Secondary battery electrode and secondary battery

Abstract

This secondary battery electrode is provided with: a sheet-shaped electrode current collector; and a first electrode mixture layer and a second electrode mixture layer, which are respectively carried on one main surface and the other main surface of the electrode current collector. An end portion of the electrode has end portions of the first electrode mixture layer and the second electrode mixture layer, and an end portion of the electrode current collector. The end portions of the first electrode mixture layer and the second electrode mixture layer both protrude from the end portion of the electrode current collector.

Description

Secondary battery electrode and secondary battery

[0001] The present disclosure relates to an electrode for a secondary battery and a secondary battery.

[0002] The secondary battery includes an electrode group including a pair of electrodes and an electrolyte. At least one of the pair of electrodes includes a sheet-shaped electrode current collector and an electrode mixture layer supported on a main surface of the electrode current collector. The electrode mixture layer includes an electrode active material that absorbs and releases lithium ions. The electrode group is formed, for example, by winding the pair of electrodes with a separator interposed therebetween.

[0003] Patent Document 1 proposes "a battery electrode characterized by comprising an active material on the surface of a current collector and an insulating material or an electrode active material mixture coated on the periphery of the current collector."

[0004] Japanese Patent Application Publication No. 11-111302

[0005] The electrode mixture layer at the end of the electrode group (electrode) may fall off due to stress generated by the expansion of the electrode mixture layer, which may result in a decrease in the cycle characteristics of the secondary battery.

[0006] One aspect of the present disclosure relates to an electrode for a secondary battery, the electrode comprising a sheet-like electrode current collector, and a first electrode mixture layer and a second electrode mixture layer carried on one main surface and the other main surface of the electrode current collector, respectively, at least one end of the electrode has an end of the first electrode mixture layer and the second electrode mixture layer and an end of the electrode current collector, and the ends of the first electrode mixture layer and the second electrode mixture layer each protrude from an end of the electrode current collector.

[0007] Another aspect of the present disclosure relates to a secondary battery including a pair of electrodes and an electrolyte, wherein at least one of the pair of electrodes is the secondary battery electrode described above.

[0008] According to the present disclosure, it is possible to suppress deterioration in the cycle characteristics of a secondary battery.

[0009] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0010] Fig. 1 is a top view schematically showing an example of an electrode for a secondary battery according to an embodiment of the present disclosure; Fig. 2 is a cross-sectional view of a main part schematically showing an example of an electrode for a secondary battery according to an embodiment of the present disclosure; Fig. 3 is a cross-sectional view of a main part schematically showing another example of an electrode for a secondary battery according to an embodiment of the present disclosure; Fig. 4 is a top view of a main part showing an example of a wide electrode current collector prepared when fabricating an electrode for a secondary battery; Fig. 5 is a schematic perspective view of a partially cutaway secondary battery according to an embodiment of the present disclosure.

[0011] Below, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties, conditions, etc. are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined as long as the lower limit is not equal to or greater than the upper limit.

[0012] An electrode for a secondary battery according to an embodiment of the present disclosure includes a sheet-like electrode current collector, and a first electrode mixture layer and a second electrode mixture layer respectively supported on one main surface and the other main surface of the electrode current collector. At least one end of the electrode includes an end of the first electrode mixture layer and the second electrode mixture layer and an end of the electrode current collector. The ends of the first electrode mixture layer and the second electrode mixture layer each protrude beyond an end of the electrode current collector.

[0013] The electrode mixture layer contains an electrode active material that absorbs and releases lithium ions. The electrode mixture layer expands due to the absorption of lithium ions. The electrode may have a current collector exposed portion where no electrode mixture layer is formed. Note that the end of the current collector exposed portion is not included in the end of the electrode current collector.

[0014] Because the ends of the first electrode mixture layer and the second electrode mixture layer each protrude from the end of the electrode current collector, a gap is formed between the end of the first electrode mixture layer and the end of the second electrode mixture layer. The gap formed at the electrode end allows stress generated by the expansion of the electrode mixture layer to escape, thereby alleviating the stress on the electrode (electrode end). This suppresses shedding of the electrode mixture layer at the end of the electrode group (electrode) due to the stress, thereby suppressing a decrease in cycle characteristics. Furthermore, the protruding end of the electrode mixture layer can protect the end of the electrode current collector. For example, in the case of a wound-type electrode group, forming gaps at both widthwise ends of the strip-shaped electrodes included in the electrode group can efficiently suppress shedding of the electrode mixture layer at the end of the electrode group.

