JP4959048B2 - Sheet-like lithium secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium ion secondary sheet battery.
[0002]
[Prior art]
Lithium secondary batteries with large discharge capacities are attracting attention as batteries for electronic devices such as portable phones and personal computers. Conventionally, as the lithium secondary battery, a three-dimensional battery mainly in a columnar shape or a box shape has been mainly used. However, recently, interest in sheet-like lithium secondary batteries has increased due to space factor and light weight.
[0003]
The advantage of a sheet-like lithium secondary battery is that it is thin, unlike a three-dimensional battery, so the heat dissipation is good and the degree of heat build-up in the battery is low. Even if a flow or a flaw due to a nail or the like occurs, an explosion accident due to lithium combustion inside the battery hardly occurs and it is safe.
[0004]
Usually, in a sheet-like lithium secondary battery, in order to increase the battery capacity, a laminated structure including a plurality of units in which a positive electrode sheet electrode and a negative electrode sheet electrode are stacked via a separator or a solid electrolyte layer is formed, and this is generated. Used in the element to increase the effective reaction area of the battery.
[0005]
However, when such a power generation element has such a laminated structure, floating occurs between the sheet electrodes in the laminated structure (a state in which a gap is left between the sheet electrodes), and the internal resistance of the battery increases, so that the battery performance (Especially rate characteristics, low temperature characteristics, cycle characteristics, etc.) may deteriorate. Therefore, in order to prevent this, usually, an adhesive tape is wound around the outer periphery of the laminated structure, and a plurality of sheet electrodes constituting the laminated structure are bound and fixed.
[0006]
In binding with the adhesive tape, the adhesive tape is wound around substantially the entire laminated structure so that the entire sheet electrode is in close contact with the separator or the solid electrolyte layer. However, when an adhesive tape is wound around substantially the entire laminated structure, the electrolytic solution is applied to the laminated structure in the step of impregnating the electrolytic solution into the laminated structure (step of impregnating the separator or the solid electrolyte layer with the electrolytic solution). As a result, it becomes difficult to permeate (inflow), and as a result, the time required for the process becomes longer, battery productivity decreases, and the electrolyte does not sufficiently permeate (impregnate) every corner of the laminated structure. In some cases, the battery performance may deteriorate.
[0007]
[Problems to be solved by the invention]
In view of the above circumstances, the present invention is capable of allowing an electrolytic solution to permeate every corner of a laminated structure in a short time during an impregnation operation of the electrolytic solution into the laminated structure during battery manufacture, and a sheet in the laminated structure It is an object of the present invention to provide a sheet-like lithium secondary battery that has no unnecessary gaps between the electrodes (each sheet electrode is in close contact with the separator or the solid electrolyte layer) and exhibits excellent battery performance.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the present invention has the following features.
(1) A laminated structure including a plurality of units in which a positive electrode sheet electrode and a negative electrode sheet electrode are stacked via a separator or a solid electrolyte layer is used as a power generation element, and tape is wound around the outer periphery of the laminated structure to constitute the laminated structure A sheet-like lithium secondary battery in which a plurality of sheet electrodes are bound and fixed,
The surface of the two sheet electrodes located at the outermost part of the laminated structure is called the main surface of the laminated structure, and the surface where the end faces of the plurality of sheet electrodes are gathered is called the side surface of the laminated structure. A sheet-like lithium secondary battery characterized in that a gap is formed between the side surface and the tape without contacting the side surface of the body.
(2) The positive electrode sheet electrode and the negative electrode sheet electrode are substantially rectangular sheets, the laminated structure has a substantially rectangular parallelepiped outer shape, and each of the set of opposing side surfaces of the substantially rectangular parallelepiped laminated structure is approximately The sheet-like lithium according to the above (1), wherein a bent plate that is bent in a letter-shape is disposed facing the convex surface outward, and the tape is wound around both main surfaces of the laminated structure and the convex surface of the bent plate. Secondary battery.
(3) The above (1) or (2) wherein a solid electrolyte layer mainly composed of a salt, a compatible solvent, and a fluoropolymer mainly composed of vinylidene fluoride is interposed between the positive electrode sheet electrode and the negative electrode sheet electrode. ) The sheet-like lithium secondary battery described.
(4) The density of the fluoropolymer of the solid electrolyte layer is 0.60 to 1.30 g / cm. ^Three The sheet-like lithium secondary battery according to the above (3), which is a porous body.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described more specifically.
The sheet-like lithium secondary battery of the present invention is obtained by winding a tape around a laminated structure as a power generation element, that is, a laminated structure including a plurality of units in which a positive electrode sheet electrode and a negative electrode sheet electrode are stacked via a separator or a solid electrolyte layer. The plurality of sheet electrodes constituting the laminated structure are bound and fixed, but the tape is not brought into contact with the side surface of the laminated structure (that is, the surface where the end surfaces of the plurality of sheet electrodes are gathered), and between the side surface and the tape. A feature is that a gap is formed.
[0010]
1 to 4 show a sheet-like lithium secondary battery according to an embodiment of the present invention. FIG. 1 is a schematic perspective view of a laminated structure as a power generation element, FIG. 2 is an enlarged cross-sectional view of a main part showing a laminated state of a positive electrode sheet electrode and a negative electrode sheet electrode in the laminated structure, and FIG. FIG. 4 is a schematic perspective view of a state in which the bending plate is attached and the adhesive tape is wound, and FIG.
