U.S. patent application number 16/380203 was filed with the patent office on 2019-08-01 for secondary battery and method for manufacturing same.
This patent application is currently assigned to NEC ENERGY DEVICES, LTD.. The applicant listed for this patent is NEC ENERGY DEVICES, LTD.. Invention is credited to Tetsuya SATO.
Application Number | 20190237745 16/380203 |
Document ID | / |
Family ID | 52460879 |
Filed Date | 2019-08-01 |
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United States Patent
Application |
20190237745 |
Kind Code |
A1 |
SATO; Tetsuya |
August 1, 2019 |
SECONDARY BATTERY AND METHOD FOR MANUFACTURING SAME
Abstract
An object of the present invention is to provide a high-quality
secondary battery having high electric characteristics and high
reliability, the secondary battery preventing a shortcut between a
positive electrode and a negative electrode by means of an
insulating material and preventing or reducing an increase in
volume and deformation of a battery electrode assembly, and a
method for manufacturing the same. Secondary battery 100 according
to the present invention includes a battery electrode assembly
including positive electrode 1 and negative electrode 6 alternately
stacked via separator 20. Positive electrode 1 and negative
electrode 2 each includes current collector 3 or 8 and active
material 2 or 7 applied to current collector 3 or 8. Active
material 2A positioned on one surface of positive electrode 1 of
current collector 3 includes flat portion 2A.sub.1 and
small-thickness portion (thin-layer portion) 2A.sub.3 positioned on
the end portion side relative to flat portion 2A.sub.1, the
small-thickness portion 2A.sub.3 having a thickness that is smaller
than that of flat portion 2A.sub.1. A portion of active material 2B
positioned on another surface of current collector 3 of positive
electrode 1, the portion facing thin-layer portion 2A.sub.3 of
active material 2A positioned on the one surface via current
collector 3 is a flat portion having a constant thickness.
Inventors: |
SATO; Tetsuya;
(Sagamihara-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NEC ENERGY DEVICES, LTD. |
Sagamihara-shi |
|
JP |
|
|
Assignee: |
NEC ENERGY DEVICES, LTD.
Sagamihara-shi
JP
|
Family ID: |
52460879 |
Appl. No.: |
16/380203 |
Filed: |
April 10, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
14904735 |
Jan 13, 2016 |
10305088 |
|
|
PCT/JP2013/083138 |
Dec 10, 2013 |
|
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16380203 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/0525 20130101;
H01M 4/139 20130101; H01M 10/0585 20130101; H01M 4/64 20130101;
H01M 4/13 20130101; H01M 2220/30 20130101; H01M 4/0404
20130101 |
International
Class: |
H01M 4/04 20060101
H01M004/04; H01M 10/0585 20060101 H01M010/0585; H01M 4/139 20060101
H01M004/139; H01M 4/64 20060101 H01M004/64; H01M 4/13 20060101
H01M004/13 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 9, 2013 |
JP |
2013-166462 |
Claims
1. A secondary battery comprising a battery electrode assembly
including a positive electrode and a negative electrode alternately
stacked via a separator, wherein: the positive electrode and the
negative electrode each includes a current collector and an active
material applied to the current collector; the active material
positioned on one surface of the current collector of the positive
electrode, consists essentially of a large-thickness portion and a
small-thickness portion, and the small-thickness portion is
positioned on an end portion side relative to the large-thickness
portion, and the small-thickness portion has a thickness that is
smaller than that of the large-thickness portion; the active
material which is positioned on the other surface of the current
collector of the positive electrode, has an end portion, and the
end portion of the active material which is positioned on the other
surface of the current collector, faces the small-thickness portion
of the active material positioned on the one surface of the current
collector, across the current collector, and consists essentially
of a flat portion; the small-thickness portion of the active
material positioned on the one surface of the current collector of
the positive electrode includes at least one from among a
thin-layer portion, an inclined portion whose thickness
continuously decreases, and a stepped portion whose thickness
intermittently decreases.
2. The secondary battery according to claim 1, wherein an
insulating material is positioned so that: the insulating material
covers a boundary portion between a coated portion to which the
active material has been applied and a non-coated portion to which
the active material has not been applied in the positive electrode;
and one end portion of the insulating material is positioned on the
small-thickness portion of the active material positioned on the
one surface of the current collector of the positive electrode.
3. The secondary battery according to claim 2, wherein a difference
in thickness between the large-thickness portion and the
small-thickness portion of the active material positioned on the
one surface of the current collector of the positive electrode is
equal to or larger than a thickness of the insulating material.
4. The secondary battery according to claim 3, wherein the
insulating material is provided on each of the opposite surfaces of
the current collector of the positive electrode, and the difference
in thickness between large-thickness portion and the
small-thickness portion of the active material positioned on the
one surface of the current collector of the positive electrode is
no less than twice the thickness of the insulating material.
5. The secondary battery according to claim 2, wherein a total sum
of the thickness of the insulating material, a thickness of a
portion of the active material on the one surface of the current
collector of the positive electrode on which the insulating
material is disposed, and a thickness of a portion of the active
material of the negative electrode, which faces the insulating
material across the separator, is equal to or smaller than a total
sum of the thickness of the large-thickness portion of the active
material of the positive electrode and a thickness of the
large-thickness portion of the active material of the negative
electrode, which faces the large-thickness portion of the active
material of the positive electrode across the separator.
6. The secondary battery according to claim 1, wherein the active
material positioned on the other surface of the current collector
of the positive electrode consists essentially of only the flat
portion.
7. The secondary battery according to claim 1, wherein: the active
material which is positioned on one surface of the current
collector of the negative electrode consists essentially of a
large-thickness portion and a small-thickness portion having a
thickness that is smaller than that of the large-thickness portion;
and the active material which is positioned on the other surface of
the current collector of the negative electrode, has an end
portion, and the end portion of the active material which is
positioned on the other surface of the current collector, faces the
small-thickness portion of the active material positioned on the
one surface, across the current collector, consists essentially of
a flat portion.
8. The secondary battery according to claim 8, wherein the
small-thickness portion of the active material positioned on the
one surface of the current collector of the negative electrode
faces an insulating material positioned on the active material of
the positive electrode, across the separator.
Description
TECHNICAL FIELD
[0001] The present invention relates to a secondary battery
including a positive electrode and a negative electrode laid over
each other with a separator interposed therebetween, and a method
for manufacturing the same.
BACKGROUND ART
[0002] Secondary batteries are becoming widely used as power
supplies for vehicles and household appliances, and not only as
power supplies for portable devices such as mobile phones, digital
cameras and laptop computers, and among others, lithium ion
secondary batteries, which have a high-energy density and are
lightweight, are energy storage devices that are indispensable in
daily life.
[0003] Secondary batteries are generally classified into a rolled
type and a stacked type. A battery electrode assembly of a rolled
type secondary battery has a structure in which a long positive
electrode sheet and a long negative electrode sheet laid on each
other via a separator are rolled a plurality of turns. A battery
electrode assembly of a stacked type secondary battery has a
structure in which positive electrode sheets and negative electrode
sheets are alternately stacked with separators interposed
therebetween. The positive electrode sheets and the negative
electrode sheets each include a current collector including a
coated portion to which active material (which may be a compound
agent containing, e.g., a binder and a conductive material) has
been applied and an non-coated portion to which active material has
not been applied in order to allow an electrode terminal to be
connected thereto.
