Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

This positive electrode for nonaqueous electrolyte secondary batteries is provided with: a positive electrode collector sheet; and a positive electrode mixture layer that is supported by the positive electrode collector sheet. The positive electrode mixture layer contains a positive electrode active material, a binder and a conductive agent. The positive electrode active material has a layered rock salt-type crystal structure, and contains a composite oxide which contains lithium and element A that is other than lithium. The element A contains at least nickel; and the atomic ratio of nickel to the element A, namely Ni / A is from 0.8 to 1.0. The binder contains a polymer binder that has a three-dimensional network structure. The mass of the positive electrode mixture layer supported per 1 m2 of the positive electrode collector sheet is 280 g or more.

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Nonaqueous electrolyte secondary batteries, typified by lithium-ion secondary batteries, are used as power sources for electronic devices such as mobile terminals and as power sources for vehicles such as electric vehicles. A positive electrode can be obtained, for example, by applying a positive electrode slurry containing a positive electrode mixture such as a positive electrode active material to the surface of a positive electrode current collector sheet, drying the coating, and rolling it to form a positive electrode mixture layer. The positive electrode active material can be, for example, a composite oxide containing lithium and nickel and having a layered rock salt crystal structure.

[0003] To increase the capacity, it is effective to increase the amount of positive electrode slurry applied to the positive electrode current collector sheet and increase the thickness of the positive electrode mixture layer. When a positive electrode and a negative electrode are wound together with a separator interposed therebetween to form an electrode assembly, increasing the thickness of the positive electrode mixture layer reduces the number of times the positive electrode is wound, and the separators and positive electrode current collector sheets that do not contribute to the capacity can be reduced, thereby increasing the amount of active material filled (for example, Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Topics, "Introduction to ABRI's Activities," FB Technical News, No. 74 (November 2018), page 34 Summary of the Invention

[0005] In non-aqueous electrolyte secondary batteries, when the amount of the positive electrode mixture layer carried on the positive electrode current collector sheet increases significantly, the direct current resistance (DCR) increases significantly. This increase in DCR is thought to be caused by an increase in the internal resistance of the positive electrode mixture layer. When the amount of the positive electrode mixture layer carried on the positive electrode current collector sheet increases significantly, the thickness of the positive electrode mixture layer increases accordingly, the electron conduction path becomes longer, and the current collection performance is thought to decrease.

[0006] In view of the above, one aspect of the present disclosure provides a positive electrode current collecting sheet, and a positive electrode mixture layer supported on the positive electrode current collecting sheet, the positive electrode mixture layer including a positive electrode active material, a binder, and a conductive agent, the positive electrode active material having a layered rock salt crystal structure and including a composite oxide including lithium and an element A other than the lithium, the element A including at least nickel, an atomic ratio of the nickel to the element A: Ni / A being 0.8 or more and 1.0 or less, the binder including a polymer binder having a three-dimensional network structure, 2 The positive electrode for a non-aqueous electrolyte secondary battery according to the present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery, wherein the mass of the positive electrode mixture layer carried per positive electrode is 280 g or more.

[0007] Another aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode is the above-described positive electrode.

[0008] According to the present disclosure, it is possible to suppress an increase in DCR while increasing the capacity of a nonaqueous electrolyte secondary battery. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic perspective view, with a portion cut away, of a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Positive electrode for non-aqueous electrolyte secondary batteries] A positive electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a positive electrode current collector sheet and a positive electrode mixture layer supported on the positive electrode current collector sheet. The positive electrode mixture layer includes a positive electrode active material, a binder, and a conductive agent. The positive electrode active material includes a nickel-based composite oxide. That is, the composite oxide has a layered rock salt crystal structure and includes lithium (Li) and an element A other than Li, the element A including at least nickel (Ni), and the atomic ratio of Ni to element A: Ni / A is 0.8 or more and 1.0 or less. The binder includes a polymer binder (hereinafter also referred to as binder P) having a three-dimensional network structure. The positive electrode mixture layer is thick. That is, the positive electrode mixture layer is thicker than the positive electrode current collector sheet by 1 m. 2 The mass of the positive electrode mixture layer carried per one carrier is 280 g or more.

[0011] The positive electrode active material is a nickel-based composite oxide with an Ni / A ratio of 0.8 or more, and the positive electrode current collector sheet is 1 m 2 By increasing the thickness of the positive electrode mixture layer by setting the amount of the positive electrode mixture layer carried per unit area to 280 g or more, the rated capacity of the battery can be increased to a level that was previously unattainable.

