coating
to form the active material layer, and electroplating the active
material layer with the electro-conductive material having low
capability of forming a lithium compound to form the surface
coating layer.
[0012] The present invention also provides another preferred
process of producing the negative electrode. The process comprises
applying a slurry comprising the active material particles, an
electro-conductive carbon material, a binder, and a diluting
solvent to a surface of the current collector, drying the coating
to form the active material layer, and depositing the
electro-conductive material having low capability of forming a
lithium compound on the active material layer by sputtering,
chemical vapor deposition or physical vapor deposition to form the
surface coating layer.
[0013] The present invention also provides still another preferred
process of producing the negative electrode. The process comprises
forming a coat of a material different from the material making up
the current collector on a carrier foil to a thickness of 0.001 to
1 .mu.m, electroplating the carrier foil having the coat with the
material making up the current collector to form the current
collector, applying a slurry comprising the active material
particles, an electro-conductive carbon material, a binder, and a
diluting solvent to the surface of the current collector, drying
the coating to form the active material layer, electroplating the
active material layer with the electro-conductive material having
low capability of forming a lithium compound to form the surface
coating layer, and separating the current collector from the
carrier foil.
[0014] The present invention also provides a nonaqueous secondary
battery having the negative electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is an electron micrograph showing the surface of a
negative electrode according to the present invention.
[0016] FIG. 2 is an electron micrograph showing a cross-section of
a negative electrode according to the present invention.
[0017] FIG. 3 is an electron micrograph showing a cross-section of
another negative electrode according to the present invention.
[0018] FIG. 4 is an electron micrograph showing a cross-section of
still another negative electrode according to the present
invention.
[0019] FIG. 5(a), FIG. 5(b), FIG. 5(c), FIG. 5(d), FIG. 5(e), and
FIG. 5(f) present a flow chart illustrating a process of preparing
a porous metal foil used as a current collector in a negative
electrode of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0020] The present invention will be described based on its
preferred embodiments with reference to the accompanying drawings.
FIG. 1 is an electron micrograph taken of the surface of a negative
electrode according to an embodiment of the present invention. FIG.
2 shows an electron micrograph taken of a cross-section of a
negative electrode according to the present invention. The negative
electrode 1 has a current collector 2 having formed on one or both
sides thereof an active material structure 5 containing an
electro-conductive material having low capability of forming a
lithium compound. The active material structure comprises active
material particles containing a material having high capability of
forming a lithium compound. More specifically, the active material
structure 5, which is formed on one or both sides of the current
collector 2, has a layer 3 of active material particles
(hereinafter referred to as an active material layer) and a surface
coating layer 4 that is provided on the layer 3 as shown in FIG.
2.
[0021] The current collector 2 is made of a metal that can serve as
a current collector of a nonaqueous secondary battery. It is
preferably made of a metal that can serve as a current collector of
a lithium secondary battery. Such metals include copper, iron,
cobalt, nickel, zinc, and silver, and their alloys. Particularly
preferred ones among them are copper, a copper alloy, nickel or a
nickel alloy. In using copper, the current collector has the form
of copper foil. Copper foil is obtained by, for example,
electrodeposition using a copper-containing solution. A preferred
copper foil thickness is 2 to 100 .mu.m, still preferably 10 to 30
.mu.m. The copper foil obtained by the method described in
JP-A-2000-90937 is particularly preferred because of its extreme
thinness with a thickness as small as 12 .mu.m or less. Use of an
electrolytic metal foil as a current collector 2 is advantageous in
that the adhesion between the current collector 2 and the active
material layer 3 is improved because of the moderate surface
roughness of an electrolytic metal foil.
[0022] The active material layer 3 is a layer containing active
material particles 7 which have a material having high capability
of forming a lithium compound. Such a material includes silicon
materials, tin materials, aluminum materials, and germanium
materials. The maximum particle size of the active material
particles 7 is preferably 50 .mu.m or smaller, still preferably 20
.mu.m or smaller. The particle size, represented in terms of
D.sub.50 value, of the active material particles 7 is preferably
0.1 to 8 .mu.m, still preferably 0.3 to 1 .mu.m. Where the maximum
particle size exceeds 50 .mu.m, the active material particles 7 are
liable to fall off, resulting in reduction of electrode life. The
lower limit of the particle size is not particularly specified. The
smaller, the better. In the light of the process of making the
active material particles 7 (described later), the lower limit
would be about 0.01 .mu.m. The particle size of the active material
particles 7 can be measured by Microtrac, electron microscopic
(SEM) observation. While it is desirable that all the active
material particles 7 fall under the recited particle size range, it
is no problem that greater active material particles 7 are present
in a small amount that does not impair the effects of the
invention.
[0023] It is preferred that voids be present in the active material
layer 4. The voids serve to relax the stress which results from
expansion and contraction of the active material particles 7 due to
intercalation and deintercalation of lithium. In this connection,
the proportion of the voids in the active material layer 4 is
preferably about 1 to 30% by volume, still preferably about 5 to
30% by volume, particularly preferably about 5 to 9% by volume. The
proportion of the voids is obtained through mapping under an
electron microscope. The proportion of the voids can be regulated
within the recited range by forming the active material layer by
the process described later, followed by mechanically pressing the
active material layer under appropriate conditions.
[0024] The active material layer 4 preferably contains an
electro-conductive carbon material in addition to the active
material particles 7. Incorporation of the conductive carbon
material adds improved electron conductivity to the active material
structure 5. From this viewpoint, the amount of the conductive
carbon material in the active material layer 3 is preferably 0.1 to
20% by weight, still preferably 1 to 10% by weight. To ensure the
improvement on electron conductivity, it is preferred for the
electro-conductive carbon material to have the shape of particles
with a particle size of 40 .mu.m or smaller, particularly 20 .mu.m
or smaller. The lower limit of the particle size is not critical,
which means the smaller, the better. In the light of the process of
making the particles, the lower limit would be about 0.01 .mu.m.
The conductive carbon material includes acetylene black and
graphite.
[0025] The surface coating layer 4 is a thick layer continuously
covering the surface of the active material layer 3, thereby the
active material particles 7 are not substantially exposed. The
surface coating layer 4 generally covers the surface of the active
material layer 3. The surface coating layer 4 has an almost uniform
thickness, but some part 4a of the surface coating layer 4 may
enter into the active material layer 3. Some part of the surface
coating layer 4 penetrating the active material layer 3 may reach
the current collector 2. In some parts, the material constituting
the surface coating layer 4 may penetrate the whole thickness of
the active material layer 3 to reach the current collector. It is
preferred that the material constituting the surface coating layer
4 penetrate the active material layer 3 deeper and deeper thereby
increasing the electrical conductivity of the negative electrode as
a whole. This is also preferred in that the penetrating material
constituting the surface coating layer 4 forms a network structure
and acts to prevent the active material particles 7 from falling
off due to expansion and contraction.
[0026] The active material particles 7 do not always need to be
covered completely with the surface coating layer 4, and part of
them may be exposed. Taking into consideration, however, that the
active material particles 7 should be prevented from falling off as
a result of pulverization due to intercalation and deintercalation
of lithium, it is desirable that the active material particles 7 be
completely covered with the surface coating layer 4. Even though
the active material particles 7 are completely covered with the
surface coating layer 4, an electrolyte and lithium are allowed to
penetrate through micropores 6 (described infra) into the inside of
the surface coating layer 4 and to react with the active material
particles 7.
[0027] FIGS. 3 and 4 represent different examples of the negative
electrode in which the active material layer 3 is completely
covered with the surface coating layer 4. In FIGS. 3 and 4, the
active material layer 3 formed on the current collector 1 that is
copper, contains silicon-copper alloy particles, and the surface
coating layer 4 that is copper is located on the active material
layer 3. The active material layer 3 is completely covered with the
surface coating layer 4. In the surface coating layer 4 are
observable fine breaks extending in the thickness direction. Voids
among alloy particles are observable in the active material layer
3. In FIG. 3, it is seen that part of the surface coating layer 4
goes into the active material layer 3 to such a degree that an
alloy particle is surrounded with copper. In FIG. 4, on the other
hand, the surface coating layer 4 is not so invasive into the
active material layer 3, and the two layers 3 and 4 are defined
relatively clearly. Such a difference in layer geometry is ascribed
to the process of producing the negative electrode.
[0028] The active material layer 3 being covered with the surface
coating layer 4, secondary batteries using the negative electrode
of the present invention have an extended life compared with
conventional ones. Even when the active material particles 7 are
pulverized due to intercalation and deintercalation of lithium,
they maintain the electrical contact with the surface coating layer
4 since they are shut away by the surface coating layer 4. As a
result, the electron conductivity is maintained, and reduction in
functions as a negative electrode is suppressed. Furthermore, the
service life as a negative electrode can be prolonged. Where, in
particular, part of the surface coating layer 4 enters into the
active material layer 3, the current collecting function is
retained more effectively. If the active material is used as formed
on the current collector, it would be pulverized when intercalating
and deintercalating lithium and get isolated from the current
collector. It would follow that the functions as a negative
electrode reduce, and such problems as increase of irreversible
capacity, reduction of charge and discharge efficiency, and
reduction of life will result.
[0029] The surface coating layer 4 is made of an electro-conductive
material having low capability of forming a lithium compound so as
to be prevented from oxidation and fall-off. Such conductive
materials include copper, silver, nickel, cobalt, chromium, iron,
indium, and alloys of these metals (for example, copper-tin
alloys). Of these metals preferred are copper, silver, nickel,
chromium, cobalt, and alloys containing these metals because of
their particularly low capability of forming a lithium compound.
Electro-conductive plastics or electro-conductive pastes are also
useful as a conductive material. The expression "low capability of
forming a lithium compound" as used herein means no capability of
forming an intermetallic compound or a solid solution with lithium
or, if any, the capability is such that the resulting lithium
compound contains only a trace amount of lithium or is very
labile.
[0030] The surface coating layer 4 has on its surface a large
number of micropores 6 that extend windingly in the thickness
direction thereof. Some of the numerous micropores 6 extend in the
thickness direction of the surface coating layer 4 to reach the
active material layer 3. The micropores 6 are so small as having a
width of about 0.1 .mu.m to about 10 .mu.m when observed on a cut
section of the surface coating layer 4. Small as they are, the
micropores 6 should have such a width as to allow a nonaqueous
electrolyte to penetrate. Be that as it may, a nonaqueous
electrolyte has a smaller surface tension than an aqueous one so
that it is capable of penetrating sufficiently through the
micropores 6 with such a small width.
[0031] When the surface coating layer 4 is observed from above
through an electron microscope, it is desirable for the micropores
6 to have an average opening area of 0.1 to 100 .mu.m.sup.2,
preferably 1 to 30 .mu.m.sup.2. Within this range of opening area,
the surface coating layer 4 effectively prevent the active material
layer 3 from falling off while securing sufficient penetration of a
nonaqueous electrolyte. For the same reason, it is preferred that
the surface coating layer 4, when seen from above, have 1 to 30,
still preferably 3 to 10, micropores 6 in every 100 .mu.m-side
square in the visual field under an electron microscope. The number
of the micropores 6 as defined above is referred to as a
distribution. For the same reason, when the surface coating layer 4
is observed from above under an electron microscope, the ratio of
the total opening area of the micropores 6 in the visual field to
the area of the visual field (i.e., the open area ratio) is
preferably 0.1 to 10%, still preferably 1 to 5%.
[0032] As can be seen from FIG. 1, the presence of the micropores 6
can be confirmed through electron microscopic observation. In some
cases, nevertheless, the micropores 6 are too tiny in their width
to observe even under an electron microscope. In such cases, the
present invention adapts the following method for confirming
micropores 6. A negative electrode to be evaluated is assembled
into a battery, and the battery is subjected to one
charge/discharge cycle. The cross-section of the negative electrode
is then observed with an electron microscope. If any change in
cross-sectional structure is observed between before and after the
cycle, the negative electrode before the charge/discharge cycle is
judged to have had micropores 6. The grounds of this judgement are
that the change of the cross-sectional structure, due to the
charge/discharge cycle, is a result from the nonaqueous
electrolyte's reaching the active material layer 3 through the
micropores 6 distributed in the negative electrode before the
charge and discharge and causing the lithium ions of the nonaqueous
electrolyte to react with the active material particles 7.
