Lithium-ion rechargeable battery

Abstract

To provide a lithium ion secondary battery, capable of improving capacity retention rate.SOLUTION: There is provided a lithium ion secondary battery which includes a positive electrode layer, an electrolyte layer, and a negative electrode layer. At least one of the positive electrode layer, the electrolyte layer, and the negative electrode layer includes a gel electrolyte. The gel electrolyte is a gel-like composite containing an electrolytic solution and a polymer. The electrolytic solution contains cyclic carbonate as a solvent and a lithium amide salt dissolved in the cyclic carbonate. The molar ratio of the lithium amide salt to the cyclic carbonate is greater than 0.25 and 0.33 or less. The positive electrode layer contains a lithium layered positive electrode active material as a positive electrode active material. The lithium layered positive electrode active material contains Ni elements, and the ratio of Ni elements in constituent metal elements other than lithium is 80 mol% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure relates to lithium-ion secondary batteries. [Background technology]

[0002] Various technologies have been proposed for lithium-ion secondary batteries, such as the one disclosed in Patent Document 1. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-138137 [Overview of the project] [Problems that the invention aims to solve]

[0004] Conventional lithium-ion rechargeable batteries have room for improvement in terms of capacity retention.

[0005] This disclosure is made in view of the above circumstances and primarily aims to provide a lithium-ion secondary battery that can improve capacity retention. [Means for solving the problem]

[0006] In other words, this disclosure includes the following aspects: <1> Lithium-ion secondary battery, The lithium-ion secondary battery includes a positive electrode layer, an electrolyte layer, and a negative electrode layer. At least one of the positive electrode layer, the electrolyte layer, and the negative electrode layer contains a gel electrolyte, The aforementioned gel electrolyte is a gel-like composite containing an electrolyte and a polymer. The electrolyte comprises a cyclic carbonate as a solvent and a lithium amide salt dissolved in the cyclic carbonate. The molar ratio of the lithium amide salt to the cyclic carbonate is greater than 0.25 and not more than 0.33. The positive electrode layer contains a lithium layered positive electrode active material as a positive electrode active material. The lithium layered positive electrode active material contains Ni element, and the ratio of Ni element among the constituent metal elements other than lithium is 80 mol% or more, a lithium ion secondary battery.

[0007] <2> The molar ratio of the electrolytic solution to the polymer in the gel electrolyte is 2 to 7, the lithium ion secondary battery according to <1>.

[0008] <3> The lithium layered positive electrode active material contains Mn element and Co element as constituent metal elements other than lithium, the lithium ion secondary battery according to <1> or <2>.

[0009] <4> The polymer is a fluoride-based polymer. The cyclic carbonate is at least one of propylene carbonate and ethylene carbonate. The lithium amide salt is at least one of lithium bis(fluorosulfonyl)amide and lithium bis(trifluoromethanesulfonyl)amide, the lithium ion secondary battery according to any one of <1> to <3>.

[0010] <5> The positive electrode layer and the electrolyte layer contain the gel electrolyte, the lithium ion secondary battery according to any one of <1> to <4>.

Advantages of the Invention

[0011] The lithium ion secondary battery of the present disclosure can improve the capacity retention rate.

Brief Description of the Drawings

[0012] [Figure 1]Figure 1 is a graph showing the relationship between the number of charge-discharge cycles and the discharge capacity retention rate for cells in Example 1 and Comparative Example 1, which use LiNi0.92Co0.04Mn0.04O2 as the positive electrode active material. [Figure 2] Figure 2 is a graph showing the relationship between the molar ratio of Ni among the constituent metal elements other than Li in Li(NCM)O2 and the capacity retention rate (%) after 50 charge-discharge cycles, for cells using a gel electrolyte and cells using a conventional electrolyte. [Figure 3] Figure 3 shows the charge-discharge curve of the cell in Example 1, which uses a gel electrolyte as the electrolyte. [Figure 4] Figure 4 shows the charge-discharge curve of the cell in Comparative Example 1, which uses a conventional electrolyte. [Modes for carrying out the invention]

