U.S. patent application number 15/518352 was filed with the patent office on 2017-10-26 for nonaqueous electrolyte secondary battery and manufacturing method therefor.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Ryo HANASAKI, Takashi HARAYAMA.
Application Number | 20170309953 15/518352 |
Document ID | / |
Family ID | 54477018 |
Filed Date | 2017-10-26 |
United States Patent
Application |
20170309953 |
Kind Code |
A1 |
HARAYAMA; Takashi ; et
al. |
October 26, 2017 |
NONAQUEOUS ELECTROLYTE SECONDARY BATTERY AND MANUFACTURING METHOD
THEREFOR
Abstract
A nonaqueous electrolyte secondary battery includes: an
electrode assembly; a nonaqueous electrolyte; and a battery case.
The electrode assembly includes a positive electrode, a negative
electrode, and a separator. The positive electrode includes a
positive electrode active material layer. The negative electrode
includes a negative electrode active material layer. The separator
is interposed between the positive electrode and the negative
electrode. The battery case accommodates the electrode assembly and
the nonaqueous electrolyte. Ends of contact faces of the negative
electrode active material layer and the separator are at least
partially bonded to each other.
Inventors: |
HARAYAMA; Takashi;
(Toyota-shi, Aichi-ken, JP) ; HANASAKI; Ryo;
(Toyota-shi, Aichi-ken, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Toyota-shi, Aichi-ken |
|
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi, Aichi-ken
JP
|
Family ID: |
54477018 |
Appl. No.: |
15/518352 |
Filed: |
October 16, 2015 |
PCT Filed: |
October 16, 2015 |
PCT NO: |
PCT/IB2015/001910 |
371 Date: |
April 11, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 2/168 20130101;
Y02T 10/70 20130101; Y02E 60/10 20130101; H01M 10/0525
20130101 |
International
Class: |
H01M 10/0525 20100101
H01M010/0525; H01M 2/16 20060101 H01M002/16 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 17, 2014 |
JP |
2014-212860 |
Claims
1. A nonaqueous electrolyte secondary battery comprising: an
electrode assembly including a positive electrode, a negative
electrode and a separator, the positive electrode including a
positive electrode active material layer, the negative electrode
including a negative electrode active material layer, the separator
being interposed between the positive electrode and the negative
electrode; a nonaqueous electrolyte; and a battery case
accommodating the electrode assembly and the nonaqueous
electrolyte, wherein ends of contact faces of the negative
electrode active material layer and the separator are at least
partially bonded to each other.
2. The nonaqueous electrolyte secondary battery according to claim
1, wherein the electrode assembly is a flat rolled electrode
assembly that is formed by stacking the long positive electrode,
the long negative electrode and the long separator and then rolling
the stacked positive electrode, negative electrode and separator
into an oval shape in cross section with a rolling axis set to a
width direction perpendicular to a longitudinal direction of the
long positive electrode, the long negative electrode and the long
separator, and the negative electrode active material layer and the
separator are bonded to each other at a flat portion of an
electrode face of the flat rolled electrode assembly in a
band-shaped region along each of ends in the width direction.
3. The nonaqueous electrolyte secondary battery according to claim
1, wherein the entire contact faces of the negative electrode
active material layer and the separator are bonded to each
other.
4. The nonaqueous electrolyte secondary battery according to claim
1, wherein the positive electrode active material layer and the
separator are not bonded to each other.
5. The nonaqueous electrolyte secondary battery according to claim
1, wherein a heat-resistant layer including an electrically
insulated inorganic filler is provided on a surface of the positive
electrode active material layer.
6. The nonaqueous electrolyte secondary battery according to claim
1, wherein the negative electrode active material layer and the
separator are bonded to each other such that a peeling strength
becomes larger than or equal to 0.2 N/m and smaller than or equal
to 1.2 N/m.
7. A manufacturing method for a nonaqueous electrolyte secondary
battery, comprising: constructing a cell by accommodating an
electrode assembly in a battery case, the electrode assembly
including a positive electrode, a negative electrode and a
separator, the positive electrode including a positive electrode
active material layer, the negative electrode including a negative
electrode active material layer, the separator being interposed
between the positive electrode and the negative electrode;
supplying a nonaqueous electrolyte into the battery case; and at
least partially bonding ends of contact faces of the negative
electrode active material layer and the separator to each other in
advance of supplying the nonaqueous electrolyte.
8. The manufacturing method according to claim 7, wherein the
negative electrode active material layer includes a negative
electrode active material and a binder, and the negative electrode
and the separator are bonded to each other by, at the time of
constructing the cell, bringing the negative electrode active
material layer of the negative electrode and the separator into
contact with each other and then heating while at least partially
pressing the ends of the contact faces.
9. The manufacturing method according to claim 7, further
comprising: drying the cell after constructing the cell, wherein
the negative electrode active material layer includes a negative
electrode active material and a binder, and the negative electrode
and the separator are bonded to each other by, in drying the cell,
heating while applying a pressure such that the ends of the contact
faces of the negative electrode active material layer and the
separator in the electrode assembly are at least partially pressed
from an outside of the battery case.
10. The manufacturing method according to claim 8, wherein the
negative electrode and the separator are bonded to each other by
applying a pressure higher than or equal to 0.01 MPa and lower than
or equal to 1 MPa between the negative electrode active material
layer and the separator in a temperature range higher than or equal
to a softening point of the binder and lower than or equal to
125.degree. C.
11. The manufacturing method according to claim 7, wherein the
negative electrode and the separator are bonded to each other such
that a peeling strength becomes larger than or equal to 0.2 N/m and
smaller than or equal to 1.2 N/m.
12. The manufacturing method according to claim 7, further
comprising: applying aging.
13. The manufacturing method according to claim 9, wherein the
negative electrode and the separator are bonded to each other by
applying a pressure higher than or equal to 0.01 MPa and lower than
or equal to 1 MPa between the negative electrode active material
layer and the separator in a temperature range higher than or equal
to a softening point of the binder and lower than or equal to
125.degree. C.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The invention relates to a nonaqueous electrolyte secondary
battery in which occurrence of a micro short circuit due to foreign
metal substances is reduced, and a manufacturing method for the
nonaqueous electrolyte secondary battery.
2. Description of Related Art
[0002] A nonaqueous electrolyte secondary battery, typically, a
lithium secondary battery, has a smaller size, a lighter weight, a
higher energy density and a more excellent power density than an
existing battery. For this reason, in recent years, the nonaqueous
electrolyte secondary battery is suitably used as a so-called
portable power supply for a personal computer, a mobile terminal,
or the like, or a power supply for propelling a vehicle.
[0003] The nonaqueous electrolyte secondary battery is typically
constructed by accommodating an electrode assembly in a battery
case in a hermetically sealed state.
[0004] The electrode assembly has such a structure that a positive
electrode and a negative electrode are opposed to each other via a
separator. Initial charging is applied to the constructed battery
in a predetermined condition in order to adjust the battery to an
actually usable state. Aging, or the like, is also applied to the
battery after initial charging mainly for the purpose of checking
for stabilization of a battery reaction, a short circuit, or the
like.
[0005] Incidentally, in manufacturing a nonaqueous electrolyte
secondary battery, for example, there is a case where foreign metal
substances are included in a battery case as impurities (inclusion)
of a constituent member or inevitable metal fine particles, and the
like, that are developed at the time of constructing the battery.
The included foreign metal substances can be ionized in an
environment that exceeds a dissolved potential as a result of
charging, and then dissolved into an electrolyte. The ionized
foreign metal substances migrate toward the negative electrode
during charging, and locally precipitate on the opposing negative
electrode. Therefore, there is a concern that the precipitate
reaches the positive electrode through the separator depending on
the size, and the like, of foreign metal substances and then causes
a micro internal short circuit to occur in the battery. If an
internal short circuit occurs, there can be an inconvenience, such
as deterioration of battery quality (for example, a decrease in
energy density or safety).
SUMMARY OF THE INVENTION
[0006] The included foreign metal substances are promptly dissolved
through the aging and caused to precipitate on the negative
electrode to be short-circuited. Thus, such a battery in which
foreign metal substances are included is detected (for example,
self-discharge test) before shipment. However, there is a
possibility that foreign metal substances not dissolved through the
aging are gradually dissolved during usage of the battery after
shipment and cause a micro short circuit to occur with a delay. The
invention provides a nonaqueous electrolyte secondary battery in
which occurrence of a micro short circuit is suppressed by
suppressing inclusion of foreign metal substances into a portion
that is hard to be dissolved through aging. The invention also
provides a method of simply manufacturing such a nonaqueous
electrolyte secondary battery.