[0012] Furthermore, by incorporating a polymer binder with a three-dimensional network structure into a thick positive electrode mixture layer, the distribution of the conductive agent coexisting with the binder is improved, leading to the development of a highly three-dimensional electron conduction path within the positive electrode mixture layer and a reduced internal resistance of the positive electrode mixture layer. The improved distribution of the conductive agent is believed to be due to the polymer binder with a three-dimensional network structure providing excellent adhesion between the composite oxide particles and the conductive agent in the positive electrode mixture layer. Furthermore, improved adhesion between the positive electrode mixture layer and the positive electrode current collector sheet also improves resistance to stress associated with the expansion and contraction of the positive electrode active material during charge and discharge. Therefore, even when a thick positive electrode mixture layer is formed, the decrease in adhesion between the positive electrode mixture layer and the positive electrode current collector sheet is suppressed. As a result, the increase in the battery's DCR is suppressed, improving cycle characteristics, etc. The effect of suppressing the increase in DCR when the loading of the positive electrode mixture layer is large is specifically achieved when the polymer binder has a three-dimensional network structure. For example, when a linear polymer binder (e.g., polyvinylidene fluoride) is used, the suppression of the increase in DCR is insufficient.

[0013] The binder P can be formed, for example, by crosslinking a polymer that functions as a binder. The crosslinking can be formed by known methods such as adding a crosslinking agent, heating, or irradiating with ultraviolet light or an electron beam. Among these, from the viewpoint of electrochemical stability at the positive electrode potential, it is preferable that the binder P contains a fluorine-containing polymer, and that the fluorine-containing polymer is crosslinked. In other words, it is preferable that a three-dimensional network structure is formed by crosslinking a fluorine-containing polymer having binding power.

[0014] The fluorine-containing polymer may contain at least one selected from the group consisting of units derived from vinylidene fluoride (VDF), units derived from hexafluoropropylene (HFP), and units derived from tetrafluoroethylene (TFE). In this case, the fluorine-containing polymer itself has excellent binding properties. In particular, from the viewpoint of electrochemical stability and the stability of the positive electrode slurry, it is preferable that the fluorine-containing polymer contains at least units derived from VDF. It is preferable that the fluorine-containing polymer contains at least one selected from the group consisting of polyvinylidene fluoride (PVDF) and copolymers containing units derived from vinylidene fluoride (VDF). The copolymer may be a block copolymer or a random copolymer.

[0015] The copolymer containing units derived from VDF may include a copolymer of VDF and a fluorine-containing monomer other than VDF (hereinafter referred to as monomer F). That is, the copolymer may contain units derived from VDF and units derived from monomer F. Monomer F may contain at least one selected from the group consisting of HFP, TFE, trifluoroethylene, and trifluorochloroethylene. Among these, from the viewpoint of ensuring the flexibility of the positive electrode plate, monomer F is preferably HFP. In the copolymer, the molar ratio of units derived from monomer F to units derived from VDF (monomer F / VDF) may be, for example, 0.01 or more and 0.5 or less, or 0.05 or more and 0.3 or less.

[0016] The fluorine-containing polymer may be crosslinked with a crosslinkable monomer (crosslinking agent). For example, the fluorine-containing polymer may undergo a dehydration condensation reaction with the crosslinkable monomer to form an amide bond or an ester bond, thereby crosslinking the fluorine-containing polymers via the crosslinkable monomer. The crosslinkable monomer may have a functional group (e.g., a hydroxy group, a carboxy group, an amino group, etc.) that contributes to the condensation reaction. Specific examples of the crosslinkable monomer include trimethylhexamethylenediamine, benzoyl peroxide, dicumyl peroxide, bisphenol A, hexamethylenediamine, ethylenediamine, isopropylethylenediamine, naphthalenediamine, 2,4,4-trimethyl-1- or 6-hexanediamine, etc. The fluorine-containing polymer may have a functional group (e.g., a hydroxy group, a carboxy group, an amino group, etc.) that contributes to the dehydration condensation reaction with the crosslinkable monomer, and the functional group may be introduced into the fluorine-containing polymer. For example, a fluorine-containing polymer having a carboxyl group introduced therein may be subjected to a dehydration condensation reaction with a crosslinkable monomer having two amino groups, and the fluorine-containing polymers may be crosslinked via an amide bond via the crosslinkable monomer.

