Lithium ion secondary 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] The present disclosure relates to a lithium-ion secondary battery.

Background Art

[0002] Various technologies have been proposed regarding lithium-ion secondary batteries as disclosed in Patent Document 1.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

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

[0005] The present disclosure has been made in view of the above circumstances, and the main object thereof is to provide a lithium-ion secondary battery capable of improving the capacity retention rate.

Means for Solving the Problems

[0006] That is, the present disclosure includes the following aspects. <1> 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 gel electrolyte is a gel-like composite containing an electrolytic solution and a polymer, The electrolytic solution contains 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 bisfluorosulfonylamide and lithium bistrifluoromethanesulfonylamide. 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 2

Figure 3

Figure 4

DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments according to the present disclosure will be described. Note that matters other than those specifically mentioned in this specification and necessary for the implementation of the present disclosure (for example, general configurations and manufacturing processes of lithium-ion secondary batteries that do not characterize the present disclosure) can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present disclosure can be implemented based on the content disclosed in this specification and common general knowledge in the relevant field. In the present disclosure, unless otherwise specified, the average particle diameter (D50) of the particles is the value of the particle diameter (median diameter) at 50% of the integrated value in the volume-based particle size distribution measured by laser diffraction / scattering particle size distribution measurement.

[0014] In the present disclosure, there is provided 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 gel electrolyte is a gel-like composite containing an electrolyte solution and a polymer, The electrolytic solution contains 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 equal to or less than 0.33. The positive electrode layer contains a lithium layered positive electrode active material as a positive electrode active substance. Provided is 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] A lithium layered compound with a high ratio of Ni element among the constituent metal elements other than lithium can obtain a high capacity, but it easily undergoes a side reaction with the organic electrolytic solution in the charged state, and Ni and Mn elute from the active material into the electrolytic solution and oxygen is released, making it prone to deterioration and combustion. There are examples of using an oxide coating or the like because it easily undergoes a side reaction, but since the crystal shrinks in the charged state, cracks occur in the particles and the effect of the coating is drastically reduced. In the present disclosure, a gel electrolyte in which a lithium layered oxide having a ratio of Ni element among the constituent metal elements other than lithium of 80 mol% or more is used as a positive electrode active material, and an electrolytic solution in which a lithium amide salt is dissolved in a cyclic carbonate and a polymer are mixed is used. In the present disclosure, since only Li ions move while the movement of the solvent and counteranion in the gel electrolyte is fixed, elution of metal and release of oxygen, which are the main causes of deterioration when the positive electrode active material is in a high-capacity charged state (high voltage), are suppressed. Thereby, good durability performance of the lithium ion secondary battery can be obtained.

[0016] A gel electrolyte (lithium ion conductive material) is a gel-like composite containing an electrolytic solution and a polymer. The composite is one in which the above electrolytic solution and polymer are complexed. The composite may be one in which the above electrolytic solution is held by the above polymer. If the amount of the electrolyte solution in the gel electrolyte is too large, it will not gel and the electrolyte solution will leak out without showing any effect. If it is too small, the desired lithium ion conductivity and flexibility cannot be ensured. Therefore, the molar ratio of the electrolyte solution to the polymer in the gel electrolyte may be 2 to 7, or may be 3 to 4. The gel electrolyte of the present disclosure may not have fluidity as a whole, that is, it may have a fixed shape. The gel electrolyte of the present disclosure may be, for example, in the form of a sheet or in the form of powder. Alternatively, the gel electrolyte of the present disclosure may be integrated with a material different from the gel electrolyte. The gel electrolyte according to one embodiment may be integrated with a solid electrolyte, for example. More specifically, at least a part of the surface of the solid electrolyte particles may be coated with the gel electrolyte of the present disclosure. The gel electrolyte according to one embodiment may be integrated with active materials such as a positive electrode active material and a negative electrode active material. More specifically, at least a part of the surface of the active material particles may be coated with the gel electrolyte of the present disclosure. The solid electrolyte and the active material will be described later.