[0013] Embodiments of this disclosure are described below. Matters other than those specifically mentioned herein but necessary for the implementation of this disclosure (e.g., general configuration and manufacturing processes of lithium-ion secondary batteries that do not characterize this disclosure) can be understood as design matters for those skilled in the art based on the prior art. This disclosure can be implemented based on the content disclosed herein and common technical knowledge in the art. In this disclosure, unless otherwise specified, the average particle size (D50) is the value of the particle size (median diameter) at 50% of the cumulative value in the volume-based particle size distribution measured by laser diffraction-scattering particle size distribution measurement.

[0014] In this disclosure, a lithium-ion secondary battery, The lithium-ion secondary battery includes a positive electrode layer, an electrolyte layer, and a negative electrode layer. At least one of the positive electrode layer, the electrolyte layer, and the negative electrode layer contains a gel electrolyte, The aforementioned gel electrolyte is a gel-like composite containing an electrolyte and a polymer. The electrolyte comprises a cyclic carbonate as a solvent and a lithium amide salt dissolved in the cyclic carbonate. The molar ratio of the lithium amide salt to the cyclic carbonate is greater than 0.25 and less than or equal to 0.33. The positive electrode layer includes a lithium layered positive electrode active material as the positive electrode active material. The present invention provides a lithium-ion secondary battery in which the lithium layered positive electrode active material contains Ni element, and the ratio of Ni element among the constituent metal elements other than lithium is 80 mol% or more.

[0015] Lithium layered compounds with a high proportion of Ni among the constituent metal elements other than lithium can achieve high capacity, but they are prone to side reactions with organic electrolytes when charged, causing Ni and Mn to dissolve from the active material into the electrolyte and release oxygen, making them susceptible to degradation and combustion. Although oxide coatings are sometimes used to address this tendency to cause side reactions, the crystals shrink when charged, causing particle cracking and drastically reducing the effectiveness of the coating. In this disclosure, a lithium layered oxide in which the ratio of Ni element among the constituent metal elements other than lithium is 80 mol% or more is used as the positive electrode active material, and a gel electrolyte is used which is a mixture of an electrolyte solution in which a lithium amide salt is dissolved in a cyclic carbonate and a polymer. In this disclosure, only Li ions move while the movement of the solvent and counteranions in the gel electrolyte is fixed, thereby suppressing the leaching of metals and the release of oxygen, which are the main causes of degradation when the positive electrode active material is charged to a high capacity state (high voltage). As a result, good durability performance of the lithium-ion secondary battery can be obtained.

[0016] Gel electrolytes (lithium ion conductive materials) are gel-like composites containing an electrolyte solution and a polymer. The composite is a compound formed by combining the above-mentioned electrolyte and polymer. The composite may also be such that the above-mentioned electrolyte is held by the above-mentioned polymer. If the amount of electrolyte in the gel electrolyte is too much, it will not form a gel and the electrolyte will seep out, rendering it ineffective. If the amount is too little, the desired lithium ion conductivity and flexibility cannot be ensured. Therefore, the molar ratio of electrolyte to polymer in the gel electrolyte may be 2 to 7, or it may be 3 to 4. The gel electrolyte of this disclosure may not be fluid as a whole, that is, it may be form-specific. The gel electrolyte of this disclosure may be, for example, in the form of a sheet or a powder. Alternatively, the gel electrolyte of this disclosure may be integrated with another material. In one embodiment, the gel electrolyte may be integrated with a solid electrolyte, for example. More specifically, for example, at least a portion of the surface of solid electrolyte particles may be coated with the gel electrolyte of this disclosure. In one embodiment, the gel electrolyte may be integrated with active materials such as positive electrode active material and negative electrode active material. More specifically, at least a portion of the surface of active material particles may be coated with the gel electrolyte of this disclosure. Solid electrolytes and active materials will be described later.