[0017] The average molecular weight of the fluorine-containing polymer (e.g., PVDF, PVDF-HFP) used together with the above-mentioned crosslinking agent is, for example, 100,000 or more and 2,000,000 or less. The above-mentioned average molecular weight is a number average molecular weight (polystyrene equivalent value) determined by gel permeation chromatography (GPC).

[0018] The binder contains at least binder P. The proportion of binder P in the entire binder is, for example, 50 mass % or more, and the entire binder may be binder P. The binder may contain a small amount of other components besides binder P. As the other components besides binder P, resin materials without a three-dimensional network structure (e.g., fluororesin, polyolefin resin, acrylic resin, etc.) can be used. Specific examples include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polyacrylic acid, polymethyl acrylate, ethylene-acrylic acid copolymer, etc. The binder P and other components besides binder P may be used alone or in combination of two or more.

[0019] The content of the binder in the positive electrode mixture layer is preferably, for example, 0.5 parts by mass or more and 2 parts by mass or less per 100 parts by mass of the positive electrode active material. In this case, the capacity is increased while the effect of suppressing an increase in DCR when the loading in the positive electrode mixture layer is large is easily obtained. In addition, the binding strength of the positive electrode mixture layer and the adhesion strength between the positive electrode mixture layer and the positive electrode current collector sheet are also easily improved. In this case, the entire binder may be binder P.

[0020] In order to increase capacity and improve cycle characteristics, the positive electrode current collector sheet is 1m 2 The amount of the positive electrode material mixture layer carried per unit area may be 280 g or more and 400 g or less, or 280 g or more and 360 g or less. In this case, the thickness of the positive electrode material mixture layer is, for example, 50 μm or more and 250 μm or less.

[0021] The density of the positive electrode mixture layer is 3.45 g / cm 3 More than 3.75g / cm 3 It may be less than 3.5 g / cm 3 More than 3.75g / cm 3 The density of the positive electrode mixture layer may be 3.45 g / cm or less. 3In the above cases, the number of contact points between the composite oxide and the conductive agent and binder increases. Furthermore, contact points between the composite oxide particles are easily formed. This allows for sufficient formation of electron conduction paths, making it easier to obtain high capacity. Furthermore, the binding strength of the positive electrode mixture layer and the adhesion strength between the positive electrode mixture layer and the positive electrode current collector sheet are also easily improved. Furthermore, the above case is advantageous in terms of improving the energy density of cylindrical batteries and is effective when constructing a wound-type electrode group using longer electrode plates.

[0022] The density of the positive electrode mixture layer is 3.75 g / cm 3 When the thickness is less than 1 / 2 mm, the increase in strain in the positive electrode mixture layer due to the expansion and contraction of the composite oxide during charge and discharge is suppressed, the stress generated between the positive electrode mixture layer and the positive electrode current collector sheet is easily reduced, and the decrease in adhesion between the positive electrode mixture layer and the positive electrode current collector sheet is easily suppressed. Furthermore, the composite oxide in the positive electrode mixture layer is easily brought into contact with the non-aqueous electrolyte, and the increase in reaction resistance is suppressed. An appropriate amount of voids is secured in the positive electrode mixture layer, and the diffusion resistance to the non-aqueous electrolyte (migration of lithium ions) is sufficiently reduced even at high charge rates, making it easy to obtain a high discharge capacity.

[0023] The positive electrode active material contains at least the above-mentioned nickel-based composite oxide. When the atomic ratio Ni / A of the nickel-based composite oxide is 0.8 or more, the proportion of Ni in element A is large, and high capacity is easily achieved. The proportion of the nickel-based composite oxide in the entire positive electrode active material is, for example, 80 mass% or more, and the entire positive electrode active material may be a nickel-based composite oxide. The positive electrode active material may contain a small amount of a composite oxide other than the nickel-based composite oxide (e.g., LiCoO2, Li2NiO2, Li5FeO4, etc.).

[0024] The element A contains at least Ni, and may further contain at least one element selected from the group consisting of cobalt (Co), manganese (Mn), aluminum (Al), magnesium (Mg), calcium (Ca), iron (Fe), copper (Cu), zinc (Zn), chromium (Cr), titanium (Ti), niobium (Nb), zirconium (Zr), vanadium (V), tantalum (Ta), molybdenum (Mo), tungsten (W), strontium (Sr), silicon (Si), and boron (B).

