Secondary battery electrode and secondary battery

Abstract

This secondary battery electrode comprises: a sheet-form electrode current collector; an electrode mixture layer supported on the main surface of the electrode current collector, the electrode mixture layer having an end surface ME that is continuous with an end surface CE of the electrode current collector; and a protective layer partially covering the end surface CE of the electrode current collector from the boundary between the electrode current collector and the electrode mixture layer in the thickness direction 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 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.

[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] There is a demand for suppressing the deterioration of the cycle characteristics of secondary batteries.

[0006] One aspect of the present disclosure relates to an electrode for a secondary battery, comprising: a sheet-like electrode current collector; an electrode composite layer supported on a main surface of the electrode current collector and having an end face ME continuous with an end face CE of the electrode current collector; and a protective layer that partially covers, in the thickness direction of the electrode current collector, from a boundary between the electrode current collector and the electrode composite layer to the end face CE 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 schematically showing a main part of an example of an electrode for a secondary battery according to an embodiment of the present disclosure; FIG. 3 is a cross-sectional view schematically showing a main part of another example of an electrode for a secondary battery according to an embodiment of the present disclosure; FIG. 4 is a cross-sectional view schematically showing a main part of yet another example of an electrode for a secondary battery according to an embodiment of the present disclosure; 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] A secondary battery electrode according to an embodiment of the present disclosure includes a sheet-shaped electrode current collector; an electrode mixture layer supported on a main surface of the electrode current collector and having an end surface ME continuous with the end surface CE of the electrode current collector; and a protective layer partially covering the end surface CE of the electrode current collector from the boundary between the electrode current collector and the electrode mixture layer in the thickness direction of the electrode current collector. The electrode mixture layer includes an electrode active material that absorbs and desorbs lithium ions. The electrode mixture layer expands as the lithium ions are absorbed. The electrode mixture layer may be supported on one main surface of the electrode current collector or on both main surfaces of the electrode current collector. When the electrode mixture layer is supported on both main surfaces of the electrode current collector, the electrode mixture layer supported on one main surface of the electrode current collector is also referred to as a first electrode mixture layer, and the electrode mixture layer supported on the other main surface of the electrode current collector is also referred to as a second electrode mixture layer. The electrode may have a current collector exposed portion where no electrode mixture layer is formed on either side of the electrode current collector. Note that the end face of the current collector exposed portion is not included in the end face CE of the electrode current collector.

[0013] The end faces of the electrode current collectors can be protected by covering them with a protective layer. For example, by covering the entire end faces of the electrode current collectors with a protective layer, the occurrence of internal short circuits when external force is applied to the battery can be suppressed. In addition, in the case of negative electrodes, lithium dendrites are likely to precipitate on the end faces of the negative electrode current collectors exposed at the negative electrode end face during charge and discharge, and internal short circuits may occur due to the dendrite precipitation. By covering the entire end faces of the negative electrode current collectors with a protective layer, the precipitation of dendrites can be suppressed. In the case of wound electrode groups, the protective effect of the protective layer can be efficiently obtained by covering the entire end faces of both ends in the width direction of the strip electrodes included in the electrode group with a protective layer. Note that wound electrode groups are constructed by winding a pair of electrodes with a separator interposed therebetween.

[0014] However, when the protective layer covers the entire end face CE of the electrode current collector in the thickness direction of the electrode current collector (when the TD coverage rate described below is 100%), the electrode mixture layer may expand during charge and discharge, causing the electrode mixture layer to come into strong contact with the protective layer, which may generate stress at the electrode end, resulting in cracks at the electrode end, causing the electrode mixture layer and / or the protective layer to fall off, and deteriorating the cycle characteristics.

[0015] In contrast, in the present disclosure, the protective layer partially covers the boundary between the electrode current collector and the electrode mixture layer and the end face CE of the electrode current collector in the thickness direction of the electrode current collector. In this case, even if stress occurs at the electrode end due to expansion of the electrode mixture layer, the stress can be alleviated by diverting the stress to the portion of the end face CE of the electrode current collector that is not covered by the protective layer. As a result, the end face of the electrode current collector is protected by the protective layer, and it is possible to suppress peeling of the electrode mixture layer and the protective layer due to stress occurring at the electrode end and the resulting deterioration of cycle characteristics.