[0017] The electrolyte solution 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 be a mixed solvent of a cyclic carbonate and a solvent other than the cyclic carbonate. Specific examples of the cyclic carbonate include at least one selected from propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), vinylene carbonate (VC) and their derivatives (for example, halides, etc.). In particular, when the cyclic carbonate is at least one of propylene carbonate and ethylene carbonate, it is easier to ensure better lithium ion conductivity and thermal stability. The cyclic carbonate may be used alone or in combination of two or more. Solvents other than cyclic carbonates (co-solvents, diluents) may be, for example, chain carbonates. Examples of the chain carbonate include at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), and derivatives thereof (for example, halides, particularly those having a perfluoroalkyl group). In the electrolyte, the molar ratio of the co-solvent to the cyclic carbonate ([co-solvent (mol)] / [cyclic carbonate (mol)]) may be 0 or more and 0.10 or less, 0 or more and 0.05 or less, or 0 or more and 0.03 or less. The molar ratio of the lithium amide salt to the cyclic carbonate is greater than 0.25 and not more than 0.33. In other words, the lithium amide salt is dissolved in the cyclic carbonate at a concentration of more than 0.25 mol and not more than 0.33 mol per 1 mol of the cyclic carbonate. The molar ratio may be 0.26 or more, 0.27 or more, or 0.28 or more, 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 composite can be specified by analyzing ions, elements, etc. constituting the composite. Examples of the lithium amide salt include at least one sulfonylamide salt selected from lithium bis(fluorosulfonyl)amide (LiFSA, LiN(SO2F)2), lithium bis(trifluoromethanesulfonyl)amide (LiTFSA, Li[N(CF3SO2)2]), lithium bis(perfluoroethylsulfonyl)amide (Li[N(C2F5SO2)2]), lithium bis(perfluorobutylsulfonyl)amide (Li[N(C4F9SO2)2]), lithium fluorosulfonyl(trifluoromethanesulfonyl)amide (Li[N(FSO2)(C2F5SO2)]), and the like. Alternatively, a silylamide salt having Si instead of S may be employed. In particular, when the lithium amide salt is at least one of lithium bis(fluorosulfonyl)amide (LiFSA, LiN(SO2F)2) and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA, Li[N(CF3SO2)2]), it is easier to ensure excellent lithium ion conductivity and thermal stability, and furthermore, the reactivity with the sulfide solid electrolyte described later is more likely to be reduced. The lithium amide salt may be used alone or in combination of two or more. In the present application, the "amide salt" is a concept including the "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 poly(vinylidene 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 copolymers containing polymerization units derived from other monomers such as hexafluoropropylene. The fluoride-based polymer may have 50 mol% or more and 100 mol% or less, 60 mol% or more and 100 mol% or less, 70 mol% or more and 100 mol% or less, 80 mol% or more and 100 mol% or less, 90 mol% or more and 100 mol% or less, or 95 mol% or more and 100 mol% or less of all the polymerization units derived from fluoride-based monomers. The non-fluoride polymer may be an ether-based polymer or a non-ether-based polymer. The ether-based polymer may be at least one selected from polyethylene oxide, polypropylene oxide, and the like. The non-ether-based polymer may be at least one selected from butadiene rubber (BR), isobutylene rubber (IIR), acrylate butadiene rubber, styrene butadiene rubber (SBR), polyimide (PI), polyacrylic acid, and the like. These non-fluoride polymers may be homopolymers obtained by polymerizing a single monomer alone, or copolymers containing polymerization units derived from other monomers. The non-fluoride polymer may have 50 mol% or more and 100 mol% or less, 60 mol% or more and 100 mol% or less, 70 mol% or more and 100 mol% or less, 80 mol% or more and 100 mol% or less, 90 mol% or more and 100 mol% or less, or 95 mol% or more and 100 mol% or less of all the polymerization units derived from non-fluoride monomers.

