Lithium ion secondary battery

An object is to provide a lithium ion secondary battery which maintains the electrical connections between each active material layer and a separator without using a firm housing, can have an increased energy density, can take an arbitrary shape, such as a thin shape, exhibits excellent charge and discharge characteristics, and has a large battery capacity. Positive and negative electrode active material particles are bonded to their respective current collectors to form a positive electrode and a negative electrode. The positive and the negative electrode active material layers are joined to a separator with a binder resin so that the joint strength between the separator and the positive and the negative electrode active material layers may be equal to or greater than the joint strength between the positive and the negative electrode active material layers and the respective current collectors to prepare a tabular laminated battery body having a plurality of electrode laminates. An electrolytic solution containing lithium ions is held in the voids possessed by the positive and the negative electrode active material layers and the separator to make an electrical connection between electrodes.

TECHNICAL FIELD
 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 which secures improved electrical connections
 between each of a positive electrode and a negative electrode and a
 separator without a firm metal-made housing so that a battery may have an
 arbitrary shape, such as a thin shape.
 BACKGROUND OF THE INVENTION
 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 a
 variety of batteries 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 currently
 available batteries, lithium ion batteries are the most promising
 secondary batteries for realizing a high voltage, a high energy density,
 and excellent resistance to high load and have been and will be given
 improvements.
 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 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 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.
 FIG. 5 schematically illustrates a cross section 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 case 1. The electrode body 2 has
 a roll form composed of a positive electrode 3 and a negative electrode 5
 having a separator 4 therebetween. 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 case 1 to apply pressure for maintaining 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.
 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 housing made of metal, etc. Without such a housing, the
 electrodes would be separated at their interface, and the battery
 characteristics would be deteriorated due to difficulty in maintaining
 electrical connections. However, occupying a large proportion in the total
 weight and volume of a battery, the housing 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.
 Under such circumstances, development of lithium ion secondary batteries
 which do not require a firm housing has been proceeding, aiming at
 reductions in weight and thickness. The key to development of batteries
 requiring no housing 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.
 Joining means requiring no outer force that have been proposed to date
 include a structure in which electrodes (a positive and a negative
 electrode) are joined with a liquid adhesive mixture (gel electrolyte) as
 disclosed in U.S. Pat. No. 5,460,904 and a structure in which an active
 material is bound with an electron conducting polymer to form a positive
 and a negative electrode, and the electrodes are joined via a
 polyelectrolyte as disclosed in U.S. Pat. No. 5,437,692.
 Conventional lithium ion secondary batteries having the above-mentioned
 structures have their several problems. Those in which a firm case is used
 for ensuring intimate contacts and electrical connections between
 electrodes and a separator have the problem that the case which does not
 participate in electricity generation has a large proportion in the total
 volume or weight of a battery, which is disadvantageous for production of
 batteries having a high energy density. On the other hand, the structure
 in which electrodes are joined with a liquid adhesive mixture needs a
 complicated production process and hardly shows sufficient adhesive
 strength for securing improved strength as a battery. The structure in
 which electrodes are joined with a polyelectrolyte is disadvantageous in
 that the polyelectrolyte layer should have a sufficient thickness for
 security, i.e., enough to prevent internal shortage between electrodes,
 failing to provide a sufficiently thin battery; a solid electrolyte is
 insufficient to join an electrolyte layer and an electrode active
 material, making it difficult to improve battery characteristics such as
 charge and discharge efficiency; and the production process is
 complicated, resulting in an increase of cost.
 Efficiency in intercalation and disintercalation of lithium ions by active
 materials occurring on charging and discharging of a battery is an
 important factor decisive of the charge and discharge efficiency of a
 battery. In a battery of ordinary structure, because mobility of lithium
 ions is equal throughout an electrolytic solution, there is a problem that
 intercalation and disintercalation of lithium ions take place
 preferentially in the portion of the active material layer in the vicinity
 of the electrode surface, i.e., in the vicinity of the separator so that
 the active material in the inside of the electrode is not made effective
 use of. As a result, desired charge and discharge characteristics are hard
 to obtain.
 Hence, in order to obtain a practical thin type lithium ion battery, it is
 required to develop a battery structure that exhibits satisfactory battery
 characteristics such as charge and discharge characteristics while easily
 securing safety and strength as a battery. That is, it is necessary that a
 separator is provided between electrodes for safety and that the separator
 and the electrodes are joined with sufficient strength and in such a
 manner that secures satisfactory battery characteristics.