[0015] The gap is formed at at least one end of the electrode. When a rectangular electrode is viewed from the normal direction to the main surface, the electrode has four sides. In this case, the gap may be formed, for example, at an end corresponding to at least one of the four sides, or at two ends corresponding to any two of the four sides, or at each of the four ends corresponding to the four sides. In the case of the strip-shaped electrode of FIG. 1 , the gap is formed, for example, at ends ES1 to ES2 (or ends ES1 to ES4). The gap may be formed continuously or intermittently along the end of the electrode. When the electrode is viewed from the normal direction to the main surface, the ratio of the total length L1 of the gaps formed at one end corresponding to one side to the length L0 of the end corresponding to the side may be, for example, 80% or more, or may be 100%.

[0016] The electrode can be produced, for example, by applying an electrode mixture slurry to both main surfaces of a current collector sheet (e.g., a metal foil or an alloy foil), drying the coating, and optionally rolling it to form a laminate of the current collector sheet and the electrode mixture layer, and then cutting the laminate to a predetermined size. The laminate may be cut using a predetermined blade (e.g., a slit blade, etc.).

[0017] Examples of cutting processes for the laminate include laser processing, plasma processing, and gas cutting. The gaps may be formed by utilizing the melting and solidification of the cut surface during laser processing. The electrode current collector (metal foil or alloy foil) melts more easily than the electrode composite layer. By appropriately adjusting the cutting conditions, the gaps can be formed during the cutting process.

[0018] The gap may be formed by heating the end of the electrode (the cut surface of the laminate) obtained by cutting to a predetermined size with a predetermined blade using a burner or the like. At this time, the gap may be formed by selectively melting the end of the electrode current collector by heating within a temperature range in which the end of the electrode mixture layer is difficult to melt and the end of the electrode current collector is easily melted. Alternatively, the gap may be formed using a current collector sheet 111 shown in FIG. 4, which will be described later.

[0019] FIG. 1 is a top view schematically illustrating an example of an electrode for a secondary battery according to an embodiment of the present disclosure. The electrode in FIG. 1 is strip-shaped. In FIG. 1, LD indicates the length direction of the strip-shaped electrode 10, and WD indicates the width direction of the strip-shaped electrode 10. FIG. 2 is a cross-sectional view schematically illustrating a main portion of an example of an electrode for a secondary battery according to an embodiment of the present disclosure. FIG. 2 shows a main portion of the II-II cross section of the electrode 10 in FIG. 1 (near the end ES1 of the electrode 10). In FIG. 2, TD indicates the thickness direction of the electrode current collector 11. FIG. 3 is a cross-sectional view schematically illustrating a main portion of another example of an electrode for a secondary battery according to an embodiment of the present disclosure.

[0020] The strip-shaped electrode 10 has a sheet-shaped electrode current collector 11 and electrode mixture layers 12a, 12b carried on both sides of the electrode current collector 11. The electrode 10 has opposite ends ES1, ES2 in the width direction (WD direction) and opposite ends ES3, ES4 in the length direction (LD direction).

[0021] As shown in Fig. 2, end ES1 of electrode 10 includes end portions 12AE and 12BE of the first and second electrode mixture layers 12a and 12b, respectively, and end portion 11E of electrode current collector 11. End portions 12AE and 12BE of first and second electrode mixture layers 12a and 12b each protrude from end portion 11E of electrode current collector 11. As a result, as shown in Figs. 1 and 2, a gap 13 is formed along end portion ES1 between end portion 12AE of first electrode mixture layer 12a and end portion 12BE of second electrode mixture layer 12b.

[0022] Similar to end ES1, end ES2 of electrode 10 also has gap 13. That is, end ES2 of electrode 10 has ends 12AE, 12BE of the first electrode mixture layer 12a and the second electrode mixture layer 12b, and end 11E of the electrode current collector 11. Similar to end ES1, end ES2 also has ends 12AE, 12BE of the first electrode mixture layer 12a and the second electrode mixture layer 12b protruding from end 11E of the electrode current collector 11. As a result, as shown in FIG. 1 , gap 13 is formed along end ES2 between end 12AE of the first electrode mixture layer 12a and end 12BE of the second electrode mixture layer 12b.