[0007] In aging for the purpose of dissolving foreign metal
substances, the positive electrode potential is increased by
adjusting the battery potential, thus dissolving the foreign metal
substances around the positive electrode. According to the detailed
study of the inventors, mostly, dissolution of foreign metal
substances included between the positive electrode (more
specifically, the positive electrode active material, the positive
electrode active material layer) and the separator is easy to
proceed when the foreign metal substances are exposed to the
positive electrode potential. However, it is found that dissolution
of foreign metal substances included between the separator and the
negative electrode (more specifically, the negative electrode
active material) is difficult to proceed because the foreign metal
substances are not exposed to a potential as high as the potential
of the positive electrode. The foreign metal substances at such a
portion gradually dissolve during later usage of the battery and
can cause a micro short circuit during usage. For example, even
when foreign metal substances have the same size and composition,
the foreign metal substances cause a micro short circuit to occur
with a delay (hereinafter, such a phenomenon may be referred to as
"delayed micro short circuit") in comparison with the case where
foreign metal substances are included between the positive
electrode and the separator. In the technique described here, the
inventors conceived that inclusion of foreign metal substances
between the separator and the negative electrode is suppressed. The
inventors found that inclusion of foreign metal substances into the
electrode assembly easily occurs mainly at the time of impregnating
the nonaqueous electrolyte, and completed the invention of the
present application.
[0008] That is, a first aspect of the invention provides a
nonaqueous electrolyte secondary battery. The nonaqueous
electrolyte secondary battery includes: an electrode assembly; a
nonaqueous electrolyte; and a battery case. The electrode assembly
includes a positive electrode, a negative electrode, and a
separator. The positive electrode includes a positive electrode
active material layer. The negative electrode includes a negative
electrode active material layer. The separator is interposed
between the positive electrode and the negative electrode. The
battery case accommodates the electrode assembly and the nonaqueous
electrolyte. Ends of contact faces of the negative electrode active
material layer and the separator are at least partially bonded to
each other. With this configuration, a gap is difficult to be
formed between the negative electrode active material layer and the
separator at the time of impregnating the electrode assembly with
the nonaqueous electrolyte. Therefore, inclusion of the foreign
metal substances that become a cause of a delayed micro short
circuit can be suitably suppressed, so it is desirable. Japanese
Patent Application Publication No. 2003-151638 (JP 2003-151638 A)
describes a lithium ion secondary battery in which one of a
positive electrode and a negative electrode is bonded to a
separator by an adhesive resin layer. The lithium ion secondary
battery can be clearly distinguished from the nonaqueous
electrolyte secondary battery described here in terms of the fact
that the positive electrode or the negative electrode is bonded to
the separator via the adhesive resin layer.
[0009] In the above aspect, the electrode assembly may be a flat
rolled electrode assembly that is formed by stacking the long
positive electrode, the long negative electrode and the long
separator and then rolling the stacked positive electrode, negative
electrode and separator into an oval shape in cross section with a
rolling axis set to a width direction perpendicular to a
longitudinal direction of the long positive electrode, the long
negative electrode and the long separator. The negative electrode
active material layer and the separator may be bonded to each other
at a flat portion of an electrode face of the flat rolled electrode
assembly (hereinafter, which may be simply referred to as plane
portion) in a band-shaped region along each of ends in the width
direction. A micro short circuit due to the foreign metal
substances more easily occurs at the plane portion than a curved
portion of the electrode face of the flat rolled electrode assembly
(hereinafter, which may be simply referred to as rolling curved
portion). Therefore, by suppressing inclusion of foreign metal
substances into the plane portion of the flat rolled electrode
assembly, a delayed micro short circuit is effectively suppressed,
so it is desirable.
[0010] In the above aspect, the entire contact faces of the
negative electrode active material layer and the separator may be
bonded to each other. With this configuration, inclusion of foreign
metal substances between the negative electrode active material
layer and the separator is reliably suppressed, so it is
desirable.
[0011] In the above aspect, the positive electrode active material
layer and the separator may not be bonded to each other. When the
positive electrode active material layer and the separator are
bonded to each other in a state where the negative electrode active
material layer and the separator are bonded to each other, there is
a possibility that the flexural rigidity of the electrode assembly
becomes excessively high, so it is undesirable.
[0012] In the above aspect, the negative electrode active material
layer and the separator may be bonded to each other such that a
peeling strength becomes larger than or equal to 0.2 N/m and
smaller than or equal to 1.2 N/m. Thus, the negative electrode
active material layer and the separator are bonded to each other in
an appropriate bonded state.
[0013] A second aspect of the invention provides a manufacturing
method for a nonaqueous electrolyte secondary battery. The
manufacturing method includes: constructing a cell by accommodating
an electrode assembly in a battery case, the electrode assembly
including a positive electrode, a negative electrode and a
separator, the positive electrode including a positive electrode
active material layer, the negative electrode including a negative
electrode active material layer, the separator being interposed
between the positive electrode and the negative electrode;
supplying a nonaqueous electrolyte into the battery case; and at
least partially bonding ends of contact faces of the negative
electrode active material layer and the separator to each other in
advance of supplying the nonaqueous electrolyte. Thus, foreign
metal substances are difficult to be included between the negative
electrode active material layer and the separator at the time of
impregnating the electrode assembly with the nonaqueous
electrolyte, so it is possible to manufacture a battery in which a
delayed micro short circuit is difficult to occur.
[0014] In the above aspect, the negative electrode active material
layer may include a negative electrode active material and a
binder. The negative electrode and the separator may be bonded to
each other by, at the time of constructing the cell, bringing the
negative electrode active material layer of the negative electrode
and the separator into contact with each other and then heating
while at least partially pressing the ends of the contact faces.
Thus, the negative electrode active material layer and the
separator are reliably bonded to each other.
[0015] In the above aspect, the negative electrode active material
layer may include a negative electrode active material and a
binder. The manufacturing method may further include drying the
cell after constructing the cell. The negative electrode and the
separator may be bonded to each other by, in drying the cell,
heating while applying a pressure such that the ends of the contact
faces of the negative electrode active material layer and the
separator in the electrode assembly are at least partially pressed
from an outside of the battery case. With this configuration, by
utilizing the cell drying step, the negative electrode active
material layer and the separator are bonded to each other, so it is
desirable.
[0016] In the above aspect, the negative electrode and the
separator may be bonded to each other by applying a pressure higher
than or equal to 0.01 MPa and lower than or equal to 1 MPa between
the negative electrode active material layer and the separator in a
temperature range higher than or equal to a softening point of the
binder and lower than or equal to 125.degree. C. Thus, the negative
electrode active material layer and the separator are reliably
bonded to each other, so it is desirable.
[0017] In the above aspect, the negative electrode and the
separator may be bonded to each other such that a peeling strength
becomes larger than or equal to 0.2 N/m and smaller than or equal
to 1.2 N/m. Thus, the negative electrode active material layer and
the separator are effectively bonded to each other, so it is
desirable.
[0018] In the above aspect, the manufacturing method may further
include applying aging. By applying aging to the above-configured
electrode assembly, dissolution and precipitation of foreign metal
substances included between the positive electrode active material
layer and the separator are promptly carried out. By extension, it
is possible to prevent a battery in which a micro short circuit due
to foreign metal substances can occur from being supplied to the
market.
[0019] As described above, the nonaqueous electrolyte secondary
battery according to the first aspect of the invention and the
manufacturing method for a nonaqueous electrolyte secondary battery
according to the second aspect of the invention can be the ones in
which a delayed micro short circuit is suppressed by suitably
suppressing inclusion of foreign metal substances into the
electrode assembly. Therefore, for example, foreign metal
substances are further reliably dissolved through ordinary aging,
so it is possible to highly accurately determine through a
self-discharge test whether a non-defective product or a defective
product. That is, it is possible to provide the market with a
high-quality and highly reliable secondary battery in which
occurrence of a micro short circuit due to foreign metal substances
is suppressed. The above secondary battery is easily manufactured
only by adding additional operation to existing manufacturing
steps, so the secondary battery can be provided at low cost.