 In order to solve these problems, the inventors of the present invention
 have conducted extensive study on a favorable method for adhering a
 separator to a positive and a negative electrode. The present invention
 has been reached as a result. Accordingly, an object of the present
 invention is to provide a compact and stable lithium ion secondary battery
 in which a positive electrode, a negative electrode, and a separator are
 brought into firm and intimate contact without using a firm battery case,
 which can have an increased energy density, a reduced thickness, and a
 plurality of electrode laminates in an arbitrary shape, exhibits excellent
 charge and discharge characteristics, and has a large battery capacity.
 DISCLOSURE OF THE INVENTION
 A first structure of the lithium ion secondary battery according to the
 present invention comprises a plurality of electrode laminates each having
 a positive electrode comprising a particulate positive electrode active
 material bound to a positive electrode current collector by a binder
 resin, a negative electrode comprising a particulate negative electrode
 active material bound to a negative electrode current collector by a
 binder resin, and a separator which is interposed between the positive
 electrode and the negative electrode and joined to the positive and the
 negative electrode active material layers, the positive and the negative
 electrode active material layers and the separator holding a lithium
 ion-containing electrolytic solution in their voids, and the joint
 strength between the separator and the positive and the negative electrode
 active material layers being equal to or greater than the joint strength
 between the positive and the negative electrode active material layers and
 the respective current collectors. This structure has the effect that a
 firm housing is no longer necessary, which makes it feasible to reduce the
 weight and thickness of a battery and to design the battery shape freely,
 and there is provided a lithium ion secondary battery having improved
 charge and discharge efficiency, excellent charge and discharge
 characteristics, and high safety. 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 is obtained. Having a plurality of electrode laminates,
 the battery obtained is light in weight, compact, and stable and has a
 large battery capacity.
 A second structure of the lithium ion secondary battery of the invention is
 the 1st structure, wherein the positive and the negative electrode active
 material layers are joined to the separator with the same binder resin
 that binds the particulate positive electrode active material and the
 particulate negative electrode active material to the respective current
 collectors. This structure brings about improved reliability.
 A third structure of the lithium ion secondary battery of the invention is
 the first structure, wherein the covering ratio of the binder resin on the
 particulate active material located on the separator side is greater than
 that on the particulate active material located on the current collector
 side. According to this structure, the difference in rate of intercalation
 and disintercalation of lithium ions between the positive or negative
 active material on the separator side and that in the inside of the active
 material layer is narrowed. As a result, the active material inside the
 electrode can be made effective use of, bringing about improved charge and
 discharge efficiency.
 A fourth structure of the lithium ion secondary battery of the invention is
 the first structure, wherein the plurality of electrode laminates are
 formed by interposing the positive electrode and the negative electrode
 alternately among a plurality of cut sheets of the separator.
 A fifth structure of the lithium ion secondary battery of the invention is
 the first structure, wherein the plurality of electrode laminates are
 formed by interposing the positive electrode and the negative electrode
 alternately between rolled separators.
 A sixth structure of the lithium ion secondary battery of the invention is
 the first structure, wherein the plurality of electrode laminates are
 formed by interposing the positive electrode and the negative electrode
 alternately between folded separators.
 According to the fourth to sixth structures, there is easily obtained a
 battery having a multilayer structure which is excellent in charge and
 discharge characteristics, light, compact, and stable with a large
 capacity.

BEST MODE FOR CARRYING OUT THE INVENTION
 The embodiments for carrying out the invention will be explained by way of
 the drawings.
 FIGS. 1 through 3 each show a schematic cross section illustrating the
 battery structure of the lithium ion secondary battery according to an
 embodiment of the invention. FIG. 1 shows a tabular laminated battery body
 having a plurality of electrode laminates 2 formed by successively
 building up a positive electrode 3, a separator 4, and a negative
 electrode 5. FIG. 2 shows a tabular roll type laminated battery body
 having a plurality of electrode laminates formed by rolling up a pair of
 separators 4 of band form having joined a positive electrode 3 of band
 form therebetween while inserting a plurality of negative electrodes 5.
 FIG. 3 shows a tabular roll type laminated battery body having a plurality
 of electrode laminates formed by disposing a positive electrode 3 of band
 form between a pair of separators 4 of band form with a negative electrode
 5 of band form being disposed on one side of the paired separators 4 and
 rolling up the laminate into an oblong ellipsoid. FIG. 4 is a schematic
 cross-sectional view of an example of the electrode laminate 2
 constituting the above-described batteries of the invention. In these
 Figures, numeral 3 indicates a positive electrode prepared by binding
 positive electrode active material particles 7a to a positive electrode
 current collector 6 by a binder resin 11; 7 a positive electrode active
 material layer made up of the positive electrode active material particles
 7a bound together by the binder resin 11; 5 a negative electrode prepared
 by binding negative electrode active material particles 9a to a negative
 electrode current collector 10 by a binder resin 11; 9 a negative
 electrode active material layer made up of the negative electrode active
 material particles 9a bound together by the binder resin 11; 4 a separator
 interposed between the positive electrode 3 and the negative electrode 5
 and joined to the positive and the negative electrode active material
 layers 7 and 9 by the binder resin 11; and 12 voids formed in the positive
 and the negative electrode active material layers 7 and 9 and the
 separator 4, in which a lithium ion-containing electrolytic solution is
 held.
