U.S. patent application number 12/644626 was filed with the patent office on 2010-07-01 for method of manufacturing lithium-ion secondary battery positive electrode, method of manufacturing lithium-ion secondary battery, lithium-ion secondary battery positive electrode, and lithium-ion secondary battery.
This patent application is currently assigned to TDK CORPORATION. Invention is credited to Kiyonori HINOKI, Masayoshi HIRANO, Kazuo KATAI, Yousuke MIYAKI.
Application Number | 20100167125 12/644626 |
Document ID | / |
Family ID | 42285346 |
Filed Date | 2010-07-01 |
United States Patent
Application |
20100167125 |
Kind Code |
A1 |
MIYAKI; Yousuke ; et
al. |
July 1, 2010 |
METHOD OF MANUFACTURING LITHIUM-ION SECONDARY BATTERY POSITIVE
ELECTRODE, METHOD OF MANUFACTURING LITHIUM-ION SECONDARY BATTERY,
LITHIUM-ION SECONDARY BATTERY POSITIVE ELECTRODE, AND LITHIUM-ION
SECONDARY BATTERY
Abstract
A method of manufacturing a lithium-ion secondary battery
positive electrode comprises a coating material preparing step of
preparing a positive electrode active material layer forming
coating material by mixing a positive electrode active material, a
binder, a conductive auxiliary, an organic solvent, and water; and
an active material layer forming step of forming a positive
electrode active material layer on a current collector by using the
positive electrode active material layer forming coating material.
The binder is polyvinylidene fluoride produced by emulsion
polymerization. The positive electrode active material layer
forming coating material is prepared in the coating material
preparing step such that the amount of water added (% by mass)
based on the total amount of the organic solvent and water and the
pH of the positive electrode active material satisfy the following
expression (1): 48.ltoreq.[the amount of water
added+(4.25.times.the pH of the positive electrode active
material)].ltoreq.52 (1)
Inventors: |
MIYAKI; Yousuke; (Tokyo,
JP) ; KATAI; Kazuo; (Tokyo, JP) ; HIRANO;
Masayoshi; (Tokyo, JP) ; HINOKI; Kiyonori;
(Tokyo, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 320850
ALEXANDRIA
VA
22320-4850
US
|
Assignee: |
TDK CORPORATION
TOKYO
JP
|
Family ID: |
42285346 |
Appl. No.: |
12/644626 |
Filed: |
December 22, 2009 |
Current U.S.
Class: |
429/217 ;
427/58 |
Current CPC
Class: |
H01M 10/0525 20130101;
H01M 4/366 20130101; H01M 4/131 20130101; H01M 4/139 20130101; H01M
4/13 20130101; H01M 4/0404 20130101; H01M 4/1391 20130101; H01M
4/624 20130101; Y02E 60/10 20130101; H01M 4/621 20130101; Y10T
29/49115 20150115; H01M 4/623 20130101 |
Class at
Publication: |
429/217 ;
427/58 |
International
Class: |
H01M 4/62 20060101
H01M004/62; H01M 4/04 20060101 H01M004/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 26, 2008 |
JP |
2008-333371 |
Claims
1. A method of manufacturing a lithium-ion secondary battery
positive electrode, the method comprising: a coating material
preparing step of preparing a positive electrode active material
layer forming coating material by mixing at least a positive
electrode active material, a binder, a conductive auxiliary, an
organic solvent, and water; and an active material layer forming
step of forming a positive electrode active material layer on a
current collector by using the positive electrode active material
layer forming coating material; wherein the binder is
polyvinylidene fluoride produced by emulsion polymerization; and
wherein the positive electrode active material layer forming
coating material is prepared in the coating material preparing step
such that the amount of water added (% by mass) based on the total
amount of the organic solvent and water and the pH of the positive
electrode active material satisfy the following expression (1):
48.ltoreq.[the amount of water added+(4.25.times.the pH of the
positive electrode active material)].ltoreq.52 (1)
2. A method of manufacturing a lithium-ion secondary battery
positive electrode according to claim 1, wherein the amount of
water added in the coating material preparing step is 4 to 10% by
mass based on the total amount of the organic solvent and
water.
3. A method of manufacturing a lithium-ion secondary battery
positive electrode according to claim 1, wherein the organic
solvent is N-methyl-2-pyrrolidone.
4. A method of manufacturing a lithium-ion secondary battery having
a step of making a positive electrode by the method of
manufacturing a lithium-ion secondary battery positive electrode
according to claim 1.
5. A lithium-ion secondary battery positive electrode comprising a
current collector and a positive electrode active material layer
formed on the current collector; wherein the positive electrode
active material layer contains a positive electrode active
material, a binder, and a conductive auxiliary; wherein the binder
is polyvinylidene fluoride produced by emulsion polymerization; and
wherein at least a part of a surface of the positive electrode
active material in the positive electrode active material layer is
covered with a coating layer formed by dispersing the conductive
auxiliary into the binder.
6. A lithium-ion secondary battery positive electrode according to
claim 5, wherein a plurality of pieces of the positive electrode
active material are connected to each other through the coating
layer in the positive electrode active material layer.
7. A lithium-ion secondary battery positive electrode made by the
method of manufacturing a lithium-ion secondary battery positive
electrode according to claim 1.
8. A lithium-ion secondary battery comprising the lithium-ion
secondary battery positive electrode according to claim 5.
9. A lithium-ion secondary battery comprising the lithium-ion
secondary battery positive electrode according to claim 7.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of manufacturing a
lithium-ion secondary battery positive electrode, a method of
manufacturing a lithium-ion secondary battery, a lithium-ion
secondary battery positive electrode, and a lithium-ion secondary
battery.
[0003] 2. Related Background Art
[0004] It is desirable for lithium-ion secondary batteries to
secure homogeneity within an electrode in order to improve the
electrode capacity, cycle characteristics, and reliability. The
homogeneity within an electrode seems to be secured when a coating
material for forming the electrode has favorable dispersibility.
When the productivity of electrodes is taken into consideration, it
will be desirable if the dispersibility of the coating material is
not deteriorated with time by aggregations of components contained
therein and the like.
[0005] Since the deterioration in dispersibility of a coating
material with time begins immediately after making the coating
material, a method of keeping a network structure in the coating
material by increasing the amount of binders contained in the
coating material has been under consideration as a measure for
suppressing the deterioration with time (see, for example, Japanese
Patent Application Laid-Open No. 2000-021408). This increases the
amount of binders which do not contribute to electric
characteristics, whereby problems in cell characteristics such as
decreases in capacity and increases in resistance value may occur.
From the viewpoint of keeping a network structure, increasing the
amount of conductive carbon may be considered. This is effective in
lowering the resistance value, but may decrease the amount of
active materials contained, thus reducing the capacity and making
the coating film structure fragile, thereby failing to improve the
reliability.