[0011]
As shown in FIGS. 1 and 2, the laminated structure 3 as a power generation element is a structure including a plurality of units in which a substantially rectangular positive electrode sheet electrode 1 and a negative electrode sheet electrode 2 are stacked with a separator or a solid electrolyte layer 4 interposed therebetween. Yes, the outer shape is a substantially rectangular parallelepiped.
[0012]
As shown in FIG. 2, each of the positive electrode sheet 1 and the negative electrode electrode 2 constituting the laminated structure 3 is an active material composition in which at least an active material and a binder are mixed, and a conductive material is further mixed as necessary. The layers (the positive electrode active material composition layer 6 and the negative electrode active material composition layer 7) are formed in layers on the sheet-like current collectors 8 and 9, and the active material composition is formed on the separator or the solid electrolyte layer 4. The layers (the positive electrode active material composition layer 6 and the negative electrode active material composition layer 7) are opposed to each other. The collectors 8 and 9 of each sheet electrode (the positive electrode sheet electrode 1 and the negative electrode sheet electrode 2) are provided with an unformed portion (not shown) of the active material composition layer, and leads (not shown) are provided there. For example, it is attached by welding or the like, and the ends of the leads of the plurality of positive electrode sheet electrodes 1 are connected to the positive electrode current collector terminal 10, and the ends of the leads of the negative electrode sheet electrodes 2 are the negative electrode current collector terminals 11 is connected.
[0013]
The surfaces 2a ′ and 2b ′ of the two negative electrode sheets 2a and 2b located on the outermost side of the multilayer structure 3 are called main surfaces of the multilayer structure, and a plurality of sheet electrodes (the positive electrode sheet electrode 1 and the negative electrode sheet electrode 2). 3) is called the side surface of the laminated structure, as shown in FIGS. 3 and 4, the side surface 3C-1 from which the current collecting terminals 10 and 11 protrude and the side surface 3C-2 opposite thereto are Without adhering the adhesive tape (without covering the side surface with the adhesive tape), a set of opposing side surfaces 3B-1 on a side different from the set of opposing side surfaces 3C-1, 3C-2, 3B-2, bending plates 12 and 13 bent substantially in a U-shape are arranged to face each other with their convex surfaces 12A and 13A facing outward, and both main surfaces 3A-1 and 3A-2 of the laminated structure 3 The adhesive tape 5 is wound around substantially the entire convex surfaces 12A and 13A of the bent plates 12 and 13, The sheet electrodes (positive electrode electrode 1 and negative electrode electrode 2) are bound and fixed, and a gap 14 is formed between a pair of opposite side surfaces 3B-1, 3B-2 and the adhesive tape 5 of the laminated structure 3. is doing.
[0014]
In the sheet-like lithium secondary battery, the laminated structure 3 on which the adhesive tape 5 is wound is impregnated with an electrolytic solution (each separator or solid electrolyte layer (its polymer substrate) 4 is impregnated with the electrolytic solution). The laminated structure 3 impregnated with the electrolytic solution is accommodated in a sheet-like exterior material (not shown), and the ends of the positive electrode current collector terminal 10 and the negative electrode current collector terminal 11 are pulled out of the exterior material. In this state, the outer packaging material is sealed.
Note that a laminated structure that has not been impregnated with the electrolytic solution may be accommodated in the exterior material together with the electrolytic solution, and the electrolytic solution may be permeated in the exterior material.
[0015]
FIG. 6 shows a laminated structure in which an adhesive tape of a conventional sheet-like lithium secondary battery is wound, and the same reference numerals as those in FIGS. 1 to 3 denote the same or corresponding parts. As can be seen from this figure, in this conventional sheet-like lithium secondary battery, the side surface 3C-1 from which the current collecting terminals 10 and 11 of the laminated structure 3 protrude and the side surface 3C-2 opposite thereto are The adhesive tape is not adhered (the side surface is not covered with the adhesive tape), and the set of opposing side surfaces 3B- on the side different from the set of opposing side surfaces 3C-1 and 3C-2 of the laminated structure 3 1, 3B-2 and a plurality of sheet electrodes (positive electrode sheet 1, negative electrode sheet) constituting the laminated structure 3 by adhering and winding the adhesive tape 5 on substantially the entire area of both main surfaces 3A-1 and 3A-2. The electrode 2) is bound and fixed, and only one set of opposing side surfaces (side surfaces 3C-1 and 3C-2) of the laminated structure 3 is an inflow region of the electrolytic solution.
[0016]
On the other hand, in the sheet-like lithium secondary battery (FIGS. 1 to 4) of the present invention, the adhesive tape is adhered to the side surfaces (a pair of opposing side surfaces 3B-1 and 3B-2) of the laminated structure 3. Without gaps (that is, without contact), the gaps 14 are provided between the side surfaces (the pair of opposing side surfaces 3B-1, 3B-2) and the adhesive tape 5, so that all of the laminated structure 3 Side surfaces (that is, a set of opposite side surfaces 3C-1, 3C-2 on one side and a set of opposite side surfaces 3B-1, 3B-2 on the other side) serve as an inflow region for the electrolyte solution, and are laminated structures The electrolyte solution can be reliably infiltrated (impregnated) to the corners of the body 3 in a short time.