[0017] The electrolyte contains a cyclic carbonate as a solvent and a lithium amide salt dissolved in the cyclic carbonate. The solvent may consist of a cyclic carbonate. The solvent may also be a mixed solvent of a cyclic carbonate and a solvent other than a cyclic carbonate. Specific examples of cyclic carbonates include at least one selected from propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), vinylene carbonate (VC), and their derivatives (e.g., halides). In particular, when the cyclic carbonate is at least one of propylene carbonate and ethylene carbonate, even better lithium-ion conductivity and thermal stability are more easily ensured. A single cyclic carbonate may be used alone, or two or more may be used in combination. Other solvents (secondary solvents, diluents) besides the cyclic carbonate may be, for example, linear carbonates. Examples of linear carbonates include at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), and their derivatives (e.g., halides, especially those having perfluoroalkyl groups). In the electrolyte, the molar ratio of the secondary solvent to the cyclic carbonate ([secondary solvent (mol)] / [cyclic carbonate (mol)]) may be 0 to 0.10, 0 to 0.05, or 0 to 0.03. The molar ratio of the lithium amide salt to the cyclic carbonate is greater than 0.25 and less than or equal to 0.33. In other words, the lithium amide salt is dissolved in the cyclic carbonate at a concentration greater than 0.25 mol per mole of cyclic carbonate and less than or equal to 0.33 mol. This molar ratio may be 0.26 or greater, 0.27 or greater, or 0.28 or greater, and may be 0.32 or less, 0.31 or less, or 0.30 or less. The molar ratio of the lithium amide salt to the cyclic carbonate in the complex can be determined by analyzing the ions and elements that constitute the complex. Examples of lithium amide salts include at least one sulfonylamide salt selected from lithium bisfluorosulfonylamide (LiFSA, LiN(SO2F)2), lithium bistrifluoromethanesulfonylamide (LiTFSA, Li[N(CF3SO2)2]), lithium bisperfluoroethylsulfonylamide (Li[N(C2F5SO2)2]), lithium bisperfluorobutylsulfonylamide (Li[N(C4F9SO2)2]), lithium fluorosulfonyltrifluoromethanesulfonylamide (Li[N(FSO2)(C2F5SO2)]), etc. Alternatively, a silylamide salt having Si instead of S may be used. In particular, when the lithium amide salt is at least one of lithium bisfluorosulfonylamide (LiFSA, LiN(SO2F)2) and lithium bistrifluoromethanesulfonylamide (LiTFSA, Li[N(CF3SO2)2]), even better lithium ion conductivity and thermal stability are more easily ensured, and furthermore, the reactivity to the sulfide solid electrolyte described later is more easily reduced. The lithium amide salt may be used alone or in combination of two or more types. In this application, the term "amide salt" also includes "imide salt".

[0018] The polymer may be a fluoride-based polymer or a non-fluoride-based polymer. The fluoride-based polymer may be at least one selected from polyvinylidene fluoride (PVdF)-based polymers, polytetrafluoroethylene (PTFE)-based polymers, and the like. These fluoride-based polymers may be homopolymers obtained by polymerizing a single monomer alone, or they may be copolymers containing polymerization units derived from other monomers such as hexafluoropropylene. In fluoride-based polymers, 50 mol% to 100 mol%, 60 mol% to 100 mol%, 70 mol% to 100 mol%, 80 mol% to 100 mol%, 90 mol% to 100 mol%, or 95 mol% to 100 mol% of the total polymerization units may be derived from fluoride-based monomers. The non-fluoride polymer may be an ether polymer or a non-ether polymer. The ether polymer may be at least one selected from polyethylene oxide, polypropylene oxide, etc. The non-ether polymer may be at least one selected from butadiene rubber (BR), butylene rubber (IIR), acrylate butadiene rubber, or styrene butadiene rubber (SBR), polyimide (PI), and polyacrylic acid, etc. These non-fluoride polymers may be homopolymers obtained by polymerizing one monomer alone, or copolymers that include polymerization units derived from other monomers. In the non-fluoride polymer, 50 mol% to 100 mol%, 60 mol% to 100 mol%, 70 mol% to 100 mol%, 80 mol% to 100 mol%, 90 mol% to 100 mol%, or 95 mol% to 100 mol% of the total polymerization units may be derived from non-fluoride monomers.