[0025] Among these, element A preferably contains Ni and at least one selected from the group consisting of Co, Mn, and Al, and more preferably contains Ni, Co, and Mn and / or Al. When element A contains Co, the phase transition of the composite oxide containing Li and Ni during charge and discharge is suppressed, the stability of the crystal structure is improved, and the cycle characteristics are likely to be improved. When element A contains Mn and / or Al, the thermal stability is improved.

[0026] Nickel-based composite oxides have the general formula: Li a Ni x Co y M 1-x-y O2, where 0.97≦a≦1.2, 0.8≦x≦1.0, and 0≦y≦0.2, and M is preferably at least one selected from the group consisting of Mn, Al, B, W, Sr, Mg, Mo, Nb, Ti, Si, and Zr.

[0027] When a, which represents the composition ratio of Li, is 0.97 or more and 1.2 or less, cation mixing in which Ni ions enter the Li site is less likely to occur, and the output characteristics are likely to improve. When x, which represents the composition ratio of Ni, is 0.8 or more and 1 or less, the proportion of Ni in element A is large, and high capacity is likely to be achieved. y may be more than 0 and 0.2 or less. In this case, the nickel-based composite oxide contains Co, so the stability of the crystal structure is likely to improve, and the cycle characteristics are likely to improve. Element M may be Al, and 0 < y ≦ 0.2, 0 < (1 - x - y) ≦ 0.05 may hold. In this case, the nickel-based composite oxide contains Al, so the thermal stability of the composite oxide is likely to improve. Note that the value of a changes during charge and discharge.

[0028] The particles of the composite oxide usually contain secondary particles in which a plurality of primary particles are aggregated. The average particle size (D50) of the secondary particles is, for example, 5 μm or more and 20 μm or less. Here, the average particle size (D50) refers to the median diameter at which the volume integration value is 50% in the volume-based particle size distribution. The average particle size (D50) of the secondary particles is determined by measuring the particle size distribution by the laser diffraction method.

[0029] When the positive electrode mixture layer contains a conductive agent, a conductive path is sufficiently formed between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector sheet. The conductive agent preferably contains carbon nanotubes (CNT). CNT is likely to entangle with a three-dimensional network structure polymer binder, and the contact points between CNT and the composite oxide are firmly maintained by the polymer binder during charge and discharge.

[0030] The average length of the CNT is preferably 0.5 μm or more, more preferably 0.5 μm or more and 10.0 μm or less, and still more preferably 0.5 μm or more and 5.0 μm or less. In this case, CNT is more likely to entangle with a three-dimensional network structure polymer binder, and the contact points between CNT and the composite oxide are likely to be firmly maintained by the polymer binder during charge and discharge. Also, CNT is likely to be interposed between the composite oxide particles, and an electron conduction path between the composite oxide particles is likely to be sufficiently formed by CNT.

[0031] From the viewpoint of improving cycle characteristics, the average diameter of the CNTs may be 0.5 nm or more and 30 nm or less, or 0.5 nm or more and 20 nm or less. When the average diameter of the CNTs is 0.5 nm or more, the strength of the CNTs is sufficiently ensured, and the CNTs are likely to maintain an electron conduction path during charge and discharge. In addition, the CNTs are likely to be interposed between the composite oxide particles.

[0032] The average length and diameter of the CNTs are determined by obtaining an image of the cross section of the positive electrode mixture layer or the CNTs using a scanning electron microscope (SEM) or a transmission electron microscope (TEM), measuring the lengths and diameters of a number of randomly selected CNTs (e.g., about 50 to 200) using the image, and averaging the lengths and diameters. The length of the CNTs refers to the length when they are linear.

[0033] The conductive agent may contain a conductive material other than CNT. Examples of conductive materials other than CNT include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black; metal fiber; and metal powder such as aluminum. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0034] The content of the conductive agent in the positive electrode mixture layer is preferably, for example, 0.01 parts by mass or more and 1.0 parts by mass or less per 100 parts by mass of the positive electrode active material. In this case, the capacity is increased while the conductive agent easily forms a sufficient electron conduction path between the positive electrode active material particles. In this case, the conductive agent may be entirely CNT.

[0035] The positive electrode current collector sheet may be, for example, 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 sheet include stainless steel, aluminum, aluminum alloy, and titanium. The thickness of the positive electrode current collector sheet is, for example, 3 to 50 μm.

