U.S. patent application number 09/809076 was filed with the patent office on 2001-07-26 for lithium ion secondary battery and process for producing the same.
This patent application is currently assigned to MITSUBISHI DENKI KABUSHIKI KAISHA. Invention is credited to Aihara, Shigeru, Aragane, Jun, Hamano, Kouji, Inuzuka, Takayuki, Murai, Michio, Shiota, Hisashi, Takemura, Daigo, Urushibata, Hiroaki, Yoshida, Yasuhiro.
Application Number | 20010009736 09/809076 |
Document ID | / |
Family ID | 23338533 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009736 |
Kind Code |
A1 |
Shiota, Hisashi ; et
al. |
July 26, 2001 |
Lithium ion secondary battery and process for producing the
same
Abstract
An object is to provide with good productivity a practical
lithium ion secondary battery which secures lightness in weight and
safety without using a firm battery case and has reduced internal
resistivity. The lithium ion secondary battery comprises a tabular
roll type laminated electrode body in which a band-formed positive
electrode (3) comprising a positive electrode active material layer
(7) and a positive electrode current collector (6) alternates with
a band-formed negative electrode (5) comprising a negative
electrode active material layer (9) and a negative electrode
current collector (10), having therebetween a band-formed rolled
separator (4) which holds a lithium ion-containing electrolytic
solution, the positive electrode (3) or the negative electrode (5)
being adhered with two separators (4) by an adhesive layer
(11).
Inventors: |
Shiota, Hisashi; (Tokyo,
JP) ; Aihara, Shigeru; (Tokyo, JP) ; Takemura,
Daigo; (Tokyo, JP) ; Aragane, Jun; (Tokyo,
JP) ; Urushibata, Hiroaki; (Tokyo, JP) ;
Hamano, Kouji; (Tokyo, JP) ; Yoshida, Yasuhiro;
(Tokyo, JP) ; Inuzuka, Takayuki; (Tokyo, JP)
; Murai, Michio; (Tokyo, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
MITSUBISHI DENKI KABUSHIKI
KAISHA
Tokyo
JP
|
Family ID: |
23338533 |
Appl. No.: |
09/809076 |
Filed: |
March 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
09809076 |
Mar 16, 2001 |
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09341665 |
Jul 19, 1999 |
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6232014 |
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09341665 |
Jul 19, 1999 |
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PCT/JP97/04200 |
Nov 19, 1997 |
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Current U.S.
Class: |
429/164 ;
429/137; 429/145; 429/249; 429/250; 429/94 |
Current CPC
Class: |
H01M 4/62 20130101; H01M
10/0431 20130101; Y02E 60/10 20130101; Y02P 70/50 20151101; H01M
10/0436 20130101; H01M 10/052 20130101 |
Class at
Publication: |
429/164 ;
429/137; 429/145; 429/250; 429/94; 429/249 |
International
Class: |
H01M 002/02; H01M
002/18 |
Claims
1. A lithium ion secondary battery characterized by comprising a
tabular roll type laminated battery body having: a band-formed
positive electrode comprising a positive electrode active material
layer and a positive electrode current collector; a band-formed
negative electrode comprising a negative electrode active material
layer and a negative electrode current collector; and band-formed
separators which hold a lithium ion-containing electrolytic
solution, wherein the positive electrode and the negative electrode
alternate with a rolled separator therebetween, either one of the
positive electrode and the negative electrode and the separators
being adhered to each other by an adhesive layer.
2. A lithium ion secondary battery according to claim 1,
characterized in that the adhesive layer is a porous adhesive resin
layer holding an electrolyte.
3. A lithium ion secondary battery according to claim 2,
characterized in that the porosity of the porous adhesive resin
layer is equal to or greater than that of the separators.
4. A lithium ion secondary battery according to claim 3,
characterized in that the ion conduction resistivity of the
adhesive resin layer holding an electrolytic solution is equal to
or smaller than that of the separators holding an electrolytic
solution.
5. A lithium ion secondary battery according to claim 2,
characterized in that the adhesive resin layer comprises a
fluorocarbon resin or a mixture mainly comprising a fluorocarbon
resin.
6. A lithium ion secondary battery according to claim 5,
characterized in that the fluorocarbon resin is polyvinylidene
fluoride.