[0019] A lithium-ion secondary battery comprises a positive electrode, a negative electrode, and an electrolyte layer between the positive electrode and the negative electrode. The positive electrode includes a positive electrode layer. The positive electrode may include a positive electrode current collector as needed. The negative electrode includes a negative electrode layer. The negative electrode may include a negative electrode current collector as needed. At least one of the positive electrode layer, electrolyte layer, and negative electrode layer may contain a gel electrolyte, the positive electrode layer and the electrolyte layer may contain a gel electrolyte, and all of the positive electrode layer, electrolyte layer, and negative electrode layer may contain a gel electrolyte.

[0020] The positive electrode layer contains a lithium layered positive electrode active material as the positive electrode active material, and may optionally contain an electrolyte, a conductive additive, a binder, and various other additives. If the positive electrode layer contains the gel electrolyte described above, the positive electrode layer may further optionally contain other electrolytes, conductive materials, binders, and various other additives in addition to the positive electrode active material and the gel electrolyte. The content of each component in the positive electrode layer may be appropriately determined according to the desired battery performance. There are no particular limitations on the thickness of the positive electrode layer. The lithium layered cathode active material contains Ni element, and the ratio of Ni element among the constituent metal elements other than lithium is 80 mol% or more, and may be 83 mol% or more. Examples of the constituent metal elements other than lithium include Ni, Mn, Co, Al, etc. Among the constituent metal elements other than lithium, the smaller the ratio of Co, the more the effect of the combination with the electrolyte is exerted (for example, not more than half of the elements other than Ni). The lithium layered cathode active material is lithium nickelate, lithium nickel cobaltate, lithium nickel manganate, lithium nickel cobalt manganate (Li , , Ni x Co y Mn z O 2±δ (for example, 0.8 ≦ x < 1, 0 < y < 0.2, 0 < z < 0.2, x + y + z = 1)), lithium nickel cobalt aluminum oxide (for example, Li 1±α Ni p Co q Al r O 2±δ (for example, p + q + r = 1)), etc., and may be at least one selected from these. Only one kind of the cathode active material may be used alone, or two or more kinds may be used in combination. The cathode active material may be cathode active material particles. The cathode active material may have voids, for example, may be porous or may be hollow. The cathode active material may be primary particles or secondary particles in which a plurality of primary particles are aggregated.

[0021] The electrolyte that can be contained in the cathode layer may be the above-mentioned gel electrolyte, may be a solid electrolyte, or may be a combination of these. Examples of the solid electrolyte include oxide-based solid electrolytes and sulfide-based solid electrolytes. The solid electrolyte may be solid electrolyte particles. Examples of sulfide-based solid electrolytes include solid electrolytes containing Li, M (where M is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In), and S. Furthermore, sulfide-based solid electrolytes may further contain at least one of O and halogen elements. Examples of sulfide-based solid electrolytes include Li2S-P2S5, Li2S-SiS2, LiX-Li2S-SiS2, LiX-Li2S-P2S5, LiX-Li2O-Li2S-P2S5, LiX-Li2S-P2O5, LiX-Li3PO4-P2S5, and Li3PS4. The term "Li2S-P2S5" above refers to a material made using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions. Furthermore, the "X" in LiX above represents a halogen element. Examples of halogen elements include F, Cl, Br, and I. The raw material composition containing LiX may contain one or more types of LiX. When two or more types of LiX are included, the mixing ratio of the two or more types is not particularly limited. The molar ratio of each element in a sulfide-based solid electrolyte can be controlled by adjusting the content of each element in the raw materials. Furthermore, the molar ratio and composition of each element in a sulfide-based solid electrolyte can be measured, for example, by ICP emission spectrometry.