[0019] The lithium-ion secondary battery includes 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 optionally includes a positive electrode current collector. The negative electrode includes a negative electrode layer. The negative electrode optionally includes a negative electrode current collector. At least one of the positive electrode layer, the electrolyte layer, and the negative electrode layer may contain a gel electrolyte. The positive electrode layer and the electrolyte layer may contain a gel electrolyte. All of the positive electrode layer, the electrolyte layer, and the negative electrode layer may contain a gel electrolyte.

[0020] The positive electrode layer contains a lithium layered positive electrode active material as a positive electrode active material, and optionally may contain an electrolyte, a conductive aid, a binder, various additives, and the like. When the positive electrode layer contains the gel electrolyte of the present disclosure, the positive electrode layer may further contain other electrolytes, conductive materials, binders, and various 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 intended battery performance. The thickness of the positive electrode layer is not particularly limited. 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, less 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 1±α 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 aluminate (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 type of the cathode active material may be used alone, or two or more types 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 solid electrolytes include solid electrolytes containing Li element, M element (M is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, In), and S element. Further, the sulfide solid electrolyte may further contain at least one of O element and halogen element. Examples of sulfide 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, etc. Note that the description of "Li2S-P2S5" means a material made using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions. In addition, "X" in the above LiX represents a halogen element. Examples of halogen elements include F element, Cl element, Br element, and I element, etc. LiX may be included in one or more kinds in the raw material composition containing LiX. When two or more kinds of LiX are included, the mixing ratio of two or more kinds is not particularly limited. The molar ratio of each element in the sulfide solid electrolyte can be controlled by adjusting the content of each element in the raw material. Also, the molar ratio and composition of each element in the sulfide solid electrolyte can be measured, for example, by ICP emission spectrometry.

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

[0023] Examples of the oxide-based solid electrolyte include substances having a garnet-type crystal structure containing, for example, Li element, La element, A element (A is at least one of Zr, Nb, Ta, and Al), and O element. Examples of the oxide-based solid electrolyte 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) etc. may also be used. The solid electrolyte can be used alone or in combination of two or more.

[0024] As the conductive material, known materials can be used, and examples include carbon materials and metal particles. Examples of the carbon materials include acetylene black (AB), furnace black, vapor grown carbon fiber (VGCF), carbon nanotube, and carbon nanofiber. Among them, from the viewpoint of electron conductivity, it may be at least one selected from the group consisting of VGCF, carbon nanotube, and carbon nanofiber. Examples of the metal particles include particles of Ni, Cu, Fe, and SUS. The content of the conductive material in the positive electrode layer is not particularly limited. Only one kind of the conductive material may be used alone, or two or more kinds may be used in combination.

[0025] Examples of the binder include acrylonitrile-butadiene rubber (ABR), butadiene rubber (BR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and the like. The content of the binder in the positive electrode layer is not particularly limited. Only 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, a known metal that can be used as a current collector of a lithium-ion secondary battery can be used. Examples of such a metal include metal 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 the positive electrode current collector include SUS, aluminum, nickel, iron, titanium, and carbon. The form of the positive electrode current collector is not particularly limited, and it can be in various forms such as foil and mesh.