Therefore, such a secondary battery is usable in various
applications, and is suitably used as a driving power supply
mounted on a vehicle, such as a vehicle, which requires high safety
and reliability. The secondary battery may be used solely (single
cell) or may be used in a mode of a battery pack in which a
plurality of the secondary batteries are connected in series with
or parallel with each other.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Features, advantages, and technical and industrial
significance of exemplary embodiments of the invention will be
described below with reference to the accompanying drawings, in
which like numerals denote like elements, and wherein;
[0021] FIG. 1 is a cross-sectional view that schematically shows
the configuration of a nonaqueous electrolyte secondary battery
according to an embodiment of the invention;
[0022] FIG. 2 is a schematic view that illustrates the
configuration of a flat rolled electrode assembly according to the
embodiment of the invention;
[0023] FIG. 3A is a flowchart of a manufacturing method for a
nonaqueous electrolyte secondary battery according to an embodiment
of the invention;
[0024] FIG. 3B is a flowchart of a manufacturing method for a
nonaqueous electrolyte secondary battery according to another
embodiment of the invention; and
[0025] FIG. 4A to FIG. 4E are views that schematically illustrate
portions at which a negative electrode and a separator are bonded
to each other in the flat rolled electrode assembly.
DETAILED DESCRIPTION OF EMBODIMENTS
[0026] In the specification, a nonaqueous electrolyte secondary
battery is a term that includes not only a so-called chemical
battery, such as a lithium secondary battery, a nickel-metal
hydride battery, a nickel cadmium battery and a lead storage
battery, but also a storage element (for example, a
pseudo-capacitance capacitor, a redox capacitor) that can be used
similarly in a similar industrial field to a chemical battery (for
example, a lithium secondary battery), a hybrid capacitor that is a
combination of the chemical battery and the storage element, a
lithium ion capacitor, and the like. The lithium secondary battery
means a secondary battery that utilizes lithium ions as
electrolytic ions and that is charged or discharged as a result of
migration of lithium ions (migration of charges) between a positive
electrode and a negative electrode. Generally, secondary batteries
called a lithium ion battery, a lithium polymer battery, and the
like, are typical examples included in the lithium secondary
battery in the specification. In the specification, an active
material means a substance (chemical compound) that is able to
reciprocally occlude or release chemical species (lithium ions in
the lithium secondary battery) that serves as an charge carriers at
the positive electrode side or the negative electrode side.
[0027] In the technique described here, foreign metal substances
that can be a dissolving target are metal (typically, metal fine
particles) that can be included into a battery case in a
manufacturing process for a nonaqueous electrolyte secondary
battery, have an oxidation-reduction potential within an operation
voltage range of the secondary battery, and can be dissolved
(ionized) at that potential. Such foreign metal substances can be,
for example, included into the battery case inevitably as
impurities of constituent raw materials of the secondary battery,
dust particles in sputtering or processing at the time of welding,
or the like. Metallic elements that constitute such foreign metal
substances (and the oxidation-reduction potentials thereof) are
typically, for example, iron (Fe; 2.6 V) and copper (Cu; 3.4 V),
and the compositions of foreign metal substances are chemical
elements of them, alloys of them, and the like. Particularly, iron
is, for example, a main ingredient of a stainless steel that is
frequently used as various manufacturing devices, constituent
members, and the like, so iron is considered as an chemical element
that is highly likely to be included in the manufacturing process.
In the specification, a potential means a lithium reference
potential (VvsLi/Li.sup.+) unless otherwise specified.
[0028] Hereinafter, the nonaqueous electrolyte secondary battery
will be described together with embodiments of a manufacturing
method for the battery described here. In the specification,
matters other than the matters particularly referred to and
required to be implemented (the general configuration, and the
like, of the nonaqueous electrolyte secondary battery, which do not
characterize the invention) can be understood as a design matter of
persons skilled in the art based on the existing art in that field.
The thus structured secondary battery may be implemented on the
basis of information described in the specification and a technical
common sense in the field. Hereinafter, a lithium secondary battery
that is a desired mode will be described as an example; however, a
target to which the invention is applied is not intended to be
limited to the battery.
[0029] FIG. 1 is a schematic cross-sectional view of the nonaqueous
electrolyte secondary battery according to the embodiment. FIG. 2
is a schematic view that illustrate the configuration of an
electrode assembly that is provided in the nonaqueous electrolyte
secondary battery. FIG. 3A and FIG. 3B are flowcharts that show
manufacturing methods for the nonaqueous electrolyte secondary
battery described here. Hereinafter, the scale ratios (length,
width, thickness, and the like) in the drawings do not always
reflect actual scale ratios. The nonaqueous electrolyte secondary
battery 100 described here essentially includes an electrode
assembly 20, a nonaqueous electrolyte (not shown), and a battery
case 10. The electrode assembly 20 includes a positive electrode
30, a negative electrode 40, and a separator 50. The positive
electrode 30 includes a positive electrode active material layer
34. The negative electrode 40 includes a negative electrode active
material layer 44. The separator 50 is interposed between these
positive electrode 30 and negative electrode 40. The electrode
assembly 20 and the nonaqueous electrolyte are accommodated in the
battery case 10. Initial charging is applied to the thus
constructed secondary battery 100 in a predetermined condition in
order to adjust the secondary battery 100 to an actually usable
(shippable) state. For example, high-temperature aging for the main
purpose of stabilizing a battery reaction, a self-discharge test
(low-temperature aging) for checking for a micro short circuit, or
the like, can be applied to the battery after initial charging.
Only the battery that has passed the test is shipped as a
product.
[0030] Incidentally, at the time of constructing the cell 100,
foreign metal substances can be included in the electrode assembly
20. In such a case, when the potential of the positive electrode 30
becomes higher than the dissolved potential (oxidation potential)
of foreign metal substances as a result of initial charging or
charging for aging, foreign metal substances near the positive
electrode 30 are oxidized (lose electrons) to become metallic ions,
and dissolve into the electrolyte. For example, when the foreign
metal substances are copper or iron, copper dissolves like
Cu.fwdarw.Cu.sup.2+, or iron dissolves like Fe.fwdarw.Fe.sup.2+.
The metallic ions are positively charged, so the metallic ions
ordinarily linearly migrate toward the negative electrode 40
through the separator 50 between the positive and negative
electrodes. The metallic ions are reduced at a position facing the
foreign metal substances on the negative electrode 40, and locally
(intensively) precipitate. Therefore, with the progress of
charging, a precipitate on the negative electrode gradually grows
toward the positive electrode 30. When the precipitate reaches the
positive electrode, a short circuit (micro short circuit) occurs in
the battery
[0031] When foreign metal substances are not subjected to the
potential of the positive electrode 30, specifically, for example,
when foreign metal substances are present between the negative
electrode 40 and the separator 50, the potential at the position at
which the foreign metal substances are present is hard to reach the
dissolved potential, so the foreign metal substances are hard to be
dissolved. However, for example, depending on the mode of usage of
the battery, dissolution of foreign metal substances can gradually
proceed because of, for example, the fact that the potential of the
foreign metal substances intermittently reaches the dissolved
potential as a result of local overcharging, or the like. For
example, in a battery in a usage mode in which a large capacity is
charged or discharged at a high rate, an overcharged state easily
occurs. During usage of the battery, there can occur a situation
that a short circuit occurs with a delay. In order to avoid such a
situation, the nonaqueous electrolyte secondary battery 100
described here employs such a configuration that foreign metal
substances are not included between the negative electrode 40 and
the separator 50.
[0032] In the technique described here, for example, specifically,
as shown in FIG. 3A, the nonaqueous electrolyte secondary battery
is manufactured in the following procedure.
(S1) Cell Construction Step
(S2) Nonaqueous Electrolyte Supply Step
[0033] In advance of the nonaqueous electrolyte supply step (S2),
(X) a bonding step of at least partially bonding the ends of
contact faces of the negative electrode active material layer and
the separator to each other is included as a characteristic.
S1. Construction of Cell
[0034] In step S1, initially, the electrode assembly including the
positive electrode 30, the negative electrode 40 and the separator
50 is prepared. The positive electrode 30 includes the positive
electrode active material layer 34. The negative electrode 40
includes the negative electrode active material layer 44. The
separator 50 is interposed between the positive electrode 30 and
the negative electrode 40.