7. A lithium ion secondary battery according to claim 2,
characterized in that the adhesive resin layer comprises polyvinyl
alcohol or a mixture mainly comprising polyvinyl alcohol.
8. A process for producing a lithium ion secondary battery
characterized by comprising the steps of adhering either one of a
band-formed positive electrode having a positive electrode active
material layer and a positive electrode current collector and a
band-formed negative electrode having a negative electrode active
material layer and a negative electrode current collector in
between a pair of band-formed separators to prepare an electrode
with separators and rolling the electrode with separators and the
other electrode in such a manner that the positive electrode and
the negative electrode may alternate with the separator interposed
therebetween.
Description
TECHNICAL FIELD
[0001] This invention relates to a lithium ion secondary battery
comprising a positive electrode and a negative electrode facing
each other via a separator holding an electrolytic solution. More
particularly, it relates to a battery structure providing improved
electrical connections between each of a positive electrode and a
negative electrode and a separator so that a battery may have an
arbitrary shape, such as a thin shape.
BACKGROUND OF THE INVENTION
[0002] There has been an eager demand for reduction in size and
weight of portable electronic equipment, and the realization relies
heavily on improvement of battery performance. To meet the demand,
development and improvement of batteries from various aspects have
been proceeding. Battery characteristics expected to be improved
include increases in voltage, energy density, resistance to high
load, freedom of shape, and safety. Of available batteries, lithium
ion batteries are the most promising secondary batteries for
realizing a high voltage, a high energy density, and resistance to
high load and have been and will be given improvements.
[0003] A lithium ion secondary battery mainly comprises a positive
electrode, a negative electrode, and an ion-conducting layer
interposed between the electrodes. The lithium ion secondary
batteries that have been put to practical use employ a positive
electrode plate prepared by applying to an aluminum current
collector a mixture comprising a powdered active material, such as
a lithium-cobalt complex oxide, a powdered electron conductor, and
a binder resin; a negative electrode plate prepared by applying to
a copper current collector a mixture of a powdered carbonaceous
active material and a binder resin; and an ion conducting layer
made of a porous film of polyethylene, polypropylene, etc. filled
with a nonaqueous solvent containing lithium ions.
[0004] FIG. 5 schematically illustrates a cross section showing the
structure of a conventional cylindrical lithium ion secondary
battery disclosed in JP-A-8-83608. In FIG. 5 reference numeral 1
indicates a battery case made of stainless steel, etc. which also
serves as a negative electrode terminal, and numeral 2 an electrode
body put into the battery case 1. The electrode body 2 is composed
of a positive electrode 3, a separator 4, and a negative electrode
5 in a rolled-up form. In order for the electrode body 2 to
maintain electrical connections among the positive electrode 3, the
separator 4, and the negative electrode 5, it is necessary to apply
pressure thereto from outside. For this purpose, the electrode body
2 is put into a firm metal-made case to maintain all the planar
contacts. In the case of rectangular batteries, an external
pressing force is imposed to a bundle of strip electrodes by, for
example, putting the bundle in a rectangular metal case.
[0005] That is, a contact between a positive electrode and a
negative electrode in commercially available lithium ion secondary
batteries has been made by using a firm case made of metal, etc.
Without such a case, the electrodes would be separated, and the
battery characteristics would be deteriorated due to difficulty in
maintaining electrical connection between electrodes via an
ion-conducting layer (separator). However, occupying a large
proportion in the total weight and volume of a battery, the case
causes reduction in energy density of the battery. Moreover, being
rigid, it imposes limitation on battery shape, making it difficult
to make a battery of arbitrary shape.
[0006] Under such circumstances, development of lithium ion
secondary batteries which do not require a case has been
proceeding, aiming at reductions in weight and thickness. The key
to development of batteries requiring no case is how to maintain an
electrical connection between each of a positive electrode and a
negative electrode and an ion conducting layer (i.e., separator)
interposed therebetween without adding an outer force. A method
comprising bringing electrodes and a separator into intimate
contact by means of a resin and the like has been proposed as a
connecting means requiring no outer force.
[0007] For example, JP-A-5-159802 teaches a method in which an ion
conducting solid electrolyte layer, a positive electrode, and a
negative electrode are heat-bonded into an integral body by use of
a thermoplastic resin binder. According to this technique,
electrodes are brought into intimate contact by uniting the
electrodes and an electrolyte layer into an integral body so that
the electrical connection between electrodes is maintained to
perform the function as a battery without applying outer force.
