Lithium secondary battery

Abstract

This lithium secondary battery is provided with: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and a nonaqueous electrolyte. The nonaqueous electrolyte includes: a fluorinated vinylene ether compound A having -CF=CH-O- bond; and a nonaqueous solvent.

Description

[Technical Field]

[0001] The present disclosure relates to lithium secondary batteries. [Background technology]

[0002] Lithium secondary batteries have high voltage and high energy density, and are therefore expected to be used in small consumer applications, power storage devices, and as power sources for electric vehicles.

[0003] Patent Document 1 discloses a lithium secondary battery that has high energy density, excellent cycle characteristics, and is less susceptible to electrolyte ignition. This lithium secondary battery contains an electrolyte that contains 60% to 99% by volume of a phosphate ester compound, 0% to 30% by volume of a fluorinated ether compound, and 1% to 35% by volume of a fluorinated carbonate compound, with the total of the phosphate ester compound and the fluorinated ether compound being 65% by volume or more. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 107242 Summary of the Invention

[0005] The lithium secondary battery disclosed in Patent Document 1 does not sufficiently improve the decrease in discharge capacity when charge / discharge cycles are repeated.

[0006] The lithium secondary battery according to the present disclosure includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the non-aqueous electrolyte contains a fluorinated vinylene ether compound A having a -CF=CH-O- bond and a non-aqueous solvent.

[0007] According to the present disclosure, the charge-discharge cycle characteristics of a lithium secondary battery can be improved. DETAILED DESCRIPTION OF THE INVENTION

[0008] The lithium secondary battery according to the present disclosure includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte.

[0009] In lithium secondary batteries, a side reaction may occur between the surface of the negative electrode and the non-aqueous electrolyte at the negative electrode, which may result in a deterioration in the charge-discharge cycle characteristics of the battery.

[0010] Here, the non-aqueous electrolyte contains a fluorinated vinylene ether compound A having a —CF═CH—O— bond and a non-aqueous solvent.

[0011] When the non-aqueous electrolyte contains fluorinated vinylene ether compound A, the fluorinated vinylene ether compound A in the non-aqueous electrolyte reacts and polymerizes on the negative electrode surface from the early stage of charging, forming a high-quality coating on the negative electrode surface. Therefore, it is believed that the high-quality coating derived from fluorinated vinylene ether compound A suppresses side reactions between the non-aqueous electrolyte and the negative electrode surface, improving the charge-discharge cycle characteristics.

[0012] The fluorinated vinylene ether compound A may be any compound having a -CF=CH-O- bond, and preferably R A1 -CF=CH-OR A2 (In the formula, R A1 , R A2 are each a hydrogen atom, an alkyl group, a halogenated alkyl group, a halogen atom, an alkenyl group, a halogenated alkenyl group, a cyano group, an amino group, a nitro group, an alkoxy group, a halogenated alkoxy group, a cycloalkyl group, a halogenated cycloalkyl group, or a silyl group. Among these, from the viewpoint of facilitating the formation of a coating derived from the fluorinated vinylene ether compound A on the negative electrode surface, it is preferable to contain at least one of CF2=CH-O-CF2-CF2H, CF3-CF=CH-O-CF2-CF2H, CF2H-CF=CH-O-CF2-CF2H, and CF2=CH-O-CF2-CF3, and it is more preferable to contain at least CF2=CH-O-CF2-CF2H.

[0013] The amount of fluorinated vinylene ether compound A contained in the non-aqueous electrolyte may be 5 ppm or more relative to the total amount of the non-aqueous electrolyte. From the viewpoint of facilitating the formation of a coating derived from fluorinated vinylene ether compound A on the surface of the negative electrode, the amount may be 10 ppm or more, 50 ppm or more, or 100 ppm or more. There is no particular upper limit for the amount of fluorinated vinylene ether compound A, but it is preferably 1500 ppm or less, and may be 1000 ppm or less, 500 ppm or less, or 250 ppm or less.

[0014] When a fluorinated vinylene ether compound A within the above range is used, the effect of suppressing the formation of a coating derived from decomposition products of the non-aqueous electrolyte is significantly achieved. However, since the fluorinated vinylene ether compound A is consumed in the battery by being utilized in reactions and polymerization on the negative electrode surface from the early stage of charging, when a non-aqueous electrolyte sampled from the battery is analyzed, the content of the fluorinated vinylene ether compound A may be less than 5 ppm. On the other hand, it is rare for the fluorinated vinylene ether compound A to be completely consumed. From the viewpoint of achieving the effects of the present disclosure, it is sufficient that the non-aqueous electrolyte sampled from the battery contains the fluorinated vinylene ether compound A at a concentration equal to or greater than the detection limit.