Positive Electrode
[0035] The positive electrode 30 typically includes a long positive
electrode current collector 32 and the positive electrode active
material layer 34. The positive electrode active material layer 34
is held on the positive electrode current collector 32. The
positive electrode current collector 32 typically includes a
portion, at which the positive electrode active material layer 34
is formed, and a positive electrode current collector exposed
portion 33. At the positive electrode current collector exposed
portion 33, no positive electrode active material layer 34 is
provided and the current collector 32 is exposed. The positive
electrode current collector exposed portion 33 is typically
provided in a band shape along one end of the long positive
electrode current collector 32 in the width direction. The positive
electrode active material layer 34 is provided on the surface of
the positive electrode current collector 32 other than the positive
electrode current collector exposed portion 33. The positive
electrode active material layer 34 may be provided on both faces of
the positive electrode current collector 32 or may be provided only
on any one of the faces. The positive electrode current collector
32 is suitably an electrically conductive member made of a metal
having a high electrical conductivity (for example, aluminum,
nickel, or the like). The positive electrode active material layer
34 at least includes a positive electrode active material, and has
a porous structure so that impregnation of a nonaqueous electrolyte
is possible.
[0036] A method of preparing the positive electrode 30 is not
specifically limited. For example, a composition in form of paste
(including slurry and ink) (hereinafter, referred to as "positive
electrode paste) is prepared by mixing a positive electrode active
material, an electrically conductive material, a binder, and the
like, in an adequate solvent (for example, NMP), and the resultant
paste is supplied onto the positive electrode current collector 32.
Thus, the positive electrode active material layer 34 is formed. A
method of supplying the positive electrode paste is, for example, a
method of applying the positive electrode paste to one face or both
faces of the positive electrode current collector with the use of
an existing known coating applicator (for example, a slit coater, a
die coater, a comma coater, a gravure coater). Alternatively, a
positive electrode active material, an electrically conductive
material and a binder are granulated into an adequate size to form
granules, and the granules are supplied and pressure-bonded onto
the positive electrode current collector 32. Thus, the positive
electrode active material layer 34 is formed.
[0037] The positive electrode active material may be a material
that is able to occlude or release lithium ions, and suitably a
lithium-containing compound (for example, lithium transition metal
composite oxide) including lithium element and one or two or more
kinds of transition metal elements. For example, a lithium
transition metal oxide having a bedded salt or spinel crystal
structure is a suitable example. The lithium transition metal oxide
can be, for example, a lithium-nickel composite oxide (for example,
LiNiO.sub.2), a lithium-cobalt composite oxide (for example,
LiCoO.sub.2), a lithium-manganese composite oxide (for example,
LiMn.sub.2O.sub.4), or a ternary system lithium-containing
composite oxide, such as a lithium-nickel-cobalt-manganese
composite oxide (for example,
LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2). A polyanionic compound
(for example, LiFePO.sub.4, LiMnPO.sub.4, LiFeVO.sub.4,
LiMnVO.sub.4, Li.sub.2FeSiO.sub.4, Li.sub.2MnSiO.sub.4,
Li.sub.2CoSiO.sub.4) expressed by the general formula LiMPO.sub.4
or LiMVO.sub.4 or Li.sub.2MSiO.sub.4 (M in the formula is at least
one or more chemical elements selected from among Co, Ni, Mn, and
Fe), or the like, may be used as the positive electrode active
material.
[0038] The positive electrode active material layer 34 can contain
one or two or more kinds of materials that can be used as the
constituent components of the positive electrode active material
layer in a general nonaqueous electrolyte secondary battery as
needed in addition to the positive electrode active material. An
example of such a material includes an electrically conductive
material and a binder. The electrically conductive material may be,
for example, suitably a carbon material, such as various carbon
blacks (for example, acetylene black or Ketjen black), activated
carbon, graphite, and carbon fiber. The binder may be, for example,
suitably a vinyl halide resin, such as polyvinylidene fluoride
(PVdF), and a polyalkylene oxide, such as a polyethylene oxide
(PEO).
[0039] The proportion of the positive electrode active material in
the entire positive electrode active material layer 34 is
adequately higher than or equal to about 60 percent by mass
(typically, 60 percent by mass to 95 percent by mass) from the
viewpoint of achieving a high energy density, and is ordinarily
suitably about 70 percent by mass to 95 percent by mass. When the
binder is used, the proportion of the binder in the entire positive
electrode active material layer may be, for example, about 0.5
percent by mass to 10 percent by mass from the viewpoint of
suitably ensuring a mechanical strength (shape retainability), and
is ordinarily suitably about 1 percent by mass to 5 percent by
mass. When the electrically conductive material is used, the
proportion of the electrically conductive material in the entire
positive electrode active material layer may be, for example, about
1 percent by mass to 20 percent by mass from the viewpoint of
achieving both output characteristics and energy density at high
level, and is ordinarily suitably about 2 percent by mass to 10
percent by mass.
[0040] The mass per unit area (weight per unit area) of the
positive electrode active material layer 34 that is provided on the
positive electrode current collector 32 just needs to be larger
than or equal to 3 mg/cm.sup.2 (for example, larger than or equal
to 5 mg/cm.sup.2, typically, larger than or equal to 7 mg/cm.sup.2)
per one-side face of the positive electrode current collector 32
from the viewpoint of achieving a high energy density. From the
viewpoint of achieving excellent output characteristics, the weight
per unit area of the positive electrode active material layer 34
just needs to be smaller than or equal to 100 mg/cm.sup.2 (for
example, smaller than or equal to 70 mg/cm.sup.2, typically,
smaller than or equal to 50 mg/cm.sup.2) per one-side face of the
positive electrode current collector 32. The average thickness of
the positive electrode active material layer 34 per one-side face
just needs to be, for example, larger than or equal to 20 .mu.m
(typically, larger than or equal to 40 .mu.m) and smaller than or
equal to 100 .mu.m (typically, smaller than or equal to 80 .mu.m).
The density of the positive electrode active material layer 34 just
needs to be, for example, larger than or equal to 1.0 g/cm.sup.3
(typically, larger than or equal to 2.0 g/cm.sup.3) and smaller
than or equal to 4.5 g/cm.sup.3 (for example, smaller than or equal
to 4.0 g/cm.sup.3).
Negative Electrode
[0041] The long negative electrode 40 typically includes a long
negative electrode current collector 42 and the negative electrode
active material layer 44. The negative electrode active material
layer 44 is formed on the negative electrode current collector 42.
The negative electrode current collector 42 includes a portion, at
which the negative electrode active material layer 44 is formed,
and a negative electrode current collector exposed portion 43. At
the negative electrode current collector exposed portion 43, no
negative electrode active material layer 44 is provided and the
current collector 42 is exposed. The negative electrode current
collector exposed portion 43 is typically provided in a band shape
along one end of the negative electrode current collector 42 in the
width direction. The negative electrode active material layer 44 is
provided on the surface of the negative electrode current collector
42 other than the negative electrode current collector exposed
portion 43. The negative electrode current collector 42 is suitably
an electrically conductive member made of a metal having a high
electrical conductivity (for example, copper, nickel, or the like).
The negative electrode active material layer 44 at least includes a
negative electrode active material, and has such a porous structure
that impregnation of a nonaqueous electrolyte is possible.
[0042] A method of preparing the negative electrode is not
specifically limited. For example, a composition in form of paste
(hereinafter, referred to as "negative electrode paste") is
prepared by mixing a negative electrode active material, a binder,
and the like, in an adequate solvent, and the resultant paste is
applied onto the negative electrode current collector to form the
negative electrode active material layer. The thus prepared
negative electrode may be used. Alternatively, the negative
electrode active material and an electrically conductive material
described below and a binder as needed are granulated into an
adequate size to form granules, and the granules are supplied and
pressure-bonded onto the negative electrode current collector 42.
Thus, the negative electrode active material layer 44 is formed. A
similar method to the case of the above-described positive
electrode may be employed as needed as a method of forming the
negative electrode active material layer 44.
[0043] The negative electrode active material may be one or two or
more kinds of known various materials that can be used as the
negative electrode active material of the nonaqueous electrolyte
secondary battery. A suitable example of the negative electrode
active material includes a carbon material, such as graphite,
non-graphitizable carbon (hard carbon), graphitizable carbon (soft
carbon), carbon nanotube, and a material having a structure that
combines any two or more of them. Among others, from the viewpoint
of energy density, a graphite material, such as natural graphite
(black lead) and artificial graphite, may be suitably used. The
graphite material may be suitably a material in which amorphous
carbon is arranged on at least part of the surface. More suitably,
almost the entire surface of granular carbon is coated with an
amorphous carbon film. Many edge surfaces are exposed on the
surface of amorphous carbon, so the acceptability of charge
carriers is high (that is, the speed of occlusion/release of charge
carriers is high). Graphite has a large theoretical capacity, and
has an excellent energy density. Therefore, by using amorphous
carbon-coated graphite as the negative electrode active material,
it is possible to achieve the nonaqueous electrolyte secondary
battery that has a high capacity, a high energy density and
excellent input/output characteristics. Other than such carbon
materials, for example, a lithium-titanium composite oxide, such as
Li.sub.4Ti.sub.5O.sub.12, or a lithium transition metal composite
compound, such as a lithium transition metal composite nitride, may
be used.