[0022] The sulfide-based solid electrolyte may be sulfide glass, crystalline sulfide glass (glass ceramics), or a crystalline material obtained by solid-phase reaction treatment of the raw material composition. The crystalline state of sulfide-based solid electrolytes can be confirmed, for example, by performing powder X-ray diffraction measurements using CuKα radiation on the sulfide-based solid electrolyte.

[0023] Examples of oxide-based solid electrolytes include materials having a garnet-type crystal structure containing elements Li, La, A (where A is at least one of Zr, Nb, Ta, and Al), and O. Examples of oxide-based solid electrolytes include Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3, Li 1.3 Al 0.3 Ti 0.7 (PO4)3, Li5La3Ta2O 12 Li7La3Zr2O 12 Li6BaLa2Ta2O 12 Li 3.6 Si 0.6 P 0.4 O4, Li4SiO4, Li3PO4, and Li 3+x PO 4-x N x (1 ≤ x ≤ 3) or similar conditions may also be acceptable. Solid electrolytes can be used individually or in combination of two or more types.

[0024] As conductive materials, known materials can be used, such as carbon materials and metal particles. Examples of carbon materials include acetylene black (AB), furnace black, vapor-processed carbon fiber (VGCF), carbon nanotubes, and carbon nanofibers. In particular, from the viewpoint of electronic conductivity, at least one selected from the group consisting of VGCF, carbon nanotubes, and carbon nanofibers may be used. Examples of metal particles include particles of Ni, Cu, Fe, and SUS. The content of conductive material in the positive electrode layer is not particularly limited. One type of conductive material may be used alone, or two or more types may be used in combination.

[0025] Examples of binders include acrylonitrile butadiene rubber (ABR), butadiene rubber (BR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), and styrene butadiene rubber (SBR). The binder content in the positive electrode layer is not particularly limited. One type of binder may be used alone, or two or more types may be used in combination.

[0026] As the positive electrode current collector, known metals usable as current collectors in lithium-ion secondary batteries can be used. Examples of such metals include metallic materials containing one or more elements selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Co, Cr, Zn, Ge, and In. Examples of positive electrode current collectors include SUS, aluminum, nickel, iron, titanium, and carbon. The form of the positive electrode current collector is not particularly limited and can take various forms such as foil or mesh.

[0027] The electrolyte layer may be a liquid electrolyte layer using an electrolyte solution, or it may be a solid electrolyte layer using a solid electrolyte. The electrolyte layer contains at least an electrolyte. The electrolyte layer may contain at least one of the solid electrolyte and the electrolyte solution, and may further optionally contain a binder or the like. If the electrolyte layer contains the gel electrolyte of this disclosure, the electrolyte layer may further contain other electrolytes, binders and various additives in addition to the gel electrolyte. The content of electrolyte and binder, etc. in the electrolyte layer is not particularly limited. Alternatively, the electrolyte layer may have a separator or the like to hold the electrolyte solution and prevent contact between the positive electrode layer and the negative electrode layer. The thickness of the electrolyte layer is not particularly limited and may be, for example, 0.1 μm or more or 1 μm or more, or 2 mm or less or 1 mm or less. Electrolytes and binders may be used individually or in combination of two or more types. The separator may be any separator commonly used in lithium-ion secondary batteries, such as those made of polyethylene (PE), polypropylene (PP), polyester, and polyamide resins. The separator may have a single-layer structure or a multi-layer structure. Examples of multi-layer separators include a two-layer PE / PP separator, or a three-layer PP / PE / PP or PE / PP / PE separator. The separator may also be made of a nonwoven fabric such as cellulose nonwoven fabric, resin nonwoven fabric, or glass fiber nonwoven fabric. The gel electrolyte of this disclosure may also be used as a separator.

[0028] The negative electrode layer contains at least a negative electrode active material. The negative electrode layer may also optionally contain the electrolyte, conductive material, binder, and various additives. If the negative electrode layer contains the gel electrolyte of this disclosure, the negative electrode layer may further optionally contain other electrolytes, conductive materials, binders, and various additives in addition to the negative electrode active material and the gel electrolyte. The content of each component in the negative electrode layer may be appropriately determined according to the desired battery performance.