[0004] In either a rolled type secondary battery or a stacked type
secondary battery, a battery electrode assembly is enclosed inside
an outer container in such a manner that: one end of a positive
electrode terminal is electrically connected to an non-coated
portion of a positive electrode sheet and another end of the
positive electrode terminal extends to the outside of the outer
container (outer case); and one end of a negative electrode
terminal is electrically connected to an non-coated portion of a
negative electrode sheet and another end of the negative electrode
terminal extends to the outside of the outer container. Inside the
outer container, in addition to the battery electrode assembly,
electrolyte is enclosed. Capacities of secondary batteries have
been increasing year by year, and along with this increase, heat
that would be generated if a shortcut occurs also increases,
causing an increase in risk, and thus, measures to ensure battery
safety are becoming increasingly important.
[0005] As an example of a safety countermeasure, a technique in
which insulating material is formed on a boundary portion between a
coated portion and an non-coated portion in order to prevent a
shortcut between a positive electrode and a negative electrode is
known (Patent Document 1).
RELATED ART DOCUMENT
Patent Document
[0006] Patent Document 1: JP2012-164470A
SUMMARY OF INVENTION
Problems to Be Solved by the Invention
[0007] In the technique disclosed in Patent Document 1, as
illustrated in FIG. 19, positive electrode 1 and negative electrode
6 are alternately stacked via separator 20, and on current
collector 3 of each positive electrode 1, insulating material 40
covering boundary portion 4 between a coated portion to which
active material 2 has been applied and an non-coated portion to
which active material 2 has not been applied is formed. In a
stacked type secondary battery, insulating materials 40 are stacked
at the same position in a planar view. Thus, the thickness of a
portion of the battery electrode assembly at the position where
insulating materials 40 are disposed, becomes thicker, which
results in reducing energy density per unit volume.
[0008] Also, in order to obtain stable electric characteristics and
high reliability, it is preferable that the battery electrode
assembly of a secondary battery be fastened via, e.g., a tape by
applying pressure uniformly. However, use of insulating materials
in a stacked type secondary battery in such a manner as in Patent
Document 1 results in failure to uniformly fasten a battery
electrode assembly due to a difference in thickness between a
portion in which insulating materials 40 are present and a portion
in which insulating materials 40 are not present, which may cause
battery quality deterioration such as variability in electric
characteristics and/or degradation of battery cycle properties.
[0009] Therefore, an object of the present invention is to solve
the aforementioned problems and provide a high-quality secondary
battery having high electric characteristics and high reliability,
the secondary battery preventing a short circuit between a positive
electrode and a negative electrode by means of insulating material
and preventing or reducing an increase in volume and deformation of
a battery electrode assembly, and a method for manufacturing the
same.
Means to Solve the Problems
[0010] A secondary battery according to the present invention
comprises a battery electrode assembly including a positive
electrode and a negative electrode alternately stacked via a
separator, and the positive electrode and the negative electrode
each includes a current collector and active material applied to
the current collector. The active material positioned on one
surface of the current collector of the positive electrode,
includes a flat portion and a small-thickness portion positioned on
an end portion side relative to the flat portion, the
small-thickness portion having a thickness that is smaller than
that of the flat portion. A portion of the active material
positioned on another surface of the current collector of the
positive electrode, which faces the small-thickness portion of the
active material positioned on the one surface, across the current
collector, is a flat portion having a constant thickness.
Advantageous Effect of Invention
[0011] The present invention enables preventing or reducing an
increase in volume of a battery electrode assembly and distortion
of the battery electrode assembly that are caused by insulating
material, and enables provision of a high-quality secondary battery
having good energy density.
BRIEF DESCRIPTION OF DRAWINGS
[0012] FIG. 1A is a plan view illustrating a basic structure of a
stacked type secondary battery according to the present
invention.
[0013] FIG. 1B is a cross-sectional view along line A-A in FIG.
1A.
[0014] FIG. 2 is an enlarged cross-sectional view illustrating a
positive electrode in an exemplary embodiment of a secondary
battery of the present invention.
[0015] FIG. 3 is an enlarged cross-sectional view illustrating a
major portion of an exemplary embodiment of a secondary battery
according to the present invention.
[0016] FIG. 4 is an enlarged cross-sectional view illustrating a
variation of a positive electrode in an exemplary embodiment of a
secondary battery of the present invention.
[0017] FIG. 5A is a schematic diagram illustrating an example of an
electrode coating apparatus.
[0018] FIG. 5B is a schematic diagram illustrating a reference
example of an electrode manufacturing method.
[0019] FIG. 6 is a schematic diagram illustrating an example of an
electrode manufacturing method according to the present
invention.
[0020] FIG. 7 is a plan view illustrating a positive electrode
forming step in a secondary battery manufacturing method according
to the present invention.
[0021] FIG. 8 is a plan view illustrating a step following FIG. 7
in the secondary battery manufacturing method according to the
present invention.
[0022] FIG. 9A is a plan view illustrating a step following FIG. 8
in the secondary battery manufacturing method according to the
present invention.
[0023] FIG. 9B is a plan view illustrating a positive electrode
formed as a result of cutting in the step illustrated in FIG.
9A.
[0024] FIG. 10 is a plan view illustrating a negative electrode
forming step in the secondary battery manufacturing method
according to the present invention.
[0025] FIG. 11A is a plan view illustrating a step following FIG.
10 in the secondary battery manufacturing method according to the
present invention.
[0026] FIG. 11B is a plan view illustrating a negative electrode
formed as a result of cutting in the step illustrated in FIG.
11A.
[0027] FIG. 12 is an enlarged cross-sectional view illustrating a
major portion of another exemplary embodiment of a secondary
battery according to the present invention.
[0028] FIG. 13 is a block diagram schematically illustrating an
example of an apparatus used for intermittent application of active
material.
[0029] FIG. 14A is a cross-sectional view schematically
illustrating an example of an apparatus used for continuous
application of active material.
[0030] FIG. 14B is an enlarged cross-sectional view along line B-B
in FIG. 14B.
[0031] FIG. 15 is a plan view illustrating another example of a
positive electrode forming step in a secondary battery
manufacturing method according to the present invention.
[0032] FIG. 16 is a plan view illustrating a step following FIG. 15
in the secondary battery manufacturing method according to the
present invention.
[0033] FIG. 17A is a plan view illustrating a step following FIG.
16 in the secondary battery manufacturing method according to the
present invention.
[0034] FIG. 17B is a plan view illustrating a positive electrode
formed as a result of cutting in the step illustrated in FIG.
17A.
[0035] FIG. 18 is a perspective diagram illustrating an electrode
roll used in the secondary battery manufacturing method illustrated
in FIGS. 15 to 17B.
[0036] FIG. 19 is an enlarged view illustrating a major portion of
a stacked type secondary battery according to a related art.