[0016] From the viewpoint of suppressing deterioration of cycle characteristics, the coverage rate of the end surface CE of the electrode current collector by the protective layer in the thickness direction (TD direction) of the electrode current collector (hereinafter also referred to as "TD coverage rate") is preferably 95% or less, more preferably 90% or less, and even more preferably 85% or less. From the viewpoint of protecting the end surface of the electrode current collector, the TD coverage rate of the end surface CE of the electrode current collector by the protective layer may be 5% or more, 20% or more, or 50% or more. In the case of a negative electrode, the TD coverage rate of the end surface CE of the electrode current collector by the protective layer may be 50% or more, 60% or more, or even 80% or more. The TD coverage rate of the end surface CE of the electrode current collector by the protective layer may be, for example, in the range of 5% or more and 95% or less, or in the range of 20% or more and 90% or less.

[0017] The coverage rate (TD coverage rate) of the end surface CE of the electrode current collector in the thickness direction (TD direction) of the electrode current collector by the protective layer can be determined by the following method. The secondary battery is disassembled, the electrode is removed, and a cross-sectional sample in the thickness direction of the electrode is obtained. The cross-section may be formed by embedding the electrode in resin and polishing it to obtain a surface at the end of the electrode perpendicular to the main surface of the current collector. The cross-section may also be formed by a cross-section polisher (CP) method. Next, an image of the cross-sectional sample is obtained using a scanning electron microscope (SEM). The length CL of the end surface CE of the electrode current collector (thickness of the electrode current collector) is determined using the SEM image of the cross-sectional sample. The length PL of the portion of the end surface CE of the electrode current collector covered by the protective layer is determined. The TD coverage rate is calculated as PL / CL x 100. Several cross-sectional samples (e.g., 3 to 7) are prepared, the TD coverage rate is determined for each, and the average value is calculated.

[0018] From the viewpoint of protecting the electrode current collector, the average thickness of the protective layer is preferably 1 μm or more, and may be 1 μm or more and 100 μm or less. The average thickness of the protective layer may be, for example, 0.06 to 6.6 times the thickness of the electrode current collector. The average thickness of the protective layer is the average value of the maximum thicknesses of the protective layer in the cross section in the thickness direction of the electrode (e.g., T in Figures 2 to 5). The average thickness of the protective layer can be determined by measuring the maximum thickness of the protective layer in the cross section in the thickness direction of the electrode using an SEM image of the cross-sectional sample, determining the maximum thickness for each of several (e.g., 3 to 7) of the cross-sectional samples, and calculating the average value thereof.

[0019] The protective layer preferably partially covers the end face CE of the electrode current collector and at least a portion of the end face ME of the electrode mixture layer. In this case, the electrode mixture layer and the protective layer are likely to come into strong contact as the electrode mixture layer expands, so the stress relief effect due to the partial coverage of the protective layer on the end face CE is significantly achieved. In this case, the end face ME of the electrode mixture layer that is continuous with the end face CE of the electrode current collector can also be protected. By covering the boundary between the end face CE and the end face ME with the protective layer, peeling of the end of the electrode mixture layer during expansion and contraction can be suppressed. The end face CE of the electrode current collector can be easily partially covered with the protective layer.

[0020] When electrode mixture layers are supported on both main surfaces of the electrode current collector, each of the electrode mixture layers (first electrode mixture layer and second electrode mixture layer) formed on both main surfaces of the electrode current collector has an end face ME that is continuous with the end face CE. The protective layer may cover the end faces ME of both the first electrode mixture layer and the second electrode mixture layer, or may cover the end face ME of either the first electrode mixture layer or the second electrode mixture layer.

[0021] When the electrode is viewed from the normal direction of the main surface, a rectangular electrode has four sides. In this case, the protective layer may be formed, for example, on an end surface corresponding to at least one of the four sides, on two end surfaces corresponding to any two of the four sides, or on each of the four end surfaces corresponding to the four sides. For example, in the case of the strip-shaped electrode of FIG. 1 , the protective layer is formed on the end surfaces of the ends ES1 to ES2 (or ends ES1 to ES4). The length of the portion of the end surface (end surface CE) corresponding to any one of the sides that is covered with the protective layer may be 0.8 times or more the length L of that side, or may be approximately 1 time the length L of that side.

[0022] The protective layer may be an electrode mixture layer or may contain the same components as the electrode mixture layer. This is advantageous in terms of increasing capacity. In this case, the protective layer may also expand, but the stress caused by the expansion of the protective layer can be released to the portion of the end surface CE not covered by the protective layer, thereby alleviating the stress. The protective layer may also be an insulating layer. The insulating layer includes a resin material. Examples of resin materials include fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene.