[0006] As the binders added to the coating material, those made by
suspension polymerization have been in wide use (see, for example,
Japanese Patent Application Laid-Open Nos. 2004-087325,
2005-310747, 2008-045096, and 2008-088330 and Japanese Domestic
Republication of PCT International Application Laid-Open No.
2005-116092).
SUMMARY OF THE INVENTION
[0007] As in the foregoing, it has conventionally been difficult to
inhibit the dispersibility of produced coating materials from
deteriorating with time without worsening characteristics of
lithium-ion secondary batteries.
[0008] In view of the problems of the prior art mentioned above, it
is an object of the present invention to provide a method of
manufacturing a lithium-ion secondary battery positive electrode
which can manufacture a positive electrode capable of improving the
cycle characteristic of the lithium-ion secondary battery, while
being excellent in productivity and able to inhibit the
dispersibility of a coating material for forming the positive
electrode from deteriorating with time. It is another object of the
present invention to provide a method of manufacturing a
lithium-ion secondary battery which can manufacture a lithium-ion
secondary battery having an improved cycle characteristic, while
being excellent in productivity and able to inhibit the
dispersibility of a coating material for forming the positive
electrode from deteriorating with time. It is a further object of
the present invention to provide a lithium-ion secondary battery
positive electrode which can improve the cycle characteristic of a
lithium-ion secondary battery and a lithium-ion secondary battery
using the same.
[0009] The inventors conducted diligent studies in order to achieve
the above-mentioned objects and, as a result, have found that the
easiness for the coating material to aggregate also depends on the
type of polymerization of binders in use, so that binders made by
suspension polymerization cause alkali components of active
materials in use to generate a hydrogen fluoride elimination
reaction with time, thereby polymerizing the binders together and
gelling them. The inventors have therefore found it important to
choose binders for use and manage the pH of active materials for
use. The inventors have further found that the deterioration of
dispersibility in a coating material with time can be suppressed
more by adding a predetermined amount of water to the coating
material rather than by eliminating moisture from within the
coating material so as to keep it from becoming alkaline, and the
cycle characteristic of the lithium-ion secondary battery can also
be improved thereby.
[0010] Hence, the present invention provides a method of
manufacturing a lithium-ion secondary battery positive electrode,
the method comprising a coating material preparing step of
preparing a positive electrode active material layer forming
coating material by mixing at least a positive electrode active
material, a binder, a conductive auxiliary, an organic solvent, and
water; and an active material layer forming step of forming a
positive electrode active material layer on a current collector by
using the positive electrode active material layer forming coating
material; wherein the binder is polyvinylidene fluoride produced by
emulsion polymerization; and wherein the positive electrode active
material layer forming coating material is prepared in the coating
material preparing step such that the amount of water added (% by
mass) based on the total amount of the organic solvent and water
and the pH of the positive electrode active material satisfy the
following expression (1):
48.ltoreq.[the amount of water added+(4.25.times.the pH of the
positive electrode active material)].ltoreq.52 (1)
[0011] By using polyvinylidene fluoride (PVDF) produced by emulsion
polymerization as a binder and adjusting the amount of water added
in the coating material and the pH of the positive electrode active
material such as to satisfy the above-mentioned expression (1), the
method of manufacturing a lithium-ion secondary battery positive
electrode in accordance with the present invention can yield a
positive electrode capable of improving the cycle characteristic of
the lithium-ion secondary battery, while being excellent in
productivity and able to inhibit the dispersibility of the coating
material for forming the positive electrode from deteriorating with
time.
[0012] Though the reason why the effect mentioned above is
exhibited by adding a predetermined amount of water to the coating
material is not definitely clear, the inventors presume as follows.
It is inferred that, while a network of the binder dispersed in the
coating material supports the positive electrode active material
and keeps this structure when there is no water added thereto, PVDF
and the conductive auxiliary increase their affinity to each other
with time, so as to dissolve the network of PVDF, thus aggregating
the coating material, whereby the viscosity changes when aggregates
are separated.
[0013] When water is added to the coating material such as to
satisfy the above-mentioned expression (1), on the other hand, it
is speculated that water molecules in the coating material are
hydrogen-bonded to the main chain of PVDF, thus making it possible
to support the positive electrode active material without breaking
the network structure formed by the binder, whereby no aggregation
occurs with time. This hydrogen bond has such a weak binding
strength as to be broken by a force as small as that of stirring
the coating material. Therefore, the coating material keeps its
fluidity when in use. When the fluidity is lost, however, hydrogen
bonds occur again, thereby making it possible to keep the structure
in the coating material and inhibit the viscosity from changing.
Since the coating material is restrained from aggregating, the
conductive auxiliary exists in the binder while being fully
dispersed therein when the positive electrode active material layer
is formed, whereby the cycle characteristic of the lithium-ion
secondary battery can be improved.
[0014] When such a large amount of water is added to the coating
material as to dissatisfy the above-mentioned expression (1), the
alkali components in the positive electrode active material makes
the coating material alkaline, so that the main chain of PVDF seems
to generate a hydrogen fluoride elimination reaction, thereby
producing a double bond. This double bond is so unstable as to
generate a chemical bond with its adjacent double bond, thereby
gelling the coating material.
[0015] Preferably, the amount of water added in the coating
material preparing step in the method of manufacturing a
lithium-ion secondary battery positive electrode in accordance with
the present invention is 4 to 10% by mass based on the total amount
of the organic solvent and water. The amount of water added falling
within the range mentioned above can more fully inhibit the
dispersibility of the positive electrode active material layer
forming coating material from deteriorating with time and further
improve the cycle characteristic of the lithium-ion secondary
battery.
[0016] Preferably, in the method of manufacturing a lithium-ion
secondary battery positive electrode in accordance with the present
invention, the organic solvent is N-methyl-2-pyrrolidone. Using
N-methyl-2-pyrrolidone as the organic solvent can further improve
the cycle characteristic of the lithium-ion secondary battery.
[0017] In another aspect, the present invention provides a method
of manufacturing a lithium-ion secondary battery having a step of
making a positive electrode by the above-mentioned method of
manufacturing a lithium-ion secondary battery positive
electrode.
[0018] This method of manufacturing a lithium-ion secondary battery
has a step of making a positive electrode by the above-mentioned
method of manufacturing a lithium-ion secondary battery positive
electrode in accordance with the present invention and thus can
manufacture a lithium-ion secondary battery having an improved
cycle characteristic, while being excellent in productivity and
able to inhibit the dispersibility of the coating material for
forming the positive electrode from deteriorating with time.
[0019] In still another aspect, the present invention provides a
lithium-ion secondary battery positive electrode comprising a
current collector and a positive electrode active material layer
formed on the current collector; wherein the positive electrode
active material layer contains a positive electrode active
material, a binder, and a conductive auxiliary; wherein the binder
is polyvinylidene fluoride produced by emulsion polymerization; and
wherein at least a part of a surface of the positive electrode
active material in the positive electrode active material layer is
covered with a coating layer formed by dispersing the conductive
auxiliary into the binder.