[0023] In FIG. 2 , TE is the thickness of the end 11E of the electrode current collector 11, and TC is the thickness of the portion 11C of the electrode current collector 11 that is inward from the end 11E. In FIG. 2 , TE is approximately the same as TC, but TE may be greater than TC. When TE is greater than TC, it is easier to ensure a sufficient distance between the end 12AE of the first electrode mixture layer 12a and the end 12BE of the second electrode mixture layer 12b, making it easier to form the gap 13. For example, when cutting a laminate of electrode current collectors and electrode mixture layers using laser processing or the like, melting and solidifying the end (cut portion) of the electrode current collector can form an end 11E having a thickness TE greater than the thickness TC. The thickness of the electrode 10 is, for example, 100 μm or more and 300 μm or less (or 200 μm or less). TC is, for example, 5 to 20 μm. TE is, for example, 1 to 5 times TC.

[0024] The protrusion length L of the ends 12AE, 12BE of the first electrode mixture layer 12a and the second electrode mixture layer 12b from the end 11E of the electrode current collector 11 is preferably 20 μm or more, and may be 20 μm or more and 180 μm or less. In this case, it is easy to ensure a sufficient gap 13 and to suppress deterioration of cycle characteristics. The protrusion length L is, for example, 1 / 10,000 to 1 / 1,000 times the length of the electrode current collector 11 in the width direction (WD direction). The protrusion length L is, for example, 2 to 15 times the thickness TE of the end 11E of the electrode current collector 11.

[0025] In FIG. 2 , the protrusion length L of the end 12AE of the first electrode mixture layer 12a from the end 11E of the electrode current collector 11 is approximately the same as, but may be different from, the protrusion length L of the end 12BE of the second electrode mixture layer 12b from the end 11E of the electrode current collector 11. In FIG. 1 , the protrusion length L on the end ES1 side is approximately the same as, but may be different from, the protrusion length L on the end ES2 side. The protrusion length L is determined by observing a cross section of the electrode in the thickness direction and measuring the protrusion length of the end of the electrode mixture layer from the end of the electrode current collector. Observation of the cross section of the electrode in the thickness direction can be performed using, for example, a scanning electron microscope (SEM).

[0026] The gap 13 formed at the ends ES1 and ES2 of the electrode 10 is formed continuously and entirely along the LD direction, but may also be formed intermittently or partially. The ratio of the LD-direction length of the portion of the end ES1 (end ES2) where the gap 13 is formed to the LD-direction length of the electrode 10 (the gap formation rate in the LD direction) is approximately 100%, but is not limited to this. The gap formation rate in the LD direction may be, for example, 80% or more.

[0027] The strip-shaped electrode 10 has two ends ES3 and ES4 in the longitudinal direction (LD direction). In a wound electrode group, one of the ends ES3 and ES4 of the electrode 10 is the end where the winding begins, and the other of the ends ES3 and ES4 of the electrode 10 is the end where the winding ends. A gap may be formed at the ends ES3 and ES4, as with the ends ES1 and ES2.

[0028] As shown in FIG. 3 , the electrode 10 may include a coating layer 14 that covers the ends ES1 and ES2 of the electrode 10, which have a gap 13. In this case, the coating layer 14 is formed to cover the opening of the gap 13. This allows the coating layer 14 to protect the end (exposed surface) of the electrode current collector 11 while maintaining the gap 13. This can suppress internal short circuits caused by the electrode current collector being exposed at the electrode end. When the electrode is a negative electrode, this can suppress lithium dendrite deposition on the end surface of the electrode current collector and the resulting internal short circuits. In Patent Document 1, the electrode end surface is coated with an insulating material or the like, but the gap 13 is not provided, making it impossible to alleviate stress on the electrode end.

[0029] The thickness of the coating layer 14 may be 1 μm or more, or may be 1 μm or more and 200 μm or less, from the viewpoint of protecting the electrode current collector 11. The thickness of the coating layer can be determined by observing a cross section of the electrode in the thickness direction using an SEM or the like, measuring the thickness of the coating layer at several points, and calculating the average value of these measurements.