[0023] The electrode can be produced, for example, by applying an electrode mixture slurry to the main surface of a current collector sheet (e.g., a metal foil or alloy foil), drying the slurry to form a coating film, and optionally rolling the coating film to form a laminate of the current collector sheet and the electrode mixture layer, which can then be cut to a predetermined size. The laminate may be cut using a predetermined blade (e.g., a slit blade, etc.). The electrode mixture layer may be formed on one or both main surfaces of the current collector sheet.

[0024] The cutting of the laminate may be performed by laser processing, plasma processing, or gas cutting. In this case, a protective layer may be formed on the cut surface by melting and solidifying the cut portion of the laminate. By appropriately adjusting the cutting conditions, the protective layer can be formed during the cutting process. In this case, the protective layer contains the same components as the electrode mixture layer.

[0025] The method for forming the protective layer is not particularly limited. In addition to the above methods, a protective layer may be formed by applying a predetermined treatment liquid to a predetermined region of the electrode end surface and drying the liquid. A predetermined region of the electrode end surface may be immersed in the treatment liquid and then dried to form the protective 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 or drying using a drying oven may be used. Examples of electrode mixtures include the positive electrode mixture or negative electrode mixture described below. Examples of insulating materials include fluororesins (e.g., polyvinylidene fluoride, polytetrafluoroethylene, etc.).

[0026] FIG. 1 is a top view schematically illustrating an example of a secondary battery electrode 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 (the winding direction when used in a wound electrode group), and WD indicates the width direction of the strip-shaped electrode 10. The shaded area in FIG. 1 is a protective layer 13 that protects the end of the electrode 10. FIG. 2 is a cross-sectional view schematically illustrating a main portion of an example of a secondary battery electrode 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 (electrode 10), T indicates the thickness (maximum thickness) of the protective layer 13, and L1 indicates the length of the protective layer 13 in the TD direction.

[0027] The strip-shaped electrode 10 has a sheet-shaped electrode current collector 11 and electrode mixture layers 12a, 12b supported 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). The end faces of the ends ES1 to ES4 each include an end face 11E (end face CE) of the electrode current collector 11 and end faces 12AE, 12BE (end faces ME) of the electrode mixture layers 12a, 12b that are continuous with the end face 11E.

[0028] The electrode 10 has a protective layer 13 formed on each of the end surfaces of the ends ES1 and ES2. The protective layer 13 formed on the end surface of the end ES1 will be described in detail below, but the protective layer 13 formed on the end surface of the end ES2 is also formed in the same manner as the protective layer 13 on the end ES1.

[0029] 2 , the electrode 10 includes a protective layer 13 that partially covers, in the thickness direction TD of the electrode current collector 11, from the boundary between the electrode current collector 11 and the electrode mixture layer 12a to an end face 11E of the electrode current collector 11. The protective layer 13 partially covers the end face 11E of the electrode current collector 11, and also covers at least a part of the end face 12AE of the electrode mixture layer 12a, but does not cover the end face 12BE of the electrode mixture layer 12b.

[0030] Even if stress occurs at the electrode end due to the expanded electrode mixture layer 12a coming into strong contact with the protective layer 13, the stress can be alleviated by dissipating it to the portion of the end face 11E that is not covered by the protective layer 13. The end face 12BE of the electrode mixture layer 12b and a portion of the end face CE of the electrode current collector 11 on the end face 12BE side are not covered by the protective layer 13. Therefore, even when the electrode mixture layer 12b expands, the electrode mixture layer 12b is unlikely to come into contact with the protective layer 13. Even if the expanded electrode mixture layer 12b comes into contact with the protective layer 13 when the coverage rate of the end face 11E in the TD direction is high, the contact is weak, and therefore the resulting stress at the electrode end is small. From the above, the protective layer 13 protects the ends ES1 and ES2 of the electrode 10 (end surface 11E of the electrode current collector 11), and the partial coverage of the protective layer 13 can alleviate the stress that occurs at the electrode ends when the electrode composite layer expands, thereby preventing the electrode composite layer (or protective layer) from falling off due to this stress.

[0031] Although end face 12BE is not covered with protective layer 13, a portion of end face 11E and at least a portion (preferably the entirety) of end face 12AE are covered with protective layer 13, thereby adequately protecting ends ES1 and ES2 of positive electrode 10. The coverage of end face 12AE of electrode mixture layer 12a by the protective layer in the thickness direction (TD direction) of electrode 10 (hereinafter also referred to as "TD coverage") may be 70% or more, or may be 90% or more. From the viewpoint of protecting ends ES1 and ES2 of electrode 10, a high TD coverage of end face 12AE is preferable.