DESCRIPTION OF EXEMPLARY EMBODIMENT
[0037] An exemplary embodiment of the present invention will be
described below with reference to the drawings.
[Basic Configuration of Secondary Battery]
[0038] FIG. 1 schematically illustrates an example of a
configuration of a stacked type lithium ion secondary battery
employing the present invention. FIG. 1A is a plan view as viewed
from the upper side perpendicular to a principal surface (planar
surface) of the secondary battery, and FIG. 1B is a cross-sectional
view along line A-A in FIG. 1A.
[0039] Lithium ion secondary battery 100 according to the present
invention includes an electrode stack (battery electrode assembly)
formed by alternately stacking positive electrodes (positive
electrode sheets) 1 and negative electrodes (negative electrode
sheets) 6 via separators 20. The electrode stack is housed together
with an electrolyte in an outer container consisting of flexible
films 30. One end of positive electrode terminal 11 is connected to
positive electrodes 1 of the electrode stack, and one end of
negative electrode terminal 16 is connected to negative electrodes
6, and another end side of positive electrode terminal 11 and
another end side of negative electrode terminal 16 extend to the
outside of the flexible films 30. In FIG. 1B, illustration of a
portion of the layers (layers positioned in the intermediate
portion in a thickness direction) included in the electrode stack
is omitted but the electrolyte is illustrated.
[0040] Each positive electrode 1 includes positive-electrode
current collector 3 and positive-electrode active materials 2
applied to positive-electrode current collector 3, and on each of a
front surface and a back surface of positive-electrode current
collector 3, a coated portion to which positive-electrode active
material 2 has been applied and an non-coated portion to which
positive-electrode active material 2 has not been applied are
positioned side by side along a longitudinal direction. Likewise,
each negative electrode 6 includes negative-electrode current
collector 8 and negative-electrode active materials 7 applied to
negative-electrode current collector 8, and on each of a front
surface and a back surface of negative-electrode current collector
8, a coated portion and an non-coated portion are positioned side
by side along the longitudinal direction. A planar position of
boundary portion 4 between the coated portion and the non-coated
portion of each positive electrode 1 and a planar position of
boundary portion 4 between the coated portion and the non-coated
portion of each negative electrode 6 may be the same or different
(not aligned in planar view) between the front surface and the back
surface of the relevant current collector.
[0041] The non-coated portion of each of positive electrodes 1 and
negative electrodes 6 is used as a tab for connection with an
electrode terminal (positive electrode terminal 11 or negative
electrode terminal 16). The positive electrode tabs connected to
respective positive electrodes 1 are bundled on positive electrode
terminal 11 and are mutually connected together with positive
electrode terminal 11 by means of, e.g. ultrasonic welding. The
negative electrode tabs connected to respective negative electrodes
6 are bundled on negative electrode terminal 16 and are mutually
connected together with negative electrode terminal 16 by means of,
e.g., ultrasonic welding. On that basis, the other end portion of
positive electrode terminal 11 and the other end portion of
negative electrode terminal 16 extend to the outside of the outer
container.
[0042] As illustrated in FIG. 2, insulating material 40 for
preventing the occurrence of a short circuit in negative electrode
terminal 16 is formed so as to cover boundary portion 4 between the
coated portion and the non-coated portion of each positive
electrode 1. Insulating material 40 is preferably formed to
straddle both the positive electrode tab and positive-electrode
active material 2 so as to cover boundary portion 4. Formation of
insulating material 40 will be described later.
[0043] Outer dimensions of the coated portion (negative-electrode
active material 7) of each negative electrode 6 are larger than
those of the coated portion (positive-electrode active material 2)
of each positive electrode 1 and are smaller than or equal to those
of each separator 20.
[0044] In the battery illustrated in FIGS. 1A and 1B, examples of
positive-electrode active material 2 include layered oxide
materials such as LiCoO.sub.2, LiNiO.sub.2,
LiNi.sub.(1-x)CoO.sub.2, LiNi.sub.x(CoAl).sub.(1-x)O.sub.2,
Li.sub.2MO.sub.3--LiMO.sub.2,
LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2, spinel materials such as
LiMn.sub.2O.sub.4, LiMn.sub.1.5Ni.sub.0.5O.sub.4,
LiMn.sub.(2-x)M.sub.xO.sub.4, olivine materials such as
LiMPO.sub.4, olivine fluoride materials such as Li.sub.2MPO.sub.4F
and Li.sub.2MSiO.sub.4F, and vanadium oxide materials such as
V.sub.2O.sub.5. One of the above materials or a mixture of two or
more selected from among the above materials may be used as
positive-electrode active material 2.
[0045] As negative-electrode active material 7, carbon materials
such as graphite, amorphous carbon, diamond-like carbon, fullerene,
carbon nanotube, carbon nanohorn, lithium metal materials, silicon-
or tin-based alloy materials, oxide-based materials such as
Nb.sub.2O.sub.5 and TiO.sub.2, or a composite of them may be
used.
[0046] A binding agent and/or a conductive assistant may
arbitrarily be added to positive-electrode active material 2 and
negative-electrode active material 7. As the conductive assistant,
carbon black or carbon fiber or graphite or the like can be used
and the combination of two or more of the above materials can be
used. As the binding agent, polyvinylidene fluoride,
polytetrafluoroethylene, carboxymethyl cellulose, modified
acrylonitrile rubber particles or the like may be used.
[0047] As positive-electrode current collector 3, aluminum,
stainless steel, nickel, titanium or an alloy containing any of
these materials can be used, and in particular, aluminum is
preferable. As negative-electrode current collector 8, copper,
stainless steel, nickel, titanium or an alloy containing any of
these materials can be used.
[0048] As the electrolyte, one organic solvent selected from among
cyclic carbonates such as ethylene carbonate, propylene carbonate,
vinylene carbonate and butylene carbonate, chain carbonates such as
ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl
carbonate (DMC) and dipropyl carbonate (DPC), aliphatic carboxylic
acid esters, .gamma.-lactones such as .gamma.-butyrolactone, chain
ethers, and cyclic ethers may be used and the mixture of two or
more of the above materials may be used. Furthermore, a lithium
salt can be dissolved in the organic solvent(s).
[0049] Separator 20 is formed mainly of a porous membrane, woven
fabric, nonwoven fabric that are made of resin. As the resin
component in separator 20, polyolefin-based resin such as
polypropylene or polyethylene, polyester resin, acrylic resin,
styrene resin, nylon resin or the like can be used, for example. A
polyolefin-based microporous membrane is particularly preferable
because the polyolefin-based microporous membrane has excellent
ion-permeating properties and excellent performance characteristics
for physically separating a positive electrode and a negative
electrode. A layer containing inorganic particles may be formed in
separator 20 as needed. Examples of the inorganic particles include
insulating oxide, silicate, nitride, and carbide. In particular,
the inorganic particles preferably contain TiO.sub.2 or
Al.sub.2O.sub.3.