[0030] The coating layer may contain the same components as the electrode mixture layer. Alternatively, the coating layer may be an insulating layer. The insulating layer contains a resin material. Examples of the resin material include fluororesins (e.g., polyvinylidene fluoride, polytetrafluoroethylene, etc.).

[0031] The method for forming the coating layer is not particularly limited. The coating layer may be formed by applying a treatment liquid to the electrode end and drying it. The electrode end may be immersed in the treatment liquid and drying it to form the coating layer. The treatment liquid contains an electrode mixture or an insulating material, and a dispersion medium or a solvent. N-methyl-2-pyrrolidone (NMP) or the like is used as the dispersion medium or solvent. The application method is not particularly limited. Examples of application methods include a dispenser method and a spray method. The drying method is not particularly limited. Natural drying may be used, or drying may be performed using a drying oven. Examples of electrode mixtures include the positive electrode mixture or negative electrode mixture described below. Examples of insulating materials include those exemplified above.

[0032] The electrode 10 shown in FIG. 1 may be produced using a wide current collector sheet 111 shown in FIG. 4. FIG. 4 is a top view of a main portion showing an example of a wide current collector sheet prepared when producing an electrode for a secondary battery. C1 shown by a dashed line in FIG. 4 indicates a cut portion that is cut together with the electrode mixture layer during slitting. C2 shown by a dashed line in FIG. 4 indicates a perforation that has been pre-formed in the current collector sheet 111. Region 112 in FIG. 4 indicates a portion that will become electrode current collector A, which will be described later. Region 113 indicates a portion that will become electrode current collector B, which will be described later.

[0033] A method for fabricating an electrode 10 using a current collector sheet 111 is described below. An electrode composite slurry is applied to both sides of the current collector sheet 111 shown in FIG. 4 , the coating is dried, and, if necessary, rolled to form an electrode composite layer. In this manner, a laminate is obtained in which an electrode composite layer is formed on both sides of the current collector sheet 111. The current collector sheet 111, together with the electrode composite layer, is cut into strips along the cutting portion C1 using a slit blade. In this manner, a plurality of strip-shaped laminates including an electrode current collector A (region 112) are obtained. Next, both widthwise end portions 112a (the portions from perforation C2 to cutting portion C1 in FIG. 4 ) of the electrode current collector A (region 112) are cut along perforation C2 to obtain a strip-shaped laminate including an electrode current collector B (region 113). That is, by cutting out both end portions 112a, a strip-shaped electrode 10 having gaps 13 is obtained.

[0034] A secondary battery according to an embodiment of the present disclosure includes a pair of electrodes and an electrolyte. At least one of the pair of electrodes is a secondary battery electrode according to an embodiment of the present disclosure. One of the pair of electrodes is a positive electrode, and the other of the pair of electrodes is a negative electrode. The pair of electrodes (the positive electrode and the negative electrode) are, for example, wound or stacked with a separator interposed therebetween.

[0035] Examples of secondary batteries include non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, lithium metal secondary batteries, and solid-state batteries containing gel electrolytes or solid electrolytes. That is, the secondary battery may be a liquid secondary battery containing an electrolytic solution as the electrolyte, or an all-solid-state secondary battery containing a solid electrolyte.

[0036] Each component of the secondary battery will be described in detail below. [Positive Electrode] The positive electrode includes a positive electrode current collector and a positive electrode composite layer supported on the positive electrode current collector. The positive electrode composite layer is made of a positive electrode composite. The positive electrode composite layer is supported on one or both main surfaces of the positive electrode current collector.

[0037] The positive electrode mixture contains a positive electrode active material as an essential component, and may contain optional components such as a binder, a conductive additive, and a thickener. The positive electrode active material may be a material that reversibly absorbs and releases lithium ions. The positive electrode active material may be, for example, a lithium-containing transition metal oxide. Examples of transition metals include Ni, Co, Mn, and the like. Representative examples of lithium-containing transition metal oxides include lithium cobalt oxide and lithium nickel oxide, which have a layered, rock-salt crystal structure.

[0038] The positive electrode mixture layer can be formed, for example, by applying a positive electrode mixture slurry containing a positive electrode mixture and a dispersion medium to the surface of a positive electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. N-methyl-2-pyrrolidone (NMP) or the like is used as the dispersion medium for the positive electrode mixture slurry.