[0015] The non-aqueous solvent contained in the non-aqueous electrolyte preferably contains an ether compound X different from the fluorinated vinylene ether compound A. The ether compound X is less likely to undergo a side reaction with the negative electrode surface. The ether compound X preferably accounts for, for example, 50% by volume or more of the entire non-aqueous solvent, but may also account for 60% by volume or more, 70% by volume or more, or 90% by volume or more. The entire non-aqueous solvent may be the ether compound X.

[0016] The content of each component of the non-aqueous electrolyte can be determined using, for example, high performance liquid chromatography, gas chromatography-mass spectrometry (GC-MS), nuclear magnetic resonance (NMR), inductively coupled plasma mass spectrometry (ICP-MS), elemental analysis, or the like.

[0017] The ether compound X is, for example, R1-(OCH2CH2) n The alkyl ether compound Y may be represented by —OR2. R1 and R2 are each an alkyl group having 1 to 5 carbon atoms, and preferably an alkyl group having 1 to 2 carbon atoms. Furthermore, n is preferably 1 to 4, and more preferably 1 or 2. By using the alkyl ether compound Y as the main component of the non-aqueous solvent, the solubility of the lithium salt in the non-aqueous electrolyte is increased, and high fluidity and high lithium ion conductivity of the non-aqueous electrolyte are ensured.

[0018] Here, the main component of the non-aqueous solvent refers to a component that occupies, for example, 20% by volume or more of the non-aqueous solvent. The alkyl ether compound Y may occupy, for example, 20% by volume or more and 80% by volume or less of the non-aqueous solvent.

[0019] Examples of the alkyl ether compound Y include tetrahydrofuran, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc. The alkyl ether compound Y may be used alone or in combination of two or more.

[0020] As the ether compound X, (C x1 H y1 F z1 )-O-(C x2 H y2 F z2 ) wherein x1, x2, and z1 are integers of 1 or greater, y1, y2, and z2 are integers of 0 or greater, and satisfy x1+x2≦10 and 1≦y1+y2. The saturated hydrofluoroether compound Z may account for, for example, 20% by volume or greater and 80% by volume or less of the non-aqueous solvent.

[0021] The fluorination rate of the saturated hydrofluoroether compound Z may be 60% or more, 80% or more, or even 100%. Here, the fluorination rate of the saturated hydrofluoroether compound Z is the ratio of the number of fluorine atoms to the total number of fluorine atoms and hydrogen atoms contained in the saturated hydrofluoroether compound Z, expressed as a percentage (%).

[0022] By using saturated hydrofluoroether compound Z, a coating derived from fluorinated vinylene ether compound A is uniformly formed on the negative electrode surface, making it easier to achieve improved charge-discharge cycle characteristics. Although the detailed mechanism is unclear, fluorinated vinylene ether compound A and saturated hydrofluoroether compound Z are both fluorinated ether compounds and therefore highly compatible. Therefore, fluorinated vinylene ether compound A is more easily dispersed uniformly in the non-aqueous electrolyte than in a mixed solution of a non-fluorinated ether compound and fluorinated vinylene ether compound A. Therefore, by mixing saturated hydrofluoroether compound Z and fluorinated vinylene ether compound A, when fluorinated vinylene ether compound A reacts and polymerizes on the negative electrode surface, the reaction proceeds more uniformly, forming a high-quality coating.

[0023] Specific examples of the saturated hydrofluoroether compound Z include 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, etc. The fluorinated ether compounds may be used alone or in combination of two or more.

[0024] The lithium secondary battery of the present disclosure will be described in more detail below, with respect to each component.

[0025] (positive electrode) The positive electrode includes a positive electrode active material. The positive electrode typically includes a positive electrode current collector and a positive electrode mixture held on the positive electrode current collector. The positive electrode mixture includes a positive electrode active material as an essential component, and may include optional components such as a binder, a thickener, and a conductive agent. The positive electrode typically includes a layer of positive electrode mixture (hereinafter referred to as a positive electrode mixture layer) held on the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the components of the positive electrode mixture are dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied layer. The dried coating may be rolled, if necessary.

[0026] The positive electrode active material may be, for example, a lithium transition metal composite oxide having a layered rock salt structure. In particular, by using a lithium transition metal composite oxide containing Ni and at least one of Co, Al, and Mn as the main component of the positive electrode active material, it is possible to achieve high capacity and high voltage.