[0017]
The bent plates 12 and 13 bent in a generally U-shape opposed to the opposite side surfaces 3B-1 and 3B-2 of the laminated structure 3 are usually molded bodies of synthetic resin (for example, polyethylene, polypropylene, nylon, etc.). Alternatively, a metal plate or the like with an insulating coating is used. The bent plates 12 and 13 are electrically insulating and need to be strong enough not to be deformed or bent against the tightening force caused by the winding of the adhesive tape. Since it does not act, it must be relatively lightweight. Therefore, the material and thickness are determined in consideration of this point. Generally, the thickness is about 0.05 to 0.5 mm, preferably about 0.1 to 0.3 mm.
[0018]
The bending plates 12 and 13 that bend in a substantially square shape so that the tightening force by the winding of the adhesive tape acts equally from both sides (in the directions of arrows Y1 and Y2 in FIG. 4) of the laminated structure. The flat plate portions 12b and 12c (13b and 13c) on both sides of the bent portion L are preferably substantially the same size. Although the degree of bending differs depending on the thickness of the laminated structure 3, the distance (D1 in FIG. 4) from the side surface 3B-1 (3B-2) to the bent portion L of the laminated structure 3 and the side surface 3B-1 ( The ratio (D1 / W1) to the width (W1 in FIG. 4) of 3B-2) is preferably 0.1 to 1.0, and particularly preferably 0.25 to 0.50. Within such a preferable range, the tightening force due to the winding of the adhesive tape acts more strongly on the laminated structure 3, and the inflow property of the electrolytic solution into the gap 14 during the impregnation treatment with the electrolytic solution is good. 3 quickly penetrates the electrolyte.
[0019]
As shown in FIGS. 5 and 5, the bent plates 12 and 13 that bend in a generally U shape may have both end portions in contact with the side surfaces of the laminated structure as shown in FIGS. Further, the projecting pieces 12a and 13a for attachment are further provided at both ends, and these are sandwiched between the edges of both main surfaces 3A-1 and 3A-2 (the outermost sheet electrodes 2a and 2b) of the laminated structure. It may be. With this configuration, when the adhesive tape is wound, there is no need to hold the bent plate, the workability is improved, and the bent plate presses both main surfaces of the laminated structure. The adhesion between them is further improved.
[0020]
In the sheet-like lithium secondary battery of one specific example described above, the plurality of sheet electrodes constituting the laminated structure 3 are substantially rectangular sheets, and the outer shape of the laminated structure 3 is a substantially rectangular parallelepiped. It is not limited to. The tape winding structure according to the present invention can be implemented as long as the laminated structure has a structure in which a plurality of sheet electrodes (for example, oval shapes) having a pair of sides facing substantially parallel are stacked. Moreover, you may use the bending board bent in other shapes other than the bending plates 12 and 13 bent in the said substantially square shape on the side surface of a laminated structure.
[0021]
In the sheet-like lithium secondary battery of the present invention, the size of one sheet electrode is appropriately selected according to the battery design, but usually 10 to 100 cm. ² Is selected from the range. Further, the number of units in which the positive electrode sheet electrode and the negative electrode sheet electrode are stacked through a separator or a solid electrolyte layer is appropriately selected according to the battery design, but is usually selected from the range of 2 to 20.
[0022]
In the present invention, the adhesive tape 5 is preferably excellent in electrical insulation and resistant to the heat generated by the power generation element and the electrolytic solution, such as vinyl chloride resin, polyester resin, polyolefin resin, cotton, fluorine resin, polyimide resin, etc. It is preferable that a tape base material made of a rubber-based adhesive, an acrylic pressure-sensitive adhesive, a silicone pressure-sensitive adhesive layer, etc. is formed on a tape base material, particularly preferably a polypropylene-based tape base material with an acrylic pressure-sensitive adhesive. An agent layer is formed. The thickness of the tape substrate is preferably about 5 to 100 μm, and the thickness of the pressure-sensitive adhesive layer is preferably about 10 to 50 μm.
[0023]
As shown in FIG. 3, the pressure-sensitive adhesive tape 5 may be thick so as to cover substantially the entire main surface of the laminated structure 3, or may be wound at a plurality of narrow widths.
[0024]
Further, instead of the pressure-sensitive adhesive tape, a tape having no pressure-sensitive adhesive layer (the above-described tape base material) may be used, and the tape may be fixed with a necessary stopper or adhesive after the tape is wound.
[0025]
In the present invention, as the positive electrode active material used for the positive electrode sheet electrode, a known positive electrode active material in a lithium secondary battery can be used, but Li-transition metal composite oxide is preferable, and Li-Co composite oxide, Li-Co composite oxide, Li -At least one compound selected from -Mn composite oxide and Li-Ni composite oxide, particularly preferably Li-Co composite oxide.