[0027] The electrolyte layer may be a liquid electrolyte layer using an electrolytic solution as the electrolyte, or a solid electrolyte layer using a solid electrolyte as the electrolyte. The electrolyte layer contains at least an electrolyte. The electrolyte layer may contain at least one of the above solid electrolyte and the above electrolytic solution, and may further optionally contain a binder or the like. When the electrolyte layer contains the gel electrolyte of the present disclosure, the electrolyte layer may further contain other electrolytes, binders, and various additives in addition to the gel electrolyte. The contents of the electrolyte and the binder or the like in the electrolyte layer are not particularly limited. Alternatively, the electrolyte layer may have a separator or the like for holding the electrolytic solution and preventing contact between the positive electrode layer and the negative electrode layer. The thickness of the electrolyte layer is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, and may be 2 mm or less or 1 mm or less. Each of the electrolyte and the binder may be used alone or in combination of two or more. The separator may be any separator commonly used in lithium-ion secondary batteries. For example, those made of resins such as polyethylene (PE), polypropylene (PP), polyester, and polyamide can be mentioned. The separator may have a single-layer structure or a multi-layer structure. Examples of the separator with a multi-layer structure include a separator with a two-layer structure of PE / PP, or a separator with a three-layer structure of PP / PE / PP or PE / PP / PE. The separator may be made of non-woven fabrics such as cellulose non-woven fabric, resin non-woven fabric, and glass fiber non-woven fabric. The gel electrolyte of the present disclosure may be used as a separator.

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

[0029] As the negative electrode active material, among known active materials, various substances with a potential (charge-discharge potential) for intercalating and deintercalating lithium ions that is lower than that of the above positive electrode active material can be adopted. 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 adopted. Only one type of negative electrode active material may be used alone, or two or more types may be combined and used. 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 aggregation of a plurality of primary particles. Alternatively, the negative electrode active material may be in the form of a sheet (foil-like, film-like) such as a lithium foil. That is, the negative electrode layer may be composed of a sheet of the 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 thereof include SUS, copper, nickel, etc. Examples of the form of the negative electrode current collector include foil shape and plate shape. The planar shape of the negative electrode current collector is not particularly limited, and examples thereof include circular shape, elliptical shape, rectangular shape, and any polygonal shape. Further, the thickness of the negative electrode current collector varies depending on the shape, but may be, for example, in the range of 1 μm to 50 μm, or may be in the range of 5 μm to 20 μm.

[0031] The lithium ion secondary battery may be provided with tabs, terminals, etc. The lithium ion secondary battery is provided with an exterior body that houses a positive electrode, a negative electrode, an electrolyte layer, etc., as necessary. The material of the exterior body is not particularly limited as long as it is stable to the electrolyte, and examples thereof include resins such as polypropylene, polyethylene, and acrylic resin. Examples of the shape of the lithium ion secondary battery include coin type, laminate type, cylindrical type, and square type.

[0032] The lithium ion secondary battery may be a liquid-based lithium ion secondary battery using an electrolytic solution as an electrolyte, or may be a solid lithium ion secondary battery using a solid electrolyte as an electrolyte. Examples of the uses of the lithium ion secondary battery include power sources for vehicles such as hybrid vehicles (HEV), plug-in hybrid vehicles (PHEV), electric vehicles (BEV), gasoline vehicles, and diesel vehicles. Among them, it may be used as a driving power source for hybrid vehicles (HEV), plug-in hybrid vehicles (PHEV), or electric vehicles (BEV). Further, the lithium ion secondary battery may be used as a power source for moving bodies other than vehicles (for example, railways, ships, airplanes), or may be used as a power source for electrical products such as information processing devices.