[0020] This lithium-ion secondary battery positive electrode has
the structure mentioned above and thus can improve the cycle
characteristic of the lithium-ion secondary battery.
[0021] Preferably, in the lithium-ion secondary battery positive
electrode of the present invention, a plurality of pieces of the
positive electrode active material are connected to each other
through the coating layer in the positive electrode active material
layer. When the positive electrode active material layer has the
structure mentioned above, the cycle characteristic of the
lithium-ion secondary battery can further be improved.
[0022] In still another aspect, the present invention provides a
lithium-ion secondary battery positive electrode made by the method
of manufacturing a lithium-ion secondary battery positive electrode
in accordance with the present invention.
[0023] Since this lithium-ion secondary battery positive electrode
is made by the method of manufacturing a lithium-ion secondary
battery positive electrode in accordance with the present
invention, the conductive auxiliary exists in the positive
electrode active material layer while being fully dispersed in the
binder without aggregating, whereby the cycle characteristic of the
lithium-ion secondary battery can be improved.
[0024] In a further aspect, the present invention provides a
lithium-ion secondary battery comprising the lithium-ion secondary
battery positive electrode of the present invention.
[0025] This lithium-ion secondary battery comprises the lithium-ion
secondary battery positive electrode of the present invention and
thus can yield an excellent cycle characteristic.
[0026] As in the foregoing, the present invention can provide a
method of manufacturing a lithium-ion secondary battery positive
electrode which can manufacture a positive electrode capable of
improving the cycle characteristic of a lithium-ion secondary
battery, while being excellent in productivity and able to inhibit
the dispersibility of a coating material for forming the positive
electrode from deteriorating with time. The present invention can
also provide a method of manufacturing a lithium-ion secondary
battery which can manufacture a lithium-ion secondary battery
having an improved cycle characteristic, while being excellent in
productivity and able to inhibit the dispersibility of a coating
material for forming the positive electrode from deteriorating with
time. The present invention can further provide a lithium-ion
secondary battery positive electrode which can improve the cycle
characteristic of a lithium-ion secondary battery and a lithium-ion
secondary battery using the same.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a front view illustrating a preferred embodiment
of the lithium-ion secondary battery in accordance with the present
invention;
[0028] FIG. 2 is a schematic sectional view of the lithium-ion
secondary battery taken along the line X-X of FIG. 1;
[0029] FIG. 3 is a schematic sectional view illustrating an example
of basic structures of a negative electrode in the lithium-ion
secondary battery;
[0030] FIG. 4 is a schematic sectional view illustrating an example
of basic structures of a positive electrode in the lithium-ion
secondary battery;
[0031] FIG. 5 is a schematic sectional view illustrating an inner
structure of a positive electrode active material layer;
[0032] FIG. 6 is an electron micrograph of a cross section of the
positive electrode produced by Example 1 (magnification:
1000.times.);
[0033] FIG. 7 is an electron micrograph of the cross section of the
positive electrode produced by Example 1 (magnification:
2000.times.);
[0034] FIG. 8 is an electron micrograph of the cross section of the
positive electrode produced by Example 1 (magnification:
5000.times.);
[0035] FIG. 9 is an electron micrograph of a cross section of the
positive electrode produced by Comparative Example 1
(magnification: 1000.times.); and
[0036] FIG. 10 is an electron micrograph of the cross section of
the positive electrode produced by Comparative Example 1
(magnification: 2000.times.).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] In the following, preferred embodiments of the present
invention will be explained in detail with reference to the
drawings as the case may be. In the drawings, the same or
equivalent parts will be referred to with the same signs, while
omitting their overlapping explanations. Positional relationships
such as upper, lower, left, and right are based on those
illustrated in the drawings, unless otherwise specified. Ratios of
dimensions in the drawings are not limited to those depicted.
[0038] Lithium-Ion Secondary Battery Positive Electrode and
Lithium-Ion Secondary Battery
[0039] FIG. 1 is a front view illustrating a preferred embodiment
of the lithium-ion secondary battery in accordance with the present
invention. FIG. 2 is a schematic sectional view of the lithium-ion
secondary battery 1 of FIG. 1 taken along the line X-X.
[0040] As illustrated in FIGS. 1 and 2, the lithium-ion secondary
battery 1 is mainly constituted by a power generating element 60
comprising a planar negative electrode 10 and a planar positive
electrode 20 which oppose each other and a planar separator 40
arranged between and adjacent to the negative electrode 10 and
positive electrode 20, an electrolytic solution (nonaqueous
electrolytic solution in this embodiment) containing lithium ions,
a case 50 accommodating them in a closed state, a negative
electrode lead 12 having one end part electrically connected to the
negative electrode 10 and the other end part projecting out of the
case 50, and a positive electrode lead 22 having one end part
electrically connected to the positive electrode 20 and the other
end part projecting out of the case 50.
[0041] In this specification, the "negative electrode", which is
based on the polarity of the battery at the time of discharging,
refers to an electrode which releases electrons by an oxidation
reaction at the time of discharging. The "positive electrode",
which is based on the polarity of the battery at the time of
discharging, refers to an electrode which receives electrons by a
reduction reaction at the time of discharging.
[0042] FIG. 3 is a schematic sectional view illustrating an example
of basic structures of the negative electrode 10 in the lithium-ion
secondary battery 1. FIG. 4 is a schematic sectional view
illustrating an example of basic structures of the positive
electrode 20 in the lithium-ion secondary battery 1.
[0043] As illustrated in FIG. 3, the negative electrode 10 is
constituted by a current collector 16 and a negative electrode
active material layer 18 formed on the current collector 16. As
illustrated in FIG. 4, the positive electrode 20 is constituted by
a current collector 26 and a positive electrode active material
layer 28 formed on the current collector 26.
[0044] The current collectors 16, 26 are not limited in particular
as long as they are good conductors which can sufficiently transfer
electric charges to the negative and positive electrode active
material layers 18, 28, respectively; known current collectors
employed in lithium-ion secondary batteries can be used. Examples
of the current collectors 16, 26 include metal foils made of copper
and aluminum, respectively.
[0045] The negative electrode active material layer 18 of the
negative electrode 10 is mainly constituted by a negative electrode
active material and a binder. Preferably, the negative electrode
active material layer 18 further contains a conductive
auxiliary.
[0046] The negative electrode active material is not limited in
particular as long as it allows occlusion and release of lithium
ions, desorption and insertion (intercalation) of lithium ions, or
doping and undoping of lithium ions to proceed reversibly; known
negative electrode active materials can be used. Examples of the
negative electrode active material include carbon materials such as
natural graphite, synthetic graphite, non-graphitizing carbon,
graphitizable carbon, and low-temperature-firable carbon; metals
such as Al, Si, and Sn which are combinable with lithium; amorphous
compounds mainly composed of oxides such as SiO, SiO.sub.2,
SiO.sub.x, and SnO.sub.2; lithium titanate
(Li.sub.4Ti.sub.5O.sub.12); and TiO.sub.2.