[0036] A method for producing a positive electrode includes, for example, a step of preparing a positive electrode slurry by dispersing a positive electrode mixture containing a positive electrode active material, a binder, and a conductive agent in a dispersion medium, and a step of applying the positive electrode slurry to the surface of a positive electrode current collector sheet and drying it to form a positive electrode mixture 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 sheet. Examples of the dispersion medium include water, alcohols such as ethanol, and N-methyl-2-pyrrolidone (NMP).

[0037] [Nonaqueous electrolyte secondary battery] One embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode is the above-described positive electrode for a non-aqueous electrolyte secondary battery.

[0038] The negative electrode includes, for example, a negative electrode current collector sheet and a negative electrode mixture layer supported on the negative electrode current collector sheet. The negative electrode can be obtained, for example, by applying a negative electrode slurry, in which a negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector sheet and drying the coating to form a negative electrode mixture layer. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one or both surfaces of the negative electrode current collector sheet. The negative electrode mixture contains a negative electrode active material as an essential component and may optionally contain a binder, a conductive agent, a thickener, etc. As the binder, components other than the binder P, such as rubber materials such as styrene-butadiene rubber, can be used. As the dispersion medium, those exemplified for the positive electrode can be used. As the conductive agent, those exemplified for the positive electrode can be used except for graphite. As the thickener, for example, carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salts) can be used.

[0039] Examples of the negative electrode active material include carbon materials, silicon, silicon-containing materials, and lithium alloys.

[0040] From the perspective of high capacity, the negative electrode active material preferably contains at least one Si-based active material of silicon and silicon-containing materials. As the silicon-containing material, for example, a composite material including a silicate phase containing at least one of an alkali metal element and a Group 2 element and silicon particles dispersed in the silicate phase can be used. In this case, the initial charge-discharge efficiency and cycle characteristics are improved. The composite material includes, for example, a lithium silicate phase and silicon particles dispersed in the lithium silicate phase. The lithium silicate phase can have a composition represented by, for example, Li 2u SiO u+2 (0 < u < 2). As the silicon-containing material, SiO z (0.5 ≦ z ≦ 1.5) including a SiO2 phase and silicon particles dispersed in the SiO2 phase may be used. The surface of the particles of the silicon-containing material may be coated with a conductive layer including a conductive material such as a carbon material.

[0041] Examples of the carbon material include graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), etc. Among them, graphite is preferable as it has excellent charge-discharge stability and little irreversible capacity. Graphite means a material having a graphite-type crystal structure and includes, for example, natural graphite, artificial graphite, graphitized mesophase carbon particles, etc. The carbon material may be used alone or in combination of two or more.

[0042] From the perspective of easily obtaining good cycle characteristics and high capacity in a well-balanced manner, it is preferable to use the Si-based active material and the carbon material in combination. From the perspective of high capacity, the proportion of the Si-based active material in the total of the Si-based active material and the carbon material is, for example, preferably 0.5 mass% or more, more preferably 1 mass% or more, and still more preferably 2 mass% or more. Also, from the perspective of improving cycle characteristics, the proportion of the Si-based active material in the total of the Si-based active material and the carbon material is, for example, preferably 30 mass% or less, more preferably 20 mass% or less, and still more preferably 15 mass% or less.

[0043] The negative electrode current collector sheet can be made of, for example, the conductive substrate exemplified for the positive electrode. Examples of materials for the negative electrode current collector sheet include stainless steel, nickel, nickel alloy, copper, copper alloy, etc. The thickness of the negative electrode current collector sheet is, for example, 1 to 50 μm.

[0044] The non-aqueous electrolyte includes a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent.

[0045] 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.

[0046] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10Examples of the lithium salt include lithium carboxylates, lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of the 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(FSO2)2), lithium bistrifluoromethanesulfonyl imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate nonafluorobutanesulfonyl imide (LiN(CF3SO2)(CF9SO2)), and lithium bispentafluoroethanesulfonyl imide (LiN(C2F5SO2)2). One lithium salt may be used alone, or two or more lithium salts may be used in combination. The concentration of the lithium salt in the nonaqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0047] It is usually 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.

[0048] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a nonaqueous electrolyte are housed in an outer casing. Alternatively, instead of a wound-type electrode group, other types of electrode groups may be used, such as a stacked-type electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The nonaqueous electrolyte secondary battery may be in any type, such as a cylindrical type, a prismatic type, a coin type, a button type, or a laminate type.