[0008] Being thus constituted, conventional lithium ion secondary
batteries have their several problems. That is, those in which a
firm case is used for ensuring intimate contacts between electrodes
and a separator and electrical connections between electrodes have
the problem that the case which does not participate in electricity
generation occupies a large proportion in the total volume or
weight of a battery, which is disadvantageous for production of
batteries having a high energy density. Where the proposed method
comprising bonding electrodes and an ion conductor with an adhesive
resin is followed, for example, where a solid electrolyte and
electrodes are merely brought into contact by an adhesive resin,
the resistance to ion conduction within a battery increases due to
the great resistance of the adhesive resin layer, resulting in
reduction of battery characteristics.
[0009] Further, the battery according to JP-A-5-159802 supra, in
which electrodes and a solid electrolyte are bonded with a binder,
is disadvantageous in terms of ion conductivity as compared with,
for example, batteries using a liquid electrolyte because the
interface between an electrode and an electrolyte is covered with
the binder. Even though an ion-conducting binder is employed, there
is no binder generally known to be equal or superior in ion
conductivity to a liquid electrolyte, and it has been difficult to
achieve battery performance equal to that of a battery using a
liquid electrolyte.
[0010] That is, a metal case is necessary for holding a liquid
electrolyte in the electrode-electrolyte interface, which is
disadvantageous for energy density. On the other hand, batteries of
electrode-electrolyte bound type do not require a metallic case but
have reduced conductivity through the electrode-electrolyte
interface as compared with batteries using a liquid electrolyte,
which is disadvantageous in terms of battery performance such as
charge and discharge characteristics at a high load.
[0011] A nonaqueous electrolyte which can generally be used in
lithium ion batteries has one-tenth or less as much conductivity as
an aqueous electrolyte. Therefore, it is necessary to increase the
battery area to reduce the internal resistivity. In order to make
large-area electrodes into a compact battery, the electrodes are
cut into strips which are laid one on top of another or the
electrodes are inserted in rolled or folded band-formed separators.
In practical battery assembly, a battery body is usually
constructed by rolling up bands of separators and electrodes. It is
possible to apply this assembly to the type of batteries in which
electrodes are joined to a separator via an adhesive layer.
However, the speed of rolling up the bands while applying an
adhesive is lower than the speed of rolling using no adhesive,
resulting in poor productivity of assembly. In a case where a
battery body rolled up with no adhesive is fastened with a tape
band, the internal resistivity is high because of insufficient
contact at the electrode-separator interface. This is problematical
in practical use particularly where a large electrical current is
needed.
[0012] In order to solve these problems, the inventors of the
present invention have conducted extensive study on a favorable
method for laminating separators and electrodes. The present
invention has been reached as a result. Accordingly, an object of
the present invention is to provide a practical lithium ion
secondary battery having low internal resistivity, in which a
separator and an electrode are brought into intimate contact
without using a firm battery case.
DISCLOSURE OF THE INVENTION
[0013] A first lithium ion secondary battery according to the
present invention comprises a tabular roll type laminated battery
body having a band-formed positive electrode comprising a positive
electrode active material layer-and a positive electrode current
collector, a band-formed negative electrode comprising a negative
electrode active material layer and a negative electrode current
collector, and band-formed separators which hold a lithium
ion-containing electrolytic solution, wherein the positive
electrode and the negative electrode alternate with a rolled
separator therebetween, either one of the positive electrode and
the negative electrode and the separators being adhered to each
other by an adhesive layer. According to this structure, either one
of a positive electrode and a negative electrode to which
separators have been adhered is rolled up together with the other
electrode to prepare a tabular roll type laminated battery body.
The time required for the adhesive to dry can be shortened as
compared with a case where rolling and adhesion operations are
carried out simultaneously. When compared with a case wherein a
positive electrode, a negative electrode, and separators are rolled
up with no adhesion, it is only two members that are rolled up
together, i.e., one of electrodes that is previously bonded between
separators and the other electrode. Therefore, the rolling
operation can be carried out easily. Further, since an electrode
and a separator hardly slip on each other while rolled, it is less
likely that an internal shortage occurs due to a contact between a
positive and a negative electrode, which leads to improved safety.