[0047] As the binder used in the negative electrode 10, known
binders can be employed without any restrictions in particular.
Examples include fluororesins such as polyvinylidene fluoride
(PVDF), polytetrafluoroethylene (PTFE),
tetrafluoroethylene/hexafluoropropylene copolymers (FEP),
tetrafluoroethylene/perfluoroalkylvinyl ether copolymers (PFA),
ethylene/tetrafluoroethylene copolymers (ETFE),
polychlorotrifluoroethylene (PCTFE),
ethylene/chlorotrifluoroethylene copolymers (ECTFE), and polyvinyl
fluoride (PVF). For more fully binding constituent materials such
as active material particles, the conductive auxiliary added when
necessary, and the like together and more fully binding these
constituent materials to the current collector, functional groups
such as carboxylic acids may be introduced in the binder.
[0048] Other examples of the binder include fluorine rubbers based
on vinylidene fluoride such as fluorine rubbers based on vinylidene
fluoride/hexafluoropropylene (VDF/HFP-based fluorine rubbers).
[0049] Still other examples of the binder include polyethylene,
polypropylene, polyethylene terephthalate, aromatic polyamides,
cellulose, styrene/butadiene rubber, isoprene rubber, butadiene
rubber, and ethylene/propylene rubber. Also employable are
thermoplastic elastomeric polymers such as
styrene/butadiene/styrene block copolymers and hydrogenated
derivatives thereof, styrene/ethylene/butadiene/styrene copolymers,
and styrene/isoprene/styrene block copolymers and hydrogenated
derivatives thereof. Further employable are syndiotactic
1,2-polybutadiene, ethylene/vinyl acetate copolymers,
propylene-.alpha.-olefin (having a carbon number of 2 to 12)
copolymers, and the like. Conductive polymers may also be used.
[0050] As the conductive auxiliary used when necessary, known
conductive auxiliaries can be employed without any restrictions in
particular. Examples include carbon blacks, carbon materials,
powders of metals such as copper, nickel, stainless steel, and
iron, mixtures of the carbon materials and metal powders, and
conductive oxides such as ITO.
[0051] The content of the negative electrode active material in the
negative electrode active material layer 18 is preferably 80 to 98%
by mass, more preferably 85 to 97% by mass, based on the total
amount of the negative electrode active material layer 18. When the
content of the negative electrode active material is less than 80%
by mass, the energy density tends to become lower than that in the
case where the content falls within the range mentioned above. When
the content of the negative electrode active material exceeds 98%
by mass, the bonding force tends to become weaker, thereby lowering
the cycle characteristic as compared with the case where the
content falls within the range mentioned above.
[0052] The positive electrode active material layer 28 of the
positive electrode 20 is mainly constituted by a positive electrode
active material, a binder, and a conductive auxiliary.
[0053] FIG. 5 is a schematic sectional view illustrating an inner
structure of the positive electrode active material layer 28. As
illustrated in FIG. 5, at least a part of a surface of the positive
electrode active material 2 in the positive electrode active
material layer 28 is covered with a coating layer 8 formed by
dispersing a conductive auxiliary 6 in a binder 4. Preferably, as
illustrated in FIG. 5, the positive electrode active material layer
28 has a structure in which a plurality of pieces of the positive
electrode active material 2 are connected together through the
coating layer 8.
[0054] The positive electrode active material 2 is not limited in
particular as long as it allows occlusion and release of lithium
ions, desorption and insertion (intercalation) of lithium ions, or
doping and undoping of lithium ions to proceed reversibly; known
positive electrode active materials can be used. Examples of the
positive electrode active material include lithium cobaltate
(LiCoO.sub.2), lithium nickelate (LiNiO.sub.2), lithium manganese
spinel (LiMn.sub.2O.sub.4), mixed metal oxides expressed by the
general formula of LiNi.sub.xCo.sub.yMn.sub.zM.sub.aO.sub.2 (where
x+y+z+a=1, 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1,
0.ltoreq.z.ltoreq.1, 0.ltoreq.a.ltoreq.1, and M is at least one
kind of element selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), a
lithium vanadium compound (LiV.sub.2O.sub.5), olivine-type
LiMPO.sub.4 (where M is at least one kind of element selected from
Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), and mixed metal
oxides such as lithium titanate (Li.sub.4Ti.sub.5O.sub.12).
[0055] As the binder 4, polyvinylidene fluoride produced by
emulsion polymerization is used. Binders other than polyvinylidene
fluoride produced by emulsion polymerization may be used therewith
as long as they fall within a range not inhibiting effects of the
present invention. As the other binders, those similar to the
binder used in the negative electrode 10 can be employed.
[0056] Known conductive auxiliaries can be used as the conductive
auxiliary 6 without being restricted in particular. Examples of the
conductive auxiliary 6 include carbon blacks, carbon materials,
powders of metals such as copper, nickel, stainless steel, and
iron, mixtures of the carbon materials and metal powders, and
conductive oxides such as ITO.
[0057] From the viewpoint of imparting a favorable cycle
characteristic to the lithium-ion secondary battery, the average
particle size of the conductive auxiliary 6 is preferably 30 to 100
nm, more preferably 35 to 60 nm.
[0058] The ratio of area occupied by the coating layer 8 in a cross
section of the positive electrode active material layer 28 is
preferably 10 to 60%, more preferably 30 to 50%. The ratio of area
occupied by the coating layer 8 is measured by electron microscopic
observation or the like. When the ratio of area is less than 10%,
the area in contact with the active material becomes less
sufficient than in the case where the ratio falls within the range
mentioned above, whereby conductivity tends to be distributed
unevenly. When the ratio of area exceeds 60%, the surface area
contributing to desorption and insertion of lithium ions tends to
become smaller than that in the case where the ratio falls within
the above-mentioned range, thereby lowering the capacity.
[0059] The content of the positive electrode active material 2 in
the positive electrode active material layer 28 is preferably 80 to
97% by mass, more preferably 85 to 96% by mass, based on the total
amount of the positive electrode active material layer 28. When the
content of the positive electrode active material 2 is less than
80% by mass, the energy density tends to become lower than that in
the case where the content falls within the range mentioned above.
When the content of the positive electrode active material 2
exceeds 97% by mass, the bonding force tends to become weaker,
thereby lowering the cycle characteristic as compared with the case
where the content falls within the above-mentioned range.
[0060] The content of the binder 4 in the positive electrode active
material layer 28 is preferably 2 to 10% by mass, more preferably 2
to 5% by mass, based on the total amount of the positive electrode
active material layer 28. When the content of the binder 4 is less
than 2% by mass, the coating film strength and the adhesion to the
current collector tend to become less sufficient, thereby lowering
the cycle characteristic as compared with the case where the
content falls within the range mentioned above. When the content of
the binder 4 exceeds 10% by mass, the internal resistance tends to
increase, thereby deteriorating characteristics as compared with
the case where the content falls within the above-mentioned
range.