[0033] The micropores 6 allow a nonaqueous electrolyte to
sufficiently penetrate into the active material layer 3 and to
sufficiently react with the active material particles 7. Fall-off
of the active material particles 7, having pulverized due to
charging and discharging, can be prevented by the thick surface
coating layer 4 covering the surface of the active material layer
3. That is, since the active material particles 7 are shut up by
the surface coating layer 4, fall-off of the active material
particles 7 attributed to lithium intercalation and deintercalation
can effectively be prevented. Generation of electrically isolated
active material particles 7 is effectively prevented and thereby
the current collecting performance can be retained. As a result,
reduction of functions as a negative electrode is suppressed.
Extension of the negative electrode life also results. In
particular, where a part 4a of the surface coating layer 4 enters
the active material layer 3, the current collecting function is
retained more effectively. A secondary battery using the negative
electrode of the present invention achieves a remarkably increased
energy density per unit volume and unit weight over conventional
ones and also enjoys a prolonged life.
[0034] The micropores 6 can be formed by various methods. For
example, they can be formed by mechanically pressing the surface
coating layer 4 under proper conditions. A method of creating the
micropores 6 in the surface coating layer 4 simultaneously with the
formation of the surface coating layer 4 by electroplating as
described later is especially preferred. In more detail, since the
active material layer 3 contains the active material particles 7 as
previously stated, it has a microscopically textured surface, that
is, a mixed profile having active sites where deposit grows easily
and sites where deposit does not grow easily. When the active
material layer 3 having such a surface condition is electroplated,
growth of the deposit differs from site to site, and the particles
of the material making up the surface coating layer 4 grow into a
polycrystalline structure. On further growth of crystals, adjacent
crystals meet, resulting in formation of voids in the meeting site.
The thus formed voids connect to each other to form the micropores
6. According to this mechanism, there are formed micropores 6
having an extremely fine structure, and micropores 6 that extend in
the thickness direction of the surface coating layer 4 can easily
be created. Not involving outer force application, such as pressing
force, to the surface coating layer 4, the method is advantageous
in that the surface coating layer 4 is not damaged, which means
that the negative electrode 1 is not damaged.
[0035] In order to effectively prevent fall-off of the active
material particles 7 and to sufficiently maintain the current
collecting function, it is preferred for the surface coating layer
4 to have a large thickness of 0.3 to 50 .mu.m, still preferably
0.3 to 10 .mu.m, particularly preferably 1 to 10 .mu.m. Even with
such a large thickness, penetration of a nonaqueous electrolyte
through the surface coating layer 4 is assured by the presence of
the micropores 6. For securing sufficient negative electrode
capacity, the thickness of the active material layer 3 is
preferably 1 to 100 .mu.m, still preferably 3 to 40 .mu.m. The
thickness of the active material structure 5 inclusive of the
surface coating layer 4 and the active material layer 3 is
preferably about 2 to 100 .mu.m, still preferably about 2 to 50
.mu.m. The total thickness of the negative electrode is preferably
2 to 200 .mu.m, still preferably 10 to 100 .mu.m, from the
viewpoint of compactness and higher energy density of the
battery.
[0036] The amount of the active material particles 7 in the active
material structure 5 inclusive of the active material layer 3 and
the surface coating layer 4 is 5% to 80% by weight, preferably 10%
to 50% by weight, still preferably 20% to 50% by weight. It is
difficult to sufficiently improve the energy density of the battery
with less than 5% by weight of the active material particles 7.
More than 80% by weight of the active material particles 7 easily
tend to suffer from fall-off, which can result in increased
irreversible capacity, reduced charge and discharge efficiency, and
reduced battery life.
[0037] The active material particles 7 include (a) particles of
single silicon or single tin, (b) mixed particles containing at
least silicon or tin and carbon, (c) mixed particles containing
silicon or tin and a metal, (d) particles of a compound of silicon
or tin and a metal, (e) mixed particles containing particles of a
compound of silicon or tin and a metal and metal particles, and (f)
single silicon or single tin particles coated with a metal. The
particles (a) to (f) can be used either individually or as a
combination of two or more kinds thereof. Compared with the
particles (a), use of the particles (b) to (f) is advantageous in
that cracking and pulverizing of the active material particles 7
due to intercalation and deintercalation of lithium is suppressed
more. This advantage is particularly conspicuous in using the
particles (f). When silicon is chosen, use of the particles (b) to
(f) is also advantageous in that poor electron conductivity of
silicon, which is semiconductive, can be compensated for.
[0038] In particular, where the mixed particles (b) containing at
least silicon and carbon are used as active material particles 7,
improved cycle life and negative electrode capacity are obtained
for the following reason. Carbon, especially graphite, which is
used in a negative electrode of nonaqueous secondary batteries,
contributes to intercalation and deintercalation of lithium,
provides a negative electrode capacity of about 300 mAh/g, and is
additionally characterized by its very small volumetric expansion
on lithium storage. Silicon, on the other hand, is characterized by
as high a negative electrode capacity as about 4200 mAh/g, 10 times
or more the negative electrode capacity of graphite. Nevertheless,
volumetric expansion of silicon on lithium storage reaches about 4
times that of graphite. Then, silicon and carbon such as graphite
are mixed at a predetermined ratio and ground by, for example,
mechanical milling to obtain uniformly mixed powder having a
particle size of about 0.1 to 1 .mu.m. When this mixed powder is
used as an active material, the volumetric expansion of silicon on
lithium storage is relaxed by graphite to provide improved cycle
life, and a negative electrode capacity ranging about 1000 to 3000
mAh/g is obtained. The amount of silicon in the mixed powder is
preferably 10 to 90% by weight. The amount of carbon in the mixed
particles is preferably 10 to 90% by weight. Increased battery
capacity and extended negative electrode life will be secured with
the mixed particles composition falling within the above range.
Moreover, no compound such as silicon carbide is formed in the
mixed particles.
[0039] The mixed particles (b) as active material particles 7 may
be a multi-component mixture containing other metal element(s) in
addition to silicon or tin and carbon. The other metal element is
at least one element selected from the group consisting of Cu, Ag,
Li, Ni, Co, Fe, Cr, Zn, B, Al, Ge, In, V, Ti, Y, Zr, Nb, Ta, W, La,
Ce, Pr, Pd, and Nd.
[0040] In using the mixed particles (c) of silicon or tin and a
metal as active material particles 7, the metal in the mixed
particles (c) includes at least one of Cu, Ag, Li, Ni, Co, Fe, Cr,
Zn, B, Al, Ge, Sn (except for cases where the particles 7 contain
tin), Si (except for cases where the particles 7 contain silicon),
In, V, Ti, Y, Zr, Nb, Ta, W, La, Ce, Pr, Pd, and Nd. Preferred of
these metals are Cu, Ag, Ni, Co, and Ce. It is particularly
desirable to use Cu, Ag or Ni for their excellent electron
conductivity and low capability of forming a lithium compound. Use
of Li as the metal is also preferred. In that case, the active
material contains metallic lithium from the beginning, which
produces advantages, such as reduction of irreversible capacity,
improvement on charge/discharge efficiency, and reduction in
volumetric change leading to improved cycle life. In the mixed
particles (c), the amount of silicon or tin is preferably 30% to
99.90/o by weight, still preferably 50% to 95% by weight,
particularly preferably 75% to 95% by weight. The amount of the
metal, such as copper, is preferably 0.1% to 70% by weight, still
preferably 5% to 50% by weight, particularly preferably 5% to 30%
by weight. Increased battery capacity and extended negative
electrode life will be secured with the mixed particles composition
falling within the above range.
[0041] The mixed particles (c) can be prepared, for example, as
follows. Silicon particles or tin particles and metal particles,
such as copper particles, are mixed and pulverized simultaneously
by use of a pulverizer, which includes an attritor, a jet mill, a
cyclon mill, a paint shaker, and a fine mill. Pulverization in
these pulverizers may be either in a dry system or a wet system.
Wet pulverization is preferred for particle size reduction. The
particle size before pulverization is preferably about 20 to 500
.mu.m. Mixing and pulverizing in a pulverizer result in formation
of uniformly mixed powder of silicon or tin and the metal. The
particle size of the resulting powder can be adjusted to, e.g., 40
.mu.m or smaller by properly controlling the operation conditions
of the pulverizer. Thus are prepared the mixed particles (c).
[0042] Where the active material particles 7 are (d) particles of a
compound of silicon or tin and a metal, the compound includes an
alloy of silicon or tin and a metal, which is any one of (i) a
solid solution of silicon or tin and the metal, (ii) an
intermetallic compound of silicon or tin and the metal, and (iii) a
composite having at least two phases selected from a single phase
of silicon or tin, a single phase of the metal, a solid solution of
silicon or tin and the metal, and an intermetallic compound of
silicon or tin and the metal. The metal can be selected from those
recited above as the metal used in the mixed particles (c).
Similarly to the mixed particles (c), the silicon or tin/metal
compound particles preferably comprise 30% to 99.9% by weight of
silicon or tin and 0.1% to 70% by weight of the metal. A still
preferred composition of the compound is selected appropriately
according to the process of producing the compound particles. For
instance, where the compound is a silicon or tin/metal binary alloy
prepared by a quenching process described infra, a preferred amount
of silicon or tin is 40% to 90% by weight, and a preferred amount
of the metal, e.g., copper, is 10% to 60% by weight.
[0043] Where the compound is a ternary or higher order alloy
containing silicon or tin and metals, the above-described binary
alloy further contains a small amount of an element selected from
the group consisting of B, Al, Ni, Co, Fe, Cr, Zn, In, V, Y, Zr,
Nb, Ta, W, La, Ce, Pr, Pd, and Nd. Such an additional component
produces an additional effect in preventing cracking and
pulverizing of the active material particles. To enhance the
effect, a preferred amount of the additional component in the
silicon or tin/metal alloy is 0.01% to 10% by weight, particularly
0.05% to 1.0% by weight.
[0044] Where the compound particles (d) are alloy particles, the
alloy particles are preferably prepared by a quenching process
hereinafter described. The quenching process is advantageous in
that the resulting alloy crystallites have a small size and are
uniformly dispersible to provide an active material layer that will
be prevented from cracking and pulverizing and maintain electron
conductivity. The quenching process starts with preparing a molten
metal of raw materials including silicon or tin and a metal, e.g.,
copper by high frequency melting. The ratio of silicon or tin and
the metal in the molten metal is selected from the above-specified
range. The molten metal temperature is preferably 1200.degree. to
1500.degree. C., still preferably 1300.degree. to 1450.degree. C.,
in connection to the quenching conditions. An alloy is made from
the molten metal by mold casting. That is, the molten metal is
poured into a copper- or iron-made mold and quenched to obtain an
alloy ingot, which is ground and sieved to obtain particles, e.g.,
of 40 .mu.m or smaller for use in the present invention. A roll
casting process can be used instead of the mold casting process. In
a roll casting process, the molten metal is injected onto the
peripheral surface of a roll which is made of copper and rotates at
a high speed. For quenching the molten metal, the rotating speed of
the roll is preferably 500 to 4000 rpm, still preferably 1000 to
2000 rpm. The rotating speed in terms of peripheral speed is
preferably 8 to 70 m/sec, particularly preferably 15 to 30 m/sec.