[0050] As the outer container, a case made of flexible film 30 or a
can case can be used. From the point of view of battery weight
reduction, using flexible film 30 is preferable. As flexible film
30, a film in which resin layers are provided on both the front and
rear surfaces of a metal layer as a base material can be used. As
the metal layer, a layer having barrier properties which may be
properties for preventing leakage of an electrolyte and
infiltration of moisture from the outside can be selected, and
aluminum, stainless steel or the like can be used. A
thermally-fusible resin layer such as modified polyolefin is
provided on at least one surface of the metal layer. The
thermally-fusible resin layers of flexible film 30 are opposite to
each other and are thermally fused to each other in the portion
that surrounds the space where the laminated electrode assembly is
stored, thereby the outer container is formed. A resin layer such
as a nylon film and a polyester film may be provided on the surface
of the outer container opposite to the surface on which the
thermally-fusible resin layer is formed.
[0051] As positive electrode terminal 11, a terminal formed of
aluminum or aluminum alloy can be used. As negative electrode
terminal 16, a terminal formed of copper, copper alloy, or
nickel-plated copper or copper alloy can be used. Each of the other
ends of terminals 11 and 16 extends to the outside of the outer
container. Thermally-fusible resin can be provided in advance at
each of the positions of terminals 11 and 16 corresponding to the
thermal-welded portions of the outer peripheral of the outer
container.
[0052] Insulating material 40 that is formed to cover boundary
portion 4a between a coated portion and an non-coated portion of
positive electrode active material 2 can be made of polyimide,
glass fiber, polyester, polypropylene, or a material including
these. Insulating material 40 may be formed by applying heat to a
tape-like resin member to weld the resin member to boundary portion
4a or by applying a gel resin to boundary portion 4a and drying the
resin.
[Detailed Configuration of Electrodes]
[0053] FIG. 2 is a schematic cross-sectional view for describing an
exemplary embodiment of lithium ion secondary battery 100 according
to the present invention, and schematically illustrates only a
portion of an electrode stack in an enlarged manner. Here, a
portion around an end portion on the positive electrode tab side of
positive-electrode active material 2 is illustrated. FIG. 3
illustrates an electrode stack including positive electrodes 1.
[0054] As illustrated in FIGS. 2 and 3, positive-electrode active
material 2 is formed on each of opposite surfaces of each
positive-electrode current collector 3, and although not
illustrated in FIGS. 1A and 1B, insulating material 40 is provided
to straddle a coated portion to which positive-electrode active
material 2 has been applied and an non-coated portion (positive
electrode tab) to which positive-electrode active material 2 has
not been applied. First positive-electrode active material layer 2A
formed on one surface (upper surface in FIG. 2) of each
positive-electrode current collector 3 includes flat portion
2A.sub.1, inclined portion 2A.sub.2 and thin-layer portion
2A.sub.3. Thin-layer portion 2A.sub.3 is a portion positioned on
the end portion side (positive electrode tab side) relative to the
flat portion 2A.sub.1, the portion having a thickness that is
smaller than that of flat portion 2A.sub.1. Inclined portion
2A.sub.2 is a portion whose thickness continuously decreases so as
to smoothly connect thick flat portion 2A.sub.1 and thin-layer
portion 2A.sub.3. However, instead of inclined portion 2A.sub.2, a
stepped portion whose thickness intermittently decreases may be
provided. On the other hand, second positive-electrode active
material layer 2B formed on another surface (lower surface in FIG.
2) of each positive-electrode current collector 3 includes only a
flat portion. One end portion 40a of insulating material 40 is
positioned on thin-layer portion 2A.sub.3 of first
positive-electrode active material layer 2A, and another end
portion 40b is positioned on the non-coated portion, that is, the
portion of positive-electrode current collector 3 on which
positive-electrode active material 2 is not formed (positive
electrode tab). As illustrated in FIG. 3, in each negative
electrode 6, also, negative-electrode active material 7 is applied
to each of a front surface and a back surface of each
negative-electrode current collector 8; however, negative-electrode
active material 7 includes only a flat portion, and includes
neither an inclined portion nor a thin-layer portion.
[0055] The difference in thickness between flat portion 2A.sub.1
and thin-layer portion 2A.sub.3 of first positive-electrode active
material 2A is preferably larger than the thickness of insulating
material 40. Also, end portion 40a of insulating material 40
positioned on second positive-electrode active material layer 2B is
preferably positioned so as to face thin-layer portion 2A.sub.3 of
first positive-electrode active material layer 2A. Such disposition
enables preventing or reducing an increase in thickness caused by
insulating materials 40 positioned on the opposite surfaces of each
positive-electrode current collector 3. In other words, adjustment
(reduction) of the thickness of an outer edge portion of each first
positive-electrode active material layer 2A (coated portion)
enables preventing or reducing an increase in thickness in the
portion of the electrode stack in which insulating materials 40 are
positioned, thereby preventing characteristics of the battery from
being affected by the thickness increase. In particular, if the
difference in thickness between thin-layer portion 2A.sub.3 and
flat portion 2A.sub.1 of each first positive-electrode active
material layer 2A is no less than twice the thickness of one
insulating material 40, such a degree of difference is effective
because an increase in thickness caused by two insulating materials
40 can be absorbed by the thickness reduction provided by
thin-layer portion 2A.sub.3 of first positive-electrode active
material layer 2A. Here, since it is not necessary that
positive-electrode active material layers 2A and 2B on opposite
surface of positive-electrode current collector 3 have the same
thickness, and thus, even where the thickness of one
positive-electrode active material layer (second positive-electrode
active material layer 2B) is less than twice the thickness of
insulating material 40, as illustrated in FIG. 4, if only flat
portion 2A.sub.1 of another positive-electrode active material
layer (first positive-electrode active material layer 2A) is made
thicker in order that a difference in thickness between flat
portion 2A.sub.1 and thin-layer portion 2A.sub.3 may be no less
than twice the thickness of insulating material 40, the thickness
increase that is caused by two insulating materials 40 can be
absorbed by reduction in the thickness provided by thin-layer
portion 2A.sub.3, and thus a sufficient effect can be provided.
[0056] At an end portion of each negative electrode 6 on the side
that is the same as the end portion of each first
positive-electrode active material layer 2A in which inclined
portion 2A.sub.2 and thin-layer portion 2A.sub.3 are provided as
described above, negative-electrode current collector 8 and flat
negative-electrode active materials 7 formed on opposite surfaces
thereof are cut and terminated. In other words, at the end portion
of each negative-electrode active material 7 on the side that is
the same as the side of each first positive-electrode active
material layer 2A in which inclined portion 2A.sub.2 and thin-layer
portion 2A.sub.3 are provided, neither an inclined portion, nor a
stepped portion nor a thin-layer portion is provided. The end
portion is located at a position facing relevant insulating
material 40 across relevant separator 20.