[0039] Examples of lithium-containing transition metal oxides include Li ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ M １－ｂ O ｃ , Li ａ Ni １－ｂ M ｂ O ｃ , Li ａ Mn ２ O ４ , Li ａ Mn ２－ｂ M ｂ O ４ , LiMPO ４ , Li ２MPO ４ F (M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B). Here, 0<a≦1.2, 0<b≦0.9, and 2.0≦c≦2.3. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0040] The lithium-containing transition metal oxides include Li ａ Ni ｂ M １－ｂ O ２ (M is at least one selected from the group consisting of Mn, Co, and Al, and 0<a≦1.2, and 0.3≦b<1). From the viewpoint of increasing capacity, it is more preferable that 0.85≦b<1 is satisfied. From the viewpoint of stability of the crystal structure, Li containing Co and Al as M is preferable. ａ Ni ｂ Co ｃ Al ｄ O ２ (0<a≦1.2, 0.85≦b<1, 0<c<0.15, 0<d≦0.1, b+c+d=1).

[0041] Examples of binders include resin materials, for example, fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF), polyolefin resins such as polyethylene and polypropylene, polyamide resins such as aramid resin, polyimide resins such as polyimide and polyamideimide, acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer, vinyl resins such as polyacrylonitrile and polyvinyl acetate, polyvinylpyrrolidone, polyethersulfone, etc. One type of binder may be used alone, or two or more types may be used in combination.

[0042] Examples of the conductive additive include carbon materials such as graphite, carbon black such as acetylene black, carbon fibers (carbon nanotubes (CNT), carbon fibers other than CNT), etc. One type of conductive material may be used alone, or two or more types may be used in combination.

[0043] The positive electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the positive electrode current collector include stainless steel, aluminum, an aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0044] [Negative Electrode] The negative electrode may be a negative electrode from which lithium metal is deposited during charging, or a negative electrode that absorbs lithium ions during charging.

[0045] The negative electrode includes a strip-shaped negative electrode current collector. The negative electrode may include the negative electrode current collector and a negative electrode composite layer supported on the negative electrode current collector. The negative electrode composite layer is made of a negative electrode composite. The negative electrode composite layer is supported on one or both main surfaces of the negative electrode current collector.

[0046] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive additive, and a thickener. The negative electrode mixture layer can be formed, for example, by applying a negative electrode mixture slurry containing a negative electrode mixture and a dispersion medium to the surface of a negative electrode current collector and drying the applied slurry. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0047] The negative electrode active material may be a material that reversibly absorbs and releases lithium ions. The negative electrode active material may be lithium metal or a lithium alloy. That is, the negative electrode mixture layer may be a foil-shaped negative electrode active material layer made of lithium metal or a lithium alloy.

[0048] Examples of negative electrode active materials that absorb and release lithium ions include carbon materials, metal materials such as Si and Sn, alloy materials containing Si, Sn, etc., metal compounds containing Si, Sn, etc., metal oxides containing lithium, etc. Examples of metal oxides containing lithium include spinel-type lithium titanium oxides, spinel-type lithium manganese oxides, etc.

[0049] The carbon material may be graphite, easily graphitizable carbon (soft carbon), hardly graphitizable carbon (hard carbon), etc. Among these, graphite is preferred because it has excellent charge / discharge stability and a small irreversible capacity.

[0050] Graphite refers to a carbon material having a (002) plane spacing d002 of, for example, 0.340 nm or less as measured by X-ray diffraction. The crystallite size Lc(002) of the graphite as measured by X-ray diffraction may be, for example, 5 nm or more, 5 nm or more and 300 nm or less, or 10 nm or more and 200 nm or less.

[0051] The negative electrode active material may also be a composite material containing Si. The composite material containing Si has high capacity and is suitable as a negative electrode active material. The composite material contains a silicon phase. Silicon can reversibly form an alloy with lithium. The composite material is a material that can reversibly absorb and release lithium ions.

[0052] The composite material includes a silicon phase and a matrix phase in which the silicon phase is dispersed. The matrix phase may be composed of any material having lithium ion conductivity. The matrix phase may include, for example, at least one phase selected from the group consisting of a silicon oxide phase and a carbon phase.