[0026]
In addition, as the conductive material used together with the positive electrode active material, natural and artificial graphites such as fibrous graphite, flaky graphite, and spherical graphite, and conductive carbon black, which have been widely used in this field, have been used. Can be used. The amount of the conductive material used is preferably 1% by weight to 10% by weight, more preferably 3% by weight to 7% by weight, per 100 parts by weight of the total amount of the positive electrode active material, the binder and the conductive material used.
[0027]
Moreover, as a binder used with a positive electrode active material, a polytetrafluoroethylene, a polyvinylidene fluoride, polyethylene, an ethylene-propylene-diene type polymer etc. are mentioned as a suitable thing, for example. The amount of the binder used is preferably 1% by weight to 7% by weight and more preferably 2% by weight to 5% by weight per 100 parts by weight of the total amount of the positive electrode active material, the binder and the conductive material.
[0028]
Examples of the positive electrode current collector 8 include a foil or an expanded metal formed of a conductive metal such as aluminum, an aluminum alloy, or titanium, and these may have holes formed therein.
[0029]
As a negative electrode active material used for the negative electrode sheet electrode, graphitized carbon is suitably used. Examples of such graphitized carbon include various natural graphites and artificial graphites, for example, graphites such as fibrous graphite, flaky graphite, and spherical graphite. Further, as the binder used together with the negative electrode active material, polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, ethylene-propylene-diene-based polymer and the like can be cited as suitable, as in the past, and in particular, polyvinylidene fluoride. Is preferred. The amount of the negative electrode active material used is preferably about 2 to 20 parts by weight per 100 parts by weight of the total amount of the negative electrode active material and the binder.
[0030]
Examples of the negative electrode current collector 9 include foils and expanded metals formed of copper, nickel, silver, stainless steel, etc., and these may have holes formed therein.
[0031]
In the case of a battery of a type in which a separator is interposed between the positive electrode sheet electrode and the negative electrode sheet electrode, a known separator can be used without limitation, and among them, a polyolefin separator is preferable, and a polyolefin porous separator is particularly preferable. . The polyolefin (porous) separator may be a single layer of a polyethylene layer or a polypropylene layer, or a composite separator in which a polyethylene layer and a polypropylene layer are laminated. The laminated structure of the composite separator is not particularly limited, and various laminated structures such as polyethylene layer / polypropylene layer, polyethylene layer / polypropylene layer / polyethylene layer, and polypropylene layer / polyethylene layer / polypropylene layer can be used.
[0032]
In general, the separator preferably has an average thickness of 5 to 100 μm, particularly preferably 10 to 30 μm. Here, the thickness of the separator is a thickness in a state of being interposed between the positive electrode and the negative electrode (a state where the battery is actually assembled), and is equal to the distance between the positive electrode and the negative electrode.
[0033]
In addition, in the case of a battery of a type in which a solid electrolyte layer is interposed between a positive electrode sheet electrode and a negative electrode sheet electrode, a known solid electrolyte layer can be used as the solid electrolyte layer without limitation, but an electrolyte solution (lithium) can be used as a polymer substrate. A solid electrolyte layer prepared so as to be impregnated with gel (salt (electrolyte) + compatible solvent) to be gelled and exhibit ionic conductivity per se is preferable. Among these, polymers using polyethylene oxide, polyacrylonitrile, polyvinyl pyrrolidone, polyvinyl pyridine, polyvinyl chloride, polyvinylidene carbonate, vinylidene fluoride as the main unit (including polyvinylidene fluoride), etc. Are particularly preferable, and those using a fluoropolymer having vinylidene fluoride as a main unit are particularly preferable from the viewpoint of battery rate characteristics, low-temperature characteristics, and the like. If a solid electrolyte layer is used, the gelation can provide better adhesion to the sheet electrode than the separator.
[0034]
The fluoropolymer having vinylidene fluoride as a main unit is a homopolymer of vinylidene fluoride (polyvinylidene fluoride (PVdF)) or a copolymer of vinylidene fluoride and another vinyl monomer having a fluorine atom. It means a combination, and these may be used alone or in combination. Examples of the vinyl monomer having a fluorine atom other than the vinylidene fluoride include hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), and tetrafluoroethylene (TFE). The form of the copolymer may be either random or block. In the case of a copolymer, the proportion of vinylidene fluoride (unit) is preferably 70 mol% or more, particularly preferably 75 mol% or more.
[0035]
In addition, fluoropolymers having vinylidene fluoride as a main unit include carboxyl groups (—COOH) and sulfonic acid groups (—SOOH). ₂ OH), carboxylate group (—COOR), amide group (—CONH) ₂ ) Or phosphate group (-PO (OH) ₂ Or the like. A vinyl monomer polymer having a functional group consisting of, for example, may be grafted (the substituent R in the carboxylic acid ester group (—COOR) has 1 carbon atom such as a methyl group, an ethyl group, or a butyl group). -4 lower alkyl groups). When a polymer containing such a functional group is grafted with a fluoropolymer, the adhesion of the solid electrolyte layer to the sheet electrode is further improved, and the resistance between the electrodes is further reduced. Characteristics and low-temperature characteristics) are further improved. As the vinyl monomer having a functional group, a monomer composed of a compound having 4 or less carbon atoms in the portion excluding the functional group is suitable. As a carboxyl group-containing monomer, one having one carboxyl group such as acrylic acid, methacrylic acid, crotonic acid, vinyl acetic acid, and allylic acetic acid, and one having two or more carboxyl groups such as itaconic acid and maleic acid are also used. Is possible. As the sulfonic acid group-containing monomer, styrene sulfonic acid, vinyl sulfonic acid and the like are suitable. As the carboxylic acid ester group-containing monomer, methyl acrylate, butyl acrylate and the like are preferable. As the amide group-containing monomer, acrylamide or the like is preferable. As the phosphate group-containing monomer, triphenyl phosphate, tricresyl phosphate and the like are suitable. Of these, acrylic acid or methacrylic acid is most preferred.