[0057] In FIG. 3, for ease of viewing, positive electrodes 1,
negative electrode 6 and separators 20 are illustrated so as not to
be in contact with one another; however, in reality, positive
electrodes 1, negative electrode 6 and separators 20 are stacked in
close contact with one another. In the configuration illustrated in
FIG. 3, as described above, the difference in thickness between
inclined portion 2A.sub.1 and thin-layer portion 2A.sub.3 of each
first positive-electrode active material layer 2A is more than
twice the thickness of each insulating material 40, and thus, when
positive electrodes 1, negative electrodes 6 and separators 20 are
brought into close contact with one another, positive electrodes 1
are curved at respective positions of thin-layer portions 2A.sub.3,
enabling preventing or reducing a partial increase in the thickness
of the electrode stack caused by insulating materials 40. As
described above, FIG. 3 illustrates a configuration in which
positive electrodes 1 are curved; however, a configuration in which
only negative electrodes 6 are curved or a configuration in which
both positive electrodes 1 and negative electrodes 6 are curved can
be employed.
[0058] Here, it is not necessary that flat portion 2A.sub.1 and
thin-layer portion 2A.sub.3 be disposed in parallel to each other
on each positive-electrode current collector 3, and an edge of
boundary portion 4 between a coated portion and an non-coated
portion of each positive electrode 1 and an edge of an end portion
of each negative electrode 6 may each have a round curve shape,
rather than a linear shape perpendicular to a direction in which
relevant current collector 3 or 8 extends. It should be understood
that each of positive-electrode active materials 2 and
negative-electrode active materials 7 may include e.g., an
unavoidable inclination, irregularities or roundness of respective
layers due to, for example, manufacturing variations and/or layer
formation capability.
[0059] Each first positive-electrode active material layer 2A may
include a stepped portion whose thickness decreases in a stepwise
fashion, instead of inclined portion 2A.sub.2 whose thickness
gently decreases as illustrated in FIG. 3. Alternatively, each
first positive-electrode active material layer 2A may include both
inclined portion 2A.sub.2 and the stepped portion. Also, it is
possible that: thin-layer portion 2A.sub.3 is not provided
independently from inclined portion 2A.sub.2 and the stepped
portion; and a portion of inclined portion 2A.sub.2 or the stepped
portion which has a decreased thickness, faces relevant insulating
material 40, whereby the thickness increase caused by insulating
material 40 is absorbed. In such a case, the portion of the
inclined portion 2A.sub.2 or the stepped portion which faces
insulating material 40, can be regarded as acting as thin-layer
portion 2A.sub.3. Inclined portions 2A.sub.2 and thin-layer
portions 2A.sub.3 illustrated in FIGS. 2 to 4 and the
non-illustrated stepped portions each have a low density compared
to flat portions 2A.sub.1.
[0060] In the configuration illustrated in FIG. 3, inclined portion
2A.sub.2 and thin-layer portion 2A.sub.3 are formed only in each
first positive-electrode active material layer 2A, rather than an
inclined portion or a stepped portion and a thin-layer portion
being provided in each of both positive-electrode active material
layers 2A and 2B, mainly because the shape of thin-layer portion
2A.sub.3 can be formed with good precision and because the
electrode capacity loss is small.
[0061] For example, if an inclined portion, a stepped portion
and/or a thin-layer portion are provided in each of both first
positive-electrode active material layers 2A and second
positive-electrode active materials 2B, and if insulating material
40 is disposed so as to face the inclined portion, the stepped
portion and/or the thin-layer portion enables preventing or
reducing a partial increase in thickness caused by insulating
materials. However, a thickness reduction causes a reduction in the
amount of active materials, which results in a decrease in battery
capacity. Also, the inventors' careful studies revealed that
provision of thin-layer portion 2A.sub.3 in each of
positive-electrode active material layers 2A and 2B may make it
impossible for thin-layer portion 2A.sub.3 to have a
sufficiently-small thickness. In such a case, the electrodes could
not be used as products and would be discarded as defective
products, which results in deterioration in productivity. Also,
provision of a thin-layer portion, an inclined portion and/or a
stepped portion in each negative-electrode active material 8 of
each negative electrode 6 facing relevant positive electrode 1
across relevant separator 20 has the effect of preventing or
reducing a partial thickness increase caused by insulating
materials 40; however, in such a case, the amount of
negative-electrode active materials 8 decreases, also unfavorably
resulting in a battery capacity decrease.
[0062] For a more detailed evaluation, it was found that a failure
to form a thin-layer portion, an inclined portion and/or a stepped
portion of positive-electrode active material 2 with good precision
and unstable formation of the thin-layer portion, the inclined
portion and/or the stepped portion are partly attributable to a
tendency of such portions being formed so as to lean toward either
first positive-electrode active material 2A or second
positive-electrode active material 2B. This will be described using
the reference example illustrated in FIGS. 5A and 5B.
[0063] FIG. 5A is a schematic diagram indicating a coating portion
of a die coater, which is a kind of apparatuses for coating
electrodes. The die coater applies slurry 200 to a current
collector between die head 500 and back roll 400. Slurry 200
containing an active material is discharged from discharge port 501
of die head 500 toward the current collector transported on the
outer peripheral surface of back roll 400. The thickness of slurry
200 on the current collector is controlled by adjusting, e.g., a
space between the current collector and discharge port 501, the
discharge amount and/or the application speed according to, e.g., a
viscosity of slurry 200. In the example illustrated in FIGS. 5A and
5B, slurry 200 containing positive-electrode active material 2 is
intermittently applied to positive-electrode current collector 3.
It should be understood that slurry 200 can continuously be applied
to positive-electrode current collector 3.
[0064] FIG. 5B illustrates a state in which after application of
first positive-electrode active material layer 2A to one surface of
positive-electrode current collector 3 and after drying of first
positive-electrode active material layer 2A, second
positive-electrode active material 2B is applied to another surface
of positive-electrode current collector 3. Each of first
positive-electrode active material layer 2A and second
positive-electrode active material layer 2B is intermittently
formed, and an inclined portion and a thin-layer portion are formed
at each opposite end (an application start end and an application
termination end) of each coated portion. When slurry 200 is
discharged from discharge port 501 of die head 500 in order to form
second positive-electrode active material layer 2B on the surface
of positive-electrode current collector 3 on the side opposite to a
surface on which first positive-electrode active material layer 2A
has already been formed, a gap is generated between the inclined
portion and the thin-layer portion of each first positive-electrode
active material 2A and back roll 400. Slurry 200 is pressurized in
die head 500, and upon discharge of slurry 200, positive-electrode
current collector 3 is pushed in a direction in which gap h is
eliminated, that is, toward the back roll 400 side, whereby the
space between the discharge port 501 and positive-electrode current
collector 3 is increased. As described above, it was found that if
an active material including an inclined portion and a thin-layer
portion is formed on one surface and then an active material is
formed on another surface, the space between discharge port 501 and
the current collector is not stable and an active material that is
subsequently formed tends to have an unstable thickness and
inclination.
[0065] Therefore, in the present invention, as illustrated in FIG.