 The lithium ion secondary battery having the above constitution is produced
 by, for example, as follows.
 Positive electrode active material particles 7a and a binder 11 are
 dispersed in a solvent to prepare active material paste. The paste is
 applied to a positive electrode current collector 6 by roll coating and
 dried to prepare a positive electrode 3. A negative electrode 5 is
 prepared in the similar manner. Then, the binder resin 11 is applied as an
 adhesive to the separator 4. The positive electrode 3 or the negative
 electrode 5 is stuck to the separator 4, and the laminate is build up
 further, rolled up, and the like, as described above to prepare a
 multilayer battery body having a plurality of electrode laminates shown in
 FIGS. 1 to 3. The multilayer battery body is impregnated with an
 electrolytic solution by soaking, and the impregnated battery body is put
 in an aluminum laminate film pack. The opening of the pack is heat-sealed
 to obtain a lithium ion secondary battery having a multilayer structure.
 The joint strength between the separator 4 and each of the positive and the
 negative electrode active material layers 7 and 9 is equal to or greater
 than the joint strength between the positive electrode current collector 6
 and the positive electrode active material layer 7 and that between the
 negative electrode current collector 10 and the negative electrode active
 material layer 9. The covering ratio of the binder resin 11 on the
 positive and the negative active material particles 7a and 9a located on
 the separator side is greater than that on the active material particles
 located on the side of the positive and the negative electrode current
 collectors 6 and 10.
 In this embodiment, the structure of the electrodes (the positive electrode
 3 and the negative electrode 5) is retained by bonding the active material
 and the current collector with the binder resin 11 as in a conventional
 battery structure. The positive electrode 3 and the negative electrode 5
 (i.e., the positive and the negative electrode active material layers 7
 and 9) are similarly bonded to the separator 4 with the same binder resin
 11. Thus, the electrical connections between the active material layers 7
 and 9 and the separator 4 can be retained without applying outer force.
 Therefore, a firm housing for maintaining the battery structure is not
 necessary any longer, which makes it feasible to reduce the weight and
 thickness of the battery and to design the shape of the battery freely.
 Further, the adhesive strength between the positive and the negative
 electrode active material layers 7 and 9 and the separator 4 is equal to
 or greater than the strength bonding the active material and the current
 collector into an integral electrode, i.e., the adhesive strength between
 the positive electrode current collector 6 and the positive electrode
 active material layer 7 and between the negative electrode current
 collector 10 and the negative electrode active material layer 9.
 Therefore, fracture of the electrode takes place in preference to
 separation between the positive and the negative active material layers 7
 and 9 and the separator 4. For example, in case some outer force that
 would deform the battery or some internal thermal stress is imposed, it is
 not the separator but the electrode structure that is broken, which is
 effective for keeping safety.
 In order to further strengthen the adhesion between the electrodes and the
 separator and to enhance the above effect, it is preferred to form a thin
 binder resin layer between each electrode and the separator.
 Intercalation and disintercalation of lithium ions usually take place
 preferentially in the portion on the separator 4 side in the positive and
 the negative active material layers 7 and 9. In the above embodiment, to
 the contrary, the difference in rate of intercalation and disintercalation
 of lithium ions between the active material on the separator side and that
 in the inside of the active material layer is narrowed because the binder
 resin of the adhesive is present in an larger amount in the portion on the
 separator 4 side (in the portion in the vicinity of the surface) of the
 positive and the negative electrode active material layers 7 and 9, i.e.,
 the positive and the negative active material particles 7a and 9a located
 in the vicinities of the separator 4 are covered with a larger amount of
 the binder resin 11 than those located in the vicinities of the positive
 and the negative electrode current collectors 6 and 10. As a result, the
 active material inside the electrode can be made effective use of, and the
 charge and discharge efficiency is improved. There is thus exerted an
 excellent effect that the charge and discharge characteristics as a
 battery can be improved.
 In addition, according to the above embodiment, impregnation with an
 electrolytic solution can be achieved with ease. That is, the tabular
 laminated battery body is soaked in an electrolytic solution under reduced
 pressure, whereby the gas in the voids 12 formed in the positive and the
 negative electrode active material layers 7 and 9 and the separator 4 are
 displaced with the electrolytic solution. The impregnated tabular
 laminated battery body is preferably dried.