Furthermore, a lithium ion secondary battery having reduced
internal resistivity can be obtained owing to the highly intimate
contact between an electrode and separators.
[0014] A second lithium ion secondary battery of the invention is
the above-described first lithium ion secondary battery, wherein
the adhesive layer is a porous adhesive resin layer holding an
electrolyte. According to this structure, the electrode and the
separator are brought into intimate contact by the adhesive resin
layer, and a liquid electrolytic solution is held in the
through-holes of the adhesive resin layer which connect the
electrode and the separator. As a result, satisfactory ion
conduction through the electrode-electrolyte interface can be
secured thereby to provide a lithium ion secondary battery which
can have an increased energy density and a reduced thickness, can
take an arbitrary shape, and exhibits excellent charge and
discharge characteristics.
[0015] A third lithium ion secondary battery of the invention is
the above-described second lithium ion secondary battery, wherein
the porosity of the porous adhesive resin layer is equal to or
greater than that of the separator. In this case, the adhesive
resin layer holding an electrolytic solution has a proper ion
conduction resistivity.
[0016] A fourth lithium ion secondary battery of the invention is
the above-described second lithium ion secondary battery, wherein
the ion conduction resistivity of the adhesive resin layer holding
an electrolytic solution is equal to or smaller than that of the
separator holding an electrolytic solution. According to this
aspect, deterioration in charge and discharge characteristics are
prevented, and excellent charge and discharge characteristics can
be maintained.
[0017] A fifth lithium ion secondary battery of the invention is
the above-described second lithium ion secondary battery, wherein
the adhesive resin layer comprises a fluorocarbon resin or a
mixture mainly comprising a fluorocarbon resin.
[0018] A sixth lithium ion secondary battery of the invention is
the above-described fourth lithium ion secondary battery, wherein
the fluorocarbon resin is polyvinylidene fluoride.
[0019] A seventh lithium ion secondary battery of the invention is
the above-described second lithium ion secondary battery, wherein
the adhesive resin layer comprises polyvinyl alcohol or a mixture
mainly comprising polyvinyl alcohol.
[0020] Where a fluorocarbon resin or a mixture mainly comprising
the same or polyvinyl alcohol or a mixture mainly comprising the
same is used as an adhesive resin layer, a lithium ion secondary
battery having the above-mentioned excellent characteristics can be
obtained.
[0021] A process for producing the first lithium ion secondary
battery according to the present invention comprises the steps of
adhering either one of a band-formed positive electrode having a
positive electrode active material layer and a positive electrode
current collector and a band-formed negative electrode having a
negative electrode active material layer and a negative electrode
current collector in between a pair of band-formed separators to
prepare an electrode with separators and rolling the electrode with
separators and the other electrode in such a manner that the
positive electrode may alternate with the negative electrode,
having the separator interposed therebetween. According to this
process, either one of the positive electrode and the negative
electrode to which separators have been adhered is rolled up
together with the other electrode so that the time for drying the
adhesive can be reduced as compared with a case where adhesion is
conducted at the time of rolling. Further, as compared with a case
where three of a positive electrode, a negative electrode, and a
separator are rolled up altogether, the workability in rolling is
improved because only two of the electrode with separators and the
other electrode are rolled up. Lightness in weight and safety are
secured with no aid of a firm battery case, and a practical lithium
ion secondary battery having reduced internal resistivity can be
obtained with good productivity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic cross section showing the structure of
the tabular roll type laminated electrode body of the lithium ion
secondary battery according to an embodiment of the present
invention. FIG. 2 is a schematic cross section illustrating the
main part of the electrode body shown in FIG. 1. FIG. 3 is a
characteristic diagram showing the discharge characteristics of the
batteries obtained in Examples 1 to 3 and a comparative battery.
FIG. 4 is a characteristic plot of internal resistivity on amount
of adhesive resin in the adhesive resin solution used in the
formation of an adhesive resin layer according to an embodiment of
the present invention. FIG. 5 is a schematic cross section of an
example of conventional lithium ion secondary batteries.