[0061] The content of the conductive auxiliary 6 in the positive
electrode active material layer 28 is preferably 1 to 10% by mass,
more preferably 1.5 to 5% by mass, based on the total amount of the
positive electrode active material layer 28. When the content of
the conductive auxiliary 6 is less than 1% by mass, the
conductivity tends to be provided less sufficiently, thereby
causing characteristics to deteriorate as compared with the case
where the content falls within the range mentioned above. When the
content of the conductive auxiliary 6 exceeds 10% by mass, the
coating film strength tends to become less sufficient, thereby
lowering the cycle characteristic as compared with the case where
the content falls within the above-mentioned range.
[0062] The current collector 26 of the positive electrode 20 is
electrically connected to one end of the positive electrode lead 22
made of aluminum, for example, while the other end of the positive
electrode lead 22 extends to the outside of the case 50. On the
other hand, the current collector 16 of the negative electrode 10
is electrically connected to one end of the negative electrode lead
12 made of copper or nickel, for example, while the other end of
the negative electrode lead 12 extends to the outside of the case
50.
[0063] The part of the negative electrode lead 12 in contact with a
seal part 50A is covered with an insulator 14 for preventing the
negative electrode lead 12 from coming into contact with a metal
layer of the case 50. The part of the positive electrode lead 22 in
contact with the seal part 50A is covered with an insulator 24 for
preventing the positive electrode lead 22 from coming into contact
with the metal layer of the case 50. The insulators 14, 24 also
serve to improve the adhesion between the innermost layer of the
case 50 and the leads 12, 22.
[0064] The separator 40 arranged between the negative electrode 10
and positive electrode 20 is not limited in particular as long as
it is formed by a porous body having ion permeability and
electronic insulativity, whereby separators used in known
lithium-ion secondary batteries can be employed. Examples include
multilayer bodies of films constituted by any of polyethylene,
polypropylene, and polyolefin, extended films of mixtures of these
polymers, and fibrous nonwovens constituted by at least one kind of
constituent material selected from the group consisting of
cellulose, polyester, and polypropylene.
[0065] The electrolytic solution (not depicted) fills the inner
space of the case 50, while being partly contained within the
negative electrode 10, positive electrode 20, and separator 40. As
the electrolytic solution, a nonaqueous electrolytic solution in
which a lithium salt is dissolved in an organic solvent is used.
Examples of the lithium salt include LiPF.sub.6, LiClO.sub.4,
LiBF.sub.4, LiAsF.sub.6, LiCF.sub.3SO.sub.3,
LiCF.sub.3CF.sub.2SO.sub.3, LiC(CF.sub.3SO.sub.2).sub.3,
LiN(CF.sub.3SO.sub.2).sub.2, LiN(CF.sub.3CF.sub.2SO.sub.2).sub.2,
LiN(CF.sub.3SO.sub.2)(C.sub.4F.sub.9SO.sub.2), and
LiN(CF.sub.3CF.sub.2CO).sub.2. These salts may be used either
singly or in combinations of two or more. The electrolytic solution
may be gelled by addition of polymers and the like thereto.
[0066] As the organic solvent, one used in known lithium-ion
secondary batteries can be employed. Preferred examples include
propylene carbonate, ethylene carbonate, and diethyl carbonate.
They may be used either singly or in mixtures of two or more at any
ratios.
[0067] As illustrated in FIG. 2, the case 50 is formed by a pair of
opposing films (first and second films 51, 52). Edge parts of films
opposing and overlapping each other are sealed with an adhesive or
by heat-sealing, so as to form the seal part 50A.
[0068] The film constituting the first and second films 51, 52 is a
flexible film. Though this film is not limited in particular as
long as it is a flexible film, it preferably has at least an
innermost layer made of a polymer in contact with the power
generating element 60 and a metal layer arranged on the side of the
innermost layer opposite from the side in contact with the power
generating element from the viewpoint of effectively preventing
moisture and air from entering the inside of the case 50 from the
outside and electrolyte components from dissipating from the inside
to the outside of the case 50, while securing sufficient mechanical
strength and lightweight of the case.
[0069] Method of Manufacturing Lithium-Ion Secondary Battery
Positive Electrode and Method of Manufacturing Lithium-Ion
Secondary Battery
[0070] Methods of manufacturing the above-mentioned lithium-ion
secondary battery positive electrode 20 and lithium-ion secondary
battery 1 will now be explained.
[0071] The positive electrode 20 is made through at least a coating
material preparing strep of preparing a positive electrode active
material layer forming coating material (slurry, paste, or the
like) by mixing at least the positive electrode active material 2,
binder 4, conductive auxiliary 6, organic solvent, and water and an
active material layer forming step of forming the positive
electrode active material layer 28 on the current collector 26 by
using the positive electrode active material layer forming coating
material. In the coating material preparing step, the positive
electrode active material layer forming coating material is
prepared such that the amount of water added (% by mass) based on
the total amount of the organic solvent and water and the pH of the
positive electrode active material 2 satisfy the following
expression (1):
48.ltoreq.[the amount of water added+(4.25.times.the pH of the
positive electrode active material)].ltoreq.52 (1)
[0072] Here, the pH of the positive electrode active material 2 is
determined by mixing the positive electrode active material 2 and
ion-exchanged water at a mass ratio of 1:100 and measuring the pH
of the resulting mixed liquid with a pH meter.
[0073] As the positive electrode active material 2, binder 4, and
conductive auxiliary 6 in the coating material preparing step,
those mentioned in the explanation of the lithium-ion secondary
battery positive electrode 20 can be used.
[0074] Here, the pH of the positive electrode active material 2 is
preferably 9.0 to 11.5, more preferably 10.0 to 11.0. Using the
positive electrode active material whose pH falls within the range
mentioned above can more fully inhibit the dispersibility of the
positive electrode active material layer forming coating material
from deteriorating with time and further improve the cycle
characteristic of the lithium-ion secondary battery.
[0075] The organic solvent used in the coating material preparing
step is not limited in particular as long as the binder 4 is
soluble therein; examples include N-methyl-2-pyrrolidone and
N,N-dimethylformamide. In these organic solvents,
N-methyl-2-pyrrolidone is preferred since it can improve the cycle
characteristic of the lithium-ion secondary battery more.
[0076] Though not restricted in particular, purified water such as
ion-exchanged water or distilled water is preferred as water used
in the coating material preparing step.
[0077] The amount of water added in the coating material preparing
step is preferably 4 to 10% by mass, more preferably 4 to 8% by
mass, based on the total amount of the organic solvent and water.
When the amount of water added is outside of the range mentioned
above, the effect of inhibiting the dispersibility of the positive
electrode active material layer forming coating material from
deteriorating with time and the effect of improving the cycle
characteristic of the lithium-ion secondary battery tend to
decrease as compared with the case where the amount falls within
the range mentioned above.