 Impregnation with an electrolytic solution can also be conducted by putting
 the tabular laminated battery body into a flexible case such as an
 aluminum laminated film pack, evacuating the case to bring the case into
 tight contact with the outer surface of the tabular laminated battery
 body, pouring an electrolytic solution into the case from the opening of
 the case to make it penetrate into at least the voids, and sealing the
 opening the case. According to this method, since the back side of the
 battery body is in tight contact with the case when the electrolytic
 solution is supplied, the electrolytic solution is prevented from going
 behind the back side of the battery body and from becoming waste that does
 not participate in electrolysis. This also contributes to reduction in
 battery weight.
 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 form.
 Particles having a particle size of 0.3 to 20 .mu.m can be used. A
 preferred particle size is 1 to 5 .mu.m. Where the particle size is too
 small, too large a surface area of the active material will be covered
 with the adhesive on being adhered, and lithium ion intercalation and
 disintercalation are not carried out efficiently at charge and discharge,
 resulting in reduction of battery characteristics. If the particle size is
 too large, it is not easy to form the active material mixture into a thin
 film, and the packing density is reduced. Further, the electrode plate
 will have a considerably uneven surface which may tend to hinder adhesion
 to the separator.
 The binder resins which can be used for binding an active material into an
 electrode plate include those which neither dissolve in an electrolytic
 solution nor undergo electrochemical reaction inside an electrode
 laminate. For example, a fluorocarbon resin or a mixture mainly comprising
 a fluorocarbon resin, polyvinyl alcohol or a mixture mainly comprising
 polyvinyl alcohol can be used. Specific examples of useful binder 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.
 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.
 Any electron-insulating separator that has sufficient strength, such as
 porous film, net, and nonwoven fabric, can be used. In some cases, for
 example, where a fluorocarbon resin is used as a separator, adhesive
 strength should be secured by a surface treatment such as a plasma
 treatment. While not particularly limiting, polyethylene or polypropylene
 is a preferred material for the separator for their adhesiveness and
 safety.
 The solvent and the electrolyte which provide 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 electrolyte
 salts used in the electrolytic solution are LiPF.sub.6, LiAsF.sub.6,
 LiClO.sub.4, LiBF.sub.4, LiCF.sub.3 SO.sub.3, LiN(CF.sub.3
 SO.sub.2).sub.2, and LiC(CF.sub.3 SO.sub.2).sub.3.
 The adhesive resins which can be used for joining a current collector and
 an electrode and the adhesive resins which can be used for joining an
 electrode 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.
 The present invention will now be illustrated in greater detail with
 reference to Examples, but the present invention is by no means limited
 thereto.
 EXAMPLE 1
 Eighty-seven parts by weight of LiCoO.sub.2, 8 parts by weight of graphite
 powder (KS-6, produced by Lonza (co.ltd)) and, as a binder resin, 5 parts
 by weight of polyvinylidene fluoride were dispersed in N-methylpyrrolidone
 (hereinafter abbreviated as NMP) to prepare positive electrode active
 material paste. The paste was applied to a 20 .mu.m-thick aluminum foil as
 a positive electrode current collector with a doctor blade to a coating
 thickness of about 100 .mu.m to form a positive electrode.
 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 as a binder resin were dispersed in N-methylpyrrolidone to
 prepare negative electrode active material paste. The paste was applied to
 a 12 .mu.m-thick copper foil as a negative electrode current collector
 with a doctor blade to a thickness of about 100 .mu.m to make a negative
 electrode.
 A 5 wt % NMP solution of polyvinylidene fluoride which was used as a binder
 resin for bonding the active material particles to the current collector
 was applied uniformly to a side each of two separators (Cellguard #2400,
 produced by Hoechst Celanese). Before the NMP solution of polyvinylidene
 fluoride dried, the negative electrode 5 was sandwiched and joined in
 between a pair of the separators 4 with their sides coated with the NMP
 solution of polyvinylidene fluoride facing inward. The laminate was dried
 in a warm air drier at 60.degree. C. for 2 hours to evaporate NMP thereby
 to bond the negative electrode 5 between the two separators 4. The pair of
 the separators 4 having the negative electrode 5 bonded therebetween was
 punched to obtain a cut piece of prescribed size. The NMP solution of
 polyvinylidene fluoride was applied uniformly to one side of the cut
 laminate, and a cut piece of the positive electrode 3 having a prescribed
 size was stuck thereto to prepare a laminate composed of the separator 4,
 the negative electrode 5, the separator 4, and the positive electrode 3 in
 this order. Another pair of separators having the negative electrode
 joined therebetween was cut to a prescribed size, and the NMP solution of
 polyvinylidene fluoride was applied to a side of the cut laminate. The
 coated side was stuck to the positive electrode of the previously prepared
 laminate. The above-described steps were repeated to build up a battery
 body having a plurality of electrode laminates each composed of the
 positive electrode and the negative electrode facing each other via the
 separator. The battery body was dried while applying pressure to prepare a
 tabular laminated battery body as shown in FIG. 1. Current collecting tabs
 each connected to the end of every positive current collectors and every
 negative current collectors of the tabular laminated battery body were
 spot-welded among the positive electrodes and among the negative
 electrodes, respectively, to establish parallel electrical connections in
 the tabular laminated battery body.