BEST MODE FOR CARRYING OUT THE INVENTION
[0023] FIG. 1 is a schematic cross section illustrating the
constitution of the tabular roll type laminated electrode body of
the lithium ion secondary battery according to an embodiment of the
invention. FIG. 2 is an enlarged schematic cross section
illustrating the main part of FIG. 1. The lithium ion secondary
battery according to the invention has a tabular roll type laminate
structure in which a positive electrode band and a negative
electrode band alternately interposed between rolled separator
bands, and either one of the positive electrode and the negative
electrode is adhered to the separators with an adhesive layer. In
these Figures, numeral 3 indicates a positive electrode made of a
positive electrode active material layer 7 bonded to a positive
electrode current collector 6; 5, a negative electrode made of a
negative electrode active material layer 9 bonded to a negative
electrode current collector 10; 4, a separator holding a lithium
ion-containing electrolytic solution, interposed between the
positive electrode 3 and the negative electrode 5; and 11, a porous
adhesive resin layer which joins the negative electrode active
material layer 9 and the separator 4. The adhesive resin layer 11
has a large number of through-holes 12 which connect the negative
electrode active material layer 9 to the separator 4. The
electrolytic solution is held in the through-holes.
[0024] The battery has a tabular roll type laminate structure in
which the positive electrode band 3 and the negative electrode band
4 alternate with the rolled separator 4 therebetween, and either
one of the positive electrode 3 and the negative electrode 5 and
the separators 4 are adhered with the adhesive layer 11. Therefore,
the tabular roll type laminated battery body can be formed by
rolling up either one of the positive electrode 3 and the negative
electrode 5 having the separator 4 adhered on each side thereof
together with the other electrode, i.e., the negative electrode 5
or the positive electrode 3. The time required for drying the
adhesive can be reduced as compared with a case in which adhesion
is carried out simultaneously with rolling. When compared with a
case in which a positive electrode, a negative electrode and a
separator are rolled up together with no adhesion, it is only the
two of the electrode with separators and the other electrode that
should be rolled up. Therefore, the workability is good, and the
apparatus for rolling can be simplified greatly. Since the
electrodes and the separator hardly slip on each other while
rolled, it is less likely that an internal shortage occurs due to a
contact between the positive electrode and the negative electrode,
leading to improved safety. A lithium ion secondary battery having
reduced internal resistivity can be obtained owing to the highly
intimate contact between the electrode and the separator.
[0025] Because the electrode layer (i.e., the active material layer
7 or 9) and the separator 4 serving as an electrolyte layer are
joined by the porous adhesive resin layer 11, the adhesive strength
between the electrode and the separator is guaranteed. An
electrolytic solution being held in the through-holes 12 that are
made through the adhesive resin layer 11 to connect the interface
with the electrode and the interface with the separator,
satisfactory ion conduction is secured through the
electrode-electrolyte interface. Reduction in ion conduction
resistance between electrodes also results. It is thus made
possible to establish substantial equality to conventional lithium
ion batteries having an outer case in terms of the quantity of ions
moving in and out of the active material of the electrodes and the
rate and quantity of migration of ions to the facing electrode.
[0026] The ending part of the separator that is the outermost layer
can be adhered to the rolled electrode body after the final turn of
rolling so as to maintain electrical connections between electrodes
without applying outer force. This excludes the necessity of a firm
battery case for retention of the battery structure and makes it
feasible to reduce the weight and thickness of a battery and to
design a battery shape freely. Further, excellent charge and
discharge characteristics and battery performance as obtained from
batteries using an electrolytic solution can be obtained.
[0027] Where the ion conduction resistivity of the adhesive resin
layer 11 holding an electrolytic solution is equal to or smaller
than that of the separator 4 holding an electrolytic solution, the
adhesive resin layer 11 would not cause deterioration in charge and
discharge characteristics, and it is possible to obtain charge and
discharge characteristics on the same level of conventional
batteries.
[0028] The ion conduction resistivity of the adhesive resin layer
11 can be controlled chiefly by varying its porosity and thickness.
The porosity can be adjusted by selecting the ratio of the adhesive
resin to, for example, N-methylpyrrolidone in the adhesive resin
solution forming the adhesive resin layer. The porosity is
preferably equal to or greater than that of the separator 4.