[0053] The silicon oxide phase contains Si and O, and may further contain a third element other than Si and O. The silicon oxide phase is SiO ２ The lithium silicate may be, for example, Li ２ｙ SiO ２＋ｙ (0<y<2). The silicon oxide phase is SiO ２ The composite material is composed of SiO ｘ (0.5≦x≦1.6).

[0054] When a carbon material and a composite material are used in combination, the proportion of the composite material in the negative electrode active material (total of the carbon material and the composite material) is, for example, 1% by mass to 20% by mass, or alternatively 3% by mass to 15% by mass, or alternatively 3% by mass to 10% by mass, which makes it easier to achieve a good balance between improved cycle characteristics and a high capacity.

[0055] Examples of binders include resin materials, such as fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). One type of binder may be used alone, or two or more types may be used in combination.

[0056] Examples of the conductive additive include carbons such as acetylene black, carbon fibers (carbon nanotubes (CNT), carbon fibers other than CNT), metal fibers, metal powders such as aluminum, etc. One type of conductive material may be used alone, or two or more types may be used in combination.

[0057] Examples of thickeners include carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salts), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), and saponified polymers having vinyl acetate units such as polyvinyl alcohol. One type of thickener may be used alone, or two or more types may be used in combination.

[0058] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0059] [Electrolyte] The electrolyte may be a liquid electrolyte (electrolytic solution), a gel electrolyte, or a solid electrolyte. The liquid electrolyte is, for example, an electrolytic solution containing a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The concentration of the salt in the electrolytic solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolytic solution may contain known additives.

[0060] The gel electrolyte contains a salt and a matrix polymer, or a salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

[0061] As the solid electrolyte, for example, a material known in all-solid-state lithium ion secondary batteries (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) can be used.

[0062] For example, a liquid non-aqueous electrolyte is prepared by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that ionizes in the electrolyte, and may include, for example, a lithium salt. The electrolyte may contain various additives. The electrolyte is usually used in its liquid state, but its fluidity may be restricted by a gelling agent or the like.

[0063] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Cyclic carbonates having an unsaturated bond, such as vinylene carbonate (VC), may also be used. Cyclic carbonates having a fluorine atom, such as fluoroethylene carbonate (FEC), may also be used. Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0064] Examples of lithium salts include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ The lithium salt may be used alone or in combination of two or more. The concentration of the lithium salt in the non-aqueous electrolyte solution is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0065] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0066] Hereinafter, the structure of a prismatic secondary battery as an example of a secondary battery according to an embodiment of the present disclosure will be described with reference to Fig. 5. Fig. 5 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away.

[0067] The battery includes a bottomed prismatic battery case 4, and an electrode group 1 and a nonaqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed therebetween to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, the positive electrode, and the separator around a flat plate-shaped winding core and then removing the winding core. At least one of the positive electrode and the negative electrode is a secondary battery electrode according to an embodiment of the present disclosure.

[0068] One end of a negative electrode lead 3 is attached to the negative electrode current collector by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate (not shown). The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of a positive electrode lead 2 is attached to the positive electrode current collector by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. That is, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5 and also separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting portion is laser-welded. In this way, the opening of the battery case 4 is sealed with the sealing plate 5. A non-aqueous electrolyte injection hole provided in the sealing plate 5 is closed with a seal 8.

[0069] <<Supplementary Notes>> The above embodiments disclose the following technologies. (Technology 1) An electrode for a secondary battery, the electrode comprising a sheet-like electrode current collector and a first electrode mixture layer and a second electrode mixture layer respectively supported on one main surface and the other main surface of the electrode current collector, at least one end of the electrode having an end of the first electrode mixture layer and an end of the second electrode mixture layer and an end of the electrode current collector, the ends of the first electrode mixture layer and the second electrode mixture layer each protruding from an end of the electrode current collector. (Technology 2) The electrode for a secondary battery according to Technology 1, wherein a gap is formed between the end of the first electrode mixture layer and the end of the second electrode mixture layer. (Technology 3) The electrode for a secondary battery according to Technology 2, the electrode comprising a coating layer covering the end of the electrode having the gap, the coating layer covering an opening of the gap. (Technology 4) The electrode for a secondary battery according to any one of Technologies 1 to 3, wherein the protrusion length L of the ends of the first electrode mixture layer and the second electrode mixture layer from the end of the electrode current collector is 20 μm or more. (Technology 5) The electrode for a secondary battery according to any one of Technologies 1 to 4, wherein TE is a thickness of the end of the electrode current collector and TC is a thickness of a portion of the electrode current collector that is inside the end, and TC is a thickness of the electrode current collector that is inside the end. (Technology 6) A secondary battery comprising a pair of electrodes and an electrolyte, at least one of the pair of electrodes being the electrode for a secondary battery according to any one of Technologies 1 to 5.