[0029] The negative electrode active material can be any of the known active materials whose potential for intercalating and releasing lithium ions (charge / discharge potential) is lower than that of the positive electrode active material. For example, silicon-based active materials such as Si, Si alloys, and silicon oxide; carbon-based active materials such as graphite and hard carbon; various oxide-based active materials such as lithium titanate; metallic lithium and lithium alloys can be used. The negative electrode active material may be used alone or in combination of two or more types. The negative electrode active material may be negative electrode active material particles. The negative electrode active material particles may be primary particles or secondary particles formed by the aggregation of multiple primary particles. Alternatively, the negative electrode active material may be in the form of a sheet (foil-like, film-like) such as lithium foil. That is, the negative electrode layer may consist of a sheet of negative electrode active material.

[0030] The material of the negative electrode current collector may be a material that does not alloy with Li, and examples include SUS, copper, and nickel. Examples of negative electrode current collector shapes include foil and plate shapes. The planar shape of the negative electrode current collector is not particularly limited, but examples include circular, elliptical, rectangular, and any polygonal shape. The thickness of the negative electrode current collector varies depending on the shape, but may be in the range of 1 μm to 50 μm, or in the range of 5 μm to 20 μm.

[0031] Lithium-ion secondary batteries may be equipped with tabs, terminals, etc. Lithium-ion secondary batteries may also be equipped with an outer casing that houses the positive electrode, negative electrode, electrolyte layer, etc., as needed. The material of the outer casing is not particularly limited as long as it is stable in electrolytes, but examples include polypropylene, polyethylene, and resins such as acrylic resin. Examples of lithium-ion secondary battery shapes include coin-type, laminate-type, cylindrical, and prismatic types.

[0032] Lithium-ion secondary batteries may be liquid-type lithium-ion secondary batteries using an electrolyte solution, or solid-type lithium-ion secondary batteries using a solid electrolyte solution. Applications of lithium-ion secondary batteries include, for example, power sources for vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), gasoline cars, and diesel cars. In particular, they may be used as power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or battery electric vehicles (BEVs). Furthermore, lithium-ion secondary batteries may be used as power sources for mobile devices other than vehicles (e.g., trains, ships, aircraft), and as power sources for electrical products such as information processing devices. [Examples]