When the molten metal having the above-specified temperature is
quenched on the roll rotating at the above-specified speed, the
cooling rate reaches 10.sup.2 K/sec or higher, preferably 10.sup.3
K/sec or higher. The injected molten metal is rapidly cooled on the
roll and made into a thin strip, which is ground and sieved to
obtain particles having a particle size, e.g., of 40 .mu.m or
smaller for use in the present invention. Particles of desired size
can also be prepared by a gas atomization process instead of the
quenching process. In a gas atomization process, a jet of an inert
gas such as argon is applied to the molten metal at 1200.degree. to
1500.degree. C. under a gas pressure of 5 to 100 atm to atomize and
quench the molten metal. An arc melting process or mechanical
milling can also be used.
[0045] Where the active material particles are the mixed particles
(e) containing particles of a compound of silicon or tin and a
metal and metal particles, the compound particles described with
respect to the particles (d) and the metal particles described with
respect to the mixed particles (c) can be used in the mixed
particles (e). The metal element contained in the compound
particles and the metal element of the metal particles may be
either the same or different. In particular, when the metal element
of the compound particles is nickel, copper, silver or iron, and
the metal element of the metal particles is nickel, copper, silver
or iron, these metals easily form a network structure in the active
material layer 3. Such a metal network structure is effective in
improving the electron conductivity and preventing fall-off of the
active material particles 7 due to expansion and contraction.
Taking these effects into consideration, it is preferred that the
metal element in the compound particles and that of the metal
particles be the same. The active material particles (e) are
obtained by first preparing compound particles in the same manner
as for those of the particles (d) and then mixing the compound
particles with metal particles in the same manner as for the
production of the mixed particles (c). The silicon or tin to metal
ratio in the compound particles can be the same as in the compound
particles (d). The compound particles to metal particles ratio can
be the same as the ratio of silicon or tin particles to metal
particles in the mixed particles (c). With respect to other
particulars of the active material particles (e), the description
given to the mixed particles (c) and the compound particles (d)
apply appropriately.
[0046] Where the active material particles 7 are (f) the single
silicon or single tin particles coated with a metal (hereinafter
referred to as "metal-coated particles"), the coating metal is
selected from the above-recited metals used in the particles (c)
and (d), for example, copper (except Li). The amount of silicon or
tin in the metal-coated particles is preferably 70% to 99.9% by
weight, still preferably 80% to 99% by weight, particularly
preferably 85 to 95. The amount of the coating metal, such as
copper, is preferably 0.1% to 30% by weight, still preferably 1% to
20% by weight, particularly preferably 5% to 15% by weight. The
metal-coated particles can be prepared by, for example, electroless
plating. In carrying out the electroless plating, a plating bath
having silicon particles or tin particles suspended therein and
containing a coating metal (e.g., copper) is prepared. The silicon
particles or tin particles are electroless plated in the plating
bath to deposit the coating metal on the surface of the silicon
particles or tin particles. A preferred concentration of the
silicon particles or tin particles in the plating bath is about 400
to 600 g/l. In electroless plating using copper as a coating metal,
the plating bath preferably contains copper sulfate, Rochelle salt,
etc. A preferred concentration of copper sulfate and that of
Rochelle salt are 6 to 9 g/l and 70 to 90 g/l, respectively, from
the viewpoint of plating rate control. From the same viewpoint, the
plating bath preferably has a pH of 12 to 13 and a temperature of
20 to 30.degree. C. The plating bath contains a reducing agent,
such as formaldehyde, in a concentration of about 15 to 30
cc/l.
[0047] Where the active material particles 7 are silicon-containing
particles, it is preferred for the particles to have an average
particle size (D.sub.50) of 0.1 to 10 .mu.m, particularly 0.3 to 8
.mu.m, especially 0.8 to 5 .mu.m, whichever of the forms (a) to (e)
the silicon-containing particles may have. In other words, the
silicon-containing active material particles are preferably fine
particles with a small diameter (hereinafter referred to as
"small-diametered active material particles"). Use of such
small-diametered active material particles results in reduced
fall-off of the active material particles from the negative
electrode and makes it feasible to extend the life of the negative
electrode. In more detail, active material particles will change
greatly in volume upon intercalating and deintercalating lithium
and are to be disintegrated into microcrystallites or finer
particles in due course of time. It follows that cracks develop,
and part of the active material particles lose electrochemical
contact among themselves, which causes reduction in
charge/discharge cycle characteristics important for a secondary
battery. For this reason, fine particles of small size are used in
the negative electrode from the very beginning thereby to suppress
further size reduction of the particles during charging and
discharging and to improve the charge/discharge cycle
characteristics. Incidentally, if the small-diametered active
material particles have an average particle size smaller than the
lower limit of the above-specified range, the particles are
susceptible to oxidation. Moreover, such small particles are costly
to produce. The particle size of the small-diametered active
material particles is measured by a laser diffraction scattering
method or under electron microscopic (SEM) observation.
[0048] Having a large surface area, small-diametered active
material particles are more susceptible to oxidation than
relatively large-diametered particles (e.g., those having a
diameter of several tens of micrometers). Oxidation of active
material particles causes increase of irreversible capacity and
reduction of charge/discharge efficiency. Irreversible capacity and
charge/discharge current efficiency are important characteristics
for secondary batteries similarly to the charge/discharge cycle
characteristics. In some detail, if much oxygen is present in
small-diametered active material particles, electrochemically
intercalated lithium ions form firm bonding with oxygen atoms. It
would follow that the lithium ions are not released in discharging.
Accordingly, small-diametered active material particles need
stricter control of oxygen concentration than relatively
large-diametered particles. Specifically, the concentration of
oxygen present in the small-diametered active material particles is
preferably less than 2.5% by weight, still preferably 1.5% by
weight or lower, particularly preferably 1% by weight or lower. In
contrast, relatively large-diametered particles, whose surface area
is not so large, do not require such severe control against
oxidation. It is desirable for the small-diametered active material
particles to have as low an oxygen concentration as possible. It is
most desirable that no oxygen be present. In the light of the
process of producing the small-diametered active material
particles, nevertheless, a presently reachable lowest oxygen
concentration would be about 0.005% by weight. The oxygen
concentration in small-diametered active material particles is
measured by gas analysis involving combustion of a sample to be
analyzed.
[0049] In addition to the preferred oxygen concentration of the
whole small-diametered active material particles, it is also
preferred that the Si concentration in the outermost surface of the
small-diametered active material particles be higher than 1/2,
still preferably not less than 4/5, particularly preferably not
less than 10 times, the oxygen concentration in the outermost
surface of the particles. As a result of investigations the present
inventors have revealed that an increase of irreversible capacity
and a decrease of charge/discharge current efficiency are affected
predominantly by the oxygen concentration of the outermost surface
of the small-diametered active material particles. This is because
the oxygen present in the outermost surface easily undergoes
reaction with lithium during charging of the secondary battery,
which can deteriorate the battery characteristics. Hence, the Si
concentration to oxygen concentration ratio in the outermost
surface of the particles is specified as described above. The
surface oxygen concentration of small-diametered active material
particles can be measured with various surface analyzers including
an electron spectroscope for chemical analysis (ESCA) and an Auger
electron spectroscope (AES).
[0050] Whichever of the particles (a) to (e) may be used, the
small-diametered active material particles are preferably produced
under conditions inhibiting contamination of oxygen, for example,
in an inert gas atmosphere.
[0051] Whichever of the particles (a) to (e) may be used, the
small-diametered active material particles are pulverized to an
average particle size within the above-recited range by a
prescribed pulverization process, typically exemplified by a dry
pulverization process and a wet pulverization process. In dry
pulverization, a jet mill is used, for example. In wet
pulverization, the particles are dispersed in an organic solvent
(grinding liquid), such as hexane or acetone, and ground together
with a grinding medium, such as alumina beads or zirconia
beads.
[0052] During the pulverization operation, the small-diametered
active material particles are often oxidized. It is therefore
preferred that the pulverized small-diametered active material
particles, the average particle size D.sub.50 of which has been
reduced to 0.1 to 10 .mu.m, be subjected to etching with an etching
solution to remove the oxide on the surface of the particles. By so
doing, the oxygen concentration of the whole small-diametered
active material particles and the oxygen concentration of the
outermost surface of the particles can easily be controlled to or
below the recited values. Useful etching solutions include aqueous
solutions of HF, buffered acids, NH.sub.4F, KOH, NaOH, ammonia or
hydrazine. The degree of etching can be controlled appropriately by
the kind and concentration of the etching solution, the temperature
of the etching solution, the etching time, and the like. As a
result, the oxygen concentration of the whole small-diametered
active material particles and the oxygen concentration of the
outermost surface of the particles can easily be controlled within
the recited ranges. Note that, however, the oxide on the particle
surface should not be removed completely in the etching step. This
is because particles from which the surface oxide has completely
been removed would be oxidized rapidly when they are exposed to the
atmosphere. Therefore, the degree of etching is preferably
controlled so that an adequate amount of the oxide may remain. Even
after being exposed to the atmosphere, those particles having an
adequate amount of the oxide remaining on the surface thereof are
capable of maintaining almost the same surface and whole oxygen
concentrations as adjusted by the etching.
[0053] When etching is effected using HF, for example, the
small-diametered active material particles are put into an HF
solution having a concentration of about 1 to 50% by weight, and
the system is stirred at room temperature for about 5 to 30
minutes, whereby the surface oxygen concentration can be reduced to
a desired level. When in using KOH or NaOH for etching, the
small-diametered active material particles are put into an aqueous
solution having a concentration of about 1 to 40% by weight, and
the system is stirred at room temperature for about 5 to 120
minutes. In using ammonia, the small-diametered active material
particles are put into an aqueous solution having a concentration
of about 1 to 20% by weight, and the system is stirred at room
temperature for about 5 to 60 minutes to carry out etching. When
NH.sub.4F is used, the small-diametered active material particles
are put into an aqueous solution having a concentration of about 1
to 50% by weight, followed by stirring at room temperature for
about 5 to 60 minutes to conduct etching. In using hydrazine, the
small-diametered active material particles are put into an aqueous
solution having a concentration of about 1 to 50% by weight,
followed by stirring at room temperature for about 5 to 60 minutes
to compete etching.
[0054] The negative electrode containing the above-described
small-diametered active material particles is less susceptible to
cracking and pulverizing with repetition of charge/discharge
cycles. As a result, charge/discharge efficiency increases, and
irreversible capacity reduces thereby to improve the
charge/discharge cycle characteristics. Furthermore, reduction in
oxygen content in the small-diametered active material particles
also brings about reduction of irreversible capacity, increase of
charge/discharge efficiency, and improvement in charge/discharge
cycle characteristics.
[0055] The small-diametered active material particles may be coated
with a thin metal coat. The thin metal coat inhibits oxidation of
the small-diametered active material particles to effectively
prevent an increase in irreversible capacity and a decrease in
charge/discharge current efficiency. In addition, the electron
conductivity is improved, and the charge/discharge cycle
characteristics are further improved.
[0056] In order to inhibit oxidation of the small-diametered active
material particles more effectively and to allow Li and Si to react
with each other more efficiently, the thickness of the thin metal
coat is preferably 0.005 to 4 .mu.m, still preferably 0.05 to 0.5
.mu.m. The thickness of the thin metal coat is measured with, for
example, ESCA or AES.
[0057] The metal making up the thin metal coat is preferably
selected from those having low capability of forming lithium. Such
metals include Ni, Cu, Co, Fe, Ag, and Au. Ni, Co, Ag, and Au are
still preferred from the standpoint of oxidation prevention. These
metals can be used either individually or in the form of an alloy
composed of two or more thereof.
[0058] In the small-diametered active material particle coated with
a thin metal coat, the oxygen concentration in the interfacial part
between the thin metal coat and the small-diametered active
material particle is such that the Si concentration exceeds 1/2 the
oxygen concentration as described with reference to the
aforementioned small-diametered active material particles. The
"interfacial part" between the thin metal coat and the
small-diametered active material particle is considered to be the
part where the concentration of the metal making up the thin metal
coat becomes the minimum in AES analysis of the metal-coated
small-diametered active material particles.