[0044] The negative electrode active material layer 44 can contain
one or two or more materials that can be used as the constituent
components of the negative electrode active material layer 44 in a
general nonaqueous electrolyte secondary battery as needed in
addition to the negative electrode active material. Examples of
such materials include a binder and various additives. A binder
similar to the binder that is used in the negative electrode of a
general lithium ion secondary battery may be employed as the binder
as needed. For example, a binder similar to that in the positive
electrode 30 may be used. As a suitable mode, when the aqueous
solvent is used to form the negative electrode active material
layer 44, a rubber, such as styrene-butadiene rubber (SBR), a
water-soluble polymer material, such a polyethylene oxide (PEO) and
a vinyl acetate copolymer, or a water-dispersible polymer material
can be suitably employed. Other than the above, various additives,
such as a thickener, a dispersant and an electrically conductive
material, may be used as needed. For example, the thickener
includes carboxymethyl cellulose (CMC), methyl cellulose (MC), a
cellulose-based polymer, such as cellulose acetate phthalate
(CAP).
[0045] The proportion of the negative electrode active material in
the entire negative electrode active material layer is adequately
higher than or equal to about 50 percent by mass, and is ordinarily
suitably 90 percent by mass to 99 percent by mass (for example, 95
percent by mass to 99 percent by mass). Thus, it is possible to
achieve a high energy density. When the binder is used, the
proportion of the binder in the entire negative electrode active
material layer may be, for example, about 1 percent by mass to 10
percent by mass, and is ordinarily suitably about 1 percent by mass
to 5 percent by mass. Thus, the mechanical strength (shape
retainability) of the negative electrode active material layer is
suitably ensured, and high durability is achieved. When the
thickener is used, the proportion of the thickener in the entire
negative electrode active material layer may be, for example, about
1 percent by mass to 10 percent by mass, and is ordinarily suitably
about 1 percent by mass to 5 percent by mass.
[0046] The mass per unit area (weight per unit area) of the
negative electrode active material layer 44 that is provided on the
negative electrode current collector 42 just needs to be larger
than or equal to 5 mg/cm.sup.2 (typically, larger than or equal to
7 mg/cm.sup.2) per one-side face of the negative electrode current
collector 42 and smaller than or equal to about 20 mg/cm.sup.2
(typically, 15 mg/cm.sup.2) from the viewpoint of achieving a high
energy density and a power density. The thickness of the negative
electrode active material layer 44 per one-side face just needs to
be, for example, larger than or equal to 40 .mu.m (typically,
larger than or equal to 50 .mu.m) and smaller than or equal to 100
.mu.m (typically, smaller than or equal to 80 .mu.m). The density
of the negative electrode active material layer 44 just needs to
be, for example, larger than or equal to 0.5 g/cm.sup.3 (typically,
larger than or equal to 1.0 g/cm.sup.3) and smaller than or equal
to 2.0 g/cm.sup.3 (typically, 1.5 g/cm.sup.3).
Separator
[0047] The separator 50 is a constituent material that electrically
insulates the positive electrode 30 and the negative electrode 40
from each other, that holds charge carriers, and that allows
passage of the charge carriers. The separator 50 may be suitably
formed of a microporous resin sheet made of various materials.
Although not specifically limited, the separator 50 may be
configured to have a shutdown function of interrupting passage of
charge carriers by softening and melting when the temperature of
the flat rolled electrode assembly 20 becomes a predetermined
temperature. For example, a microporous sheet made of a polyolefin
resin, typically, polyethylene (PE) or polypropylene (PP), is
suitable as the separator 50 because the microporous sheet is able
to relatively suitably set a shutdown temperature (for example,
softening temperature). For example, the shutdown temperature
(softening temperature) of the separator 50 made of polyethylene
may be set to 120.degree. C. to 145.degree. C., suitably
125.degree. C. to 140.degree. C., and may be, for example, set to
130.degree. C. to 135.degree. C.
[0048] In the separator 50, in order to suitably endure the
oxidation atmosphere of the positive electrode, the microporous
resin sheet is used as a base material, and the surface (one-side
face) of the base material, facing the positive electrode 30, may
be formed of an oxidation-resistant resin, such as polyamide and
polyamide-imide, having a higher oxidation resistance than the
polyolefin resin. Alternatively, the microporous resin sheet is
used as a base material, and a heat-resistant layer (HRL) made of
inorganic filler (inorganic aggregate) having a heat resistance
property and an electrical insulation property may be provided on
one-side face or both faces of the base material. Suitably, the HRL
may be provided on the surface (one-side face) facing the positive
electrode 30. For example, a fine powder having an average particle
size larger than or equal to about 0.1 .mu.m and smaller than or
equal to 3 .mu.m and made of alumina (Al.sub.2O.sub.3), zirconia
(ZrO.sub.2), ceria (CeO.sub.2), yttria (Y.sub.2O.sub.3), boehmite
(Al.sub.2O.sub.3.cndot.H.sub.2O), mullite
(Al.sub.6O.sub.13Si.sub.2), magnesia (MgO), silica (SiO.sub.2),
titania (TiO.sub.2), or the like, may be used as the inorganic
filler.
[0049] Thus, for example, even when the temperature of the flat
rolled electrode assembly 20 becomes higher than the melting point
of the separator 50 and then the separator 50 shrinks or breaks, it
is possible to prevent a short circuit between the positive
electrode 30 and the negative electrode 40. By providing the
inorganic filler on the surface facing the positive electrode 30,
bonding of the negative electrode 40 (described later) with the
separator 50 is suitably achieved. Although the average thickness
of the entire separator 50 is not specifically limited, the average
thickness may be ordinarily larger than or equal to 10 .mu.m,
typically, larger than or equal to 15 .mu.m, and, for example,
larger than or equal to 17 .mu.m. The upper limit of the average
thickness may be smaller than or equal to 40 .mu.m, typically,
smaller than or equal to 30 .mu.m, and, for example, smaller than
or equal to 25 .mu.m. Because the average thickness falls within
the above range, the high permeability of charge carriers is kept,
and a micro short circuit (leakage current) becomes more difficult
to occur. Therefore, both the input/output density and safety are
achieved at high level.
Electrode Assembly
[0050] For example, the flat rolled electrode assembly 20 shown in
FIG. 2 is formed by using the prepared positive electrode 30,
negative electrode 40 and separator 50. That is, the long positive
electrode 30, the long negative electrode 40 and the two long
separators 50 are stacked such that one of the separators 50 is
interposed between the long positive electrode 30 and the long
negative electrode 40 and the other one of the long separators 50
is stacked on the long negative electrode 40 across the long
negative electrode 40 from the one of the long separators 50, and
then rolled into an oval shape in cross section with a rolling axis
set to the width direction perpendicular to the longitudinal
direction. The flat rolled electrode assembly 20 may be shaped by
crushing and squashing the cylindrical rolled electrode assembly 20
laterally or may be formed by rolling the stacked long positive
electrode 30, long negative electrode 40 and long separators 50
into a flat shape in cross section from the beginning. The shape of
the flat rolled electrode assembly 20 may be appropriately adjusted
commensurately with the shape of the battery case 10 to be
used.
[0051] At the time of stacking the positive electrode 30, the
negative electrode 40 and the separators 50, the positive electrode
30 and the negative electrode 40 just need to overlap each other
with a slight offset in the width direction such that the positive
electrode current collector exposed portion 33 of the positive
electrode 30 and the negative electrode current collector exposed
portion 43 of the negative electrode 40 respectively project from
one end and the other end of the separators 50 in the width
direction. As a result, in the rolling axis W direction of the flat
rolled electrode assembly 20, the positive electrode current
collector exposed portion 33 and the negative electrode current
collector exposed portion 43 respectively extend outward from a
rolling core portion (that is, a portion at which the positive and
negative electrode active material layers 34, 44 face each other).