[0027] Here, the main component of the positive electrode active material refers to a component that accounts for, for example, 50% by mass or more of the positive electrode active material. The lithium transition metal composite oxide preferably accounts for, for example, 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more of the positive electrode active material.

[0028] The lithium transition metal composite oxide used as the positive electrode active material is Li α Ni 1-a1-a2-a3-b Co a1 Mn a2 Al a3 M b O 2+β It is preferable that the lithium transition metal oxide A contains a lithium transition metal oxide A represented by the formula: where 0.95≦α≦1.05, 0.8≦1-a1-a2-a3-b≦0.99, 0≦a1≦0.1, 0≦a2≦0.1, 0≦a3≦0.1, 0≦b≦0.1, −0.05≦β≦0.05, and M is at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y. Lithium transition metal oxide A has an especially high capacity among lithium transition metal oxides.

[0029] a3, which indicates the ratio of Al (atomic ratio), preferably satisfies 0 < a3 ≤ 0.1 from the viewpoints of thermal stability and durability, and preferably satisfies 0.01 ≤ x3 ≤ 0.1.

[0030] a1, which indicates the ratio of Co (atomic ratio), preferably satisfies 0 < a1 ≤ 0.1 from the viewpoints of output characteristics and durability, and preferably satisfies 0.01 ≤ a3 ≤ 0.1.

[0031] (1 - a1 - a2 - a3 - b), which indicates the ratio of Ni (atomic ratio), preferably satisfies 0.8 ≤ 1 - a1 - a2 - a3 - b ≤ 0.99 from the viewpoints of high capacity and stability, and more preferably satisfies 0.9 ≤ 1 - a1 - a2 - a3 - b ≤ 0.95.

[0032] The higher the ratio of Ni, the more lithium ions can be extracted from the lithium transition metal composite oxide during charging, and the capacity can be increased. However, when the ratio of Ni increases, the structure of the lithium transition metal composite oxide becomes unstable. Therefore, side reactions with the non-aqueous electrolyte easily occur on the surface of the lithium transition metal composite oxide particles. As a result, decomposition products due to side reactions between the surface of the lithium transition metal composite oxide particles and the non-aqueous electrolyte further react with the surface of the negative electrode, and the charge-discharge cycle characteristics of the lithium secondary battery are likely to deteriorate. In the lithium secondary battery of the present disclosure, since the non-aqueous electrolyte contains the fluorinated vinylene ether compound A, by forming a high-quality film on the surface of the negative electrode, side reactions between decomposition products due to side reactions between the surface of the lithium transition metal composite oxide particles and the non-aqueous electrolyte and the surface of the negative electrode can be suppressed. As a result, excellent cycle characteristics can be ensured.

[0033] As the material of the positive electrode current collector, for example, foils of metals such as stainless steel, aluminum, aluminum alloy, titanium, and films having the metal disposed on the surface layer can be used. The positive electrode current collector may further include a protective layer containing inorganic particles and a binder.

[0034] Examples of the conductive agent include carbon powders such as carbon black, acetylene black, ketjen black, graphite, and carbon nanotubes. These may be used alone or in combination of two or more.

[0035] Examples of binders include fluororesins (e.g., polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resins (e.g., polyethylene, polypropylene), polyamide resins (e.g., aramid resin), polyimide resins (e.g., polyimide, polyamideimide), acrylic resins (e.g., polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymer, ethylene-acrylic acid copolymer, or salts thereof), vinyl resins (e.g., polyvinyl acetate), and rubber-like materials (e.g., styrene-butadiene rubber (SBR)).

[0036] Examples of thickeners include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modified forms, methyl cellulose, etc. Modified forms of CMC also include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts.

[0037] The dispersion medium for the positive electrode slurry may be, for example, N-methyl-2-pyrrolidone (NMP).

[0038] From the viewpoint of increasing the capacity of the battery, the density of the positive electrode mixture layer may be 2.5 g / cc or more, or may be 3.0 g / cc or more. A protective layer containing inorganic particles, a binder, etc. may be formed on the positive electrode mixture layer.

[0039] (Negative electrode) The negative electrode includes a negative electrode current collector. The shape and thickness of the negative electrode current collector can be selected from the shapes and ranges described for the positive electrode current collector. Examples of materials for the negative electrode current collector (metal foil) include stainless steel, nickel, nickel alloy, copper, and copper alloy.