[0036]
The method for grafting is not particularly limited, but a radiation method is preferred. For example, the polymer chain substrate (the polymer on the side to be grafted) and the graft monomer material may be coexisting, and radiation may be irradiated continuously or intermittently, or more preferably before both are coexisting. Pre-irradiate. As the radiation, electron beam, X-ray or γ-ray is used. Upon irradiation, the polymer substrate is activated by generating free radicals.
[0037]
The degree of grafting can be determined by several factors, but most importantly, the length of time the activated substrate is in contact with the grafting monomer, the degree of preactivity of the substrate by radiation, the grafting monomer The degree to which the material can penetrate the substrate and the temperature at which the substrate and monomer are in contact. For example, when the graft monomer is an acid, the degree of grafting can be monitored by sampling the solution containing the monomer, titrating against a base, and measuring the residual monomer concentration. The degree of grafting is preferably from 2 to 20% of the final weight, particularly preferably from 3 to 12%, particularly preferably from 5 to 10%. The grafting may be performed by a method in which the activation of the polymer substrate (generation of free radicals) is performed by light irradiation or heat.
[0038]
The fluoropolymer having vinylidene fluoride as a main unit preferably has a melt flow index at 230 ° C. and 10 kg of 1.0 g / 10 min or less, more preferably 0.2 to 0.7 g / 10 min. The melt flow index is a value measured by the method described in standard ASTM D 1238. When the melt flow index is 1.0 g / 10 min or less, the mechanical strength of the solid electrolyte layer is improved, and ion conductivity at room temperature is further improved.
[0039]
Moreover, rate characteristics, low-temperature characteristics, and the like are further improved by using a fluoropolymer porous material having vinylidene fluoride as a main unit. The density of the porous body is 0.60 g / cm ^Three ~ 1.30 g / cm ^Three Is particularly preferable, 0.70 to 1.00 g / cm ^Three It is. The density of the porous body is 0.60 g / cm ^Three If it is less than the above, there is a concern that the mechanical strength is reduced, making it difficult to handle the laminated structure when it is produced, and the density is 1.30 g / cm. ^Three If it is larger, it becomes difficult to obtain the effect of improving the desired rate characteristics and low temperature characteristics.
[0040]
The average pore diameter of the pores in the porous body is preferably from 0.01 to 10 μm, particularly preferably from 0.1 to 5.0 μm. The “average pore diameter” was determined by taking an arbitrary 10 pores by SEM observation and calculating the average value of the pore diameters of the 10 pores.
[0041]
As the lithium salt (electrolyte) constituting the electrolytic solution (lithium salt (electrolyte) + compatible solvent) used for the solid electrolyte layer, LiClO _Four , LiBF _Four , LiPF ₆ , LiAsF ₆ LiAlCl _Four And Li (CF _Three SO ₂ ) ₂ One or more selected from the group consisting of N are used. The compatible solvents include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl sulfoxide, sulfolane, γ-butyrolactone, 1,2-dimethoxyethane, N, N-dimethylformamide, tetrahydrofuran, 1 , 3-dioxolane, 2-methyltetrahydrofuran, diethyl ether and the like are exemplified, and any one or a mixture of two or more selected from these is used. When a mixed solvent is used, a mixture containing at least one selected from diethyl carbonate and ethyl methyl carbonate, and further containing ethylene carbonate, propylene carbonate, and dimethyl carbonate is particularly preferable. The mixing ratio of each component constituting the mixture is preferably 25% by volume to 50% by volume and at least 30% by volume to 35% by volume in at least one selected from diethyl carbonate and ethyl methyl carbonate. More preferred. In ethylene carbonate, the mixing ratio is preferably 4% by volume to 20% by volume, and more preferably 6% by volume to 18% by volume. In propylene carbonate, the mixing ratio is preferably 3% by volume to 17% by volume, and more preferably 5% by volume to 15% by volume. Moreover, in dimethyl carbonate, it is preferable that a mixing ratio exceeds 40 volume% and is 60 volume% or less, and it is more preferable that it is 45 volume%-55 volume%.
[0042]
The concentration of the lithium salt (electrolyte) in the electrolytic solution is preferably 0.1 mol / L to 2 mol / L, particularly preferably 0.5 mol / L to 1.5 mol / L. If the salt (electrolyte) concentration is such a preferable concentration, more preferable results can be obtained in terms of rate characteristics and low-temperature characteristics.