[0059] The lower the oxygen concentration of the outermost surface
of the thin metal coat, the more desirable for increasing the
electrical conductivity of the metal-coated small-diametered active
material particles.
[0060] The small-diametered active material particles having a thin
metal coat are preferably prepared as follows. Active material
particles are pulverized to powder of prescribed size in a dry or
wet process in accordance with the above-described process for
preparing small-diametered active material particles. The oxide
present on the surface of the particles is removed by etching. The
etched particles are thoroughly rinsed with water and then
subjected to electroless plating to form a thin metal film thereon.
Prior to the electroless plating, the particles may be subjected to
a surface sensitizing treatment and a surface activating treatment
in a usual manner. The electroless plating conditions are selected
appropriately according to the plating metal. For instance, the
plating bath composition shown below is useful for Ni plating. In
this case, the bath has a temperature of about 40 to 60.degree. C.
and a pH of about 4 to 6, and the plating time is 0.5 to 50
minutes.
[0061] NiSO.sub.4.6H.sub.2O 15-35 g/l
[0062] NaH.sub.2PO.sub.2.H.sub.2O 10-30 g/l
[0063] Na.sub.3C.sub.6H.sub.5O.sub.7 15-35 g/l
[0064] NaC.sub.3H.sub.5O.sub.2 5-15 g/l
[0065] The thin metal coat formed on the small-diametered active
material particles does not always need to cover the individual
particles completely. For example, the thin metal coat covering the
whole particle uniformly may have a large number of micropores
extending through the thickness thereof. Such micropores allow an
electrolyte to pass through and reach the inside of the
small-diametered active material particle, so that the
electrochemical reactivity essentially possessed by the
silicon-containing particle may surely be manifested. The thin
metal coat may also be provided in the form of islands on the
particle surface.
[0066] A preferred process for producing the negative electrode of
the present invention will be described. This process starts with
preparation of a slurry to be applied to the surface of a current
collector. The slurry comprises active material particles,
electro-conductive carbon material particles, a binder, and a
diluting solvent. The active material particles and the
electro-conductive carbon materials have previously been described.
The binder that can be used includes polyvinylidene fluoride
(PVDF), polyethylene (PE), and ethylene-propylene-diene monomer
(EPDM). The diluting solvent includes N-methylpyrrolidone and
cyclohexane.
[0067] The amount of the active material particles in the slurry is
preferably about 14% to 40% by weight. The amount of the
electro-conductive carbon material is preferably about 0.4% to 4%
by weight. The amount of the binder is preferably about 0.4% to 4%
by weight. The amount of the diluting solvent is preferably about
60% to 85% by weight.
[0068] The slurry is applied to the surface of a current collector.
The current collector may be prepared separately or in the same
line for producing the negative electrode of the invention. In the
latter case, the current collector is preferably prepared by
electrodeposition. The spread of the slurry on the current
collector is preferably such that the dry thickness of the active
material layer is about one to three times the thickness of a
finally obtained active material structure. After the coating of
the slurry dries to form an active material layer, the current
collector having the active material layer formed thereon is
immersed in a plating bath containing an electro-conductive
material having low capability of forming a lithium compound and
electroplated in this state with the conductive material to form a
surface coating layer on the active material layer. By using this
process, a surface coating layer with a large number of micropores
can easily be formed. In more detail, since the active material
layer 3 has a microscopically textured surface as described supra,
there are active sites where deposit grows easily and sites where
deposit does not grow easily in a mixed state. When the active
material layer 3 having such a surface condition is electroplated,
the deposit grows non-uniformly, and the particles of the material
making up the surface coating layer 4 grow into a polycrystalline
structure. On further growth of crystals, adjacent crystals meet,
resulting in formation of microvoids in the meeting site. The
following is recommended electroplating conditions taking copper,
for instance, as an electro-conductive material. In using a copper
sulfate-based solution, electroplating is performed at a copper
concentration of 30 to 100 g/l, a sulfuric acid concentration of 50
to 200 g/l, a chlorine concentration of 30 ppm or lower, a bath
temperature of 30 to 80.degree. C., and a current density of 1 to
100 A/dm.sup.2. Under these electrolysis conditions, it is easy to
form a surface coating layer part of which enters into the active
material layer or penetrates and reaches the current collector or a
surface coating layer penetrating throughout the active material
layer. In another electrolysis system, a copper pyrophosphate-based
solution can be used. In this case, electroplating is conducted at
a copper concentration of 2 to 50 g/l, a potassium pyrophosphate
concentration of 100 to 700 g/l, a bath temperature of 30 to
60.degree. C., a pH of 8 to 12, and a current density of 1 to 10
A/dm.sup.2.
[0069] After the surface coating layer is thus formed on the active
material layer, the active material layer as covered with the
surface coating layer may be subjected to mechanical pressing
thereby densify the active material layer. As a result of
densification, the voids among the active material particles and
the conductive carbon material particles are filled with the
conductive material constituting the surface coating layer to make
a structure in which the active material particles and the
conductive carbon material particles are dispersed. Furthermore,
these particles and the surface coating layer come into closer
contact to improve electron conductivity. Additionally, the void
volume of the active material layer is adjusted appropriately to
relax the stress resulting from the active material particles'
expansion and contraction due to intercalation and deintercalation
of lithium. In order to obtain sufficient electron conductivity,
the densification by mechanical pressing is preferably such that
the total thickness of the active material layer and the surface
coating layer after mechanical pressing may be 90% or less,
particularly 80% or less, of that before mechanical pressing.
Mechanical pressing can be carried out with, for example, a roll
press.
[0070] In this process of production, it is possible to
mechanically press the active material layer before the
electroplating. For the sake of distinguishing from the
above-mentioned mechanical pressing, the mechanical pressing before
the electroplating will be called prepressing. Prepressing is
effective in preventing separation between the active material
layer and the current collector and preventing the active material
particles from being exposed on the surface coating layer. As a
result, deterioration of battery cycle life due to fall-off of the
active material particles can be averted. The prepressing
conditions are preferably such that the thickness of the active
material layer after prepressing is 95% or less, particularly 90%
or less, of that before prepressing.
[0071] The electroplating used in the process for forming the
surface coating layer may be replaced with sputtering, chemical
vapor deposition or physical vapor deposition. The surface coating
layer may also be formed by rolling an electro-conductive foil, for
example, by rolling a metal foil, a metal mesh foil or an
electro-conductive plastic film. In using these materials, creation
of micropores in the surface coating layer can be achieved by
pressing under so controlled conditions.
[0072] In another preferred process of producing the negative
electrode of the present invention, a dispersion plating technique
is employed. To conduct dispersion plating, a plating bath having
suspended therein active material particles and containing an
electro-conductive material having low capability of forming a
lithium compound is prepared. In order to incorporate a sufficient
amount of the active material particles into the active material
structure, the amount of the active material particles in the
plating bath is preferably 200 to 600 g/l, still preferably 400 to
600 g/l. In using copper as an electro-conductive material having
low capability of forming a lithium compound and copper sulfate as
a copper source, the plating bath preferably has the following
formulation in view of plating rate controllability and capability
of building up a surface coating layer to a thickness enough to
sufficiently hold an active material layer of the active material
particles. A preferred copper concentration is 30 to 100 g/l. A
preferred sulfuric acid concentration is 50 to 200 g/l. A preferred
chlorine concentration is 300 ppm or lower. A preferred
cresolsulfonic acid concentration is 40 to 100 .mu.l. A preferred
gelatin concentration is 1 to 3 g/l. A preferred .beta.-naphthol
concentration is 0.5 to 2 g/l.
[0073] A current collector is immersed in the plating bath, and
electroplating is started in this state. The current density used
in the electrolysis is preferably about 1 to 15 A/dm.sup.2 from the
standpoint of plating rate control. The plating bath temperature is
room temperature, which is around 20.degree. C. By this
electroplating, the metal in the plating bath is reduced to form a
surface coating layer and, at the same time, an active material
layer covered with the surface coating layer is formed on the
surface of the current collector. In order to form the active
material layer uniformly, the electrolysis may be effected while
stirring the plating bath.
[0074] While the current collectors that can be used in the present
invention have been described supra, a current collector formed of
the following porous metal foil is also preferably used. The porous
metal foil (hereinafter simply called a metal foil) has a great
number of micropores. It has both micropores piercing therethrough
in the thickness direction thereof and micropores that are closed
within the thickness thereof. The term "micropores" as used herein
is intended to indicate those holes which pierce the foil in the
thickness direction. This does not mean that a metal foil having
micropores that are closed within the thickness of the foil is
excluded, nor that such a metal foil is unfavorable.
[0075] The above-described metal foil, when used as a current
collector of a nonaqueous secondary battery, secures sufficient
passageways therethrough for an electrolyte thereby bringing about
a further increase in battery capacity. Moreover, the active
material is more effectively prevented from falling off from the
electrode as a result of intercalating and deintercalating
lithium.
[0076] The micropores of the metal foil preferably have a diameter
of 0.01 to 200 .mu.m, still preferably 0.05 to 50 .mu.m,
particularly preferably 0.1 to 10 .mu.m. Micropores with a diameter
less than 0.01 .mu.m can fail to secure passage of a nonaqueous
electrolyte sufficiently. Where the pore diameter exceeds 200
.mu.m, the metal foil strength tends to reduce in relation to the
foil thickness described below, the active material tends to fall
off with intercalating and deintercalating of lithium, and the
resulting nonaqueous secondary battery tends to have reduced cycle
characteristics. Not all the pores piercing the metal foil are
required to have a diameter falling within the recited range. It is
acceptable that the metal foil has a very small number of
micropores with diameters out of that range that are unavoidably
created in the course of metal foil manufacturing.
[0077] The number of micropores whose diameter is in the recited
range per unit area (pore density) is preferably 5 to
10000/cm.sup.2, still preferably 10 to 5000/cm.sup.2, particularly
preferably 100 to 2000/cm.sup.2, in every part of the metal foil.
Metal foil with a pore density of less than 1/cm.sup.2 can fail to
supply a sufficient amount of a nonaqueous electrolyte to the
active material. A pore density exceeding 10000/cm.sup.2 can reduce
the strength of the metal foil in relation to the upper limit of
the pore diameter.
[0078] The diameter and density of the micropores are measured as
follows. A metal foil is photographed with its back side irradiated
with light in a dark room, and the photograph is analyzed by image
processing to obtain the diameter and density of the
micropores.
[0079] The metal foil preferably has a thickness of 1 to 100 .mu.m,
still preferably 2 to 20 .mu.m, particularly preferably 3 to 10
.mu.m. Metal foil with a thickness less than 1 .mu.m brings about
increased energy density but has insufficient mechanical strength
and is often difficult to produce. With a thickness greater than
100 .mu.m, formation of piercing micropores is not easy, which
makes it difficult to increase the energy density and hinders
smooth passage of an electrolyte.
[0080] The metal foil can be of various metallic materials. For
example, the metal foil contains at least one metal selected from
Cu, Ni, Co, Fe, Cr, Sn, Zn, In, Ag, and Au. In other words, the
metal foil can be of a single substance selected from these metals,
an alloy of two or more metals selected from these elements, or a
material containing at least one of these elements and other
element(s). A metal foil made of Cu, Ni, Co, Fe, Cr or Au is
preferred for its low reactivity with lithium.
[0081] A preferred process of preparing a metal foil is described
with reference to FIGS. 5(a) through 5(f). First of all, a carrier
foil 11 is prepared as shown in FIG. 5(a). The material of the
carrier foil 11 is not particularly limited. The carrier foil 11 is
preferably electro-conductive. The carrier foil 11 does not need to
be made of metal as long as it is electro-conductive. Nevertheless,
use of a metal-made foil as the carrier foil 11 is advantageous in
that the carrier foil 11, which is left after making metal foil,
can be melted and recycled into foil. In using a metal-made carrier
foil 11, it is preferred to use a carrier foil 11 containing at
least one metal selected from Cu, Ni, Co, Fe, Cr, Sn, Zn, In, Ag,
Au, Al, and Ti. Considering that the carrier foil 11 is used as a
support for making metal foil, it is desirable for the carrier foil
11 to have sufficient strength not to bunch up in the production of
the metal foil. Accordingly, the carrier foil 11 preferably has a
thickness of about 10 to 50 .mu.m.