[0032] The protective layer 13 disposed on the ends ES1 and ES2 of the electrode 10 shown in FIG. 1 is formed continuously and entirely in the LD direction, but may also be formed intermittently or partially. When the electrode 10 is viewed from the normal direction of its main surface, the ratio of the LD-direction length of the protective layer 13 covering the current collector end surface CE of the end ES1 (end ES2) to the LD-direction length of the electrode (hereinafter referred to as the "LD coverage") is approximately 100%, but is not limited to this. The LD coverage may be, for example, in the range of 50% to 100%, or in the range of 70% to 100%.

[0033] Although the thickness of the protective layer 13 in FIG. 2 is substantially constant, the protective layer is not limited to the protective layer 13 in FIG. 2 . The protective layer may be the protective layer 13 shown in FIGS. 3 to 5 . Note that the protective layer is not limited to the protective layer 13 shown in FIGS. 2 to 5 . The protective layer 13 in FIG. 3 has a smaller thickness at the end on the electrode current collector 11 side than at the end on the electrode mixture layer 12a side. The protective layer 13 in FIG. 3 makes it easier to ensure a large thickness of the protective layer covering the electrode current collector 11, thereby reducing the risk of the electrode current collector being exposed. The protective layer 13 in FIG. 4 has a larger thickness at the end on the electrode current collector 11 side than at the end on the electrode mixture layer 12a side. The protective layer 13 in FIG. 4 makes it easier to improve the ability to follow electrode deformation and to prevent the protective layer from falling off the electrode current collector during electrode deformation. The protective layer 13 in FIG. 5 has a larger thickness at the center of the electrode current collector 11 in the thickness direction (TD) than at both ends. In the case of the protective layer 13 shown in FIG. 5, the protective layer has high shape stability, and therefore the protective layer is unlikely to fall off.

[0034] 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 protective layer may also be formed on the ends ES3 and ES4 of the electrode 10, as with the ends ES1 and ES2.

[0035] The electrode 10 includes the protective layer 13, and may also include a second protective layer that partially covers from the boundary between the electrode current collector 11 and the electrode mixture layer 12b to the end face 11E of the electrode current collector 11 in the thickness direction (TD direction) of the electrode current collector 11. The second protective layer may partially cover the end face 11E of the electrode current collector 11 and also cover at least a part of the end face 12BE of the electrode mixture layer 12b.

[0036] The thickness of the electrode 10 is, for example, 100 to 300 μm. The thickness of the electrode current collector 11 is, for example, 5 to 30 μm. The average length of the protective layer 13 in the thickness direction (TD direction) of the electrode current collector 11 may be 0.3 to 40 times the thickness of the electrode current collector 11. The average length of the protective layer 13 in the thickness direction (TD direction) of the electrode current collector 11 may be 10 μm or more and 200 μm or less. The average length of the protective layer in the TD direction can be determined by observing the protective layer covering the electrode end face using an SEM or the like, measuring the TD length (L1 in FIGS. 2 to 5) of multiple arbitrary points (10 to 20 points) of the protective layer, and calculating the average value. Furthermore, the average TD length of the portion of the protective layer 13 covering the end face 12AE of the electrode mixture layer 12a may be 0.8 to 1 times the TD length of the end face 12AE.

[0037] 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 positive electrode and the negative electrode are, for example, wound or stacked with a separator interposed therebetween.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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 Oｃ , Li ａ Mn ２ O ４ , LiaMn ２－ｂ M ｂ O ４ , LiMPO ４ , Li ２ MPO 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.

[0043] 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).

[0044] 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.

[0045] 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.

[0046] 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.

[0047] [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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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).

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] [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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] [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.

[0069] 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. 6. Fig. 6 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away.

[0070] 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.

[0071] 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.