6, after flat second positive-electrode active material layer 2B,
that does not include a thin-layer portion, an inclined portion,
and a stepped portion are formed on positive-electrode current
collector 3; first positive-electrode active material layer 2A,
that includes thin-layer portion 2A.sub.3 and inclined portion
2A.sub.2 are formed on a surface opposite to the surface on which
positive-electrode active material layer 2B has been formed. Then,
the portion of positive-electrode current collector 3, to which
first positive-electrode active material layer 2A will be
subsequently applied, is a portion on the opposite side of a
portion in which flat second positive-electrode active material
layer 2B comes into close contact with back roll 400 with no gap
therebetween. Since no gap is generated between second
positive-electrode active material 2B and back roll 400, when first
positive-electrode active material 2A is formed on
positive-electrode current collector 3, the space between the
discharge port 501 and positive-electrode current collector 3 is
extremely stable, enabling formation of inclined portion 2A.sub.2
and thin-layer portion 2A.sub.3 with very good precision.
Therefore, the difference in thickness between flat portion
2A.sub.1 and thin-layer portion 2A.sub.3 of first
positive-electrode active material layer 2A can be made to be no
less than twice the thickness of insulating material 40 with good
precision. Even if the thickness of positive-electrode active
material 2 is so small such that a thin-layer portion having a
thickness decreased by no less than twice the thickness of the
insulating material cannot be formed and such that a thickness
increase caused by insulating materials 40 cannot completely be
absorbed by first positive-electrode active material layer 2A
alone, the thickness of first positive-electrode active material 2A
can be controlled with good precision, enabling preventing or
reducing a partial increase in the thickness of the electrode stack
by reducing a thickness of either or both of the negative-electrode
active materials to the minimum necessary at respective positions
facing the insulating materials.
[0066] As described above, provision of a thin-layer portion, an
inclined portion and/or a stepped portion in the active material
provided on one surface of a current collector effectively prevents
or reduces a partial increase in the thickness at a position where
insulating materials are provided, and in addition, providing
neither a thin-layer portion, nor an inclined portion, nor a
stepped portion in the active material provided on another surface
of the current collector enables productivity enhancement.
[Electrode Manufacturing Method]
[0067] First, as described above, in the step illustrated in FIG.
6, positive-electrode active material 2 is intermittently applied
to each of the opposite surfaces of long band-like
positive-electrode current collector 3 for manufacturing a
plurality of positive electrodes (positive electrode sheets) 1. In
FIG. 7, a surface on the first positive-electrode active material
layer 2A side of positive-electrode current collector 3 with
positive-electrode active material 2 applied to each of the
opposite surfaces thereof is illustrated. Although not clearly
illustrated in FIG. 7, each first positive-electrode active
material layer 2A includes inclined portion 2A.sub.2 and thin-layer
portion 2A.sub.3 in the vicinity of boundary portion 4, which
serves as a positive electrode tab. Then, as illustrated in FIG. 8,
insulating material 40 is formed so as to cover boundary portion 4.
As illustrated in FIGS. 2 and 3, one end portion 40a of insulating
material 40 is positioned on thin-layer portion 2A.sub.3, and
another end portion 40b of insulating material 40 is positioned on
an non-coated portion. If the thickness of insulating material 40
is too small, a sufficient insulating property cannot be ensured
and thus the thickness is preferably no less than 10 .mu.m. Also,
if the thickness of the insulating material 40 is excessively
large, the effect of preventing or reducing an increase in the
thickness of the electrode stack, which is provided by the present
invention, cannot be sufficiently realized, and thus, insulating
material 40 is preferably smaller in thickness than the flat
portion of positive-electrode active material 2. The thickness of
insulating material 40 is preferably no more than 90% of the
thickness of the flat portion of positive-electrode active material
2, more preferably no more than 60% of the thickness of flat
portion 2b. Although the end portion of each coated portion
(positive-electrode active material 2) at boundary portion 4
between the coated portion and the relevant non-coated portion may
rise substantially perpendicularly to relevant positive-electrode
current collector 3 as illustrated in FIGS. 2 to 4, the end portion
may be slightly inclined as illustrated in FIG. 19. Also, in each
negative electrode 6, the end portion of each coated portion
(negative-electrode active material 7) may be slightly inclined or
rise substantially perpendicular to relevant negative-electrode
current collector 8.
[0068] Subsequently, in order to obtain positive electrodes 1 used
for individual stacked type batteries, positive-electrode current
collector 3 is cut along each cutting line 90 indicated by a dashed
line in FIG. 9A to obtain positive electrodes 1 of a desired size,
one of which is illustrated in FIG. 9B. The cutting lines 90 are
imaginary lines and thus not actually formed.
[0069] Meanwhile, with a method that is similar to the step
illustrated in FIG. 6, negative-electrode active material 7 is
intermittently applied to each of the opposite surfaces of large
negative-electrode current collector 8, which is provided for
manufacturing a plurality of negative electrodes (negative
electrode sheets) 6. In FIG. 10, negative-electrode current
collector 8 with negative-electrode active material 7 applied on
each of the opposite surfaces thereof is illustrated. If the
difference in thickness between flat portion 2A.sub.1 and
thin-layer portion 2A.sub.3 of each first positive-electrode active
material layer 2A is no less than twice the thickness of each
insulating material 40 as illustrated in FIGS. 2 and 3,
negative-electrode active material 7 may include a flat portion
alone in which neither an inclined portion, nor a thin-layer
portion, nor a stepped portion are present.
[0070] Subsequently, in order to obtain negative electrodes 6 to be
used for individual stacked type batteries, negative-electrode
current collector 8 is divided by cutting negative-electrode
current collector 8 along each cutting line 91 indicated by a
dashed line in FIG. 11A to obtain negative electrodes 6 having a
desired size, one of which is illustrated in FIG. 11B. Cutting
lines 91 are imaginary lines and thus are not actually formed.
[0071] Positive electrodes 1 illustrated in FIG. 9B and negative
electrodes 6 illustrated in FIG. 11B formed as described above are
alternately stacked via separators 20, and positive electrode
terminal 11 and negative electrode terminal 16 are connected to the
stacked electrodes, whereby the electrode stack illustrated in FIG.
3 is formed. The electrode stack is housed and sealed together with
electrolyte in an outer container including flexible films 30,
whereby secondary battery 100 illustrated in FIGS. 1A and 1B is
formed. In secondary battery 100 according to the present
invention, which has been formed as described above, one end
portion 40a of each insulating material 40 is positioned on
thin-layer portion 2A.sub.3 of relevant first positive-electrode
active material layer 2A.
[0072] According to secondary battery 100, the amount of thickness
increase caused by each insulating material 40 formed so as to
cover boundary portion 4 between the coated portions and the
non-coated portion of relevant positive electrode 1 is absorbed
(cancelled out) by the thickness reduction provided by thin-layer
portion 2A.sub.3 and inclined portion 2A.sub.2 of relevant first
positive-electrode active material layer 2A, preventing or reducing
a partial increase in the thickness of the electrode stack, and
thus, the electrode stack can be uniformly fastened and held in
place, thereby preventing or reducing a deterioration in product
quality as regards, for example, variability in the electric
characteristics and battery cycle degradation.