 The tabular laminated battery body was impregnated with an electrolytic
 solution consisting of ethylene carbonate and 1,2-dimethoxyethane as a
 mixed solvent and lithium hexafluorophosphate as an electrolyte and was
 put in an aluminum laminated film pack. The opening of the pack was
 heat-sealed to complete a lithium ion secondary battery.
 The resulting battery stably retained its shape to maintain the electrical
 connections among electrodes without imposing pressure from outside. When
 the aluminum laminate film was removed from the assembled battery, and the
 electrode was stripped off the separator, the active material layer came
 off while being adhered to the separator, proving that the adhesive
 strength between the separator and the active material layer in the
 vicinity of the electrode surface was greater than the adhesive strength
 between the active material layer and the current collector within the
 electrode. This is considered to be because the binder resin was present
 as an adhesive in a larger amount in the portion of the positive and the
 negative active material layers on the separator side than in the portion
 on the current collector side. Because fracture of the electrode occurs in
 preference to the separation between the positive and the negative active
 material layers and the separator, safety can be secured.
 As a result of evaluation of the battery characteristics, a weight energy
 density of about 100 Wh/kg was obtained owing to effective utilization of
 the active materials inside the electrodes. The charge capacity after 200
 charge and discharge cycles at a current of C/2 was as high as 75% of the
 initial one. This seems to be because the binder resin existed in the
 portion on the separator side in a greater amount. In other words, the
 active material particles present in the portion on the separator side
 were covered with the binder resin over a wider area more than the active
 material particles present in the portion on the current collector side so
 that the difference in rate of intercalation and disintercalation of
 lithium ions between the active material on the separator side and that in
 the inside of the active material layer was reduced, and the active
 material inside the electrode could be utilized effectively.
 Thus, since a firm housing is unnecessary, it is possible to reduce the
 weight and thickness of the battery; the battery can take an arbitrary
 shape; and the charge and discharge efficiency is improved. As a result, a
 lithium ion secondary battery which is excellent in charge and discharge
 characteristics and highly safe and can have a great capacity is obtained.
 Example 1 may be carried out by repeating the steps of bonding the positive
 electrode 3 between a pair of separators 4, applying the adhesive resin
 solution to a side of the paired separators 4 having the positive
 electrode 3 therebetween, adhering the negative electrode 5 to the coated
 side, and adhering another paired separators having the positive electrode
 therebetween onto the negative electrode 5.
 EXAMPLE 2
 A battery having a multilayer structure was prepared in the same manner as
 in Example 1, except for changing the thickness of the positive and the
 negative electrode to about 200 .mu.m. Similarly to Example 1, the
 resulting battery stably retained its shape and the electrical connections
 among electrodes without applying pressure from outside. When the aluminum
 laminate film was removed from the assembled battery, and the electrode
 was stripped off the separator, the active material layer was separated
 while being adhered to the separator, proving that the adhesive strength
 between the separator and the active material layer in the vicinity of the
 electrode surface was greater than the adhesive strength between the
 active material layer and the current collector within the electrode. As a
 result of evaluation of the battery characteristics, a weight energy
 density of 113 Wh/kg was obtained, and the charge capacity after 200
 charge and discharge cycles at a current of C/2 was as high as 60% of the
 initial one. Similarly to Example 1, there was obtained a large capacity
 lithium ion secondary battery having excellent charge and discharge
 characteristics which could have a reduced thickness and an arbitrary
 shape.
 EXAMPLE 3
 A positive electrode and a negative electrode were prepared in the same
 manner as in Example 1. A 12 wt % NMP solution of polyvinylidene fluoride
 was used for adhesion of the separator and the electrode. Similarly to
 Example 1, the resulting battery stably retained its shape and the
 electrical connections among electrodes without imposing pressure from
 outside. When the aluminum laminate film was removed from the assembled
 battery, and the electrode was stripped off the separator, the active
 material layer was separated while being adhered to the separator, proving
 that the adhesive strength between the separator and the active material
 layer in the vicinity of the electrode surface was greater than the
 adhesive strength between the active material layer and the current
 collector within the electrode. The high concentration solution of
 polyvinylidene fluoride provided a thin polyvinylidene fluoride layer
 between the separator and the electrode. Thus, the adhesive strength was
 further enhanced, and the electrical connections were maintained stably.