[0029] The adhesive resins which can be used for joining an active
material layer and a separator include those which neither dissolve
in the electrolytic solution nor undergo electrochemical reaction
inside a battery and are capable of forming a porous film, such as
a fluorocarbon resin or a mixture mainly comprising a fluorocarbon
resin and polyvinyl alcohol or a mixture mainly comprising
polyvinyl alcohol. Specific examples of useful resins include
polymers or copolymers containing a fluorine atom in the molecular
structure thereof, e.g., vinylidene fluoride or
tetrafluoroethylene, polymers or copolymers having vinyl alcohol in
the molecular skeleton thereof, and their mixtures with polymethyl
methacrylate, polystyrene, polyethylene, polypropylene,
polyvinylidene chloride, polyvinyl chloride, polyacrylonitrile or
polyethylene oxide. Polyvinylidene fluoride, which is a
fluorocarbon resin, is particularly suitable.
[0030] The lithium ion secondary battery having the above-mentioned
constitution can be produced by applying an adhesive to a side each
of a pair of separators 4, inserting a band-formed positive
electrode 3 (or negative electrode) between the adhesive-coated
sides of the paired separators 4, and rolling the resulting
electrode with separators and the other electrode, i.e., a negative
electrode 5 (or positive electrode), into an oblong ellipsoid so
that the positive electrode 3 and the negative electrode 5 may
alternate with the separator 4 therebetween.
[0031] The active materials which can be used in the positive
electrode include complex oxides of lithium and a transition metal,
such as cobalt, nickel or manganese; chalcogen compounds containing
lithium; or complex compounds thereof; and these complex oxides,
Li-containing chalcogen compounds or complex compounds thereof that
contain various dopant elements. While any substance capable of
intercalating and disintercalating lithium ions, which take the
main part of a battery operation, can be used as a negative
electrode active material, preferred active materials for use in
the negative electrode include carbonaceous compounds, such as
graphitizing carbon, non-graphitizing carbon, polyacene, and
polyacetylene; and aromatic hydrocarbon compounds having an acene
structure, such as pyrene and perylene. These active materials are
used in a particulate state. Particles having a particle size of
0.3 to 20 .mu.m can be used. A preferred particle size is 0.3 to 5
.mu.m.
[0032] Any binder resin that is insoluble in an electrolytic
solution and undergoes no electrochemical reaction in the electrode
laminate can be used for binding an active material into an
electrode plate. Examples of useful binder resins are homo- or
copolymers of vinylidene fluoride, ethylene fluoride,
acrylonitrile, and ethylene oxide, and ethylene propylenediamine
rubber.
[0033] Any metal stable within a battery can be used as a current
collector. Aluminum is preferred for a positive electrode, and
copper is preferred for a negative electrode. The current collector
can be foil, net, expanded metal, etc. Those presenting a large
void area, such as net and expanded metal, are preferred from the
standpoint of ease of holding an electrolytic solution after
adhesion.
[0034] Similarly to the adhesive resin used for adhesion of an
electrode and a separator, adhesive resins which can be used for
adhesion between a current collector and an electrode include those
which neither dissolve in an electrolytic solution nor undergo
electrochemical reaction inside a battery and are capable of
forming a porous film. Examples include polymers having a fluorine
molecule, e.g., vinylidene fluoride or tetrafluoroethylene, in the
molecular structure thereof or mixtures thereof with polymethyl
methacrylate, polystyrene, polyethylene, polypropylene, etc.;
polymers or copolymers having vinyl alcohol in the molecular
skeleton thereof or mixtures thereof with polymethyl methacrylate,
polystyrene, polyethylene, polypropylene, polyvinylidene chloride,
polyvinyl chloride, polyacrylonitrile, polyethylene oxide, etc.
Polyvinylidene fluoride or polyvinyl alcohol is particularly
suitable.
[0035] Any electron-insulating separator that has sufficient
strength, such as porous film, net, and nonwoven fabric, can be
used. While not limiting, polyethylene or polypropylene is a
preferred material for the separator for their adhesiveness and
safety.
[0036] The solvent and the electrolyte which make an electrolytic
solution serving as an ion conductor can be any of nonaqueous
solvents and any of lithium-containing electrolyte salts that have
been employed in conventional batteries. Examples of useful
solvents include ethers, such as dimethoxyethane, diethoxyethane,
diethyl ether, and dimethyl ether; esters, such as propylene
carbonate, ethylene carbonate, diethyl carbonate, and dimethyl
carbonate; and mixed solvents consisting of two members selected
from the ether solvents or the ester solvents or mixed solvents
consisting of one member selected from the former group and one
member selected from the latter group. Examples of useful
electrolytes are LiPF.sub.6, LiAsF.sub.6, LiClO.sub.4, LiBF.sub.4,
LiCF.sub.3SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, and
LiC(CF.sub.3SO.sub.2).sub.3.