[0049] FIG. 1 is a schematic perspective view of a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure, with a portion cut away.

[0050] The battery includes a bottomed prismatic battery case 4, an electrode group 1, and a non-aqueous electrolyte 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 between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat plate-shaped winding core and then removing the winding core.

[0051] One end of a negative electrode lead 3 is attached to the negative electrode current collector sheet of the negative electrode 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. 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 sheet of the positive electrode 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. In other words, 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. The injection hole for the non-aqueous electrolyte provided in the sealing plate 5 is closed by a sealing plug 8 .

[0052] [Example] The present disclosure will be specifically described below based on examples, but the present invention is not limited to the following examples.

[0053] Example 1 (Preparation of composite oxides) Ni obtained by coprecipitation method 0.8 Co 0.17 Al 0.03 (OH)2 and Li2CO3 were mixed so that the atomic ratio of Li to the total of Ni, Co, and Al: Li / (Ni+Co+Al) was 1.05 / 1, and the mixture was fired in an oxygen atmosphere to obtain a composite oxide. The composition of the obtained composite oxide was Li 1.05 Ni 0.8 Co0.17 Al 0.03 O2 (Ni / A = 0.8). The composition of the composite oxide was determined by ICP emission spectroscopy. After pulverization and classification using a sieve, a composite oxide powder with an average particle size of 12 μm was obtained.

[0054] (Preparation of cross-linked fluorine-containing polymers) A copolymer of vinylidene fluoride and hexafluoropropylene: PVDF-HFP (Sigma-Aldrich) and trimethylhexamethylenediamine (Tokyo Chemical Industry Co., Ltd.) as a crosslinking agent were dissolved in methyl isobutyl ketone to obtain a mixed solution. The mixed solution was cast to produce a film (solution casting method). This film was heated at 110°C to produce a crosslinked fluorine-containing polymer (binder P). The amount of trimethylhexamethylenediamine added was 0.1 parts by mass per 100 parts by mass of PVDF-HFP. The film-like binder P was pulverized to obtain a powder.

[0055] To confirm the three-dimensional crosslinking, we performed dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and evolved gas analysis (EGA). DMA confirmed the storage modulus, confirming that the three-dimensional crosslinking resulted in a high storage modulus. DSC confirmed that the glass transition temperature (Tg) of the PVDF polymer had increased. EGA confirmed that the onset temperature of the VDF-derived 1,3,5-trifluorobenzene peak at m / z = 132 had shifted to a higher temperature. These analyses confirmed that the resulting crosslinked fluorine-containing polymer had a three-dimensional network structure in which the fluorine-containing polymer of PVDF-HFP was crosslinked.

[0056] (Preparation of positive electrode) To 100 parts by mass of the positive electrode active material, 1 part by mass of binder, 1 part by mass of conductive agent, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) were added and stirred to prepare a positive electrode slurry. The positive electrode active material used was the composite oxide prepared above. The binder used was the crosslinked fluorine-containing polymer prepared above. Carbon nanofibers (average length 1 μm, average diameter 10 nm) were used as the conductive agent.

[0057] The positive electrode slurry was applied to the surface of an aluminum foil (positive electrode current collector sheet), the coating was dried, and then the aluminum foil was rolled to form a positive electrode mixture layer (density 3.5 g / cm) on both sides of the aluminum foil. 3 In this way, a positive electrode was obtained. 2 The amount of the positive electrode slurry applied was adjusted so that the amount of the positive electrode mixture layer carried per electrode was 280 g.

[0058] (Preparation of negative electrode) To 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethyl cellulose sodium (CMC-Na), and an appropriate amount of water were added, and the mixture was stirred to prepare a negative electrode slurry.

[0059] The negative electrode active material was a mixture of Si-containing material and graphite (average particle size (D50) 25 μm). The mass ratio of the Si-containing material to the graphite was 10:90. The Si-containing material contained SiO 2 whose surface was coated with a conductive layer containing conductive carbon. x The conductive layer was coated with SiO particles (x=1, average particle size (D50) 5 μm). x The amount was 5 parts by mass per 100 parts by mass of the total of the particles and the conductive layer.

[0060] The negative electrode slurry was applied to the surface of the copper foil (negative electrode current collector sheet), the coating was dried, and then rolled to form a negative electrode mixture layer (thickness 200 μm, density 1.4 g / cm) on both sides of the copper foil. 3 ) was formed. In this way, a negative electrode was obtained.