[0043]
For the purpose of preventing crystallization of the electrolyte at the battery operating temperature (especially at low temperatures), together with the above compatible solvent, tetraethylene glycol dimethyl ether, N-methyl-pyrrolidone (1-methyl-2-pyrrolidone) It is preferable to use a plasticizer such as ethylene glycol dimethyl ether or diethylene glycol dimethyl ether. The amount of the plasticizer used is preferably about 1% by weight to 50% by weight with respect to the compatible solvent. By adding the plasticizer, crystallization of the electrolyte solution infiltrated (impregnated) into the fluoropolymer hardly occurs, and sufficient ionic conductivity of the solid electrolyte layer can be ensured.
[0044]
The thickness of the solid electrolyte layer varies depending on the shape, size, etc. of the sheet electrode (positive electrode sheet electrode and negative electrode sheet electrode), but generally the average thickness is preferably 5 to 100 μm, particularly preferably 8 to 50 μm, and particularly preferably 10 ˜30 μm. Here, the thickness of the solid electrolyte layer is a thickness in a state of being interposed between the positive electrode sheet electrode and the negative electrode sheet electrode (a state where the battery is actually assembled), and a separation distance between the positive electrode sheet electrode and the negative electrode sheet electrode. be equivalent to.
[0045]
In the present invention, the method for forming the solid electrolyte layer is not particularly limited. For example,
(A) A polymer substrate material is formed into a film by a known molding method such as extrusion, or a coating liquid (paste) is prepared by mixing a polymer substrate material and an appropriate solvent. The coating liquid (paste) is coated on the surface of the peeling substrate with an appropriate coater to form a coating film, and the coating film is heated, dried, and then peeled off from the peeling substrate to form a film. A method of gelling by immersing the obtained film in an electrolytic solution (a solution in which a lithium salt is dissolved in a compatible solvent) (including a case of immersing in a solution together with a positive electrode and a negative electrode in a battery manufacturing process), (b ) Dissolve lithium salt for electrolyte and compatible solvent in a suitable solvent, add polymer matrix material, and dissolve polymer matrix material while heating as necessary. Prepare the substrate for peeling A method of forming a coating film by coating with a suitable coater on the surface, heating and drying the coating film in stages, evaporating the solvent, and peeling the solid electrolyte layer from the peeling substrate (C) A coating film is formed directly on at least one surface of the positive electrode sheet electrode and / or the negative electrode by the coating solution (paste) in which the lithium salt, the compatible solvent, and the polymer substrate material are dissolved, and the solvent is evaporated. And a method of forming a solid electrolyte layer.
[0046]
In the above methods (a) to (c), if a foaming agent is further added to and mixed with the coating liquid (paste) to generate bubbles along with the evaporation of the solvent, the solid electrolyte in which the polymer substrate exists in a porous body A layer can be obtained.
[0047]
As the solvent in the methods (b) and (c), for example, tetrahydrofuran (THF), dimethylacetamide, dimethylformamide and the like are preferable.
[0048]
Moreover, as a foaming agent used when making the said polymer substrate porous, any of a decomposable foaming agent, a gas foaming agent, and a volatile foaming agent can be used, but in the method of said (a), a gas foaming agent is used. Or a volatile foaming agent is preferable, and as a gaseous foaming agent, nitrogen, carbon dioxide, propane, neopentane, methyl ether, dichloromethane difluoride, n-butane, isobutane, etc. are suitable, and a volatile foaming agent. N-octanol, 1-pentanol, 3-methyl-1-butanol, 2-methyl-1-butanol, 2-pentanol, 3-pentanol, 2-methyl-2-butanol, 3-methyl- 2-Butanol and the like are preferred. In the above methods (b) and (c), a volatile foaming agent is preferable, and n-octanol, 1-pentanol, 3-methyl-1-butanol and the like are particularly preferable, and n-octanol is particularly preferable. .
[0049]
The density of the porous body when the polymer substrate is made into a porous body is adjusted by changing the amount of the foaming agent and various conditions during production. For example, in the case of the method (a), the production conditions that greatly affect the density (foaming degree) other than the amount of the foaming agent are molding temperature, molding speed, molding pressure, and the like. In the case of the methods (b) and (c), the production conditions that greatly affect the density (foaming degree) other than the amount of the foaming agent include the coating speed, the drying temperature profile, the degree of exhaust, and the molding speed. is there.
[0050]
As the electrolytic solution used in the battery of the type in which the separator is interposed between the sheet electrodes, the electrolytic solution (lithium salt (electrolyte) + compatible solvent) impregnated in the polymer substrate of the solid electrolyte layer is used.
[0051]
As an exterior material in the sheet-like lithium secondary battery of the present invention, a thermoplastic resin laminate metal sheet or a thermoplastic resin laminate foil having a thermoplastic resin laminate on one side or both sides is preferable. The exterior material having such a thermoplastic resin laminate is used to further melt the contents by using the thermoplastic resin layer from the viewpoint of excellent permeation prevention against water and gas permeation and electrical insulation. This is preferable because it can be sealed.
[0052]
【Example】
EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples.
Example 1
[Production of negative electrode sheet electrode]
A negative electrode active material composition was obtained by mixing fibrous graphite as a negative electrode active material and polyvinylidene fluoride as a binder in N-methylpyrrolidone to form a slurry. In the negative electrode active material composition, the binder was 10% by weight. The slurry was applied on both sides of a 15 μm thick copper foil serving as a negative electrode current collector, and then dried and rolled to form a negative electrode active material layer. A nickel plate was welded to the negative electrode current collector to form a lead to obtain a rectangular negative electrode sheet electrode having the following dimensions.