[0037] Methods for applying the adhesive resin include coating with
a bar coater, spraying with a spray gun, and dip coating.
[0038] The present invention will now be illustrated with reference
to Examples, but the present invention is by no means limited
thereto.
EXAMPLE 1
[0039] Preparation of Positive Electrode
[0040] Eighty-seven parts by weight of LiCoO.sub.2, 8 parts by
weight of graphite powder, and 5 parts of polyvinylidene fluoride
were dispersed in N-methylpyrrolidone to prepare positive electrode
active material paste. The paste was applied to 20 .mu.m thick
band-formed aluminum foil as a positive electrode current collector
with a doctor blade to a coating thickness of 150 .mu.m to form an
active material film. The coated aluminum foil was allowed to stand
in a drier kept at 60.degree. C. for 60 minutes to dry the paste.
The resulting laminate was pressed to reduce the thickness of the
positive electrode active material layer to 100 .mu.m to prepare a
band formed positive electrode 3 comprising the aluminum foil
current collector 6 having formed thereon a 100 .mu.m-thick
positive electrode active material layer 7.
[0041] Preparation of Negative Electrode
[0042] Ninety-five parts by weight of Mesophase Microbead Carbon (a
trade name, produced by Osaka Gas Co., Ltd.) and 5 parts by weight
of polyvinylidene fluoride were dispersed in N-methylpyrrolidone
(hereinafter abbreviated as NMP) to prepare negative electrode
active material paste. The paste was applied to 20 .mu.m-thick
copper band-formed foil as a negative electrode current collector
with a doctor blade to a thickness of 150 .mu.m to form an active
material film. The laminate was dried by allowing to stand in a
drier at 60.degree. C. for 60 minutes and pressed to reduce the
thickness of the negative electrode active material layer to 100
.mu.m to prepare a band-formed negative electrode 5 comprising the
copper foil negative electrode current collector 10 having formed
thereon a 100 .mu.m-thick negative electrode active material layer
9.
[0043] Preparation of Adhesive Resin Solution
[0044] Five parts by weight of polyvinylidene fluoride and 5 parts
by weight of finely powdered alumina (Aerosil C, produced by
Aerosil) were suspended and dissolved in N-methylpyrrolidone
(hereinafter abbreviated as NMP), and the mixture was thoroughly
stirred to prepare a viscous and uniform adhesive resin
solution.
[0045] Preparation of Battery
[0046] The adhesive resin solution thus prepared was uniformly
applied to a side each of two porous polyethylene sheets (ME 9630,
produced by Asahi Chemical Industry Co., Ltd.) of band form which
serve as a pair of separators 4. Before the adhesive dried, the
above-prepared band-formed negative electrode 5 (or positive
electrode) was sandwiched and stuck in between the separators with
their coated sides inward. The separators 4 used here were slightly
wider and longer than the negative electrode 5 (or positive
electrode). The resulting laminate, i.e., the negative electrode 5
(or positive electrode) having the separators 4 stuck on each side
thereof, was put in a warm air drier at about 80.degree. C. to
evaporate NMP, whereby NMP escaped to leave through-holes 12 in the
adhesive layer 11.
[0047] Then, the band-formed positive electrode 3 (or negative
electrode) was put on the outer side of one of the separators 4
having the negative electrode 5 (or positive electrode)
therebetween, with a prescribed length of the starting end of the
positive electrode 3 sticking out over the end of that separator 4.
The sticking end of the positive electrode 3 (or negative
electrode) was folded to come into contact with the other separator
4. The separators 4 having the negative electrode 5 (or positive
electrode) therebetween were rolled up in such a manner that the
folded part of the positive electrode 3 (or negative electrode)
might be wrapped in, making an oblong ellipsoid. After the final
turn of rolling, the margin of the separators was fixed to the
rolled electrode body with an adhesive to prepare a tabular roller
type laminated electrode body.