By utilizing the positive electrode current collector exposed
portion 33 and the negative electrode current collector exposed
portion 43, current is collected at high efficiency.
Assembling of Cell
[0052] Next, the electrode assembly 20 is accommodated in the
appropriate battery case 10. The secondary battery (cell) 100 is
constructed by hermetically sealing the case 10. Although not
specifically distinguished strictly, in the specification, the
battery up to completion of treatment, such as initial charging and
aging, in the state where the electrode assembly is accommodated in
the battery case may be referred to as cell. A material and a shape
that are conventionally used for a secondary battery may be used
for the battery case 10. For example, a relatively light metal
material, such as aluminum and steel, or a resin material, such as
polyphenylene sulfide resin and polyimide resin, may be used as the
material of the battery case. Among others, from the viewpoint of,
for example, improving heat radiation property and energy density,
the battery case 10 made of a relatively light metal (for example,
aluminum or an aluminum alloy) can be suitably employed. The shape
of the case 10 (the outer shape of the casing) is also not
specifically limited. For example, the shape of the case 10 can be
a circular shape (a cylindrical shape, a coin shape, a button
shape), a hexahedron shape (box shape), a shape deformed by
processing any one of them, or the like.
[0053] The battery case 10 illustrated in this drawing includes a
flat rectangular parallelepiped (box-shaped) case body 12 and a
sealing lid 14. The upper end of the case body 12 is open. The
sealing lid 14 closes the opening. A positive electrode terminal 60
and a negative electrode terminal 70 are provided at the upper face
(that is, the sealing lid 14) of the battery case 10. The positive
electrode terminal 60 is electrically connected to the positive
electrode 30 of the flat rolled electrode assembly 20. The negative
electrode terminal 70 is electrically connected to the negative
electrode 40 of the electrode assembly 20. For example, a positive
electrode current collecting plate 62 is connected to the exposed
end of the positive electrode current collector 32 of the electrode
assembly 20 by a welded portion, or the like, and a negative
electrode current collecting plate 72 is connected to the exposed
end of the negative electrode current collector 42 by a welded
portion, or the like. The positive electrode current collecting
plate 62 and the negative electrode current collecting plate 72 are
respectively electrically connected to the positive electrode
terminal 60 and the negative electrode terminal 70. In the battery
case 10, a safety mechanism, such as a current interrupting
mechanism 80 (a mechanism that can interrupt current in response to
an increase in internal pressure at the time of overcharging of the
battery) may be provided as needed in an electrically conductive
path between the positive electrode terminal 60 and the electrode
assembly 20 or an electrically conductive path between the negative
electrode terminal 70 and the electrode assembly 20. The sealing
lid 14, as well as the battery case 10 of an existing secondary
battery, includes a safety valve 82, a filling port 84, and the
like. The safety valve 82 is used to emit gas, generated inside the
battery case, to the outside of the case. The filling port 84 is
used to fill a nonaqueous electrolyte.
[0054] Sealing work may be carried out in a similar method to a
method that is conventionally used for a secondary battery. For
example, when the battery case 10 made of a metal is used, a
technique, such as laser welding, resistance welding and electron
beam welding, may be used. When the battery case 10 made of a
nonmetal (for example, a resin material) is used, a technique, such
as bonding using an adhesive agent and ultrasonic welding, may be
used.
X. Bonding
[0055] It is presumable that inclusion of foreign metal substances
between the negative electrode 40 and each of the separators 50
occurs at the time when a nonaqueous electrolyte is filled into the
cell constructed as described above and then the nonaqueous
electrolyte is impregnated between the negative electrode 40 and
each separator 50 in the flat rolled electrode assembly 20. Thus,
in the technique described here, in advance of supplying a
nonaqueous electrolyte in the next step S2, this step X is carried
out, and the ends of the contact faces of the negative electrode
active material layer 44 and each of the separators 50 are at least
partially bonded to each other.
X-1. Bonding in Drying Step
[0056] Depending on the application of the nonaqueous electrolyte
secondary battery, a cell drying step can be included after the
cell is constructed and before a nonaqueous electrolyte is
supplied. In this cell drying step, in order to reduce moisture in
the electrode assembly 20 (for example, a solvent contained in the
positive electrode paste or the negative electrode paste, or the
like) to a predetermined level or lower, typically, for example,
the cell can be heated to about 105.degree. C..+-.5.degree. C. In
the technique described here, as shown in FIG. 3A, by utilizing the
cell drying step, the negative electrode 40 is bonded to each of
the separators 50.
[0057] Specifically, the negative electrode 40 is bonded to each of
the separators 50 by heating the cell while pressing the battery
case 10 from the outside at the time of drying the cell. At this
time, the ends of the contact faces of the negative electrode
active material layer 44 and each separator 50 of the electrode
assembly 20 are at least partially pressed via the battery case 10.
Thus, the binder contained in the negative electrode active
material layer 44 softens or melts at that portion, and adheres to
each separator 50 at the pressed portion. In this state, when the
binder is solidified by cooling the negative electrode active
material layer 44 and each separator 50, the negative electrode
active material layer 44 and each separator 50 are bonded to each
other.
[0058] As long as the negative electrode active material layer 44
and each separator 50 are able to prevent insertion of foreign
metal substances between these layers, a certain advantageous
effect is obtained. From the above viewpoint, the negative
electrode active material layer 44 and each separator 50 just need
to be at least partially bonded to each other at a plane portion P
of the flat rolled electrode assembly 20 in each of the band-shaped
regions along both ends in the rolling axis W direction (width
direction). Both ends in the rolling axis W direction become inlets
between the layers of the flat rolled electrode assembly 20. For
example, as shown in FIG. 4A, both ends of the plane portion P in
the width direction may be suitably bonded respectively in the
band-shaped regions B. However, the negative electrode active
material layer 44 and each separator 50 do not need to be bonded
all over each of the band-shaped regions. For example, as shown in
FIG. 4B, the negative electrode active material layer 44 and each
separator 50 may be bonded to each other in regions B, each of
which is located near the center of the corresponding band-shaped
region in the longitudinal direction. For example, as shown in FIG.
4C, bonding may be carried out in a plurality of regions B in each
of the band-shaped regions. Alternatively, for example, as shown in
FIG. 4D, bonding may be reliably carried out substantially over the
entire region B of the plane portion P. For example, according to
the mode shown in FIG. 4D, it is suitable in terms of making it
possible to reliably prevent inclusion of foreign metal substances
between the negative electrode active material layer 44 and each
separator 50. In the examples shown in FIG. 4A to FIG. 4E, the
bonding regions B at one end and the other end of the plane portion
P in the width direction are provided at the same positions;
however, bonding regions B at one end and the other end may be
provided at different positions. In the specification, "partially",
for example, means at a part of ends of contact faces of the
negative electrode active material 44 and the separator 50.
[0059] The heating temperature in bonding may be set to a
temperature higher than or equal to the softening point (for
example, higher than or equal to 60.degree. C.) of the binder
contained in the negative electrode active material layer 44. More
suitably, the heating temperature is suitably higher than or equal
to the softening point of the binder and higher than or equal to
100.degree. C., more suitably higher than or equal to 105.degree.
C., and particularly suitably higher than or equal to 110.degree.
C. Although the upper limit of the heating temperature is not
specifically limited, for example, when the separator has a
shutdown function, the heating temperature is suitably a
temperature lower than or equal to the softening temperature (or
lower than the softening temperature) of the separator. For
example, the upper limit of the heating temperature may be lower
than or equal to 125.degree. C., suitably lower than or equal to
120.degree. C. and particularly suitably lower than or equal to
115.degree. C. For example, the upper limit of the heating
temperature may be a temperature range up to about 105.degree. C.
at which moisture can be volatized at normal pressure.
[0060] Although not always required, bonding may be carried out in
a reduced pressure in order to suitably dry or remove moisture in
the battery case 10. The pressure in this case is suitably reduced
to, for example, 3 kPa (abs) or higher and 18 kPa (abs) or lower,
more suitably 5 kPa (abs) or higher and 15 kPa (abs) or lower, and
particularly suitably 8 kPa (abs) or higher and 12 kPa (abs) or
lower, in absolute pressure.