[0078] From the viewpoints of yielding favorable coating film
formability and more fully inhibiting the coating material from
deteriorating with time, the solid content of the positive
electrode active material layer forming coating material in the
coating material preparing step is preferably 60 to 72% by mass,
more preferably 62 to 70% by mass, based on the total amount of the
coating material.
[0079] In the active material layer forming step, the positive
electrode active material layer forming coating material is applied
onto the surface of the current collector 26, dried, and extended
and so forth as necessary, so as to form the positive electrode
active material layer 28 on the current collector 26, thereby
yielding the positive electrode 20. The technique for applying the
positive electrode active material layer forming coating material
to the surface of the current collector 26 is not limited in
particular, but may be determined as appropriate according to the
material, form, and the like of the current collector 26. Examples
of the coating method include metal mask printing, electrostatic
coating, dip coating, spray coating, roll coating, doctor blading,
gravure coating, and screen printing.
[0080] Making the positive electrode 20 through the above-mentioned
coating material preparing step and active material layer forming
step can form the positive electrode active material layer 28
having a structure in which at least a part of a surface of the
positive electrode active material 2 is covered with the coating
layer 8 formed by sufficiently dispersing the conductive auxiliary
6 in the binder 4 without aggregation as illustrated in FIG. 5.
[0081] The method of making the negative electrode 10 is not
restricted in particular. For example, constituents of the negative
electrode 10 mentioned above are mixed and dispersed in a solvent
which can dissolve the binder, so as to make a negative electrode
active material layer forming coating material (slurry, paste, or
the like). The solvent is not limited in particular as long as the
binder is soluble therein. Its examples include
N-methyl-2-pyrrolidone and N,N-dimethylformamide.
[0082] Subsequently, the negative electrode active material layer
forming coating material is applied onto a surface of the current
collector 16, dried, and extended and so forth as necessary, so as
to form the negative electrode active material layer 18 on the
current collector 16, thereby yielding the negative electrode 10.
An example of the technique for applying the negative electrode
active material layer forming coating material onto the surface of
the current collector 16 is one similar to the method of applying
the positive electrode active material layer forming coating
material mentioned above.
[0083] After making the negative electrode 10 and positive
electrode 20 as mentioned above, the negative and positive leads
12, 22 are electrically connected to the negative and positive
electrodes 10, 20, respectively.
[0084] Subsequently, the separator 40 is arranged between and in
contact with the negative electrode 10 and positive electrode 20
(preferably in an unbonded state), so as to complete the power
generating element 60 (multilayer body in which the negative
electrode 10, separator 40, and positive electrode 20 are laminated
in sequence in this order). Here, a surface F2 of the negative
electrode 10 facing the negative electrode active material layer 18
and a surface F2 of the positive electrode 20 facing the positive
electrode active material layer 28 are arranged in contact with the
separator 40.
[0085] Next, the edge parts of the first and second films 51, 52
overlaid on each other are sealed with an adhesive or by heat
sealing, so as to make the case 50. Here, for securing an opening
for introducing the power generating element 60 into the case 50 in
a later step, a part of the edge parts is left unsealed. This
yields the case 50 having the opening.
[0086] Subsequently, the power generating element 60 having the
negative and positive electrode leads 12, 22 electrically connected
thereto is inserted into the case 50 having the opening, and the
electrolytic solution is injected therein. Then, while the negative
and positive electrodes 12, 22 are partly inserted in the case 50,
the opening of the case 50 is sealed, whereby the lithium-ion
secondary battery 1 is completed.
[0087] Though a preferred embodiment of the present invention is
explained in the foregoing, the present invention is not limited
thereto.
[0088] For example, though the above-mentioned embodiment explains
the lithium-ion secondary battery 1 comprising one each of the
negative and positive electrodes 10, 20, two or more each of the
negative and positive electrodes 10, 20 may be provided while
always arranging one separator 40 between each pair of the negative
and positive electrodes 10, 20. The lithium-ion secondary battery 1
is not limited to the form illustrated in FIG. 1, but may have a
cylindrical form, for example.
[0089] The lithium-ion secondary battery of the present invention
can also be used for power supplies for self-propelled
micromachines, IC cards, and the like and decentralized power
supplies placed on or within printed boards.
EXAMPLES
[0090] The present invention will now be explained more
specifically with reference to examples and comparative examples.
However, the present invention is not limited to the following
examples. In the following examples and comparative examples, the
pH of each positive electrode active material was determined by
mixing the positive electrode active material and ion-exchanged
water at a mass ratio of 1:100 and measuring the pH of the
resulting mixed liquid with a pH meter.
Example 1
Preparation of a Positive Electrode Active Material Layer Forming
Coating Material
[0091] A slurry-like positive electrode active material layer
forming coating material was prepared by adding 64.0 parts by mass
of LiCoO.sub.2 (product name: SE-02, manufactured by Seimi Chemical
Co., Ltd., having a pH of 10.0) as a positive electrode active
material, 1.5 parts by mass of polyvinylidene fluoride (PVDF)
(product name: Solef 6020 manufactured by Solvay S.A.) produced by
emulsion polymerization as a binder, and 1.5 parts by mass of
carbon (product name: Super-P manufactured by TIMCAL) as a
conductive auxiliary to 33.0 parts by mass of a mixed solvent in
which N-methyl-2-pyrrolidone (NMP) and ion-exchanged water were
mixed such that the amount of ion-exchanged water added was 8.5% by
mass based on their total amount, and mixing them. In thus obtained
coating material, the value of [the amount of water
added+(4.25.times.the pH of the positive electrode active
material)] was 51, which satisfied the above-mentioned expression
(1).
[0092] Evaluation of the Coating Material State and Viscosity
Change
[0093] The coating material state of thus produced positive
electrode active material layer forming coating material (whether
or not it was aggregated or gelled) immediately after its
production was observed. As a result, the produced coating material
was neither aggregated nor gelled, and thus was favorable.
[0094] Using a B-type viscometer, the viscosity of the produced
positive electrode active material layer forming coating material
was measured at an initial stage (immediately after the
production), after being left for 24 hr at 25.degree. C., and after
being left for 48 hr at 25.degree. C. Since the viscosity decreases
when the dispersibility of the coating material deteriorates with
time, the dispersibility of the coating material is more inhibited
from deteriorating with time as the viscosity decreases less. Table
2 lists the results.
[0095] Making of a Positive Electrode
[0096] The produced positive electrode active material layer
forming coating material was applied by doctor blading onto an Al
foil serving as a current collector and dried, so as to yield a
positive electrode in which a positive electrode active material
layer having a thickness of 100 .mu.m was formed on the current
collector having a thickness of 15 .mu.m.