[0033] (Example 1) [Positive electrode fabrication] LiNi 0.92 Mn 0.04Co 0.04 O2, VGCF as a conductive material, and a copolymer of polyvinylidene fluoride and hexafluoropropylene (KF8300: manufactured by Kureha Corporation) powder were mixed in a mass ratio of 83:7:10. The mixed powder was transferred to a stirring vessel, and dimethyl carbonate (DMC: manufactured by Kishida Chemical Co., Ltd.) was weighed out in an amount equivalent to eight times the mass of the polymer and added as a diluent, and the mixture was stirred. Subsequently, an electrolyte was prepared such that the molar ratio of lithium bisfluorosulfonylamide (LiFSA, manufactured by Kishida Chemical Co., Ltd.) as the lithium salt to propylene carbonate (PC, manufactured by Kishida Chemical Co., Ltd.) as the solvent was 0.33 (PC:LiFSA = 3:1). Then, the electrolyte was added to a stirring vessel so that the molar ratio of the solvent, lithium salt, and polymer was 3:1:1, and the mixture was stirred while heating. Once a uniform positive electrode slurry was obtained, the slurry was applied onto aluminum foil, which served as the positive electrode current collector, and cast. Subsequently, the diluent was dried at 80°C to obtain a positive electrode having a positive electrode layer on the positive electrode current collector. [Negative electrode fabrication] Graphite was used as the negative electrode active material, and a copolymer of polyvinylidene fluoride and hexafluoropropylene (KF8300: manufactured by Kureha Corporation) was used as the polymer. The graphite powder and polymer powder were mixed in a mortar in a mass ratio of 90:10. The mixed powder was transferred to a stirring container, and dimethyl carbonate (DMC: manufactured by Kishida Chemical Co., Ltd.) was weighed out in an amount equivalent to 8 times the mass of the polymer and added as a diluent, and the mixture was stirred. Subsequently, an electrolyte was prepared such that the molar ratio of lithium bisfluorosulfonylamide (LiFSA, manufactured by Kishida Chemical Co., Ltd.) as the lithium salt to propylene carbonate (PC, manufactured by Kishida Chemical Co., Ltd.) as the solvent was 0.33 (PC:LiFSA = 3:1). Then, the electrolyte was added to a stirring vessel so that the molar ratio of the solvent, lithium salt, and polymer was 3:1:1, and the mixture was stirred while heating. After heating and mixing the diluent, solvent, lithium salt, and polymer to obtain a uniform negative electrode slurry, the slurry was applied to a copper foil serving as the negative electrode current collector, cast, and then dried at 80°C to obtain a negative electrode having a negative electrode layer on the negative electrode current collector. [Separator] The electrolyte was prepared so that the molar ratio of lithium bisfluorosulfonylamide (LiFSA, manufactured by Kishida Chemical Co., Ltd.) as the lithium salt to propylene carbonate (PC, manufactured by Kishida Chemical Co., Ltd.) as the solvent was 0.33 (PC:LiFSA = 3:1). The gel electrolyte was obtained by adding and mixing the electrolyte with polyvinylidene fluoride as the polymer, so that the molar ratio of solvent, lithium salt, and polymer was 3:1:1. The resulting gel electrolyte was used as a separator. [Cell creation] The negative electrode, separator, and positive electrode were stacked in this order, tabs were attached to the positive and negative electrodes, and the cell was fabricated by vacuum sealing with lamination. [Evaluation of battery characteristics] The above cell was constrained at 0.5 MPa, and at 25°C, the upper limit was 4.25 V - lower limit was 2.5 V vs. Li / Li + A 50-cycle charge-discharge test was performed under a 0.2C condition.

[0034] (Example 2) In the fabrication of the positive electrode, LiNi is used as the positive electrode active material. 0.83 Mn 0.12 Co 0.05 Cells were prepared and evaluated in the same manner as in Example 1, except that O2 was used.

[0035] (Example 3) The cells were fabricated and evaluated in the same manner as in Example 1, except that LiNiO2 was used as the cathode active material.

[0036] (Comparative Example 1) [Positive electrode fabrication] LiNi 0.92 Mn 0.04 Co 0.04O2, VGCF as a conductive material, and PVDF (Kureha #7305) as a binder were mixed in a mass ratio of 85:10:5. N-methylpyrrolidone (NMP) was added as a solvent in an amount equal to 0.85 times the amount of the positive electrode active material, and the mixture was stirred and kneaded in a planetary mixer. After obtaining a uniform positive electrode slurry, the slurry was applied to aluminum foil as a positive electrode current collector, cast, and then dried at 80°C to evaporate the solvent, thereby obtaining a positive electrode with a positive electrode layer on the positive electrode current collector. [Negative electrode fabrication] Graphite was used as the negative electrode active material, and carboxymethylcellulose (CMC) was used as a thickener. Graphite powder and CMC were mixed in a mortar in a mass ratio of 98:1, and water was added as a solvent in an amount equal to 2 / 3 of the negative electrode active material. The mixture was then stirred and kneaded in a planetary mixer. Subsequently, SBR was used as a binder, and graphite and the binder were mixed in a mass ratio of 98:1 to obtain a uniform negative electrode slurry. The negative electrode slurry was applied onto copper foil, which served as the negative electrode current collector, and cast. After that, it was dried at 80°C to evaporate the moisture, thereby obtaining a negative electrode with a negative electrode layer on the negative electrode current collector. [Electrolyte] The electrolyte used was 1.1M LiPF6 as the lithium salt and EC / EMC / DMC (=30 / 30 / 40 vol%) as the solvent, with the lithium salt dissolved in the solvent. [Cell creation] The negative electrode, separator, and positive electrode were stacked in this order, the electrolyte was injected, the positive and negative electrodes were tabbed, and the cell was fabricated by vacuum sealing with laminate. [Evaluation of battery characteristics] The cells were evaluated in the same manner as in Example 1.