[0061] (Preparation of non-aqueous electrolyte) A non-aqueous electrolyte was obtained by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 3:7).

[0062] (Fabrication of non-aqueous electrolyte secondary battery) 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. The positive and negative electrodes were wound with a polyethylene separator interposed therebetween to prepare a wound electrode assembly. The electrode assembly was vacuum dried at 105°C for 2 hours and then housed in a cylindrical battery case with a bottom that also served as the negative electrode terminal. An iron case (outer diameter 18 mm, height 65 mm) was used as the battery case. Next, a nonaqueous electrolyte was injected into the battery case, and the opening of the battery case was closed with a metal sealing member that also served as the positive electrode terminal. At this time, a resin 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 way, an 18650-type cylindrical nonaqueous electrolyte secondary battery (Battery A1) was prepared.

[0063] The battery A1 was evaluated as follows.

[0064] [Rating 1: Initial capacity] The aging-treated battery was charged at a constant current of 0.5 C (1800 mA) until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 C (180 mA). It was then discharged at a constant current of 0.1 C until the voltage reached 2.5 V, and the discharge capacity at this point was recorded as the initial capacity. The rest time between charge and discharge was 10 minutes. Charge and discharge were performed in an environment of 25°C. The initial capacity was expressed as an index, with the initial capacity of Battery B1 set to 100.

[0065] [Evaluation 2: Cycle capacity retention rate] A charge-discharge cycle test was conducted under the following conditions. The rest time between charge and discharge was 10 minutes. The charge-discharge cycle was conducted at 25°C.

[0066] <charging> The battery was charged at a constant current of 0.5 C (1800 mA) until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 C (180 mA).

[0067] <Discharge> A constant current discharge was carried out at a current of 0.5 C until the voltage reached 2.5 V.

[0068] Charge and discharge were repeated under the above conditions. The ratio (percentage) of the discharge capacity at the 100th cycle to the discharge capacity at the 1st cycle was calculated as the cycle capacity retention rate (%). The cycle capacity retention rate was expressed as an index, with the cycle capacity retention rate of Battery B1 set to 100.

[0069] [Rating 3: DC resistance (DCR)] The battery was charged and discharged twice under the same conditions as in Evaluation 2. After the second discharge cycle, the battery was charged at a constant current of 0.2 C (720 mA) at 25°C until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 C (72 mA). In this way, a fully charged battery (SOC 100%) was obtained.

[0070] A fully charged battery was subjected to constant current discharge at a current I of 1C. The difference ΔV between the voltage immediately before the start of discharge and the voltage 10 seconds after the start of discharge was calculated, and the value (ΔV / I) obtained by dividing ΔV by the current I was used to calculate the DC resistance (Ω). The DCR was expressed as an index, with the DCR of battery B1 set at 100.

[0071] Example 2 Positive electrode current collecting sheet 1m 2 Battery A2 of Example 2 was produced and evaluated in the same manner as Battery A1 of Example 1, except that the amount of positive electrode slurry applied was adjusted so that the amount of the positive electrode mixture layer carried per battery was 330 g.

[0072] Comparative Examples 1 and 3 PVDF-HFP was used as the binder. Positive electrode current collector sheet 1 m 2The amount of positive electrode slurry applied was adjusted so that the amount of the positive electrode mixture layer per unit area was the value shown in Table 1. Except for the above, batteries B1 and B3 of Comparative Examples 1 and 3 were produced and evaluated in the same manner as battery A1 of Example 1.

[0073] Comparative Example 2 Battery B3 of Comparative Example 3 was produced and evaluated in the same manner as battery A1 of Example 1, except that PVDF-HFP was used as the binder.

[0074] Comparative Example 4 Battery B4 of Comparative Example 4 was produced and evaluated in the same manner as Battery B1 of Comparative Example 1, except that a crosslinked fluorine-containing polymer was used as the binder.

[0075] Comparative Example 5 Battery B5 of Comparative Example 5 was fabricated and evaluated in the same manner as battery A1 of Example 1, except that LiCoO2 (Ni / A=0) was used as the positive electrode active material.

[0076] Table 1 shows the evaluation results of the batteries A1 to A2 and the batteries B1 to B5.

[0077] [Table 1]

[0078] Batteries A1 and A2 had a small DCR, a large initial capacity, and excellent cycle characteristics.