[0070] Hereinafter, the present disclosure will be specifically described based on examples, but the present disclosure is not limited to the following examples.

[0071] <Secondary Batteries A1 to A3> (Fabrication of Positive Electrode) LiCoO ２ 100 parts by mass of the cathode mixture was mixed with 4 parts by mass of polyvinylidene fluoride (PVDF) as a binder and an appropriate amount of N-methyl-2-pyrrolidone (NMP) to prepare a cathode mixture slurry. The obtained cathode mixture slurry was applied to both sides of an aluminum foil (thickness 15 μm) as a cathode current collector, and the coating was dried and rolled to form a cathode mixture layer (thickness 148 μm). The laminate of the cathode mixture layer and the cathode current collector was cut to obtain a strip-shaped cathode.

[0072] (Preparation of Negative Electrode) A negative electrode mixture slurry was prepared by mixing 100 parts by mass of natural graphite as a negative electrode active material, 1 part by mass of styrene-butadiene copolymer rubber (SBR) as a binder, 1 part by mass of carboxymethyl cellulose sodium salt (CMC-Na) as a thickener, and an appropriate amount of water.

[0073] An electrolytic copper foil (thickness: 8 μm) was prepared as a negative electrode current collector.

[0074] The obtained negative electrode composite slurry was applied to both sides of an electrolytic copper foil (current collector sheet 111 in FIG. 4 ), the coating was dried at 110°C, and rolled with a roller to form a negative electrode composite layer (thickness 148 μm). In this way, a laminate of a negative electrode composite layer and copper foil was obtained. The laminate was cut into a strip by slitting. Then, both ends of the copper foil in the width direction (both ends 112a in FIG. 4 ) were cut along perforations previously formed in the copper foil. As a result, at both ends of the width direction of the strip-shaped laminate, the ends of the negative electrode composite layer protruded beyond the ends of the negative electrode current collector, and a strip-shaped negative electrode (electrode 10 in FIG. 1 ) having gaps at both ends in the width direction was produced.

[0075] The width of the cutout of both ends of the copper foil (the width of both end portions 112a in FIG. 4 ) was appropriately adjusted to set the protrusion length L of the end of the negative electrode composite layer from the end of the negative electrode current collector to the value shown in Table 1. The thickness TE of the end of the negative electrode current collector was approximately the same as the thickness TC of the portion inside the end of the negative electrode current collector, which was 8 μm. The ratio of the protrusion length L to the thickness TE of the end of the negative electrode current collector: L / TE, was the value shown in Table 1.

[0076] (Preparation of non-aqueous electrolyte) LiPF 6 was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1. ６ The non-aqueous electrolyte solution contained 3% by weight of vinylene carbonate.

[0077] (Battery Fabrication) One end of an aluminum positive electrode lead was attached to the exposed current collector portion (the portion where no composite layer was formed) of the obtained positive electrode. One end of a nickel negative electrode lead was attached to the exposed current collector portion (the portion where no composite layer was formed) of the obtained negative electrode. A wound electrode assembly was fabricated by winding the positive electrode and negative electrode with a separator interposed therebetween. A polyethylene microporous film was used as the separator. The electrode assembly was housed in a cylindrical battery case with a bottom that also served as a negative electrode terminal. An upper insulating plate and a lower insulating plate were placed on the top and bottom of the electrode assembly, respectively. Next, a nonaqueous electrolyte was injected into the battery case, and the opening of the battery case was closed by placing a metal sealing member, which also served as a positive electrode terminal, in the opening. A resin insulating gasket was interposed between the sealing member and the open end of the battery case. The other end of the positive electrode lead was connected to the sealing member, and the other end of the negative electrode lead was connected to the inner bottom surface of the battery case. In this manner, a cylindrical non-aqueous electrolyte secondary battery (diameter 18 mm, height 65 mm) was fabricated.