[0061] The pressure at the time of pressing is set so as to apply a
pressure higher than or equal to 0.01 MPa and lower than or equal
to 1 MPa between the negative electrode active material layer 44
and each separator 50. When the pressure higher than or equal to
0.01 MPa is applied, a bonding strength is achieved to such an
extent that, for example, the negative electrode active material
layer 44 and each separator 50 do not easily peel off from each
other because of expansion or shrinkage of the negative electrode
active material layer 44 resulting from charging or discharging.
The pressure is suitably higher than or equal to 0.05 MPa and more
suitably higher than or equal to 0.1 MPa. Although the upper limit
of the pressure to be applied is not specifically limited, a
bonding force to such an extent that it is possible to prevent
inclusion of foreign metal substances between the negative
electrode active material layer 44 and each separator 50 just needs
to be provided in the invention described here. The upper limit of
the pressure may be, for example, lower than or equal to 1 MPa, may
be lower than or equal to 0.75 MPa, and may be, for example, lower
than or equal to 0.7 MPa.
[0062] The local pressing from the outside of the battery case as
shown in FIG. 4A to FIG. 4E may be, for example, suitably carried
out by using a constrained plate (which can be a spacer) that is
used at the time when a plurality of nonaqueous electrolyte
secondary batteries are constrained. Specifically, in order to
obtain the effect of facilitating aging, or the like, there is a
case where a plurality of batteries are constrained by interposing
a constrained plate between the plurality of batteries and then
aging, or the like, is treated in this constrained state. By using
the constrained plate, for example, it is possible to press the
plane portion P of the flat rolled electrode assembly 20 inside the
battery case by applying confined pressure uniformly to the plane
portion P without reducing the strength of the battery case. For
example, by providing desired protruded portions having shapes
corresponding to the regions B on the face that the constrained
plate contacts the battery case 10, it is possible to locally apply
pressure to the regions B of the electrode assembly with the
protruded portions. Thus, at selected positions at the ends of the
plane portion P of the flat rolled electrode assembly 20 in the
width direction, the negative electrode active material layer 44
and each separator 50 are bonded to each other.
[0063] In the specification, "bonding", for example, means the
state where the negative electrode active material layer 44 and
each separator 50 are bound to each other by physical or chemical
binding force via a component (typically, a binder or a resin)
having a binder ability intrinsically contained in the negative
electrode active material layer 44 and/or each separator 50. For
example, the case where the negative electrode active material
layer 44 and each separator 50 are in contact with (adjacent to)
each other via the binder component of the negative electrode
active material layer 44 or the case where both are not bound to
each other even when, for example, electrostatically adhere to each
other is not included in bonding described here.
[0064] Whether sufficient bonding is achieved is, for example,
determined by measuring the peeling strength between the negative
electrode active material layer 44 and each separator 50 subjected
to bonding in the same condition. The peeling strength between the
negative electrode active material layer 44 and each separator 50
can be quantitatively evaluated by, for example, "90 degree peeling
test" defined in JIS K6854-1: 1999. In one suitable mode, the 90
degree peeling strength between the negative electrode 40 and each
separator 50 after bonding is suitably larger than or equal to 0.2
N/m, typically, larger than or equal to 0.3 N/m, and can be, for
example, larger than or equal to 0.4 N/m. The upper limit of the
peeling strength is not specifically limited. For example, it is
determined to be sufficient as long as the upper limit of the
peeling strength is smaller than or equal to about 2 N/m. Because
bonding stronger than required can lead to breakage of an
electrode, the peeling strength may be, for example, smaller than
or equal to about 1.2 N/m, suitably smaller than or equal to 0.7
N/m, typically smaller than or equal to 0.6 N/m, and can be, for
example, smaller than or equal to 0.5 N/m. Thus, the negative
electrode active material layer 44 and each separator 50 are
reliably bonded to each other, so it is possible to prevent
inclusion of foreign metal substances.
[0065] The positive electrode 30 (positive electrode active
material layer 34) and each separator 50 are not limited to this
configuration. As in the case of an existing cell, even when the
positive electrode 30 and each separator 50 contact each other, the
positive electrode 30 and each separator 50 are suitably not bonded
to each other. This is because, if the positive electrode 30 is
bonded in addition to the negative electrode 40 (negative electrode
active material layer 44) and each separator 50, there is a concern
that the durability decreases because of a loss of the flexibility
of the flat rolled electrode assembly 20 or suppressing
impregnation of a nonaqueous electrolyte. At the surface of the
negative electrode 40 (negative electrode active material layer
44), gas can be generated as a result of decomposition of the
electrolyte or overcharged additive during ordinary battery use or
during overcharging. When the positive electrode 30 and each
separator 50 are bonded to each other, emission of gas generated in
the negative electrode 40 to the outside of the electrode assembly
20 can be impaired, so it is not suitable. Although not always
required, from such a viewpoint, each separator 50 more suitably
has the HRL on the surface facing the positive electrode.
X-2. Bonding During Construction of Electrode Assembly
[0066] As another mode, the timing of bonding between the negative
electrode 40 and each separator 50 is, for example, not limited to
the timing after the cell construction step S1 and before the
nonaqueous electrolyte supply step S2 as described above. For
example, as shown in FIG. 3B, bonding may be carried out together
when the flat rolled electrode assembly 20 is constructed. That is,
for example, in constructing the flat rolled electrode assembly 20,
the long negative electrode 40 is placed between the two separators
50 (or the one separator 50 having a double length and folded into
two), the long positive electrode 30 is laid over them, and the
stacked components are rolled in the longitudinal direction about
the rolling axis W. For example, at this time, when the negative
electrode 40 is placed between the separators 50 and the negative
electrode active material layer 44 is brought into contact with the
separators 50, both ends of the contact faces of the negative
electrode active material layer 44 and each separator 50 in the
width direction may be at least partially bonded to each other. The
heating condition and pressing condition for bonding may be
determined similarly to the case where the battery case 10 is
pressed from the outside. For example, heating and pressing may be
carried out by utilizing a flat press machine, a roll press
machine, or the like. By utilizing an appropriate pressing jig, for
example, the negative electrode active material layer 44 and each
separator 50 are bonded to each other in the regions B as shown in
FIG. 4A to FIG. 4D with the above configuration. For example, as
shown in FIG. 4E, the negative electrode active material layer 44
and each separator 50 may be bonded to each other at not only the
plane portion P of the flat rolled electrode assembly 20 but also a
curved portion R of the flat rolled electrode assembly 20.
S2. Supply of Nonaqueous Electrolyte
[0067] Subsequently, in step S2, a nonaqueous electrolyte is
supplied into the battery case 10. In the technique described here,
in advance of the nonaqueous electrolyte supply step S2, the ends
of the negative electrode 40 and each separator 50 are at least
partially bonded to each other. Thus, at the time of supplying the
nonaqueous electrolyte, inclusion of foreign metal substances,
included in the battery case 10, between the negative electrode 40
and each separator 50 together with the nonaqueous electrolyte is
suppressed. Thus, for example, it is possible to suppress the rate
of occurrence of delayed fracture to a lower rate. The delayed
fracture is that dissolution of foreign metal substances gradually
proceeds after shipment of the battery and then a micro short
circuit occurs.
[0068] Typically, a supporting electrolyte (for example, a lithium
salt in a lithium ion secondary battery) dissolved or dispersed in
a nonaqueous solvent can be employed as the nonaqueous electrolyte.
Alternatively, a so-called solid electrolyte in which a polymer is
added to a liquid nonaqueous electrolyte to form a gel may be
employed as the nonaqueous electrolyte. Various organic solvents,
such as carbonates, ethers, esters, nitriles, sulfones and
lactones, that are used as an electrolyte in a general lithium ion
secondary battery may be used as the nonaqueous solvent without any
particular limitations. For example, specifically, the aqueous
solvent includes ethylene carbonate (EC), propylene carbonate (PC),
diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl
carbonate (EMC), and the like. One of such nonaqueous solvents may
be used solely or two or more of such nonaqueous solvents may be
used as a mixed solvent.
[0069] Various salts that are used in a general lithium ion
secondary battery may be selected and employed as needed as the
supporting electrolyte. For example, using a lithium salt, such as
LiPF.sub.6, LiBF.sub.4, ClO.sub.4, LiAsF.sub.6,
Li(CF.sub.3SO.sub.2).sub.2N and LiCF.sub.3SO.sub.3, is illustrated.
One of such supporting salts may be used solely or two or more of
such supporting salts may be used in combination. The supporting
salt is suitably prepared such that the concentration in the
nonaqueous electrolyte falls within the range of 0.7 mol/L to 1.3
mol/L.