[0073] In the example illustrated in FIG. 11B, the coated portion
at each of the opposite surfaces of each negative electrode 6 is
cut and terminated at a position facing the non-coated portion
(positive electrode tab) of relevant positive electrode 1, and as
illustrated in FIG. 3, at a position facing the non-coated portion
of each positive electrode 1, negative-electrode active material 7
exists on the front and back of negative-electrode current
collector 8 with no non-coated portion provided. However, each
negative electrode 6 may also be configured in such a manner that
an non-coated portion is present at a position in negative
electrode 6, the position facing the non-coated portion of positive
electrode 1. As illustrated in FIG. 11B, at an end portion of each
negative electrode 6, the end portion not facing the non-coated
portion of relevant positive electrode 1, an non-coated portion,
which serves as a negative electrode tab, is provided. If
insulating material (not illustrated) is provided on a boundary
portion between the coated portion and the non-coated portion of
each negative electrode 6, as in the case in which a thickness
increase caused by insulating material 40 is cancelled out by means
of each positive electrode 1, a thin-layer portion, an inclined
portion and/or a stepped portion having a small thickness may be
provided in each negative-electrode active material or in each
positive-electrode active material, and insulating material may be
disposed at a position facing the thin-layer portion, the inclined
portion and/or the stepped portion.
[0074] As illustrated in FIG. 12, an inclined portion 7a can be
provided in at least one of negative-electrode active materials 7
in each negative electrode 6 to further reduce the possibility of
battery distortion due to insulating materials 40 provided on
positive electrodes 1. Each insulating material 40 with one end
portion 40a positioned on thin-layer portion 2A.sub.3 of relevant
first positive-electrode active material layer 2A is preferably
formed in such a manner that the total thickness of two insulating
materials 40 is no larger than the difference in thickness between
flat portion 2A.sub.1 and thin-layer portion 2A.sub.3 of each first
positive-electrode active material layer 2A. However, manufacturing
variations may prevent the difference in thickness between flat
portion 2A.sub.1 and thin-layer portion 2A.sub.3 of each first
positive-electrode active material layer 2A from becoming the
desired size. Even if such manufacturing variations occur, the
presence of inclined portion 7a of each negative-electrode active
material 7 enables the thickness increase caused by manufacturing
variations of positive electrodes 1 to be absorbed (cancelled out).
In FIG. 12, a configuration in which inclined portion 7a of each
negative-electrode active material 7 is positioned facing
insulating material 40 on relevant first positive-electrode active
material layer 2A, which includes inclined portion 2A.sub.1 and
thin-layer portion 2A.sub.3, across relevant separator 20 is
illustrated as an example. However, inclined portion 7a may be
disposed so as to face insulating material 40 on relevant second
positive-electrode active material layer 2B, which has neither an
inclined portion nor a thin-layer portion, across relevant
separator 20.
[0075] Unless otherwise specified, each of thicknesses, distances,
etc., of the respective members in the present invention means the
average value of values measured at three or more arbitrary
positions.
EXAMPLES
Example 1
[0076] According to the manufacturing method described with
reference to FIGS. 6 to 12, a lithium ion secondary battery was
manufactured.
[0077] First, a mixed active material of LiMn.sub.2O.sub.4 and
LiNi.sub.0.8Co.sub.0.1Al.sub.0.1O.sub.2 was used as
positive-electrode active material, carbon black was used as a
conductive agent, and PVdF was used as binder, and slurry 200 in
which a compound agent consisting of these materials is dispersed
in an organic solvent was prepared. This slurry 200 was
intermittently applied to positive-electrode current collector 3
having a thickness of 20 .mu.m and mainly consisting of aluminum
and then dried, whereby second positive-electrode active material
layers 2B having a thickness of 80 .mu.m was formed. As a result of
the intermittent application of positive-electrode active material
2, coated portions coated with positive-electrode active material 2
and non-coated portions not coated with positive-electrode active
material 2 are alternately present along a longitudinal direction
of positive-electrode current collector 2. Next, as illustrated in
FIGS. 6 and 7, positive-electrode active material 2 was
intermittently applied to the surface of positive-electrode current
collector 3 on the side that is opposite to the side on which
second positive-electrode active material layers 2B was formed, and
then dried, whereby first positive-electrode active material layers
2A were formed. Each first positive-electrode active material layer
2A was configured so as to include flat portion 2A.sub.1 having a
thickness of 80 .mu.m, thin-layer portion 2A.sub.3 having a
thickness 20 .mu.m and inclined portion 2A.sub.2 whose thickness
continuously decreases between flat portion 2A.sub.1 and thin-layer
portion 2A.sub.3.
[0078] A method of applying an active material to a current
collector will be described. As an apparatus that applies active
material, any device that utilizes various coating methods
including transfer methods or vapor deposition methods, such as
doctor blades, die coaters and gravure coaters, may be used. In the
present invention, in order to control the position of an end
portion of applied active material, it is particularly preferable
to use a die coater such as illustrated in FIG. 6. Methods of
coating active material that use a die coater are generally
classified into two methods: a continuous application method in
which active material is continuously formed in the longitudinal
direction of a long current collector, and an intermittent coating
method in which coated portions coated with active material and
non-coated portions not coated with the active material are
alternately formed along a longitudinal direction of a current
collector.
[0079] FIG. 13 is a diagram illustrating an example of a
configuration of a die coater that intermittently applies active
material. As illustrated in FIG. 13, a slurry flow path of a die
coater that performs intermittent coating includes die head 500,
coating valve 502 connected to die head 500, pump 503, and tank 504
that stores slurry 200. Also, return valve 505 is provided between
tank 504 and coating valve 502. In this configuration, at least for
coating valve 502, it is preferable to use a motor valve. A motor
valve can vary an open/closed state of the valve with good
precision even while slurry 200 is being applied. Therefore, e.g.,
the flow path of slurry 200 is controlled by coating valve 502,
which includes a motor valve, in combination with the operation of
return valve 505, enabling a coated portion (flat portion 2A.sub.1,
inclined portion 2A.sub.2 or a stepped portion and thin-layer
portion 2A.sub.3) of each active material, an non-coated portion
and a boundary portion therebetween to be formed into respective
desired shapes.
[0080] Also, active material can be formed by being continuously
applied using the die coater schematically illustrated in FIGS. 14A
and 14B. At each of opposite end portions of discharge port 501 of
die head 500 of the die coater, shim 501b including a tapered
portion or stepped portion 501a whose thickness decreases toward
the center portion of discharge port 501 is provided. Shims 501b
enable formation of active material in such a manner that a stepped
portion or an inclined portion and a thin-layer portion are formed
at an end portion of each coated portion.
[0081] After the coating positive-electrode active material 2 on
positive-electrode current collector 3 as described above, as
illustrated in FIG. 8, polypropylene insulating tape (insulating
materials) 40 having a thickness of 30 .mu.m was attached to
boundary portion 4 between the coated portion and the non-coated
portion of each positive electrode 1. Here, insulating tape 40,
provided so as to cover boundary portion 4 on one surface of each
positive-electrode active material 2, was formed so that end
portion 40a is positioned on thin-layer portion 2A.sub.3 of
relevant first positive-electrode active material layer 2A.