[0082] A coat 12 is formed on one side of the carrier foil 11 by a
prescribed method as shown in FIG. 5(b). Before formation of the
coat, it is preferred that the surface of the carrier foil 11 be
cleaned by a pretreatment such as acid cleaning. The coat 12 serves
to make the carrier foil surface, on which metal foil is to be
formed, non-uniform in electron conductivity thereby to form a
large number of micropores in the resulting metal foil. The coat 12
is preferably applied to a thickness of 0.001 to 1 .mu.m, still
preferably 0.002 to 0.5 .mu.m, particularly preferably 0.005 to 0.2
.mu.m. Applied to a thickness in that range, the coat 12 covers the
surface of the carrier foil 11 discontinuously, for example in the
form of islands. Discontinuous formation of the coat 12 is
advantageous for forming the micropores with the aforementioned
diameter and density more easily. In FIG. 5(b), the size of the
coat 12 is exaggerated for the sake of better understanding.
[0083] The coat 12 is made of a material different from the
material which makes up the metal foil, whereby the resulting metal
foil can successfully be peeled from the carrier foil 11 in the
step of peeling hereinafter described. It is preferred for the
material of the coat 12 to differ from the material which makes up
metal foil and to contain at least one element of Cu, Ni, Co, Mn,
Fe, Cr, Sn, Zn, In, Ag, Au, C, Al, Si, Ti, and Pd.
[0084] The process of forming the coat 12 is not particularly
restricted. For example, the process of forming the coat 12 can be
selected in relation to the process of forming metal foil described
infra. More specifically, where the metal foil is to be formed by
electroplating, it is preferred to form the coat 12 also by
electroplating from the standpoint of production efficiency and the
like. The coat 12 can also be formed by other processes, such as
electroless plating, sputtering, physical vapor deposition (PVD),
chemical vapor deposition (CVD), a sol-gel process, and ion
plating.
[0085] Where the coat 12 is formed by electroplating, a proper
plating bath and plating conditions are decided according to the
constituent material of the coat 12. For example, in making the
coat 12 of tin, a plating bath having the composition shown below
or a tin borofluoride bath can be used. In using these plating
baths, the bath temperature is preferably about 15 to 30.degree.
C., and the current density is preferably about 0.5 to 10
A/dm.sup.2.
1 SnSO.sub.4 30 to 70 g/l H.sub.2SO.sub.4 60 to 150 g/l
Cresolsulfonic acid 70 to 100 g/l
[0086] As stated above, the coat 12 is used to provide the surface
of the carrier foil 11, on which metal foil is to be formed, with
non-uniform electron conductivity. When the constituent material of
the coat 12 is largely different from the carrier foil 11 in
electron conductivity, application of the coat 12 immediately
imparts non-uniformity of electron conductivity to the surface on
which metal foil is to be formed. Use of carbon nitrogen-containing
compounds preferably include triazole compounds, such as
benzotriazole (BTA), carboxybenzotriazole (CBTA), tolyltriazole
(TTA), N',N'-bis(benzotriazolylmethyl)urea (BTD-U), and
3-amino-1H-1,2,4-triazole (ATA). The sulfur-containing compounds
include mercaptobenzothiazole (MBT), thiocyanuric acid (TCA), and
2-benzimidazolethiol (BIT). Considering that the purpose of
applying a release agent is just to facilitate peeling the formed
metal foil off the carrier foil 11 in the hereinafter described
step of peeling, a porous metal foil can be formed without the step
of applying a release agent.
[0087] A material for making metal foil is then deposited on the
release layer 13 by electroplating to form a metal foil 14 as shown
in FIG. 5(e). The resulting metal foil 14 contains a great number
of micropores. While FIG. 5(e) shows that the micropores are formed
at positions on the top of the individual islands (the coat 12),
the aim of this depiction is only for the sake of convenience. In
fact, the micropores are not always formed at positions on the top
of the individual islands (the coat 12). The plating bath and
plating conditions are chosen appropriately according to the
material of the metal foil. In making a metal foil 14 of Ni, for
instance, a Watts bath having the composition shown below or a
sulfamic acid bath can be used as a plating bath. In using these
baths, the bath temperature is preferably about 40 to 70.degree.
C., and the current density is preferably about 0.5 to 20
A/dm.sup.2.
2 NiSO.sub.4.6H.sub.2O 150 to 300 g/l NiCl.sub.2.6H.sub.2O 30 to 60
g/l H.sub.3BO.sub.3 30 to 40 g/l
[0088] The above-described process of preparing a metal foil 14 is
advantageous in that the diameter and the density of micropores can
be controlled with ease for the following reason. In the process, a
fresh carrier foil is used for every lot of metal foil. That is, a
metal foil is always electrodeposited on a fresh surface so that
the condition of the surface on which metal foil is formed can be
maintained as constant.
[0089] Being so thin as previously stated, the resulting metal foil
14 is often difficult to handle by itself. This being the case, it
is advisable to leave the metal foil 14 remaining on the carrier
foil 11 until prescribed processing operations (such as as a
material of the coat 12 is an example of that case. On the other
hand, when in using, as a constituent material of the coat 12, a
material whose electron conductivity is about the same as that of
the carrier foil 11, such as various metallic materials including
tin, application of the coat 12 does not immediately result in
non-uniform electron conductivity of the surface for forming metal
foil. Then, in case where the coat 12 is made of such a material,
it is preferred that the carrier foil 11 having the coat 12 formed
thereon be exposed to an oxygen-containing atmosphere, such as the
air, in a dry condition, thereby to oxidize the surface of the coat
12 (and the exposed area of the carrier foil 11) (see FIG. 5(c)).
By this operation, the electron conductivity on the surface for
forming metal foil becomes non-uniform. When electroplating
(described infra) is performed on the surface with the thus created
non-uniformity of electron conductivity, there is produced a
difference in electrodeposition rate between the surface of the
coat 12 and the exposed area of the carrier foil 11. As a result, a
metal foil having micropores which have the above-recited diameter
at the above-recited density can easily be formed. The degree of
oxidation is not critical in the present invention. According to
the present inventors' study, it has been confirmed that allowing
the carrier foil 11 having the coat 12 formed thereon, for example,
in the atmosphere for about 10 to 30 minutes is sufficient.
However, the carrier foil 11 having the coat 12 formed thereon may
be forcibly oxidized.
[0090] The reason why the exposure of the carrier foil 11 having
the coat 12 formed thereon to an oxygen-containing atmosphere is
carried out in a dry condition, is for the sake of oxidation
efficiency. For example, where the coat 12 is formed by
electroplating, it is sufficient to take the carrier foil 11 out of
the plating bath, to dry the carrier foil 11 by means of a dryer,
etc. and to allow it to stand in the atmosphere for a given time.
Where the coat 12 is formed by dry processes, such as sputtering
and various vacuum deposition techniques, the drying operation is
unnecessary and the foil 11 having the coat 12 formed thereon is
allowed to stand in the atmosphere as it is.
[0091] Oxidizing the coat 12 is followed by applying a release
agent 13 thereon as shown in FIG. 5(d). The release agent 13 is
used to facilitate peeling the metal foil off the carrier foil in
the peeling step which will be hereinafter described. An organic
compound is preferably applied as the release agent 13.
Nitrogen-containing compounds or sulfur-containing compounds are
particularly preferred. The formation of an active material layer
as described later) on the metal foil 14 complete. The metal foil
14 is peeled from the carrier foil 11 as depicted in FIG. 5(f)
after completion of the prescribed processing operations. Since the
release agent 13 has been applied between the carrier foil 11 and
the metal foil 14 as described, peeling of the metal foil 14 from
the carrier foil 11 can be achieved very smoothly. Although FIG.
5(f) shows that the coat 12 remains on the side of the carrier foil
11 after peeling, it depends on the circumstances whether the coat
12 actually remains on the carrier foil side or the metal foil
side. The same applies to the release agent. On whichever side the
coat 12 remains, the coat and the release agent give no adverse
influences on the metal foil in view of their very small
amounts.
[0092] Instead of the above-described process, the metal foil 14
can also be prepared by the following process (hereinafter called
an alternative process (1)). In the alternative process (1), a
coating, e.g., paste, containing carbonaceous material particles is
prepared. Useful carbonaceous materials include acetylene black. In
order to easily form micropores with the recited diameter at the
recited density, it is preferred for the carbonaceous material to
have an average particle size D.sub.50 (determined by a laser
diffraction scattering method combined with scanning electron
microscopic observation) of about 2 to 200 nm, particularly about
10 to 100 nm. The coating is applied to a prescribed support. The
coating thickness is preferably about 0.001 to 1 .mu.m, still
preferably about 0.05 to 0.5 .mu.m. A material of the metal foil is
then deposited on the coating layer by electroplating to form metal
foil. The conditions of the electroplating can be the same as those
used in the above-described process.
[0093] The support used here typically includes, but is not limited
to, the aforementioned carrier foil.
[0094] After the formation of metal foil, the metal foil may be
either separated from the support or left on the support as formed.
For example, when the process is applied to the manufacture of a
negative electrode for a nonaqueous secondary battery, there is no
need to separate the metal foil. In contrast, where the metal foil
is to be separated, it is advisable to apply a release agent on the
coating layer formed by applying the carbonaceous
material-containing paste and then to electrodeposit the metal foil
thereon so as to facilitate peeling. The release agent that can be
used includes those usable in the above-described process.
[0095] The metal foil can also be obtained by the following process
(hereinafter called an alternative process (2)) instead of the
alternative process (1). In the alternative process (2), a plating
bath containing the material of metal foil is prepared. In making
Ni foil, for instance, the aforementioned Watts bath or sulfamic
acid bath is prepared. Particles of a carbonaceous material are
added and suspended in the plating bath. The kind and the particle
size of the carbonaceous material can be chosen from those useful
in the alternative process (1). For easy formation of micropores
with the recited diameter at the recited density, the amount of the
carbonaceous material to be suspended in the plating bath is
preferably about 0.5 to 50 g/l, still preferably about 1 to 10
g/l.
[0096] A prescribed support is electroplated in the plating bath
while stirring the bath to keep the carbonaceous material
suspended. The material constituting metal foil is thus
electrodeposited to form a metal foil 14. The same support as
useful in the alternative process (1) is usable as well. The metal
foil thus formed can be handled in the same manner as for the one
obtained by the alternative process (1). If desired, the support
may have a release agent applied thereto to successfully separate
the formed metal foil.
[0097] The negative electrode using the porous metal foil 14 is
produced as follows. The negative electrode can be produced by
taking advantage of the above-described processes of preparing
metal foil. For example, metal foil is prepared in accordance with
the process shown in FIGS. 5(a) through 5(e). With the metal foil
remaining on the carrier foil, an active material layer is formed
on the metal foil. The active material layer is formed by applying,
for example, paste containing active material particles and
electro-conductive material particles. The metal foil having the
active material layer formed thereon is immersed in a plating bath
containing an electro-conductive material having low capability of
forming a lithium compound. In this state, the active material
layer is electroplated with the conductive material to form a
surface coating layer. Finally, the metal foil is separated from
the carrier foil as shown in FIG. 5(f) to obtain a negative
electrode.