Example

[0033] (Example 1) [Fabrication of positive electrode] As the positive electrode active material, LiNi 0.92 Mn 0.04Co 0.04 Powders of O2, VGCF as a conductive material, and a copolymer of polyvinylidene fluoride and hexafluoropropylene (KF8300, manufactured by Kuraray) as a polymer were mixed at a mass ratio of 83:7:10. The mixed powder was transferred to a stirring container, and dimethyl carbonate (DMC, manufactured by Kishida Chemical Co., Ltd.) as a diluent was weighed and added in an amount eight times the mass of the polymer, followed by stirring. Subsequently, an electrolytic solution was prepared such that the molar ratio of lithium bis(fluorosulfonyl)amide (LiFSA, manufactured by Kishida Chemical Co., Ltd.) as a lithium salt to propylene carbonate (PC, manufactured by Kishida Chemical Co., Ltd.) as a solvent was 0.33 (PC:LiFSA = 3:1). Then, the electrolytic solution was added to the stirring container 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 positive electrode slurry was applied onto an aluminum foil as a positive electrode current collector, cast into a film, and then 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 preparation] Graphite was used as a negative electrode active material, and a copolymer of polyvinylidene fluoride and hexafluoropropylene (KF8300, manufactured by Kuraray) was used as a polymer. The graphite powder and the polymer powder were mixed in a mortar at a mass ratio of 90:10. The mixed powder was transferred to a stirring container, and dimethyl carbonate (DMC, manufactured by Kishida Chemical) as a diluent was weighed and added in an amount eight times the mass of the polymer, followed by stirring. Subsequently, an electrolytic solution was prepared such that the molar ratio of lithium bis(fluorosulfonyl)amide (LiFSA, manufactured by Kishida Chemical Co., Ltd.) as a lithium salt to propylene carbonate (PC, manufactured by Kishida Chemical Co., Ltd.) as a solvent was 0.33 (PC:LiFSA = 3:1). Then, the electrolytic solution was added to the stirring container so that the molar ratio of the solvent, lithium salt, and polymer was 3:1:1, and the mixture was stirred while heating. When a diluent, a solvent, a lithium salt, and a polymer were heated and mixed to obtain a uniform negative electrode slurry, the negative electrode slurry was applied onto a copper foil as a negative electrode current collector, cast into a film, and then the diluent was dried at 80 °C to obtain a negative electrode having a negative electrode layer on the negative electrode current collector. [Separator] The electrolytic solution was prepared such that the molar ratio of lithium bis(fluorosulfonyl)amide (LiFSA, manufactured by Kishida Chemical Co., Ltd.) as a lithium salt to propylene carbonate (PC, manufactured by Kishida Chemical Co., Ltd.) as a solvent was 0.33 (PC:LiFSA = 3:1). And the gel electrolyte was obtained by adding and mixing the electrolytic solution and poly(vinylidene fluoride) as a polymer so that the molar ratio of the solvent, the lithium salt, and the polymer was 3:1:1. The obtained gel electrolyte was used as the separator. [Cell Fabrication] The above-mentioned negative electrode, separator, and positive electrode were stacked in this order, the positive electrode and the negative electrode were each tabbed, and the cell was fabricated by vacuum sealing with lamination. [Evaluation of Battery Characteristics] The above-mentioned cell was constrained at 0.5 MPa and subjected to 50-cycle charge and discharge at 25 °C under the conditions of an upper limit of 4.25 V - a lower limit of 2.5 V vs. Li / Li + and 0.2C.

[0034] (Example 2) In the fabrication of the positive electrode, a cell was fabricated and evaluated in the same manner as in Example 1, except that LiNi 0.83 Mn 0.12 Co 0.05 O2 was used.

[0035] (Example 3) In the fabrication of the positive electrode, a cell was fabricated and evaluated in the same manner as in Example 1, except that LiNiO2 was used as the positive electrode active material.