[0097] Making of a Negative Electrode
[0098] A slurry-like negative electrode active material layer
forming coating material was prepared by adding 46.0 parts by mass
of graphite (product name: FNSC-1 manufactured by Shenglin, China)
as a negative electrode active material, 1.2 parts by mass of
styrene/butadiene copolymer (SBR) (product name: SN307N
manufactured by Nippon A&L Inc.) as a binder, 0.7 part by mass
of carboxymethylcellulose (CMC) (product name: WS-C manufactured by
Dai-Ichi Kogyo Seiyaku Co., Ltd.) as a thickener, and 0.7 part by
mass of carbon (product name: Super-P manufactured by TIMCAL) as a
conductive auxiliary to 51.4 parts by mass of ion-exchanged water
and mixing them. This coating material was applied by doctor
blading to a copper foil serving as a current collector and dried,
so as to yield a negative electrode in which a negative electrode
active material layer having a thickness of 100 .mu.m was formed on
the current collector having a thickness of 15 .mu.m.
[0099] Making of a Lithium-Ion Secondary Battery
[0100] The negative electrode was punched out into a size of 17.5
mm.times.34.5 mm, the positive electrode was punched out into a
size of 17 mm.times.34 mm, and a separator made of polyethylene was
arranged between and laminated with the negative and positive
electrodes, so as to make a battery element. Thus obtained battery
element was put into an aluminum-laminated film, an electrolytic
solution was injected therein, and the film was sealed under
vacuum, so as to yield a lithium-ion secondary battery. Employed as
the electrolytic solution was one in which lithium
hexafluorophosphate (LiPF.sub.6) was dissolved at a concentration
of 1.5 moldm.sup.-3 in a mixed solvent made by mixing 20 parts by
volume of propylene carbonate (PC), 10 parts by volume of ethylene
carbonate (EC), and 70 parts by volume of diethyl carbonate.
Examples 2 to 4
[0101] Lithium-ion secondary batteries of Examples 2 to 4 were made
as in Example 1 except that the amount of ion-exchanged water added
in the mixed solvent was changed as listed in Table 1 in the
preparation of the positive electrode active material layer forming
coating material. The coating material state and viscosity change
of each positive electrode active material layer forming coating
material were also evaluated as in Example 1. Table 2 lists the
results.
Example 5
[0102] The lithium-ion secondary battery of Example 5 was made as
in Example 1 except that the amount of ion-exchanged water added in
the mixed solvent was changed as listed in Table 1 and LiCoO.sub.2
(product name: LIII, manufactured by Yuyan, China, having a pH of
10.2) was used as the positive electrode active material in the
preparation of the positive electrode active material layer forming
coating material. The coating material state and viscosity change
of the positive electrode active material layer forming coating
material were also evaluated as in Example 1. Table 2 lists the
results.
Example 6
[0103] The lithium-ion secondary battery of Example 6 was made as
in Example 1 except that the amount of ion-exchanged water added in
the mixed solvent was changed as listed in Table 1 and
LiCoNiMnO.sub.2 (product name: 01ST, manufactured by Toda Kogyo
Corp., having a pH of 10.7) was used as the positive electrode
active material in the preparation of the positive electrode active
material layer forming coating material. The coating material state
and viscosity change of the positive electrode active material
layer forming coating material were also evaluated as in Example 1.
Table 2 lists the results.
Example 7
[0104] The lithium-ion secondary battery of Example 7 was made as
in Example 1 except that N,N-dimethylformamide (DMF) was used as
the organic solvent, the amount of ion-exchanged water added in the
mixed solvent was changed as listed in Table 1, and LiCoNiMnO.sub.2
(product name: S-600, manufactured by Yuyan, China, having a pH of
10.9) was used as the positive electrode active material in the
preparation of the positive electrode active material layer forming
coating material. The coating material state and viscosity change
of the positive electrode active material layer forming coating
material were also evaluated as in Example 1. Table 2 lists the
results.
Example 8
[0105] The lithium-ion secondary battery of Example 8 was made as
in Example 1 except that the amount of ion-exchanged water added in
the mixed solvent was changed as listed in Table 1 and
LiCoNiMnO.sub.2 (product name: S-600, manufactured by Yuyan, China,
having a pH of 10.9) was used as the positive electrode active
material in the preparation of the positive electrode active
material layer forming coating material. The coating material state
and viscosity change of the positive electrode active material
layer forming coating material were also evaluated as in Example 1.
Table 2 lists the results.
Example 9
[0106] The lithium-ion secondary battery of Example 9 was made as
in Example 1 except that the amount of ion-exchanged water added in
the mixed solvent was changed as listed in Table 1 and
LiCoNiMnO.sub.2 (product name: S-600, manufactured by Yuyan, China,
having a pH of 10.9) was used as the positive electrode active
material in the preparation of the positive electrode active
material layer forming coating material. The coating material state
and viscosity change of the positive electrode active material
layer forming coating material were also evaluated as in Example 1.
Table 2 lists the results.
Comparative Examples 1 to 3
[0107] Lithium-ion secondary batteries of Comparative Examples 1 to
3 were made as in Example 1 except that the amount of ion-exchanged
water added in the mixed solvent was changed as listed in Table 1
(no ion-exchanged water was added in Comparative Example 1) in the
preparation of the positive electrode active material layer forming
coating material. The coating material state and viscosity change
of each positive electrode active material layer forming coating
material were also evaluated as in Example 1. Table 2 lists the
results.
Comparative Example 4
[0108] The positive electrode active material layer forming coating
material of Comparative Example 4 was made as in Example 1 except
that the amount of ion-exchanged water added in the mixed solvent
was changed as listed in Table 1 and polyvinylidene fluoride (PVDF)
produced by suspension polymerization (product name: KF-1300
manufactured by Kureha Corp.) was used as the binder in the
preparation of the positive electrode active material layer forming
coating material. When evaluating the coating material state as in
Example 1, gelling occurred immediately after the production.
Therefore, no viscosity change could be evaluated, and no positive
electrode could be made.
Comparative Example 5
[0109] The positive electrode active material layer forming coating
material of Comparative Example 5 was made as in Example 1 except
that the amount of ion-exchanged water added in the mixed solvent
was changed as listed in Table 1 and LiCoNiMnO.sub.2 (product name:
01ST, manufactured by Toda Kogyo Corp., having a pH of 10.7) was
used as the positive electrode active material in the preparation
of the positive electrode active material layer forming coating
material. When evaluating the coating material state as in Example
1, gelling occurred immediately after the production. Therefore, no
viscosity change could be evaluated, and no positive electrode
could be made.
Comparative Example 6
[0110] The positive electrode active material layer forming coating
material of Comparative Example 6 was made as in Example 1 except
that the amount of ion-exchanged water added in the mixed solvent
was changed as listed in Table 1, LiCoNiMnO.sub.2 (product name:
01ST, manufactured by Toda Kogyo Corp., having a pH of 10.7) was
used as the positive electrode active material, and polyvinylidene
fluoride (PVDF) produced by suspension polymerization (product
name: KF-1300 manufactured by Kureha Corp.) was used as the binder
in the preparation of the positive electrode active material layer
forming coating material. When evaluating the coating material
state as in Example 1, gelling occurred immediately after the
production. Therefore, no viscosity change could be evaluated, and
no positive electrode could be made.