 As a result of evaluation of the battery characteristics, a weight energy
 density of about 100 Wh/kg was obtained, and the charge capacity after 200
 charge and discharge cycles at a current of C/2 was as high as 60% of the
 initial one. Similarly to Example 1, a lithium ion secondary battery
 having excellent charge and discharge characteristics which can have a
 reduced thickness and an arbitrary shape was obtained.
 EXAMPLE 4
 Eighty-seven parts by weight of LiCoO.sub.2, 8 parts by weight of graphite
 powder (KS-6, produced by Lonza) and, as a binder resin, 5 parts by weight
 of polystyrene powder were mixed, and adequate amounts of toluene and
 2-propanol were added thereto to prepare positive electrode active
 material paste. The paste was applied to a 20 .mu.m-thick aluminum foil as
 a positive electrode current collector with a doctor blade to a coating
 thickness of about 100 .mu.m to form a positive electrode.
 Ninety-five parts by weight of Mesophase Microbead Carbon (a trade name,
 produced by Osaka Gas Co., Ltd.) and, as a binder resin, 5 parts by weight
 of polystyrene powder were mixed, and adequate amounts of toluene and
 2-propanol were added thereto to prepare negative electrode active
 material paste. The paste was applied to a 12 .mu.m-thick copper foil as a
 negative electrode current collector with a doctor blade to a thickness of
 about 100 .mu.m to make a negative electrode.
 A tabular laminated battery body as shown in FIG. 1 was prepared in the
 same manner as in Example 1, except for using a nitrocellulose porous film
 (pore size: 0.8 .mu.m) as a separator and a 5 wt % toluene solution of
 polystyrene, which was used as a binder resin, as an adhesive for joining
 the positive and the negative electrodes (i.e., the positive and the
 negative electrode active material layers) to the separator. Current
 collecting tabs connected to the positive and the negative current
 collectors of the tabular laminated battery body were spot-welded among
 the positive electrodes and among the negative electrodes to establish
 parallel electrical connections in the tabular laminated battery body.
 Subsequently, the tabular laminated battery body was impregnated with an
 electrolytic solution consisting of ethylene carbonate and
 1,2-dimethoxyethane as a mixed solvent and lithium hexafluorophosphate as
 an electrolyte and was put in an aluminum laminated film pack. The opening
 of the pack was heat-sealed to complete a lithium ion secondary battery.
 The resulting battery stably maintained its shape and the electrical
 connections without imposing pressure from outside. When the aluminum
 laminate film was removed from the assembled battery, and the electrode
 was stripped off the separator, the active material layer was separated
 while being adhered to the separator, proving that the adhesive strength
 between the separator and the active material layer in the vicinity of the
 electrode surface was greater than the adhesive strength between the
 active material layer and the current collector within the electrode. As a
 result of evaluation of the battery characteristics, a weight energy
 density of about 90 Wh/kg was obtained, and the charge capacity after 100
 charge and discharge cycles at a current of C/10 was as high as about 60%
 of the initial one.
 Similarly to Example 1, a lithium ion secondary battery having excellent
 charge and discharge characteristics which could have a reduced thickness
 and an arbitrary shape was obtained.
 EXAMPLE 5
 A positive electrode and a negative electrode were prepared in the same
 manner as in Example 1. A battery having a tabular laminate structure was
 prepared in the same manner as in Example 1, except for using a 10 wt %
 toluene solution of a 1:2 (by weight) mixture of polyvinylidene fluoride,
 which was the binder resin used for adhering the active material particles
 to the current collector, and polymethacrylic acid for adhering the
 separator and the electrode. Drying after adhesion was carried out by
 heating at 80.degree. C. in vacuo. Similarly to Example 1, the resulting
 battery stably maintained its shape without imposing pressure from
 outside. When the aluminum laminate film was removed from the assembled
 battery, and the electrode was stripped off the separator, the active
 material layer was separated while being adhered to the separator, proving
 that the adhesive strength between the separator and the active material
 layer in the vicinity of the electrode surface was greater than the
 adhesive strength between the active material layer and the current
 collector within the electrode. As a result of evaluation of the battery
 characteristics, a weight energy density of 95 Wh/kg was obtained, and the
 charge capacity after 100 charge and discharge cycles at a current of C/2
 was as high as about 80% of the initial one. Similarly to Example 1, a
 lithium ion secondary battery having excellent charge and discharge
 characteristics which could have a reduced thickness and an arbitrary
 shape was obtained.