[0037] (Comparative Example 2) In the fabrication of the positive electrode, LiNi is used as the positive electrode active material. 0.83 Mn 0.12 Co 0.05 Except for using O2, the cells were prepared and evaluated in the same manner as in Comparative Example 1.

[0038] (Comparative Example 3) Except for using LiNiO2 as the cathode active material, the cell was prepared and evaluated in the same manner as in Comparative Example 1.

[0039] [Discussion of the results] Figure 1 shows LiNi as the positive electrode active material. 0.92 Co 0.04 Mn 0.04 This graph shows the relationship between the number of charge / discharge cycles and the discharge capacity retention rate of cells in Example 1 and Comparative Example 1, which use O2. As shown in Figure 1, the cell in Example 1, which uses a gel electrolyte, has a higher discharge capacity retention rate than the cell in Comparative Example 1, which uses a conventional electrolyte. Figure 2 is a graph showing the relationship between the molar ratio of Ni among the constituent metal elements other than Li in Li(NCM)O2 and the capacity retention rate (%) after 50 charge-discharge cycles, for cells using a gel electrolyte and cells using a conventional electrolyte. As shown in Figure 2, even when the molar ratio of Ni among the constituent metal elements other than Li in Li(NCM)O2 is 80% or more, cells using gel electrolytes have a higher capacity retention rate than cells using conventional electrolytes. Figure 3 shows the charge-discharge curve of the cell in Example 1, which uses a gel electrolyte as the electrolyte. As shown in Figure 3, the capacity retention rate of the cell in Example 1 after 50 charge-discharge cycles is 96.1%. Figure 4 shows the charge-discharge curve of the cell in Comparative Example 1, which uses a conventional electrolyte. As shown in Figure 4, the capacity retention rate of the cell in Comparative Example 1 after 50 charge-discharge cycles is 87.5%. This shows that the cell in Example 1 has a higher capacity retention rate after 50 charge-discharge cycles than the cell in Comparative Example 1.

Claims

1. Lithium-ion secondary battery, The lithium-ion secondary battery includes a positive electrode layer, an electrolyte layer, and a negative electrode layer. At least one of the positive electrode layer, the electrolyte layer, and the negative electrode layer contains a gel electrolyte, The aforementioned gel electrolyte is a gel-like composite containing an electrolyte and a polymer. The electrolyte comprises a cyclic carbonate as a solvent and a lithium amide salt dissolved in the cyclic carbonate. The molar ratio of the lithium amide salt to the cyclic carbonate is greater than 0.25 and less than or equal to 0.

33. The positive electrode layer includes a lithium layered positive electrode active material as the positive electrode active material. The lithium layered positive electrode active material contains Ni element, and the ratio of Ni element to the constituent metal elements other than lithium is 80 mol% or more, in a lithium-ion secondary battery.

2. The lithium-ion secondary battery according to claim 1, wherein the molar ratio of the electrolyte to the polymer in the gel electrolyte is 2 to 7.

3. The lithium-ion secondary battery according to claim 1, wherein the lithium layered positive electrode active material includes Mn and Co as constituent metal elements other than lithium.

4. The aforementioned polymer is a fluoride-based polymer, The cyclic carbonate is at least one of propylene carbonate and ethylene carbonate. The lithium-ion secondary battery according to claim 1, wherein the lithium amide salt is at least one of lithium bisfluorosulfonylamide and lithium bistrifluoromethanesulfonylamide.

5. The lithium-ion secondary battery according to claim 1, wherein the positive electrode layer and the electrolyte layer include the gel electrolyte.

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