[0078] Secondary Battery B1: In the preparation of the negative electrode, the laminate was cut into strips by slitting, and then both ends of the copper foil in the width direction were not cut off. That is, no gaps were provided at both ends of the negative electrode in the width direction. Secondary Battery B1 was prepared in the same manner as Secondary Battery A1, except for the above.

[0079] [Evaluation] Each battery was evaluated as follows.

[0080] (Cycle Test) Each of the obtained batteries was subjected to 500 cycles of charge and discharge under the following conditions: The cycle test was carried out in an environment of 25° C. A 20-minute break was allowed between charge and discharge.

[0081] (Charging) The battery was charged at a constant current of 700 mA until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 35 mA.

[0082] (Discharge) A constant current discharge was carried out at a current of 700 mA until the voltage reached 3V.

[0083] The ratio of the discharge capacity at 500 cycles to the discharge capacity at 1 cycle was determined as the capacity retention rate at 500 cycles.

[0084] (Thickness change and expansion coefficient of negative electrode) Each battery obtained was charged under the above conditions to obtain a battery in an initial charged state. The cross section of the electrode group was observed using a CT image of the battery cross section, and the thickness of 10 arbitrary points on the initial negative electrode was measured. The average value was calculated to obtain the initial negative electrode thickness T0. Each battery after 500 cycles was charged under the above conditions to obtain a battery in a charged state after 500 cycles. Similarly, the negative electrode thickness T1 after 500 cycles was calculated. T1 - T0 was calculated as the thickness change. (T1 - T0) / T0 × 100 was calculated as the expansion coefficient. When the stress caused by the expansion of the electrode mixture layer is alleviated, the change in electrode thickness before and after the cycle test is small.

[0085] (Short-Circuit Defect Rate) 1,000 batteries were produced and the above cycle test was performed for 500 cycles. After 500 cycles, the batteries were inspected by X-ray to check for the occurrence of buckling of the negative electrode on the inner circumferential side of the electrode group. Batteries in which buckling of the negative electrode occurred were considered to have a short circuit. Based on the results of examining the occurrence of short circuits for the 1,000 batteries, the short-circuit defect rate (predicted value) was calculated using the process capability index.

[0086] The evaluation results are shown in Table 1. In Table 1, A1 to A3 are examples, and B1 is a comparative example.

[0087]

[0088] In the secondary batteries A1 to A3, the thickness change and expansion rate were smaller, the short circuit defect rate was smaller, and the capacity retention rate was significantly improved compared to the secondary battery B1.

[0089] The secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0090] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0091] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug, 10: electrode, 11: electrode current collector, 12a, 12b: electrode mixture layer, 13: gap, 14: coating layer

Claims

1. An electrode for a secondary battery comprising a sheet-shaped electrode collector, and a first electrode mixture layer and a second electrode mixture layer respectively supported on one main surface and the other main surface of the electrode collector, at least one end of the electrode has an end of the first electrode mixture layer and the second electrode mixture layer and an end of the electrode collector, and the ends of the first electrode mixture layer and the second electrode mixture layer each protrude beyond the end of the electrode collector.

2. The electrode for a secondary battery according to claim 1, wherein a gap is formed between an end of the first electrode mixture layer and an end of the second electrode mixture layer.

3. The electrode for a secondary battery according to claim 2, wherein the electrode is provided with a coating layer that covers an end of the electrode having the gap, the coating layer covering an opening of the gap.

4. An electrode for a secondary battery described in any one of claims 1 to 3, wherein the protruding length L of the ends of the first electrode mixture layer and the second electrode mixture layer from the end of the electrode collector is 20 μm or more.

5. The electrode for a secondary battery according to any one of claims 1 to 3, wherein TE is a thickness of an end of the electrode current collector, and TC is a thickness of a portion of the electrode current collector that is on the inside of the end, and TE is greater than TC.

6. A secondary battery comprising a pair of electrodes and an electrolyte, at least one of the pair of electrodes being an electrode for a secondary battery according to any one of claims 1 to 3.

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