[0036] (Comparative Example 1) [Positive Electrode Fabrication] As the positive electrode active material, LiNi 0.92 Mn 0.04 Co 0.04O2, VGCF as the conductive material, and PVDF (Kureha #7305) as the binder were mixed in a powder form at a mass ratio of 85:10:5. N-methylpyrrolidone (NMP) was added as a solvent in an amount 0.85 times that of the positive electrode active material, and the mixture was stirred and kneaded using a planetary mixer. After that, when a uniform positive electrode slurry was obtained, the positive electrode slurry was applied onto an aluminum foil as the positive electrode current collector, cast into a film, and then dried at 80 °C to volatilize the solvent, thereby obtaining a positive electrode having a positive electrode layer on the positive electrode current collector. [Preparation of Negative Electrode] Graphite was used as the negative electrode active material, and carboxymethyl cellulose (CMC) was used as the thickening agent. The graphite powder and CMC were mixed in a mortar at a mass ratio of 98:1, and water was added as a solvent in an amount of 2 / 3 of the negative electrode active material, and the mixture was stirred and kneaded using a planetary mixer. After that, SBR was used as the binder, and graphite and the binder were mixed at a mass ratio of 98:1 to obtain a uniform negative electrode slurry. The negative electrode slurry was applied onto a copper foil as the negative electrode current collector, cast into a film, and then dried at 80 °C to volatilize the moisture, thereby obtaining a negative electrode having a negative electrode layer on the negative electrode current collector. [Electrolyte Solution] As the electrolyte solution, 1.1 M LiPF6 as the lithium salt and EC / EMC / DMC (=30 / 30 / 40 vol%) as the solvent were used, and a solution obtained by dissolving the lithium salt in the solvent was used. [Cell Preparation] The above negative electrode, separator, and above positive electrode were stacked in this order, the above electrolyte solution was injected, the positive electrode and the negative electrode were each tabbed, and the cell was vacuum-sealed by lamination. [Evaluation of Battery Characteristics] The cell was evaluated in the same manner as in Example 1.

[0037] (Comparative Example 2) In the preparation of the positive electrode, except that LiNi 0.83 Mn 0.12 Co 0.05 O2 was used, the cell was prepared and evaluated in the same manner as in Comparative Example 1.

[0038] (Comparative Example 3) In the production of the positive electrode, a cell was produced and evaluated in the same manner as in Comparative Example 1, except that LiNiO2 was used as the positive electrode active material.

[0039] [Discussion on the Results] Figure 1 is a graph showing the relationship between the number of charge-discharge cycles and the discharge capacity retention rate of the cells of Example 1 using LiNi 0.92 Co 0.04 Mn 0.04 O2 and Comparative Example 1. As shown in Figure 1, it can be seen that the cell of Example 1 using a gel electrolyte as the electrolyte has a higher discharge capacity retention rate than the cell of Comparative Example 1 using a conventional electrolyte solution as the 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-cycle charge-discharge durability of the cell when using a gel electrolyte as the electrolyte and the cell when using a conventional electrolyte solution as the electrolyte. As shown in Figure 2, it can be seen that even when the molar ratio of Ni among the constituent metal elements other than Li in Li(NCM)O2 is 80% or more, the cell when using a gel electrolyte as the electrolyte has a higher capacity retention rate than the cell when using a conventional electrolyte solution as the electrolyte. Figure 3 is the charge-discharge curve of the cell of Example 1 using a gel electrolyte as the electrolyte. As shown in Figure 3, the capacity retention rate of the cell of Example 1 after 50-cycle charge-discharge durability is 96.1%. Figure 4 is the charge-discharge curve of the cell of Comparative Example 1 using a conventional electrolyte solution as the electrolyte. As shown in Figure 4, the capacity retention rate of the cell of Comparative Example 1 after 50-cycle charge-discharge durability is 87.5%. It can be seen that the cell of Example 1 has a higher capacity retention rate after 50-cycle charge-discharge durability than the cell of Comparative Example 1.

Claims

1. A lithium-ion secondary battery, wherein 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 gel electrolyte is a gel-like composite including an electrolytic solution and a polymer, the electrolytic solution includes a cyclic carbonate as a solvent and a lithium amide salt dissolved in the cyclic carbonate, a molar ratio of the lithium amide salt to the cyclic carbonate is greater than 0.25 and equal to or less than 0.33, the positive electrode layer includes a lithium layered positive electrode active material as a positive electrode active material, the lithium layered positive electrode active material contains Ni element, and a ratio of the Ni element among constituent metal elements other than lithium is 80 mol% or more, the lithium-ion secondary battery.

2. The lithium-ion secondary battery according to Claim 1, wherein a molar ratio of the electrolytic solution 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 contains Mn element and Co element as constituent metal elements other than lithium.

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 Claim 1.

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

Images