[0040] The negative electrode may further include a negative electrode mixture supported on a negative electrode current collector. The negative electrode mixture includes a negative electrode active material as an essential component, and may include a binder, a thickener, a conductive agent, etc. as optional components. The negative electrode may include a layer of a negative electrode mixture (hereinafter referred to as a negative electrode mixture layer) supported on the negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the components of the negative electrode mixture are dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the slurry. The dried coating film may be rolled as necessary. Examples of the dispersion medium include water and NMP.

[0041] Examples of negative electrode active materials include carbonaceous materials, Si-containing materials, and Sn-containing materials. The negative electrode may contain one type of negative electrode active material or a combination of two or more types. In particular, when a carbonaceous material and a Si-containing material are contained in combination, the mass ratio of the carbonaceous material to the total of the carbonaceous material and the Si-containing material is, for example, preferably 80 mass% or more, and more preferably 90 mass% or more.

[0042] Examples of carbonaceous materials include graphite, artificial graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon).

[0043] Examples of Si-containing materials include simple Si, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which silicon particles (fine Si phases) are dispersed in a lithium ion conductive phase (matrix). Examples of silicon oxides include SiO x The x may be, for example, 0.5≦x<2, or 0.8≦x≦1.6.

[0044] Examples of Sn-containing materials include simple Sn and alloys of silicon and Sn.

[0045] As the binder, conductive agent, etc., for example, those exemplified for the positive electrode can be used.

[0046] The negative electrode may also be of a type in which lithium metal precipitates on the negative electrode surface during charging, and the lithium metal precipitated on the negative electrode surface dissolves in the non-aqueous electrolyte during discharge. While a negative electrode in which lithium metal precipitates on the negative electrode surface during charging is advantageous for increasing the capacity of the battery, it may also produce dendritic deposits of lithium metal on the negative electrode during charging. When lithium metal precipitates on the negative electrode in a dendritic form, the specific surface area of ​​the lithium metal increases. This further increases side reactions between the lithium metal and the non-aqueous electrolyte. As a result, the discharge capacity decreases significantly, and cycle characteristics tend to deteriorate significantly. In the lithium secondary battery disclosed herein, even when a negative electrode in which lithium metal precipitates on the negative electrode surface is used, a high-quality coating can be formed on the lithium metal surface by the fluorinated vinylene ether compound A contained in the non-aqueous electrolyte. Because lithium metal precipitates mainly between the coating and the negative electrode current collector, the coating presses the lithium metal, suppressing the extension of the dendritic deposits. Therefore, when a negative electrode in which lithium metal precipitates on the surface during charging is used, the fluorinated vinylene ether compound A suppresses the formation of dendritic deposits on the negative electrode, thereby significantly improving the charge-discharge cycle characteristics.

[0047] (non-aqueous electrolyte) The term "non-aqueous electrolyte" encompasses liquid non-aqueous electrolytes (i.e., non-aqueous electrolyte solutions), gel electrolytes, and solid electrolytes, but excludes aqueous electrolytes. The gel electrolyte and solid electrolyte may be non-fluidic electrolytes formed by combining a non-aqueous electrolyte solution with a gelling agent or a matrix material.

[0048] The non-aqueous electrolyte includes a fluorinated vinylene ether compound A and a non-aqueous solvent. For example, the non-aqueous electrolyte solution includes a fluorinated vinylene ether compound A, a non-aqueous solvent, and a lithium salt dissolved in the non-aqueous solvent.

[0049] Lithium salts include, for example, LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB 10 Cl10 , lower aliphatic carboxylate lithium, lithium borate, lithium imide, oxalate complex lithium salt, etc. These may be used alone or in combination of two or more.

[0050] Examples of lithium imide include lithium bisfluorosulfonylimide (LiN(FSO2)2) (hereinafter also referred to as LiFSI), lithium bistrifluoromethanesulfonyl imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate fluorosulfonylimide (LiN(CF3SO2)(FSO2)), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF3SO2)(C4F9SO2)), and lithium bispentafluoroethanesulfonate imide (LiN(C2F5SO2)2).

[0051] Examples of the oxalate complex lithium salt include lithium bisoxalate borate (LiB(C2O4)2), lithium difluorooxalate borate (LiBF2(C2O4)) (hereinafter also referred to as LiFOB), LiPF4(C2O4), LiPF2(C2O4)2, etc. Among them, LiFOB is more preferred in that it forms a stable coating on the negative electrode surface even at high temperatures.

[0052] The total concentration of lithium salts in the non-aqueous electrolyte may be, for example, 0.05 mol / L or more and 3 mol / L or less.