[0098] The thus obtained negative electrode of the invention is
assembled together with a known positive electrode, separator, and
nonaqueous electrolyte into a nonaqueous secondary battery. A
positive electrode is obtained as follows. A positive electrode
active material and, if necessary, a conductive material and a
binder are suspended in an appropriate solvent to prepare a
positive electrode active material mixture, which is applied to a
current collector, dried, rolled, and pressed, followed by cutting
and punching. The positive electrode active material includes
conventionally known ones, such as lithium-nickel composite oxide,
lithium-manganese composite oxide, and lithium-cobalt composite
oxide. Preferred separators include a nonwoven fabric of synthetic
resins and a porous film of polyethylene or polypropylene. The
nonaqueous electrolyte used in a lithium secondary battery, for
example, is a solution of a lithium salt, which is a supporting
electrolyte, in an organic solvent. The lithium salt includes
LiClO.sub.4, LiAlCl.sub.4, LiPF.sub.6, LiAsF.sub.6, LiSbF.sub.6,
LiSCN, LiCl, LiBr, LiI, LiCF.sub.3SO.sub.3, and
LiC.sub.4F.sub.9SO.sub.3.
[0099] The present invention is not limited to the aforementioned
embodiments. For example, punching metal or expanded metal having a
great number of openings or metal foam, such as nickel foam, can be
used as a current collector. In using punching metal or expanded
metal, the opening area is preferably 0.0001 to 4 mm.sup.2, still
preferably 0.002 to 1 mm.sup.2. Where punched metal or expanded
metal is used, the active material layer is formed preferentially
in the openings, and the surface coating layer is formed on the
surface of the thus formed active material layer and the surface of
the punching metal or expanded metal. On the other hand, where
metal foam is used, the cells of the foamed body are filled with
the active material layer, and the surface coating layer is formed
on the surface of the active material layer and the surface of the
metal foam.
[0100] While the cross-sectional photographs shown in FIGS. 2 to 4
represent an embodiment in which the active material structure 5 is
formed on only one side of the current collector 2, the active
material structure may be formed on both sides of the current
collector.
[0101] The present invention will now be illustrated in greater
detail with reference to Examples, but it should be understood that
the invention is not construed as being limited thereto. Unless
otherwise noted, all the percents are by weight.
EXAMPLE 1-1
[0102] (1) Preparation of Active Material Particles
[0103] A molten metal at 1400.degree. C. containing 90% of silicon
and 10% of nickel was cast into a copper-made mold and quenched to
obtain an ingot of a silicon-nickel alloy. The ingot was pulverized
and sieved to obtain silicon-nickel alloy particles having particle
sizes of 0.1 to 10 .mu.m. The silicon-nickel alloy particles and
nickel particles (particle size: 30 .mu.m) were blended at a rate
of 80%:20% and mixed and pulverized simultaneously in an attritor
to obtain uniformly mixed particles of silicon-nickel particles and
nickel. The mixed particles had the maximum particle size of 1
.mu.m and a D.sub.50 value of 0.8 .mu.m.
[0104] (2) Preparation of Slurry
[0105] A slurry having the following composition was prepared.
3 Mixed particles obtained in (1) above 16% Acetylene black
(particle size: 0.1 .mu.m) 2% Binder (polyvinylidene fluoride) 2%
Diluting solvent (N-methylpyrrolidone) 80%
[0106] (3) Formation of Active Material Layer
[0107] The above prepared slurry was applied to a 35 .mu.m thick
copper foil and dried to form an active material layer having a dry
thickness of 60 .mu.m. The active material layer was densified by
prepressing.
[0108] (4) Formation of Surface Coating Layer
[0109] The copper foil having the active material layer formed
thereon was immersed in a plating bath having the following
composition to carry out electroplating.
4 Nickel 50 g/l Sulfuric acid 60 g/l Bath temperature 40.degree.
C.
[0110] After formation of the surface coating layer, the copper
foil was taken out of the plating bath, and both the active
material layer and the surface coating layer were densified by roll
pressing. The thickness of the thus formed active material
structure was found to be 23 .mu.m as a result of electron
microscopic observation. As a result of chemical analysis, the
amounts of the active material particles and acetylene black were
found to be 40% and 5%, respectively. Presence of micropores in the
resulting negative electrode was confirmed by observation under an
electron microscope.
EXAMPLES 1-2 TO 1-4
[0111] A negative electrode was produced in the same manner as in
Example 1-1, except for using the active material particles shown
in Table 1-1 below. The same electron microscopic observation as in
Example 1-1 revealed presence of micropores in the resulting
negative electrode.
EXAMPLE 1-5
[0112] A 35 .mu.m thick copper foil was plated with nickel to a
deposit thickness of 2 .mu.m to prepare a current collector. An
active material layer and a surface coating layer were formed on
the nickel layer in the same manner as in Example 1-1, except for
using the active material particles shown in Table 1-1 in the
active material layer. The same electron microscopic observation as
in Example 1-1 revealed presence of micropores in the resulting
negative electrode.
EXAMPLES 1-6
[0113] A 400 .mu.m thick nickel foam was used as a current
collector. The nickel foam had an average cell diameter of 20
.mu.m. A slurry was prepared in the same manner as in Example 1-1,
except for using the active material particles shown in Table 1-1.
The nickel foam was impregnated with the slurry. The impregnated
foam was immersed in the same plating bath as used in Example 1-1
to carry out electroplating. The same electron microscopic
observation revealed presence of micropores in the resulting
negative electrode.
EXAMPLE 1-7
[0114] A 40 .mu.m thick expanded copper metal sheet was used as a
current collector. The area of the individual openings of the
expanded metal was 0.01 mm.sup.2. A slurry was prepared in the same
manner as in Example 1-1, except for using the active material
particles shown in Table 1-1, and the expanded metal was
impregnated with the slurry. The impregnated expanded metal was
immersed in the same plating bath as used in Example 1-1 to carry
out electroplating. The same electron microscopic observation
revealed presence of micropores in the resulting negative
electrode.
[0115] M-306[0051]
COMPARATIVE EXAMPLE 1-1
[0116] Graphite powder having a particle size of 10 .mu.m, a binder
(PVDF), and a diluting solvent (N-methylpyrrolidone) were kneaded
to prepare a slurry. The slurry was applied to a 30 .mu.m thick
copper foil, dried, and pressed to obtain a negative electrode. The
pressed graphite layer was 20 .mu.m thick.
COMPARATIVE EXAMPLE 1-2
[0117] A negative electrode was obtained in the same manner as in
Comparative Example 1-1, except for replacing graphite powder with
silicon powder having a particle size of 5 .mu.m.
[0118] Evaluation of Performance:
[0119] A nonaqueous secondary battery was assembled using each of
the negative electrodes prepared in Examples and Comparative
Examples as follows. The battery was evaluated in irreversible
capacity, capacity density per unit volume when charged,
charge/discharge efficiency in the 10th cycle, and capacity
retention in the 50th cycle in accordance with the following
methods. The results of evaluation are shown in Table 1-1.
[0120] 1) Preparation of Nonaqueous Secondary Battery
[0121] A metallic lithium as a counter electrode and the negative
electrode obtained above as a working electrode were placed to face
each other with a separator between them and assembled into a
nonaqueous secondary battery in a usual manner by using an
LiPF.sub.6 solution in a mixture of ethylene carbonate and diethyl
carbonate (1:1 by volume) as a nonaqueous electrolyte.
[0122] 2) Irreversible Capacity
[0123] An irreversible capacity, represented by equation shown
below, indicates the part of the charge capacity that is not
discharged and remains in the active material.
Irreversible capacity (%)=(1-first discharge capacity/first charge
capacity).times.100
[0124] 3) Capacity Density
[0125] The first discharge capacity (mAh/g).
[0126] 4) Charge/Discharge Efficiency in the 10th Cycle
Charge/discharge efficiency in 10th cycle (%)=discharge capacity in
10th cycle/charge capacity in 10th cycle.times.100
[0127] 5) Capacity Retention in the 50th Cycle
Capacity retention (50th cycle) (%)=discharge capacity (50th
cycle)/maximum discharge capacity.times.100
5 TABLE 1-1 Active Material Structure Active Material Layer Charge/
Surface Coating Active Material Particles Discharge Layer Size
Content in Irreversible Capacity Efficiency Capacity Thickness
Thickness D.sub.50 Structure Capacity Density at 10th cycle
Retention in (.mu.m) Material (.mu.m) (.mu.m) (wt %) Material (%)
(mAh/g) (%) 50th Cycle (%) Example 1-1 3 Ni 20 0.8 40 [Si90/Ni10 4
3100 99.9 98 (cast)]80/Ni20 Example 1-2 3 Ni 20 0.8 40 [Si90/Ni10 5
3100 99.9 98 (cast)]80/Cu20 Example 1-3 3 Ni 20 0.8 40 [Si80/Cu20 4
2800 99.9 97 (cast)]80/Ni20 Example 1-4 3 Ni 20 0.8 40 [Si80/Cu20 5
2800 99.9 96 (cast)]80/Cu20 Example 1-5 3 Ni 20 0.8 40 [Si80/Ni20 4
2800 99.9 99 (cast)]80/Ni20 Example 1-6 3 Ni 20 0.8 40 [Si80/Ni20 4
2800 99.9 99 (cast)]80/Ni20 Example 1-7 3 Ni 20 0.8 40 [Si80/Ni20 4
2800 99.9 99 (cast)]80/Ni20 Compara. -- 10 80 graphite 10 310 99.7
100 Example 1-1 Compara. -- 5 80 pure Si 60 2000 85.0 7 Example
1-2
[0128] As is apparent from the results shown in Table 1-1, the
secondary batteries using the negative electrodes of Examples each
have a lower irreversible capacity and a higher capacity density
and charge/discharge efficiency than those using the negative
electrodes of Comparative Examples. They also exhibit high capacity
retention. While not shown in the Table, the negative electrodes of
Examples 1-1 to 1-7 had a structure as shown in FIG. 2 under
observation with an electron microscope.
EXAMPLE 2-1
[0129] (1) Preparation of Slurry
[0130] A slurry having the following composition was prepared.
6 Tin particles (particle size D.sub.50: 2 .mu.m) 16% Acetylene
black (particle size: 0.1 .mu.m) 2% Binder (polyvinylidene
fluoride) 2% Diluting solvent (N-methylpyrrolidone) 80%
[0131] (2) Formation of Coating
[0132] The above prepared slurry was applied to a 30 .mu.m thick
copper foil and dried. The thickness of the dried coating was 60
.mu.m.
[0133] (3) Formation of Coating Layer
[0134] The copper foil having the coating formed thereon was
immersed in a plating bath having the following composition to
carry out electroplating.
7 Copper 50 g/l Sulfuric acid 60 g/l Bath temperature 40.degree.
C.
[0135] After formation of the coating layer, the copper foil was
taken out of the plating bath, and the coating layer, including the
coating, was densified by roll pressing. The thickness of the
resulting coating layer was found to be 20 .mu.m as a result of
electron microscopic observation. As a result of chemical analysis,
the amounts of tin particles and acetylene black in the coating
layer were found to be 70% and 5%, respectively.
EXAMPLES 2-2 AND 2-3
[0136] A negative electrode was obtained in the same manner as in
Example 2-1, except that the coating layer was formed of nickel
(Example 2-2) or cobalt (Example 2-3).
EXAMPLE 2-4
[0137] A molten metal containing 60% tin and 40% copper at
1000.degree. C. was injected onto the peripheral surface of a
copper roll rotating at a high speed (1000 rpm). The injected
molten roll is quenched on the roll into a thin tin-copper alloy
strip. The cooling rate was 10.sup.3 K/sec or higher. The strip was
ground and sieved to obtain particles having particle sizes of 0.1
to 10 .mu.m. A negative electrode was obtained in the same manner
as in Example 2-1, except for using the resulting alloy
particles.
EXAMPLES 2-5 AND 2-6
[0138] A negative electrode was obtained in the same manner as in
Example 2-4, except for using tin-copper alloy particles having the
composition shown in Table 2-1 below.