[0070] The nonaqueous electrolyte may include various additives, or
the like, unless the nonaqueous electrolyte impairs the advantage
of the lithium ion secondary battery described here. The additives
include a film-forming agent, such as vinylene carbonate (VC) and
fluoroethylene carbonate (FEC), a gas-forming agent, such as
biphenyl (BP) and cyclohexylbenzene (CHB), and the like.
S3. Initial Charging
[0071] In the technique described here, although not an
indispensable step, in preparing the nonaqueous electrolyte
secondary battery 100, after the electrolyte supply step S2,
initial charging may be suitably carried out. Initial charging is a
step of charging the constructed secondary battery (cell) 100, for
example, up to an operating upper limit voltage value of the
battery in a normal temperature range. Typically, charging
(typically, constant-current charging) just needs to be carried out
up to a predetermined voltage by connecting an external power
supply (not shown) between the positive electrode 30 (positive
electrode terminal 60) of the cell and the negative electrode 40
(negative electrode terminal 70) of the cell. The normal
temperature range in this initial charging step is typically a
temperature range considered to be a normal temperature, and can be
20.degree. C..+-.15.degree. C. The voltage (typically, reachable
maximum voltage) between the positive and negative electrode
terminals in initial charging can vary depending on the types of
active materials and nonaqueous solvent used, and the like;
however, the voltage just needs to be a voltage range that can be
indicated at the time when the state of charge (SOC) of the battery
assembly is higher than or equal to about 80% (typically, 90% to
105%) of a full charge capacity (typically, the rated capacity of
the battery). For example, in a battery that becomes a full charge
state at 4.2 V, the battery is adjusted within the range of about
3.8 to 4.2 V. A charging rate in initial charging may be similar to
an existing known charging rate that can be generally employed when
an existing battery assembly is initially charged, and may be, for
example, about 0.1 to 10 C. The charging may be carried out once or
may be, for example, repeatedly carried out twice or more with
discharging in between.
S4. Aging
[0072] An aging step is typically a step of holding (typically,
left standing) the secondary battery (cell) 100, subjected to the
initial charging step, in a predetermined temperature range at a
predetermined SOC for a set time. The temperature at which the cell
is held may be a low temperature range (for example, about the
normal temperature range to 40.degree. C.) or may be a high
temperature range (for example, about 40.degree. C. to 80.degree.
C.). The SOC of the cell may also be any one of the state from 0%
to 100% or the state higher than or equal to 100%. These
temperature and SOC may be variously changed in order to achieve a
predetermined advantageous effect through aging.
[0073] In this aging, foreign metal substances included between the
positive electrode 30 and each separator 50 can be exposed to a
high positive electrode potential. Thus, electrons are exchanged
between the positive electrode 30 and the foreign metal substances,
and the foreign metal substances are dissolved (that is, ionized).
The ions of the dissolved foreign metal substances are positively
charged, so the ions are attracted toward the negative electrode 40
having a lower potential in the electrolyte. When there is a
portion at which the potential becomes lower than or equal to the
reduction potential of the foreign metal substances on the surface
of the negative electrode 40, the ions of the foreign metal
substances can precipitate at that portion as a precipitate. When
the aging completes and the potential of the negative electrode
becomes lower than or equal to the reduction potential of the
foreign metal substances, the ions of the foreign metal substances
precipitate as a precipitate on the surface of the negative
electrode 40. The battery 100 in which a micro short circuit has
occurred because of such precipitation of metal may be, for
example, determined and collected through a self-discharge test
step described below.
[0074] In the manufacturing method described here, foreign metal
substances included between the positive electrode 30 and each
separator 50 can be promptly dissolved because of the high
potential of the positive electrode. Therefore, for example, even
when foreign metal substances are iron having a low dissolution
rate, the foreign metal substances are suitably dissolved in a
relatively short time. Inclusion of foreign metal substances
between the negative electrode 40 and each separator 50 is
prevented. Therefore, for example, inclusion of iron, or the like,
having a low dissolution rate between each separator 50 and the
negative electrode 40 where dissolution is hard to be facilitated,
is prevented. Thus, it is possible to suppress presence of
non-dissolved foreign metal substances to an extremely small amount
to such an extent that a micro short circuit does not occur over a
predetermined aging time.
S5. Self-Discharge Test Step
[0075] Although not an indispensable step, in a suitable mode,
after the above aging step S4, a self-discharge test may be further
carried out. The self-discharge test is a test to determine a
defective product (a battery in which an internal short circuit has
occurred) by measuring the voltage drop amount of the cell. The
internal short circuit to be tested is a micro short circuit due to
foreign metal substances remaining at the positive electrode side,
so, to accurately measure whether there is a micro short circuit,
at least about five days in the existing art and, where necessary,
a test time of about 10 days is required. This is mainly because,
it is assumed that foreign metal substances having a low
dissolution rate and a high resistance (typically, iron) remain
inside the cell, a test time is set on the basis of the remark that
five or more days are required in order to test the influence of
the foreign substances. However, in the aging, even when the
foreign metal substances are not completely dissolved, dissolution
can be suitably facilitated to such an extent that a micro short
circuit can occur. Therefore, the self-discharge test may be
carried out by shortening the test time as compared to the existing
one. For example, a required time of the self-discharge test may
be, for example, within 24 hours (typically, within 15 hours, for
example, within 10 hours, suitably for example, two hours to five
hours). Therefore, a time that is required for the self-discharge
test is shortened, so improvement of productivity may be
achieved.
[0076] In the above-described embodiment, the technique is
described on the assumption that the electrode assembly 20 is the
flat rolled electrode assembly 20. However, the electrode assembly
20 is not limited to the flat rolled electrode assembly. For
example, the electrode assembly 20 may be a plate stacked electrode
assembly 20 in which the plate-shaped positive electrode 30 and the
plate-shaped negative electrode 40 are stacked via the separator 50
in multiple sets. Although not specifically shown, the
configuration including a plate stacked electrode assembly may also
be similarly considered as in the case of the flat rolled electrode
assembly 20. That is, the plate stacked electrode assembly is
formed by stacking a plurality of the rectangular positive
electrodes 30 and a plurality of the rectangular negative
electrodes 40. The entire face of the negative electrode active
material layer 44 contacts the separator 50. Therefore, the ends of
the contact faces of the negative electrode active material layer
44 and the separator 50 are arranged all around in a band shape
along the outer periphery of the rectangular negative electrode
active material layer 44. The band-shaped outer peripheral portions
(ends) of the negative electrode active material layer 44 and the
separator 50 just need to be at least partially bonded to each
other.
[0077] With the technique described here, the configuration for not
causing foreign metal substances to be included is intrinsically
implemented at the portion at which dissolution of foreign metal
substances is hard to be facilitated. Therefore, for example, it is
possible to significantly reduce the probability of occurrence of a
delayed micro short circuit for the battery provided to the market
as a result of passing the above-described self-discharge test.
Thus, the high-quality and highly reliable nonaqueous electrolyte
secondary battery in which an inconvenience, such as a decrease in
capacity due to a micro short circuit and a decrease in safety
during overcharging, is reduced is provided.
[0078] The nonaqueous electrolyte secondary battery 100 described
here may be utilized in various applications, and can be provided
as, for example, the one having a high capacity retention rate and
a high safety (reliability) as described above in comparison with
an existing product. Thus, for example, both the advantage, such as
a high energy density, which can be intrinsically provided in the
nonaqueous electrolyte secondary battery 100, and safety are
achieved at high level. Such a configuration may also be suitably
applied to a battery that requires a relatively large capacity (for
example, a battery capacity of larger than or equal to 20 Ah,
typically, larger than or equal to 25 Ah, for example, larger than
or equal to 30 Ah). Therefore, by taking advantage of such a
feature, the technique described here may be particularly suitably
applied to a battery in an application that requires a high
capacity, a high energy density, a cycle characteristic, and the
like, and a battery in an application that requires high
reliability. Such applications include, for example, a driving
power supply mounted on a vehicle, such as a plug-in hybrid vehicle
(PHV), a hybrid vehicle (HV) and an electric vehicle (EV). The
lithium ion secondary battery can be typically used in a mode of a
battery pack in which a plurality of lithium secondary batteries
are connected in series with or parallel with each other.
[0079] Specific examples of the invention are described in detail;
however, these are only illustrative and do not limit the appended
claims. The techniques described in the appended claims encompass
various modifications and changes from the specific examples
illustrated above.
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