[0072] <<Supplementary Notes>> The above embodiments disclose the following techniques. (Technology 1) A secondary battery electrode comprising: a sheet-like electrode current collector; an electrode mixture layer supported on a main surface of the electrode current collector and having an end face ME continuous with the end face CE of the electrode current collector; and a protective layer that partially covers, in the thickness direction of the electrode current collector, from the boundary between the electrode current collector and the electrode mixture layer to the end face CE of the electrode current collector. (Technology 2) The secondary battery electrode according to Technology 1, in which, in the thickness direction of the electrode current collector, a coverage rate of the end face CE by the protective layer is 95% or less. (Technology 3) The secondary battery electrode according to Technology 1 or 2, in which the protective layer has an average thickness of 1 μm or more. (Technology 4) The secondary battery electrode according to any one of Technology 1 to 3, in which the protective layer partially covers the end face CE and at least a portion of the end face ME. (Technology 5) The secondary battery electrode according to Technology 4, wherein the protective layer is thicker at an end on the electrode current collector side than at an end on the electrode mixture layer side. (Technology 6) The secondary battery electrode according to Technology 4, wherein the protective layer is thinner at an end on the electrode current collector side than at an end on the electrode mixture layer side. (Technology 7) The secondary battery electrode according to Technology 4, wherein the protective layer is thicker at a center portion in the thickness direction of the electrode current collector than at both ends thereof. (Technology 8) The secondary battery electrode according to any one of Technology 4 to 7, wherein the average length of the protective layer in the thickness direction of the electrode current collector is 10 μm or more and 200 μm or less. (Technology 9) The secondary battery electrode according to any one of Technology 1 to 8, wherein the protective layer contains the same components as the electrode mixture layer. (Technology 10) A secondary battery comprising a pair of electrodes and an electrolyte, wherein at least one of the pair of electrodes is the secondary battery electrode according to any one of Technology 1 to 9.

[0073] Hereinafter, the present disclosure will be specifically described based on examples, but the present disclosure is not limited to the following examples.

[0074] Secondary Batteries A1 to A3, B1 (Preparation of Positive Electrode) A rock-salt lithium-containing transition metal oxide (NCA: positive electrode active material) having a layered structure containing Li, Ni, Co, and Al (the molar ratio of Li to the total of Ni, Co, and Al was 1.0) was prepared. This lithium-containing transition metal oxide (NCA), acetylene black (AB: conductive material), and polyvinylidene fluoride (PVdF: binder) were mixed in a mass ratio of NCA:AB:PVdF = 95:2.5:2.5, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and stirred to prepare a positive electrode mixture slurry.

[0075] The positive electrode composite slurry was applied to both sides of a strip-shaped Al foil (positive electrode current collector) and then dried to form a coating film of the positive electrode composite. Next, the coating film was rolled using a roller to form a positive electrode composite layer. The resulting laminate of the positive electrode current collector and the positive electrode composite layer was cut into a strip of a predetermined size using a cutting blade. In this way, a positive electrode was obtained.

[0076] (Formation of Protective Layer) Next, a positive electrode composite slurry was applied to both end surfaces ES1 and ES2 in the width direction of the strip-shaped positive electrode, followed by drying to form a protective layer containing the same components as the positive electrode composite layer. In this manner, a positive electrode having a protective layer was obtained. The area of ​​the positive electrode composite slurry applied to the end surfaces (end surface 11E of the positive electrode current collector 11 and end surface 12AE of the positive electrode composite layer 12a in FIG. 2) was appropriately adjusted so that the protective layer 13 shown in FIGS. 1 and 2 was formed. The TD coverage of the protective layer on the current collector end surface CE at ends ES1 and ES2 was set to the values ​​shown in Table 1. The LD coverage of the protective layer on the current collector end surface CE at ends ES1 and ES2 was set to approximately 100%. The TD coverage of the protective layer on the composite layer end surface 12AE at ends ES1 and ES2 was set to approximately 100%. The LD coverage of the composite material layer end surface 12AE of the ends ES1 and ES2 by the protective layer was set to approximately 100%. The composite material layer end surface 12BE of the ends ES1 and ES2 was not covered with the protective layer.

[0077] The thickness of the positive electrode mixture layer per side was 80 μm. The thickness of the positive electrode current collector was 15 μm. The average thickness of the protective layer (average of T in FIG. 2 ) was 7.5 μm, which was 0.5 times the thickness of the positive electrode current collector. In secondary batteries A1 to A3, in which the TD coverage was 5 to 95%, the average length of the protective layer in the thickness direction of the positive electrode current collector (average of L1 in FIG. 2 ) was in the range of 1 to 150 μm, which was in the range of 0.06 to 10 times the thickness of the positive electrode current collector.

[0078] (Preparation of Negative Electrode) 100 parts by mass of artificial graphite, 1 part by mass of styrene-butadiene copolymer rubber (SBR), 1 part by mass of carboxymethyl cellulose (CMC), and an appropriate amount of water were kneaded to prepare a negative electrode mixture slurry.