EXAMPLES 2-7 AND 2-8
[0139] A negative electrode was obtained in the same manner as in
Example 2-4, except for using tin-nickel alloy particles having the
composition shown in Table 2-1.
EXAMPLES 2-9 AND 2-10
[0140] A negative electrode was obtained in the same manner as in
Example 2-4, except for using tin-copper-nickel alloy particles
having the composition shown in Table 2-1.
EXAMPLES 2-11 TO 2-16
[0141] A negative electrode was obtained in the same manner as in
Example 2-4, except for using tin-based ternary alloy particles
having the composition shown in Table 2-1 prepared by a quenching
process.
EXAMPLE 2-17
[0142] Tin particles (particle size: 30 .mu.m) 90% and copper
particles (particle size: 30 .mu.m) 10% were mixed and pulverized
simultaneously in an attritor to obtain uniformly mixed tin/copper
powder having particle sizes of 0.1 to 10 .mu.m (D.sub.50: 2
.mu.m). A negative electrode was obtained in the same manner as in
Example 2-1, except for using the resulting mixed powder.
EXAMPLES 2-18 TO 2-31
[0143] A negative electrode was obtained in the same manner as in
Example 2-17, except for using tin-copper mixed powder of the
composition and particle size shown in Table 2-2 below or changing
the thickness of the coating layer or the content of the mixed
powder in the coating layer as shown in Table 2-2.
EXAMPLES 2-32 TO 2-39
[0144] A negative electrode was obtained in the same manner as in
Example 2-17, except for using tin-based mixed powder of the
composition shown in Table 2-2.
EXAMPLE 2-40
[0145] A molten metal at 1000.degree. C. containing 75% tin and 25%
copper was injected onto the peripheral surface of a copper roll
rotating at a high speed (1000 rpm). The injected molten roll is
quenched on the roll into a thin tin-copper alloy strip. The
cooling rate was 10.sup.3 K/sec or higher. The strip was ground and
sieved to obtain particles having particle sizes of 0.1 to 10
.mu.m. The resulting alloy particles 99% and silver particles
(particle size: 30 .mu.m) 1% were mixed and pulverized
simultaneously in an attritor to obtain uniformly mixed tin-copper
alloy/silver powder having particle sizes of 0.1 to 10 .mu.m
(D.sub.50: 2 .mu.m). A negative electrode was obtained in the same
manner as in Example 2-1, except for using the resulting mixed
powder.
EXAMPLES 2-41 TO 2-48
[0146] A negative electrode was obtained in the same manner as in
Example 2-40, except for using mixed powder obtained by mixing the
tin-copper alloy particles shown in Table 2-3 below and silver or
copper particles in the mixing ratio shown in the Table.
EXAMPLE 2-49
[0147] Tin particles having particle sizes of 0.1 to 10 .mu.m were
electroless plated in a plating bath having the tin particles
suspended therein and containing copper sulfate and Rochelle salt
to obtain copper-coated tin particles. The concentrations of the
tin particles, copper sulfate, and Rochelle salt in the plating
bath were 500 g/l, 7.5 g/l, and 85 g/l, respectively. The plating
bath had a pH of 12.5 and a temperature of 25.degree. C.
Formaldehyde was used as a reducing agent and its concentration is
22 cc/l. A negative electrode was obtained in the otherwise same
manner as in Example 2-1.
EXAMPLES 2-50 TO 2-53
[0148] A negative electrode was obtained in the same manner as in
Example 2-41, except for using copper-coated tin particles
(Examples 2-50 and 2-51) or nickel-coated tin particles (Examples
2-52 and 2-53) each having the composition shown in Table 2-3 and
obtained by electroless plating.
COMPARATIVE EXAMPLE 2-1
[0149] A negative electrode was obtained in the same manner as in
Comparative Example 1-1, except for replacing the graphite powder
with tin particles having a particle size of 5 .mu.m.
[0150] Performance Evaluation:
[0151] Nonaqueous secondary batteries were assembled using each of
the negative electrodes prepared in Examples and Comparative
Examples in the same manner as described supra. The battery was
evaluated in irreversible capacity, capacity density per unit
volume when charged, charge/discharge efficiency in the 10th cycle,
and capacity retention in the 50th cycle in accordance with the
methods described supra. The results of evaluation are shown in
Tables 2-1 to 2-3.
8 TABLE 2-1 Negative electrode Active Material Charge/ Capacity
Coating Layer Particle Content in Material Irreversible Capacity
Discharge Retention at Example Thickness Plating Size Coating Layer
(Kind of Active Capacity Density Efficiency at 10th 50th Cycle No.
(.mu.m) Material D.sub.50 (.mu.m) (wt %) Material)*.sup.1 (%)
(mAh/g) Cycle (%) (%) 2-1 20 Cu 2 70 pure Sn 9 950 99.6 99 2-2 20
Ni 2 70 pure Sn 14 910 99.3 96 2-3 20 Co 2 70 pure Sn 13 900 99.1
96 2-4 20 Cu 2 70 Sn60/Cu40(alloy) 6 450 99.9 98 2-5 20 Cu 2 70
Sn75/Cu25(alloy) 6 650 99.9 94 2-6 20 Cu 2 70 Sn90/Cu10(alloy) 6
850 99.9 99 2-7 20 Cu 2 70 Sn80/Ni20(alloy) 6 500 99.9 92 2-8 20 Cu
2 70 Sn95/Ni15(alloy) 6 850 99.9 99 2-9 20 Cu 2 70 Sn80/Cu10/Ni10 6
750 99.9 97 (alloy) 2-10 20 Cu 2 70 Sn85/Cu10/Ni5 6 800 99.9 97
(alloy) 2-11 20 Cu 2 70 Sn80/Cu19.5/A10.5 8 870 99.9 92 (alloy)
2-12 20 Cu 2 70 Sn80/Cu19.5/Ni0.5 9 810 99.9 96 (alloy) 2-13 20 Cu
2 70 Sn80/Cu19.5/Co0.5 9 800 99.9 98 (alloy) 2-14 20 Cu 2 70
Sn80/Cu19.5/Ti0.5 8 820 99.9 94 (alloy) 2-15 20 Cu 2 70
Sn80/Cu19.5/La0.5 9 860 99.9 97 (alloy) 2-16 20 Cu 2 70
Sn80/Cu19.5/Ce0.5 9 860 99.9 98 (alloy) *.sup.1Figures indicate %
by weight.
[0152]
9 TABLE 2-2 Negative electrode Active Material Charge/ Coating
Layer Particle Content in Material Irreversible Capacity Discharge
Capacity Example Thickness Plating Size Coating Layer (Kind of
Active Capacity Density Efficiency at 10th Retention at No. (.mu.m)
Material D.sub.50 (.mu.m) (wt %) Material)*.sup.1 (%) (mAh/g) Cycle
(%) 50th Cycle (%) 2-17 20 Cu 2 70 Sn90+Cu10 7 880 99.9 97 (mixed
powder) 2-18 20 Cu 0.5 70 Sn90+Cu10 8 880 99.9 97 (mixed powder)
2-19 20 Cu 10 70 Sn90+Cu10 7 880 99.9 92 (mixed powder) 2-20 20 Cu
0.2 70 Sn90+Cu10 7 880 99.10 93 (mixed powder) 2-21 20 Cu 1 70
Sn90+Cu10 7 880 99.11 97 (mixed powder) 2-22 20 Cu 5 70 Sn90+Cu10 7
880 99.12 95 (mixed powder) 2-23 20 Cu 20 70 Sn90+Cu10 7 880 99.13
98 (mixed powder) 2-24 20 Cu 2 30 Sn90+Cu10 7 880 99.9 94 (mixed
powder) 2-25 20 Cu 2 50 Sn90+Cu10 7 880 99.9 99 (mixed powder) 2-26
5 Cu 1 70 Sn90+Cu10 8 880 99.9 99 (mixed powder) 2-27 10 Cu 2 70
Sn90+Cu10 7 880 99.9 96 (mixed powder) 2-28 15 Cu 2 70 Sn90+Cu10 7
880 99.9 97 (mixed powder) 2-29 20 Cu 2 70 Sn60+Cu40 8 590 99.9 98
(mixed powder) 2-30 20 Cu 2 70 Sn75+Cu25 8 740 99.9 98 (mixed
powder) 2-31 20 Cu 2 70 Sn95+Cu5 8 890 99.9 92 (mixed powder) 2-32
20 Cu 2 70 Sn99+Ag1 5 900 99.9 93 (mixed powder) 2-33 20 Cu 2 70
Sn95+Ag5 5 890 99.9 96 (mixed powder) 2-34 20 Cu 2 70 Sn90+Ag10 5
870 99.9 94 (mixed powder) 2-35 20 Cu 2 70 Sn80+Ag20 5 850 99.9 92
(mixed powder) 2-36 20 Cu 2 60 Sn90+Si10 7 980 99.9 92 (mixed
powder) 2-37 20 Cu 2 60 Sn50+Si50 7 2500 99.9 93 (mixed powder)
2-38 20 Cu 2 60 Sn50+Si40+Cu10 7 2200 99.9 96 (mixed powder) 2-39
20 Cu 2 60 Sn50+Si40+Cu10 7 2200 99.9 97 (mixed powder)
*.sup.1Figures indicate % by weight.
[0153]
10 TABLE 2-3 Negative electrode Active Material Charge/ Coating
Layer Particle Content in Material Irreversible Capacity Discharge
Capacity Example Thickness Plating Size Coating Layer (Kind of
Active Capacity Density Efficiency at 10th Retention at No. (.mu.m)
Material D.sub.50 (.mu.m) (wt %) Material)*.sup.1 (%) (mAh/g) Cycle
(%) 50th Cycle (%) 2-40 20 Cu 2 70 [Sn75/Cu25]99+Ag1 5 690 99.9 94
(mixed powder) 2-41 20 Cu 2 70 [Sn75/Cu25]95+Ag5 5 680 99.9 98
(mixed powder) 2-42 20 Cu 2 70 [Sn75/Cu25]90+Ag10 5 670 99.9 99
(mixed powder) 2-43 20 Cu 2 70 [Sn75/Cu25]80+Ag20 5 640 99.9 99
(mixed powder) 2-44 20 Cu 2 70 [Sn75/Cu25]99+Cu1 5 680 99.9 93
(mixed powder) 2-45 20 Cu 2 70 [Sn75/Cu25]95+Cu5 5 670 99.9 96
(mixed powder) 2-46 20 Cu 2 70 [Sn75/Cu25]90+Cu10 5 620 99.9 97
(mixed powder) 2-47 20 Cu 2 70 [Sn75/Cu25]80+Cu20 5 600 99.9 93
(mixed powder) 2-48 20 Cu 2 70 [Sn75/Cu25]60+Cu40 5 450 99.9 94
(mixed powder) 2-49 20 Cu 2 70 Sn80/Cu20 5 790 99.9 99 (electroless
plating) 2-50 20 Cu 2 70 Sn95/Cu5 5 910 99.9 98 (electroless
Plating) 2-51 20 Cu 2 70 Sn99/Cu1 6 930 99.9 96 (electroless
Plating) 2-52 20 Cu 2 70 Sn99/Ni1 11 900 99.9 96 (electroless
plating) 2-53 20 Cu 2 70 Sn99.5/Ni0.05 7 930 99.9 95 (electroless
plating) Comp. no plating 5 80 pure Sn 20 950 95.0 7 Ex. 2-1
*.sup.1Figures indicate % by weight.
[0154] As is apparent from the results shown in Tables 2-1 to 2-3,
the secondary batteries using the negative electrodes obtained in
Examples retain the same levels of irreversible capacity,
charge/discharge efficiency and capacity retention as the
comparative secondary battery using the comparative negative
electrode and also have extremely higher capacity density than the
comparative battery.