[0048] The tabular roll type laminated electrode body, after
thoroughly dried, was evacuated to 50 Torr and immersed in an
electrolytic solution prepared by dissolving lithium
hexafluorophosphate in a 1:1 (by mole) mixed solvent of ethylene
carbonate and dimethyl carbonate in a concentration of 1.0
mol/dm.sup.3. The impregnated electrode body was heat-sealed into
an aluminum laminate film pack to complete a lithium ion secondary
battery having a tabular laminated battery body.
[0049] FIG. 3 shows a graph of discharge characteristics of the
lithium ion secondary battery thus obtained in Example 1 in
comparison with those of a comparative battery which was prepared
by rolling up the same electrodes and separators as used in Example
1 but without using the adhesive, fastening the rolled electrode
body with a tape band, impregnating the electrode body in the same
electrolytic solution as used above, and sealing the electrode body
into an aluminum laminate film pack. It is seen from the graph that
the battery of Example 1 retains a sufficient capacity for
discharging at a greater current because of its smaller internal
resistivity.
[0050] The characteristic plot of FIG. 4 shows the internal
resistivity of the battery with the amount of the adhesive resin in
the adhesive resin solution being varied; 5 parts, 7 parts and 10
parts by weight per NMP. It is seen that the resistivity increases
abruptly with an increase from 5 parts to 7 parts by weight. Seeing
that the thickness of the adhesive resin layer 11 is proportional
to the amount of the adhesive resin in the adhesive resin solution,
it is considered that retention and distribution of the
electrolytic solution in the adhesive resin layer 11 change
abruptly in that range, resulting in a steep rise of resistivity.
The resistivity at 5 parts by weight was almost equal to the
resistivity as measured on a battery in which no adhesive resin
layer 11 was provided but a sufficient planar pressure was applied
to electrodes 3 and 5 and separators 4.
EXAMPLE 2
[0051] A battery having the tabular roll type laminated electrode
body shown in FIG. 1 was prepared in the same manner as in Example
1, except for changing the adhesive resin solution used in Example
1.
[0052] Preparation of Adhesive Resin Solution
[0053] A viscous adhesive resin solution was prepared by mixing
N-methylpyrrolidone with polytetrafluoroethylene, a vinylidene
fluoride-acrylonitrile copolymer, a mixture of polyvinylidene
fluoride and polyacrylonitrile, a mixture of polyvinylidene
fluoride and polyethylene oxide, a mixture of polyvinylidene
fluoride and polyethylene terephthalate, a mixture of
polyvinylidene fluoride and polymethyl methacrylate, a mixture of
polyvinylidene fluoride and polystyrene, a mixture of
polyvinylidene fluoride and polypropylene, or a mixture of
polyvinylidene fluoride and polyethylene at the same mixing
ratio.
[0054] A battery having a tabular roll type laminated electrode
body was prepared by using each of the adhesive resin solutions in
the same manner as in Example 1. Compared with the comparative
battery, the resulting battery exhibited excellent discharging
current vs. capacity characteristics as shown in FIG. 3.
EXAMPLE 3
[0055] A battery having the tabular roll type laminated electrode
body shown in FIG. 1 was prepared in the same manner as in Example
1, except for changing the adhesive resin solution shown in Example
1.
[0056] Preparation of Adhesive Resin Solution
[0057] A viscous adhesive resin solution was prepared from
polyvinyl alcohol, a mixture of polyvinyl alcohol and
polyvinylidene fluoride, a mixture of polyvinyl alcohol and
polyacrylonitrile, or a mixture of polyvinyl alcohol and
polyethylene oxide by mixing with or dissolving in NMP.
[0058] A battery having a tabular roll type laminated battery body
was prepared by using each of these adhesive resin solutions in the
same manner as in Example 1. Compared with the comparative battery,
the resulting battery exhibited excellent discharging current vs.
capacity characteristics as shown in FIG. 3.
[0059] While in the foregoing examples the adhesive resin solution
was applied by a bar coating method, it may be applied with a spray
gun.
[0060] While the foregoing examples have shown embodiments in which
electrodes having an active material layer adhered to a current
collector are used as a positive electrode 3 and a negative
electrode 5, electrodes in which an active material layer itself
functions as a current collector are also useful.
[0061] Industrial Applicability
[0062] The present invention provides batteries which can have
reduced size and weight and an arbitrary shape as well as improved
performance and can be used in portable electronic equipment, such
as portable personal computers and cellular phones.
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