Comparative Example 7
[0111] The positive electrode active material layer forming coating
material of Comparative Example 7 was made as in Example 1 except
that the amount of ion-exchanged water added in the mixed solvent
was changed as listed in Table 1 and LiNiCoAlO.sub.2 (product name:
503LP, manufactured by JFE Mineral Co., Ltd., having a pH of 12.0)
was used as the positive electrode active material in the
preparation of the positive electrode active material layer forming
coating material. When evaluating the coating material state as in
Example 1, gelling occurred immediately after the production.
Therefore, no viscosity change could be evaluated, and no positive
electrode could be made.
Comparative Example 8
[0112] The positive electrode active material layer forming coating
material of Comparative Example 8 was made as in Example 1 except
that no ion-exchanged water was added in the mixed solvent,
LiNiCoAlO.sub.2 (product name: 503LP, manufactured by JFE Mineral
Co., Ltd., having a pH of 12.0) was used as the positive electrode
active material, and polyvinylidene fluoride (PVDF) produced by
suspension polymerization (product name: KF-1300 manufactured by
Kureha Corp.) was used as the binder in the preparation of the
positive electrode active material layer forming coating material.
When evaluating the coating material state as in Example 1, gelling
occurred immediately after the production. Therefore, no viscosity
change could be evaluated, and no positive electrode could be
made.
[0113] Observation of Positive Electrode Cross Section
[0114] FIGS. 6, 7, and 8 illustrate electron micrographs of a cross
section of the positive electrode produced by Example 1 (at
magnifications of 1000.times., 2000.times., and 5000.times.,
respectively). FIGS. 9 and 10 illustrate electron micrographs of a
cross section of the positive electrode produced by Comparative
Example 1 (at magnifications of 1000.times. and 2000.times.,
respectively).
[0115] As illustrated in FIGS. 6 to 8, the positive electrode
produced by Example 1 had a structure in which the surface of the
positive electrode active material 2 was covered with the coating
layer 8 formed by dispersing the conductive auxiliary in the binder
without aggregation, while a plurality of pieces of the positive
electrode active material 2 were connected to each other through
the coating layer 8 (see, for example, the part indicated by broken
line A in FIG. 7).
[0116] On the other hand, as illustrated in FIGS. 9 and 10, the
positive electrode produced by Comparative Example 1 was not formed
with the coating layer 8 in which the conductive auxiliary was
dispersed in the binder, but the conductive auxiliary formed
aggregates (see, for example, the part indicated by broken line B
in FIG. 10).
[0117] Cross sections of the positive electrodes obtained by
Examples 2 to 9 and Comparative Examples 2 and 3 were observed, so
as to see whether the coating layer 8 exists or not. Table 2 lists
the results.
[0118] Evaluation of Cycle Characteristic
[0119] Each of the lithium-ion secondary batteries obtained by
Examples 1 to 9 and Comparative Examples 1 to 3 was electrically
charged at a rate of 1 C at 25.degree. C. by constant-current,
constant-voltage charging at 4.2 V, Thereafter, constant-current
discharging to 2.5 V was performed at a rate of 1 C at 25.degree.
C. Counting them as 1 cycle, 100 cycles were carried out. The ratio
(%) of the discharge capacity at the 100th cycle to that of the 1st
cycle was determined as a cycle characteristic. Table 2 lists the
results.
TABLE-US-00001 TABLE 1 pH of positive Amount of [Amount of water
electrode Polymerization water added added + (4.25 .times. active
material type of PVDF Solvent (% by mass) pH)] value Example 1 10.0
emulsion NMP 8.50 51.0 polymerization Example 2 10.0 emulsion NMP
7.50 50.0 polymerization Example 3 10.0 emulsion NMP 9.50 52.0
polymerization Example 4 10.0 emulsion NMP 5.50 48.0 polymerization
Example 5 10.2 emulsion NMP 7.65 51.0 polymerization Example 6 10.7
emulsion NMP 5.00 50.5 polymerization Example 7 10.9 emulsion DMF
4.50 50.8 polymerization Example 8 10.9 emulsion NMP 3.00 49.3
polymerization Example 9 10.9 emulsion NMP 4.50 50.8 polymerization
Comp. Ex. 1 10.0 emulsion NMP 0 42.5 polymerization Comp. Ex. 2
10.0 emulsion NMP 5.00 47.5 polymerization Comp. Ex. 3 10.0
emulsion NMP 10.5 53.0 polymerization Comp. Ex. 4 10.0 suspension
NMP 5.00 47.5 polymerization Comp. Ex. 5 10.7 emulsion NMP 8.00
53.5 polymerization Comp. Ex. 6 10.7 suspension NMP 5.00 50.5
polymerization Comp. Ex. 7 12.0 emulsion NMP 1.50 52.5
polymerization Comp. Ex. 8 12.0 suspension NMP 0 51.0
polymerization
TABLE-US-00002 TABLE 2 Coating Coating material state material
viscosity (immediately change (MPa) Cycle after After After Coating
characteristic production) Initial 24 hr 48 hr layer (%) Example 1
good 3000 2950 2950 yes 97.6 Example 2 good 2790 2750 2770 yes 97.8
Example 3 good 3150 3120 3090 yes 95.4 Example 4 good 3120 3070
3030 yes 95.8 Example 5 good 2850 2900 2900 yes 96.6 Example 6 good
3100 3050 3040 yes 95.3 Example 7 good 3300 3100 2980 yes 93.2
Example 8 good 3380 3290 3290 yes 92.6 Example 9 good 3070 3000
2950 yes 96.7 Comp. Ex. 1 good 3800 1800 1200 no 90.2 Comp. Ex. 2
good 3800 2400 1700 no 91.7 Comp. Ex. 3 aggregated 2400 1500 1270
no 74.5 Comp. Ex. 4 gelled -- -- -- -- -- Comp. Ex. 5 gelled -- --
-- -- -- Comp. Ex. 6 gelled -- -- -- -- -- Comp. Ex. 7 gelled -- --
-- -- -- Comp. Ex. 8 gelled -- -- -- -- --
[0120] As clear from the results listed in Table 2, it was seen
that the positive electrode active material layer forming coating
materials produced by the examples yielded favorable coating
material states and little viscosity change, and were fully
inhibited from deteriorating with time. On the other hand, the
positive electrode active material layer forming coating materials
produced by the comparative examples were seen to be aggregated or
gelled or change their viscosity so much as to deteriorate with
time. The positive electrode active material layers in the examples
also exhibited bending strengths higher than those of the positive
electrode active materials in the comparative examples.
[0121] Further, as clear from the results listed in Table 2, it was
seen that the lithium-ion secondary batteries produced by the
examples had cycle characteristics superior to those of the
lithium-ion secondary batteries produced by the comparative
examples.
* * * * *