EXAMPLE 3-1
[0155]
11 (1) Preparation of plating bath Silicon particles (particle size
D.sub.50: 5 .mu.m) 600 g/l Copper sulfate 50 g/l Sulfuric acid 70
g/l Cresolsulfonic acid 70 g/l Gelatin 2 g/l .beta.-Naphthol 1.5
g/l
[0156] (2) Dispersion Plating
[0157] A 30 .mu.m thick copper foil was immersed in the plating
bath, in which the silicon particles were suspended, at 20.degree.
C. and electroplated at a current density of 10 A/dm.sup.2. There
was thus formed an active material layer having silicon particles
uniformly dispersed therein and a surface coating layer covering
the active material layer. As a result of electron microscopic
observation, the active material structure containing the active
material layer and the surface coating layer was found to be 35
.mu.m. Chemical analysis revealed that the silicon powder content
in the active material structure was 30%.
EXAMPLE 3-2
[0158] (1) Preparation of Slurry
[0159] A slurry having the following composition was prepared.
12 Silicon particles (D.sub.50: 5 .mu.m) 16% Acetylene black
(particle size: 0.1 .mu.m) 2% Binder (polyvinylidene fluoride) 2%
Diluting solvent (N-methylpyrrolidone) 80%
[0160] (2) Formation of Active Material Layer
[0161] The above prepared slurry was applied to a 30 .mu.m thick
copper foil and dried to form an active material layer having a dry
thickness of 60 .mu.m.
[0162] (3) Formation of Surface Coating Layer
[0163] The copper foil having the active material layer formed
thereon was immersed in a plating bath having the following
composition to carry out electroplating.
13 Copper 50 g/l Sulfuric acid 60 g/l Bath temperature 40.degree.
C.
[0164] After forming the surface coating layer, the copper foil was
taken out of the plating bath, and both the active material layer
and the surface coating layer were densified by roll pressing. The
thickness of the thus formed active material structure was found to
be 30 .mu.m as a result of electron microscopic observation. As a
result of chemical analysis, the amounts of the silicon particles
and acetylene black in the active material structure were found to
be 35% and 5%, respectively.
EXAMPLES 3-3 AND 3-4
[0165] A negative electrode was obtained in the same manner as in
Example 3-2, except for forming the coating layer of nickel
(Example 3-3) or cobalt (Example 3-4).
EXAMPLE 3-5
[0166] A molten metal at 1400.degree. C. containing 50% of silicon
and 50% of copper was cast into a copper-made mold and quenched to
obtain an ingot of a silicon-copper alloy. The ingot was pulverized
and sieved to obtain alloy particles having particle sizes of 0.1
to 10 .mu.m. A negative electrode was obtained in the same manner
as in Example 3-2, except for using the resulting alloy
particles.
EXAMPLES 3-6 TO 3-8
[0167] A negative electrode was obtained in the same manner as in
Example 3-5, except for using silicon-copper alloy particles of the
composition shown in Table 3-1 below.
EXAMPLES 3-9 TO 3-11
[0168] A negative electrode was obtained in the same manner as in
Example 3-5, except for using silicon-nickel alloy particles of the
composition shown in Table 3-1.
EXAMPLES 3-12 AND 3-13
[0169] A negative electrode was obtained in the same manner as in
Example 3-5, except for using silicon-copper-nickel alloy particles
of the composition shown in Table 3-1.
EXAMPLES 3-14
[0170] Silicon particles (particle size: 100 .mu.m) 80% and copper
particles (particle size: 30 .mu.m) 20% were mixed and pulverized
simultaneously in an attritor to obtain uniformly mixed
silicon/copper powder having particle sizes of 2 to 10 .mu.m
(D.sub.50: 5 .mu.m). A negative electrode was obtained in the same
manner as in Example 3-2, except for using the resulting mixed
powder.
EXAMPLES 3-15 TO 3-26
[0171] A negative electrode was obtained in the same manner as in
Example 3-14, except for using silicon-copper mixed powder of the
composition and particle size shown in Table 3-2 below and changing
the thickness of the active material structure as shown in the
Table.
EXAMPLE 3-27
[0172] Silicon particles having particle sizes of 0.2 to 8 .mu.m
were electroless plated with copper in a plating bath containing
copper sulfate and Rochelle salt, in which the silicon particles
were suspended, to obtain copper-coated silicon particles. The
concentrations of the silicon particles, copper sulfate, and
Rochelle salt in the plating bath were 500 g/l, 7.5 g/l, and 85
g/l, respectively. The plating bath had a pH of 12.5 and a
temperature of 25.degree. C. As a reducing agent formaldehyde was
used in a concentration of 22 cc/l. A negative electrode was
obtained in the otherwise same manner as in Example 3-2.
EXAMPLES 3-28 TO 3-31
[0173] A negative electrode was obtained in the same manner as in
Example 3-18, except for using copper-coated silicon particles
(Examples 3-28 and 3-29) or nickel-coated silicon particles
(Examples 3-30 and 3-31) each having the composition shown in Table
3-2 and obtained by electroless plating.
EXAMPLES 3-32 TO 3-37
[0174] A negative electrode was obtained in the same manner as in
Example 3-5, except for using silicon-based ternary alloy particles
having the composition shown in Table 3-3 below and prepared by a
quenching process.
EXAMPLE 3-38
[0175] Silicon particles (particle size: 100 .mu.m) 20% and
graphite particles (D.sub.50: 20 .mu.m) 80% were mixed and
pulverized simultaneously by mechanical milling to obtain uniformly
mixed silicon/graphite powder having a particle size (D.sub.50) of
0.5 .mu.m. A negative electrode was obtained in the same manner as
in Example 3-2, except for using the resulting mixed powder and
forming the surface coating layer of nickel.
EXAMPLES 3-39 TO 3-42
[0176] A negative electrode was obtained in the same manner as in
Example 3-38, except for using mixed powder of the composition
shown in Table 3-3.
EXAMPLE 3-43
[0177] A negative electrode was obtained in the same manner as in
Example 3-5, except for using alloy powder composed of silicon 80%,
copper 19%, and lithium 1% and forming the surface coating layer of
nickel.
[0178] Evaluation of Performance:
[0179] A nonaqueous secondary battery was assembled using each of
the negative electrodes prepared in Examples and Comparative
Examples in the same manner as described supra. The battery was
evaluated in irreversible capacity, capacity density per unit
volume when charged, charge/discharge efficiency in the 10th cycle,
and capacity retention in the 50th cycle in accordance with the
methods described supra. The results of evaluation are shown in
Tables 3-1 through 3-3.
14 TABLE 3-1 Active Material Structure Surface Coating Charge/
Layer Si-based Active Material Layer Discharge Capacity Thick-
Thick- Thick- Size Content in Irreversible Capacity Efficiency at
Retention in Example ness ness ness D.sub.50 Structure Capacity
Density 10th Cycle 50th Cycle No. (.mu.m) (.mu.m) Material (.mu.m)
(.mu.m) (wt %) Material (%) (mAh/g) (%) (%) 3-1 30 5 Cu 25 5 30
pure Si 12 4010 99.6 95 3-2 30 5 Cu 25 5 35 pure Si 9 4010 99.7 95
3-3 30 5 Ni 25 5 35 pure Si 14 3700 99.5 96 3-4 30 5 Co 25 5 35
pure Si 13 3600 99.5 96 3-5 30 5 Cu 25 5 45 Si50/Cu50 5 2000 99.7
99 3-6 30 5 Cu 25 5 40 Si60/Cu40 6 2400 99.7 99 3-7 30 5 Cu 25 5 40
Si70/Cu30 6 2800 99.7 99 3-8 30 5 Cu 25 5 38 Si80/Cu20 6 3200 99.7
99 3-9 30 5 Cu 25 5 45 Si50/Ni50 6 500 99.7 99 3-10 30 5 Cu 25 5 40
Si65/Ni35 6 800 99.7 99 3-11 30 5 Cu 25 5 35 Si80/Ni20 6 1300 99.7
99 3-12 30 5 Cu 25 5 40 Si60/Cu20/Ni20 6 2000 99.7 99 3-13 30 5 Cu
25 5 40 Si70/Cu15/Ni15 6 2300 99.7 99 3-14 30 5 Cu 25 5 38
Si80/Cu20 7 3200 99.7 99 3-15 30 5 Cu 25 0.8 38 Si80/Cu20 8 3200
99.9 100 3-16 30 5 Cu 25 10 38 Si80/Cu20 7 3200 99.7 96 3-17 30 5
Cu 25 20 38 Si80/Cu20 7 3200 99.6 95 3-18 30 5 Cu 25 5 10 Si80/Cu20
7 3200 99.8 99 3-19 30 5 Cu 25 5 20 Si80/Cu20 7 3200 99.7 99 3-20 5
1 Cu 4 1 38 Si80/Cu20 8 3200 99.8 99 3-21 10 2 Cu 8 5 38 Si80/Cu20
7 3200 99.7 99 3-22 15 3 Cu 12 5 38 Si80/Cu20 7 3200 99.7 99 3-23
20 4 Cu 16 5 38 Si80/Cu20 7 3200 99.7 99 3-24 30 5 Cu 25 5 37
Si90/Cu10 8 3600 99.7 98 3-25 30 5 Cu 25 5 36 Si95/Cu5 8 3600 99.7
98 3-26 30 5 Cu 25 5 36 Si95/Ni5 9 3500 99.7 98 3-27 30 5 Cu 25 5
35 Si80/Cu20 5 3200 99.7 99 3-28 30 5 Cu 25 5 35 Si80/Cu20 5 3800
99.7 99 3-29 30 5 Cu 25 5 35 Si99/Cu1 6 3900 99.7 99 3-30 30 5 Cu
25 5 35 Si99/Ni1 11 3900 99.7 99 3-31 30 5 Cu 25 5 35 Si99.5/Ni0.05
7 4000 99.7 99 3-32 30 5 Cu 25 5 38 Si80/Cu19.5/A10.5 8 3100 99.7
99 3-33 30 5 Cu 25 5 38 Si80/Cu19.5/Ni0.5 9 3100 99.7 99 3-34 30 5
Cu 25 5 38 Si80/Cu19.5/Co0.5 9 3000 99.7 99 3-35 30 5 Cu 25 5 38
Si80/Cu19.5/Ti0.5 8 3100 99.7 99 3-36 30 5 Cu 25 5 38
Si80/Cu19.5/La0.5 9 3060 99.7 99 3-37 30 5 Cu 25 5 38
Si80/Cu19.5/Ce0.5 9 3070 99.7 99 3-38 30 5 Ni 25 0.5 35 Si20/C80 6
1040 99.8 100 3-39 30 5 Ni 25 0.5 35 Si40/C60 6 1740 99.8 97 3-40
30 5 Ni 25 0.5 35 Si40/C40/Cu20 6 1600 99.8 97 3-41 30 5 Ni 25 0.5
35 Si60/C40 7 2510 99.7 94 3-42 30 5 Ni 25 0.5 35 Si80/C20 8 3230
99.7 92 3-43 30 5 Ni 25 5 38 Si80/Cu19/Li1 0 3200 100 100
[0180] As is apparent from the results shown in Tables 3-1 to 3-3,
the secondary batteries using the negative electrodes obtained in
Examples retain the same levels of irreversible capacity,
charge/discharge efficiency and capacity retention as the
comparative secondary batteries using the comparative negative
electrodes and also have extremely higher capacity density than the
comparative batteries.
INDUSTRIAL APPLICABILITY
[0181] The negative electrode for nonaqueous secondary batteries
according to the present invention provides a secondary battery
having higher energy density than conventional negative electrodes.
The negative electrode for nonaqueous secondary batteries according
to the present invention prevents the active material from falling
off the current collector so that the current collecting
performance of the active material is maintained against repetition
of charge/discharge cycles. Furthermore, the secondary battery
using the negative electrode is less susceptible to deterioration
against repetition of charge/discharge cycles and therefore enjoys
a greatly extended service life and an increased charge/discharge
efficiency.
* * * * *