[0079] In battery B1, the amount of support in the positive electrode mixture layer was small, resulting in a low initial capacity. In batteries B2 and B3, the amount of support in the positive electrode mixture layer was increased, but PVDF-HFP was used as the binder, resulting in an increased DCR and poor cycle performance. In battery B4, a cross-linked fluorine-containing polymer was used as the binder, but the amount of support in the positive electrode mixture layer was small, resulting in a low initial capacity. In battery B5, LiCoO2 was used as the positive electrode active material, resulting in a low initial capacity.

[0080] Examples 3 to 5 Batteries A3 to A5 of Examples 3 to 5 were produced and evaluated in the same manner as for Battery A1 of Example 1, except that the degree of compression of the dried coating film was adjusted so that the density of the positive electrode mixture layer would be the value shown in Table 2. The evaluation results of Batteries A3 to A5 are shown in Table 2. Table 2 also shows the evaluation results of Battery A1.

[0081] [Table 2]

[0082] All of the batteries A1, A3 to A5 had small DCR, large initial capacity, and excellent cycle characteristics. In particular, the density of the positive electrode mixture layer was 3.45 g / cm 3 More than 3.75g / cm 3 In the following batteries A1 and A4, the DCR was further reduced and the cycle characteristics were further improved.

[0083] Examples 6 to 8 Batteries A6 to A8 of Examples 6 to 8 were produced and evaluated in the same manner as battery A1 of Example 1, except that CNTs with an average length shown in Table 3 were used as the conductive agent. The evaluation results of batteries A6 to A8 are shown in Table 3. Table 3 also shows the evaluation results of battery A1.

[0084] [Table 3]

[0085] Batteries A1, A6 to A8 all had small DCR, large initial capacity, and excellent cycle characteristics. In particular, Batteries A1, A7 to A8, which had CNTs with an average length of 0.5 μm or more, had further reduced DCR and further improved cycle characteristics. [Industrial Applicability]

[0086] The positive electrode for a non-aqueous electrolyte secondary battery according to the present disclosure is suitable for use in, for example, non-aqueous electrolyte secondary batteries that require high capacity and excellent cycle characteristics. [Explanation of symbols]

[0087] 1 electrode group 2 positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Seal

Claims

1. a positive electrode current collecting sheet; and a positive electrode mixture layer supported on the positive electrode current collecting sheet, the positive electrode mixture layer includes a positive electrode active material, a binder, and a conductive agent, the positive electrode active material has a layered rock salt crystal structure and includes a composite oxide containing lithium and the element A other than lithium, The element A contains at least nickel, the atomic ratio of nickel to the element A: Ni / A is 0.8 or more and 1.0 or less; the binder includes a polymer binder having a three-dimensional network structure, the polymeric binder comprises a fluorine-containing polymer; the fluorine-containing polymer is crosslinked with a crosslinkable monomer; the fluorine-containing polymer and the crosslinkable monomer form an amide bond or an ester bond, 1 m of the positive electrode current collecting sheet ２ a mass of the positive electrode mixture layer carried per positive electrode for a non-aqueous electrolyte secondary battery is 280 g or more.

2. The composite oxide has the general formula: Li ａ Ni ｘ Co ｙ M １－ｘ－ｙ O ２ is expressed as 2. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein in the formula, 0.97≦a≦1.2, 0.8≦x≦1.0, and 0≦y≦0.2, and M is at least one element selected from the group consisting of Mn, Al, B, W, Sr, Mg, Mo, Nb, Ti, Si, and Zr.

3. The density of the positive electrode mixture layer is 3.45 g / cm ３ Above, 3.75g / cm ３ 3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein:

4. the conductive agent includes carbon nanotubes; The average length of the carbon nanotubes is 0.5 μm or more. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3.

5. 5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the fluorine-containing polymer comprises at least one selected from the group consisting of polyvinylidene fluoride and a copolymer containing a unit derived from vinylidene fluoride.

6. The copolymer containing units derived from vinylidene fluoride includes a copolymer of vinylidene fluoride and a fluorine-containing monomer other than vinylidene fluoride, 6. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 5, wherein the fluorine-containing monomer other than vinylidene fluoride includes at least one selected from the group consisting of hexafluoropropylene, tetrafluoroethylene, trifluoroethylene, and trifluorochloroethylene.

7. A positive electrode, a negative electrode, and a non-aqueous electrolyte, A non-aqueous electrolyte secondary battery, wherein the positive electrode is the positive electrode according to any one of claims 1 to 6.

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