Insulating tape 40, provided so as to cover boundary portion 4 on
another surface of each positive-electrode active material 2, was
disposed so that one end portion 40a faces thin-layer portion
2A.sub.3 of relevant first positive-electrode active material layer
2A across positive-electrode current collector 3. Then, as
illustrated in FIGS. 9A and 9B, positive-electrode current
collector 3 was cut along each cutting line 90 to obtain individual
positive electrodes 1.
<Negative Electrodes>
[0082] Graphite with a surface coated with an amorphous material
was used as negative-electrode active material 7 and PVdF was used
as a binder, and a slurry in which a compound agent of these
materials is dispersed in an organic solvent was prepared. As
illustrated in FIG. 10, the slurry was intermittently applied to a
copper foil having a thickness of 15 .mu.m, which is
negative-electrode current collector 8, and then dried to fabricate
a negative electrode roll including coated portions coated with
negative-electrode active material 7 and non-coated portions not
coated with negative-electrode active material 7 as with positive
electrodes 1. Each negative-electrode active material 7 includes
only a flat portion having a thickness of 55 .mu.m. A specific
method for applying negative-electrode active material 7 is similar
to the aforementioned method for applying positive-electrode active
material 2, and active material may be intermittently applied using
the die coater illustrated in FIG. 13 or may be continuously
applied using the die coater illustrated in FIGS. 14A and 14B.
Then, as illustrated in FIGS. 11A and 11B, negative-electrode
current collector 8 was cut along each cutting line 91 to obtain
individual negative electrodes 6. Each negative electrode 6
includes a negative electrode tab, which is an non-coated portion
not coated by negative-electrode active material 7, at a position
that does not face a positive electrode tab, and negative-electrode
current collector 8 was cut at a portion that faces a positive
electrode tab and that has negative-electrode active materials 7 on
both surfaces thereof. Insulating material is not provided at a
boundary portion between the coated portion and the non-coated
portion of each negative electrode 6.
<Manufacturing of Stacked Type Battery>
[0083] Obtained positive electrodes 1 and negative electrodes 6
were alternately stacked via separators 20 having a thickness of 25
.mu.m, each separator 20 made of polypropylene, and negative
electrode terminal 16 and positive electrode terminal 11 were
attached to the stack, which was then housed in an outer container
consisting of flexible films 30, whereby a stacked type secondary
battery having a thickness of 8 mm was obtained.
Example 2
[0084] Using a compound agent containing LiMn.sub.2O.sub.4, which
is active material 2, carbon black, which is a conductive agent,
and PVdF, which is a binder, positive-electrode active material 2
was formed on each of the opposite surfaces of positive-electrode
current collector 3. Each first positive-electrode active material
layer 2A according to the present example includes a flat portion
2A.sub.1 having a thickness of 35 .mu.m, a thin-layer portion
2A.sub.3 having a thickness of 5 .mu.m, and an inclined portion
2A.sub.2 whose thickness continuously decreases between flat
portion 2A.sub.1 and thin-layer portion 2A.sub.3. Each second
positive-electrode active material layer 2B includes only a flat
portion having a thickness of 35 .mu.m. Next, as in example 1,
polypropylene insulating tapes (insulating materials) 40 having a
thickness of 30 .mu.m were attached and then positive-electrode
current collector 3 was cut to obtain individual positive
electrodes 1.
[0085] Also, using hardly (barely) graphitizable carbon as
negative-electrode active material 7, negative-electrode active
material 7 was formed on each of the opposite surfaces of
negative-electrode current collector 8. Negative-electrode active
materials 7 according to the present example, as with first
positive-electrode active material 2A, were configured so as to
each include a flat portion having a thickness of 35 .mu.m, a
thin-layer portion having a thickness of 5 .mu.m and an inclined
portion whose thickness continuously decreases from the flat
portion to the thin-layer portion. Then, the inclined portion and
the thin-layer portion of each negative electrode 6 was disposed so
as to face inclined portion 2A.sub.2 and thin-layer portion
2A.sub.3 of relevant first positive-electrode active material layer
2A via a relevant separator. The rest of the conditions was made to
be similar to those of example 1, and a stacked type secondary
battery having a thickness of 3 mm was obtained.
Comparative Example
[0086] A positive-electrode active material on each of the opposite
surfaces of positive-electrode current collector 3 was formed as a
layer having a uniform thickness, the layer having neither a
thin-layer portion, nor an inclined portion, nor a stepped portion,
and was configured so as to consist essentially of a flat portion
with no inclined portion provided. The rest of the conditions was
made to be similar to those of example 1, and a stacked type
secondary battery was obtained. The thickness of the stacked type
battery was approximately 8 mm at the center portion and
approximately 9 mm around an end portion.
<Evaluation>
[0087] To evaluate the discharge capacities and the cycle
characteristics of the stacked type batteries obtained in the above
manner, 10 stacked type batteries for each of examples and
comparative example were evaluated. It was found that the stacked
type batteries according to examples 1 and 2 provide a very stable
discharge capacity and cycle characteristics, and that the
discharge capacity and cycle characteristics of the battery
according to the comparative example are unstable compared to those
of the batteries according to examples 1 and 2. The stable battery
characteristics can be considered as resulting from preventing or
reducing an increase in the thickness of a portion of the stacked
type battery in which insulating materials 40 are positioned from
being increased so as to be larger than a thickness of the rest of
the portions and thus enabling the stacked type battery to be held
in place while uniform pressure is applied to it.
[0088] In each of the above examples, positive-electrode active
materials 2 and negative-electrode active materials 7 are formed by
being intermittently applied; however, as illustrated in FIGS. 15
to 17B, positive-electrode active materials 2 and
negative-electrode active materials 7 may be formed by being
continuously applied so as to form active material layer with no
intervals over a plurality of electrode forming portions. Where
active materials are formed by means of being continuously applied,
before the electrodes are cut out along each cutting line 90 in
FIG. 17A, they can be kept in the form of an electrode roll as
illustrated in FIG. 18, and in such a case, extreme distortion of
portions in which insulating materials 40 are disposed can be
prevented, thereby enhancing the product quality of the
electrodes.
[0089] The present invention is useful for manufacturing electrodes
for a lithium ion secondary battery and manufacturing a lithium ion
secondary battery using such electrodes, and is also effectively
employed for a secondary battery other than a lithium ion
battery.
[0090] The present application claims priority from Japanese Patent
Application No. 2013-166462 filed on Aug. 9, 2013, and the entire
disclosure of Japanese Patent Application No. 2013-166462 is
incorporated herein by reference.
REFERENCE NUMERALS
[0091] 1 positive electrode
[0092] 2 positive-electrode active material
[0093] 2A first positive-electrode active material layer
[0094] 2A.sub.1 flat portion
[0095] 2A.sub.2 inclined portion
[0096] 2A.sub.3 thin-layer portion (small-thickness portion)
[0097] 2B second positive-electrode active material layer
[0098] 3 positive-electrode current collector
[0099] 4 boundary portion
[0100] 6 negative electrode
[0101] 7 negative-electrode active material
[0102] 8 negative-electrode current collector
[0103] 20 separator
[0104] 40 insulating material
[0105] 100 secondary battery
* * * * *