Sheet electrode size: 5cm x 3cm
Sheet electrode thickness: 160 μm
Similarly, negative electrode sheets of the same size having a negative electrode active material layer only on one side of the copper foil were produced.
[0053]
[Preparation of positive electrode sheet electrode]
LiCoO as positive electrode active material ₂ Then, a conductive material and polyvinylidene fluoride serving as a binder were mixed in N-methylpyrrolidone and uniformly dispersed to obtain a slurry-formed positive electrode active material composition. In the positive electrode active material composition, the conductive material was 5% by weight and the binder was 4% by weight. The slurry was applied on both surfaces of an aluminum plate serving as a positive electrode current collector, and then dried and rolled to form a positive electrode active material layer. An aluminum plate was welded to the positive electrode current collector to form a lead to obtain a rectangular positive electrode sheet electrode having the following dimensions.
Sheet electrode size: 4.8 cm × 2.8 cm
Sheet electrode thickness: 150 μm
[0054]
[Preparation of porous polyvinylidene fluoride film]
A coating solution is prepared by mixing polyvinylidene fluoride, dimethylformamide (DMF) and n-octanol. The coating solution is applied onto a substrate (aluminum foil), and the coating is heated and dried. Density of 0.75 g / cm by peeling from the material ^Three A porous film having an average thickness of 25 μm was obtained. And this film was cut | disconnected to the predetermined dimension, and the bag of the magnitude | size which can accommodate the said one positive electrode sheet electrode was produced using the hot press machine whose hot press metal fitting is a broken line shape.
[0055]
[Assembly of laminated structure]
A positive electrode sheet electrode accommodated in a porous polyvinylidene fluoride film bag (eight) and a negative electrode sheet electrode (seven having an active material layer on both sides and two having an active material layer on one side) Are stacked alternately, the lead of each positive electrode sheet electrode is welded to the positive electrode current collector terminal made of Ni plate, and the lead of each negative electrode sheet electrode is welded to the negative electrode current collector terminal made of Ni plate to assemble the laminated structure. .
As shown in FIG. 3, an insulating bent plate that is bent in a substantially square shape made of polyethylene is provided on each of a pair of opposing side surfaces (a set of side surfaces having a larger area) as shown in FIG. The convex surfaces were placed facing each other so that they face outward. In addition, the insulating bending plate was used in which the ratio of D1 and W1 (D1 / W1) shown in FIG. 4 is 0.35. Then, the surfaces of the current collectors (both main surfaces of the laminated structure) of the two negative electrode sheets positioned on the outermost side of the laminated structure are opposed to a pair of opposing side surfaces of the laminated structure. A plurality of sheet electrodes constituting a laminated structure by adhering an adhesive tape (base: polypropylene, adhesive layer: acrylic adhesive) 45 mm wide and 40 μm thick to the convex surface of the insulative bending plate and winding three times. Was fixed. Next, a mixed structure obtained by mixing ethylene carbonate and ethylmethyl carbonate at a ratio of 50% by volume: 50% by volume to the laminated structure bound with this adhesive tape, LiPF ₆ Was immersed in a solution in which the concentration was 1.0 mol / L (concentration after preparation), and the porous polyvinylidene fluoride film was gelled to form a solid electrolyte layer. At this time, all the porous polyvinylidene fluoride films were LiPF. ₆ It took about 5 minutes for the solution to permeate every corner and for all the porous polyvinylidene fluoride films to become a solid electrolyte layer with uniform properties. Moreover, the thickness of the solid electrolyte layer formed between each sheet electrode was substantially the same thickness within the range of 22-28 micrometers.
The electrolyte solution permeation (injection) time is the first charge for a plurality of batteries manufactured by changing the time (working time) for actually infiltrating the electrolyte solution, and the normal capacity (maximum capacity) is It is the time of the penetration time that was minimal among the ones obtained. The thickness of the solid electrolyte layer was measured with a micrometer.
[0056]
[Assembly of sheet-like lithium secondary battery]
The laminated structure bundled with the tape is housed in an exterior material (bag-like) made of a laminate film composed of a laminated structure of a heat seal layer, an electrolytic solution-resistant insulating layer, an aluminum layer, and an insulating layer in order from the inside. Then, the positive electrode current collector terminal and the negative electrode current collector terminal protruded from the opening, and the opening of the exterior material was heat-sealed and sealed to complete a sheet-like lithium secondary battery.
[0057]
Comparative example
Implemented without attaching an insulating bent plate to the laminated structure, except that the laminated structure was taped and bonded to both main surfaces of the laminated structure and a pair of opposing side surfaces of the laminated structure. In the same manner as in Example 1, a sheet-like lithium secondary battery was produced. In addition, LiPF to every corner of all porous polyvinylidene fluoride films in the laminated structure ₆ It took about 60 minutes for the solution to penetrate and all the porous polyvinylidene fluoride films to become a solid electrolyte layer with uniform properties.