 EXAMPLE 6
 A negative electrode 5 and a positive electrode 3 were prepared in the same
 manner as in Example 1. A 5 wt % NMP solution of polyvinylidene fluoride,
 which was the binder resin used for adhering the active material particles
 to the current collector, was uniformly applied to a side each of two
 separators (Cellguard #2400, produced by Hoechst Celanese) of band form.
 The positive electrode of band form was sandwiched and stuck between the
 two separators with their coated sides inward, and the laminate was put in
 a warm air drier at 60.degree. C. for 2 hours to evaporate NMP from the
 resin solution, whereby the positive electrode was joined between the pair
 of separators. The 5 wt % NMP solution of polyvinylidene fluoride was then
 applied uniformly to one of the paired separators of band form having the
 positive electrode therebetween. One end of the coated separator was
 folded back at a prescribed length while inserting a cut piece of the
 negative electrode 5 having a prescribed size into the fold, and the
 laminate was passed through a laminator. Subsequently, the 5 wt % NMP
 solution of polyvinylidene fluoride was uniformly applied to the other
 separator of band form, and another piece of the negative electrode having
 a prescribed size was stuck thereto at the position corresponding to the
 negative electrode having been inserted into the fold. The paired
 separators were rolled up by half turn to make an oblong ellipsoid in such
 a manner that the negative electrode might be wrapped in. The separators
 were repeatedly rolled up while inserting a cut piece of the negative
 electrode for every half turn to form a battery body having a plurality of
 electrode laminates. The battery body was dried while applying pressure to
 obtain a tabular roll type laminated battery body as shown in FIG. 2.
 Current collecting tabs connected to the end of every negative electrode
 current collector of the tabular roll type laminated battery body were
 spot-welded to achieve parallel electrical connections. The tabular roll
 type laminated battery body was impregnated with an electrolytic solution
 and sealed to complete a lithium ion secondary battery in the same manner
 as in Example 1.
 Similarly to Example 1, the resulting tabular roll type laminated battery
 body stably maintained its shape without imposing pressure from outside.
 When the aluminum laminate film was removed from the assembled battery,
 and the electrode was stripped off the separator, the active material
 layer was separated while being adhered to the separator, proving that the
 adhesive strength between the separator and the active material layer in
 the vicinity of the electrode surface was greater than the adhesive
 strength between the active material layer and the current collector
 within the electrode. As a result of evaluation of the battery
 characteristics, a weight energy density of 90 Wh/kg was obtained, and the
 charge capacity after 100 charge and discharge cycles at a current of C/2
 was as high as about 80% of the initial one.
 Similarly to Example 1, a lithium ion secondary battery having excellent
 charge and discharge characteristics which can have a reduced thickness
 and an arbitrary shape was obtained.
 While Example 6 has shown an embodiment in which a pair of separators 4 of
 band form having the positive electrode 3 of band form therebetween are
 rolled up while inserting and adhering the negative electrode 5 of
 prescribed size for every half turn, the battery body may be such that is
 prepared by rolling up a pair of separators of band form having the
 negative electrode 5 of band form therebetween while inserting and
 adhering a cut piece of the positive electrode 3 having a prescribed size
 for every half turn.
 While Example 6 has shown a method in which the separators 4 are rolled up,
 the method may be replaced with a method comprising folding a pair of
 separators 4 of band form having joined therebetween the negative
 electrode 5 or the positive electrode 3 of band form while inserting and
 adhering a cut piece of the positive electrode 3 or the negative electrode
 5 of prescribed size in every fold.
 EXAMPLE 7
 A negative electrode 5 and a positive electrode 3 were prepared in the same
 manner as in Example 1. The positive electrode 3 of band form was set
 between a pair of separators of band form (Cellguard #2400, produced by
 Hoechst Celanese), and the negative electrode 5 of band form was placed on
 the outer side of one of the separators 4 with a prescribed length of its
 starting end sticking out over the end of that separator 4. The inner
 sides of the paired separators 4 and the outer side of the separator 4 on
 which the negative electrode 5 was to be arranged had been uniformly
 coated with a 5 wt % NMP solution of polyvinylidene fluoride, which was
 the binder resin used for adhering the active material particles to the
 current collector. The sticking end of the negative electrode 5 was first
 sent to a laminator, and the negative electrode 5, the separator 4, the
 positive electrode 3, and the separator 4 were then passed through the
 laminator to form a laminate of band form. The outer side of the other
 separator of the laminate was uniformly coated with a 5 wt % NMP solution
 of polyvinylidene fluoride, and the sticking end of the negative electrode
 5 was folded back and stuck to the coated surface. The laminate was rolled
 up in such a manner that the folded negative electrode 5 might be wrapped
 in to make an oblong ellipsoid to form a battery body having a plurality
 of electrode laminates as shown in FIG. 3. The battery body was dried
 under pressure to join the negative electrode, the separator, and the
 positive electrode simultaneously to prepare a tabular roll type laminated
 battery body. The battery body was impregnated with an electrolytic
 solution and sealed in the same manner as in Example 1 to complete a
 battery.