[0079] The negative electrode composite slurry was applied to both sides of a strip-shaped Cu foil (negative electrode current collector) and then dried to form a coating film of the negative electrode composite. Next, the coating film was rolled using a roller to form a negative electrode composite layer. Finally, the resulting laminate of the negative electrode current collector and the negative electrode composite layer was cut to a predetermined size to obtain a negative electrode.

[0080] (Preparation of non-aqueous electrolyte) Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:DMC=30:70. LiPF ６ The concentration of LiBF becomes 1 mol / L. ２ (C ２ O ４ ) were dissolved in water so that the concentration of the non-aqueous electrolyte was 0.1 mol / L to prepare a liquid non-aqueous electrolyte.

[0081] (Battery Fabrication) One end of an aluminum positive electrode lead was attached to the positive electrode obtained above. One end of a nickel negative electrode lead was attached to the negative electrode obtained above. A wound electrode assembly was fabricated by winding the positive and negative electrodes with a polyethylene separator interposed therebetween. The electrode assembly was housed in a cylindrical battery case with a bottom that also served as the 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 poured into the battery case, and the opening of the battery case was closed by placing a metal sealing member that also served as the 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 nonaqueous electrolyte secondary battery (diameter 18 mm, height 65 mm) was fabricated. Note that A1 to A3 in Table 1 are examples, and B1 is a comparative example.

[0082] [Evaluation] Each battery was evaluated as follows.

[0083] (Crushing Test) Each battery was charged at a constant current of 0.5 It (1675 mA) until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 67 mA. Each charged battery was subjected to a crushing test at 25°C.

[0084] Specifically, the battery was placed horizontally between two flat surfaces. Next, a force was applied to the battery from the two surfaces until it reached 15 kN. After the battery was crushed, the battery was checked for heat generation. Five batteries were prepared, and the number of batteries that generated heat was counted.

[0085] (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.

[0086] (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.

[0087] (Discharge) A constant current discharge was carried out at a current of 700 mA until the voltage reached 3V.

[0088] 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.

[0089] The evaluation results are shown in Table 1.

[0090]

[0091] In the secondary batteries A1 to A3 and B1 in which a protective layer was formed on the positive electrode end surface, none of the secondary batteries generated heat during the crush test. The secondary batteries A1 to A3, in which the TD coverage was 95% or less, showed a significantly improved capacity retention rate compared to the secondary battery B1, in which the TD coverage was 100%.

[0092] In the above examples, protective layers were formed on the positive electrode ends ES1 and ES2, but protective layers may also be formed on the positive electrode ends ES3 and ES4 as well as the positive electrode ends ES1 and ES2. In secondary batteries in which the TD coverage rates of the ends ES1 to ES4 were set to 5%, 50%, and 95%, respectively, the capacity retention rate was significantly improved, as in the case of secondary batteries A1 to A3.

[0093] 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.

[0094] 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.

[0095] 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: protective layer

Claims

1. An electrode for a secondary battery comprising: a sheet-like electrode collector; an electrode mixture layer supported on a main surface of the electrode collector and having an end face ME continuous with an end face CE of the electrode collector; and a protective layer partially covering, in the thickness direction of the electrode collector, from the boundary between the electrode collector and the electrode mixture layer to the end face CE of the electrode collector.

2. The electrode for a secondary battery as described in claim 1, wherein the coverage of the end face CE by the protective layer in the thickness direction of the electrode collector is 95% or less.

3. The secondary battery electrode according to claim 1, wherein the protective layer has an average thickness of 1 μm or more.

4. The electrode for a secondary battery as described in claim 1, wherein the protective layer partially covers the end face CE and at least a portion of the end face ME.

5. The secondary battery electrode according to claim 4, wherein the protective layer has a greater thickness at the end on the electrode current collector side than at the end on the electrode mixture layer side.

6. The secondary battery electrode according to claim 4, wherein the protective layer has a smaller thickness at the end on the electrode current collector side than at the end on the electrode mixture layer side.

7. The secondary battery electrode according to claim 4, wherein the protective layer is thicker at the center of the electrode current collector than at both ends in the thickness direction of the electrode current collector.

8. The secondary battery electrode according to any one of claims 4 to 7, wherein the average length of the protective layer in the thickness direction of the electrode current collector is 10 µm or more and 200 µm or less.

9. The secondary battery electrode according to any one of claims 1 to 7, wherein the protective layer contains the same components as the electrode mixture layer.

10. 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 7.

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