[0058]
The batteries produced in the above Examples and Comparative Examples were subjected to the following low temperature characteristic test and rate characteristic test. As a result, the battery of the example and the battery of the comparative example showed equivalent battery performance.
Example (low temperature characteristics (discharge capacity change rate) 85%, rate characteristics 97%)
Comparative example (low temperature characteristics (discharge capacity change rate) 86%, rate characteristics 97%)
Therefore, in the present invention, it is possible to obtain a high-performance sheet-like lithium secondary battery in which the time for allowing the electrolytic solution to penetrate is significantly shortened compared to the conventional case, and there is no unnecessary gap between the sheet electrodes in the laminated structure. It could be confirmed.
[0059]
[Low temperature characteristics test]
The prepared lithium ion secondary battery is charged at room temperature, and then left in an atmosphere of −20 ° C. for 24 hours. The charging was performed by flowing a current at a constant voltage of 1 C (600 mA) until the voltage reached 4.2 V, and then flowing a current at a constant voltage of 4.2 V until the total charging time reached 2.5 hours. . Next, discharge is performed at 0.5 C (300 mAh) /2.5 V cut-off voltage in the air atmosphere at −20 ° C., and the discharge capacity [mA · H] at that time is obtained. Further, charging and discharging are performed under the same conditions at room temperature (20 ° C.), and the discharge capacity [mA · H] is obtained. Furthermore, the discharge capacity change rate was calculated by dividing the discharge capacity at −20 ° C. by the discharge capacity at room temperature.
[0060]
[Rate characteristics test]
2C discharge was performed under room temperature (20 degreeC), and the ratio with respect to the total capacity of the discharge capacity was computed. 2C refers to a constant current of 1200 mA with respect to the discharge capacity (600 mA) of the lithium ion secondary battery.
[0061]
【Effect of the invention】
As is apparent from the above description, according to the present invention, a laminated structure (a laminated structure including a plurality of units in which a positive electrode sheet electrode and a negative electrode sheet electrode are stacked via a separator or a solid electrolyte layer) as a power generation element is provided. Although the tape is wound around the outer periphery and the plurality of sheet electrodes constituting the laminated structure are bound and fixed, the electrolyte can be permeated to every corner of the laminated structure in a short time. There is no unnecessary gap between the sheet electrodes in the body (each sheet electrode is in close contact with the separator or solid electrolyte layer), and excellent battery characteristics, particularly good low temperature characteristics and rate characteristics. A battery can be manufactured efficiently.
[Brief description of the drawings]
FIG. 1 is a schematic perspective view of a laminated structure (power generation element) used in a sheet-like lithium secondary battery according to an example of the present invention.
2 is an enlarged cross-sectional view of a main part showing a laminated state of a positive electrode sheet electrode and a negative electrode sheet electrode in the laminated structure of FIG. 1. FIG.
3 is a schematic perspective view of a state in which a bending plate is attached to the laminated structure of FIG. 1 and an adhesive tape is wound thereon. FIG.
4 is an enlarged view of a main part showing a state in which the bent plate shown in FIG. 3 is attached to the laminated structure.
FIG. 5 is a view showing another example of the bent plate used in the present invention (attached state to the laminated structure).
FIG. 6 is a schematic perspective view of a laminated structure in which an adhesive tape is wound in a conventional sheet-like lithium secondary battery.
[Explanation of symbols]
1 Positive electrode sheet electrode
2 Negative electrode
3 Laminated structure (power generation element)
3A-1, 3A-2 Main surface of laminated structure
3B-1, 3B-2, 3C-1, 3C-2 Side surface of laminated structure
4 Separator or solid electrolyte layer
5 Adhesive tape
14 Clearance

Claims (4)

A plurality of sheet electrodes laminated structure of the positive electrode sheet electrode and the negative electrode sheet electrode comprises a plurality of units stacked via a separator or a solid electrolyte layer and the power generating element, wrapped around the tape laminate structure, constituting the laminated structure A sheet-like lithium secondary battery in which
The surface of the two sheet electrodes located at the outermost part of the laminated structure is called the main surface of the laminated structure, and the surface where the end faces of the plurality of sheet electrodes are gathered is called the side surface of the laminated structure. A sheet-like lithium secondary battery characterized in that a gap is formed between the side surface and the tape without contacting the side surface of the body. A positive electrode sheet electrode and the sheet negative electrode sheet electrode is rectangular-shaped, has layered structure the outer shape of the straight rectangular parallelepiped, in each of a pair of opposite sides of the laminated structure of the straight rectangular parallelepiped, dogleg 2. A sheet-like lithium secondary battery according to claim 1, wherein the bent plate is bent oppositely with its convex surface facing outward, and the tape is wound around both main surfaces of the laminated structure and the convex surface of the bent plate. . The sheet-like lithium according to claim 1 or 2, wherein a solid electrolyte layer mainly composed of a salt, a compatible solvent, and a fluoropolymer mainly composed of vinylidene fluoride is interposed between the positive electrode sheet electrode and the negative electrode sheet electrode. Secondary battery. The sheet-like lithium secondary battery according to claim ³ , wherein the fluoropolymer of the solid electrolyte layer is a porous body having a density of 0.60 to 1.30 g / cm ³ .

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