 Similarly to Example 1, the resulting tabular roll type laminated battery
 body stably maintained its shape without imposing pressure from outside.
 When the aluminum laminate film was removed from the assembled battery,
 and the electrode was stripped off the separator, the active material
 layer was separated while being adhered to the separator, proving that the
 adhesive strength between the separator and the active material layer in
 the vicinity of the electrode surface was greater than the adhesive
 strength between the active material layer and the current collector
 within the electrode. As a result of evaluation of the battery
 characteristics, a weight energy density of 80 Wh/kg was obtained, and the
 charge capacity after 100 charge and discharge cycles at a current of C/2
 was as high as about 80% of the initial one.
 Similarly to Example 1, a lithium ion secondary battery having excellent
 charge and discharge characteristics which could have a reduced thickness
 and an arbitrary shape was obtained.
 While Example 7 has shown an embodiment in which a pair of separators 4 of
 band form having the positive electrode 3 of band form therebetween and
 the negative electrode 5 of band form on the outer side of one of the
 separators 4 are rolled up, the same type of a battery could be prepared
 by arranging the negative electrode 5 of band form in between the
 separators 4 of band form and the positive electrode 3 on one of the
 separators 4, and rolling up the laminate.
 In Example 7, batteries were prepared with a varied number of the
 laminates. The battery capacity increased with the number of the
 laminates.
 COMATIVE EXAMPLE
 Eighty-seven parts by weight of LiCoO.sub.2, 8 parts by weight of graphite
 powder (KS-6, produced by Lonza ) and, as a binder resin, 5 parts by
 weight of polyvinylidene fluoride were dispersed in. N-methylpyrrolidone
 (hereinafter abbreviated as NMP) to prepare positive electrode active
 material paste. The paste was applied to a 20 .mu.m-thick aluminum foil as
 a positive electrode current collector with a doctor blade to a coating
 thickness of about 100 .mu.m.
 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 as a binder resin were dispersed in N-methylpyrrolidone to
 prepare negative electrode active material paste. The paste was applied to
 a 12 .mu.m-thick copper foil as a negative electrode current collector
 with a doctor blade to a thickness of about 100 .mu.m.
 Before the applied paste dried, the aluminum foil coated with the positive
 electrode active material and the copper foil coated with the negative
 electrode active material were piled up alternately with a separator
 (Cellguard #2400, produced by Hoechst Celanese) being interposed
 therebetween. The laminate was pressed from both sides and dried to
 prepare a tabular laminated battery body having a plurality of electrode
 laminates as shown in FIG. 1. The positive electrodes and the negative
 electrodes were spot-welded among themselves to electrically connect them
 in parallel. The tabular laminated battery body was impregnated with an
 electrolytic solution consisting of ethylene carbonate and
 1,2-dimethoxyethane as a mixed solvent and lithium hexafluorophosphate as
 an electrolyte and was put in an aluminum laminated film pack. The opening
 of the pack was heat-sealed to complete a lithium ion secondary battery.
 The resulting battery stably maintained its shape without applying pressure
 from outside. The aluminum laminate film was removed from the assembled
 battery, and the electrode was stripped off the separator. The active
 material was found to remain on the separated separator only sparsely,
 proving that the adhesive strength between the separator and the active
 material layer in the vicinity of the electrode surface was extremely
 lower than the strength between the active material layer and the current
 collector within the electrode. It turned out that the active material
 layer had scarcely adhered to the separator. As a result of evaluation of
 the battery characteristics, a weight energy density of 70 Wh/kg was
 obtained, and the charge capacity after 200 charge and discharge cycles at
 a current of C/2 was as low as 40% of the initial one.
 Compared with the foregoing Examples, the battery characteristics were
 considerably inferior. It has now been verified that the battery
 characteristics are improved by joining the positive and the negative
 electrodes to the separator with an adhesive. In other words, it is seen
 that distribution of the adhesive, i.e., the binder resin makes a great
 contribution to the improvement of battery characteristics.
 The binder to be used does not always need to be the same as the binder
 used for adhesion of the active material layer. Different binders may be
 employed.
 Industrial Applicability
 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.