[0053] Although the ether compound X already mentioned can be used as the non-aqueous solvent, other compounds may also be used. For example, cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, chain or cyclic ethers other than ether compound X, etc. may be used as at least a part of the non-aqueous solvent. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). 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 (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

[0054] (separator) It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be, for example, a microporous thin film, a woven fabric, a nonwoven fabric, or a laminate of at least two selected from these. The separator is preferably made of polyolefin (e.g., polypropylene, polyethylene).

[0055] (others) An example of the structure of a lithium secondary battery is a structure in which an electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween and a nonaqueous electrolyte are housed in an outer casing. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween. The lithium secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0056] [Example] The present disclosure will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0057] Example 1 (1) Preparation of the positive electrode.

[0058] Positive electrode active material particles (composition: LiNi 0.9 Co 0.05 Al 0.05 100 parts by mass of acetylene black, 1 part by mass of polyvinylidene fluoride, and an appropriate amount of NMP were mixed to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to both sides of an aluminum foil (positive electrode current collector). The coating was dried and then rolled to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm) on both sides of the aluminum foil. 3 ) was formed to obtain a positive electrode. (2) Preparation of the negative electrode.

[0059] The negative electrode (negative electrode current collector) was prepared by cutting electrolytic copper foil (thickness: 10 μm) to a predetermined size. (3) Preparation of non-aqueous electrolyte.

[0060] A non-aqueous electrolyte was prepared by adding CF2=CH-O-CF2-CF2H as a fluorinated vinylene ether compound A at a concentration of 50 ppm to a non-aqueous solvent mixture containing propylene carbonate (PC) and 1,2-dimethoxyethane (DME) as an ether compound X at a volume ratio of 1:2, and dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1 mol / L and lithium difluorooxalatoborate (LiFOB) at a concentration of 100 mmol / L as Li salts. (4) Fabrication of lithium secondary batteries.

[0061] An aluminum tab was attached to the positive electrode, and a nickel tab was attached to the negative electrode. Next, the positive electrode, negative electrode, and separator were arranged so that the separator was disposed between the positive electrode and negative electrode, and then they were spirally wound together. The wound electrode group was housed in a bag-shaped exterior body formed of a laminate sheet including an aluminum layer. After injecting a nonaqueous electrolyte into the exterior body, the exterior body was sealed to obtain a cell A1 for evaluation of a lithium secondary battery.

[0062] Example 2 A cell A2 was fabricated in the same manner as in Example 1 except that the concentration of CF2=CH-O-CF2-CF2H used in the non-aqueous electrolyte in Example 1 was changed to 100 ppm.

[0063] Example 3 A cell A3 was fabricated in the same manner as in Example 1, except that the concentration of CF2=CH-O-CF2-CF2H used in the non-aqueous electrolyte in Example 1 was changed to 250 ppm.

[0064] Example 4 A cell A4 was fabricated in the same manner as in Example 1, except that the concentration of CF2=CH-O-CF2-CF2H used in the non-aqueous electrolyte in Example 1 was changed to 1000 ppm.

[0065] Example 5 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (HFE) was used instead of DME used as the non-aqueous solvent in Example 1. A cell A5 was produced in the same manner as in Example 1 except for this.

[0066] Example 6 CF2H-CF=CH-O-CF2-CF2H was used in place of CF2=CH-O-CF2-CF2H used in the nonaqueous electrolyte in Example 1. A cell A6 was fabricated in the same manner as in Example 1 except for this.

[0067] Example 7 The following negative electrode was used instead of the negative electrode prepared in Example 1. Water was added to the negative electrode mixture and stirred to prepare a negative electrode slurry. The negative electrode mixture was a mixture of artificial graphite (average particle size 25 μm), styrene-butadiene rubber (SBR), and carboxymethyl cellulose sodium (CMC-Na). In the negative electrode mixture, the mass ratio of the artificial graphite, SBR, and CMC-Na was 100:1:1. The negative electrode slurry was applied to the surface of copper foil, the coating was dried, and then rolled to obtain a laminate in which a negative electrode mixture layer was formed on both sides of the copper foil. The laminate was cut to a predetermined size to prepare a negative electrode. A cell A7 was prepared in the same manner as in Example 1 except for this.

[0068] Example 8 The composition of the positive electrode active material particles used in Example 1 was LiNi 0.5 Co 0.2 Mn 0.3 The temperature was changed to O2. A cell A8 was produced in the same manner as in Example 1 except for this.

[0069] (Comparative Example 1) The non-aqueous electrolyte did not contain CF2=CH-O-CF2-CF2H, which was used in the non-aqueous electrolyte of Example 1. A cell B1 was fabricated in the same manner as in Example 1 except for this.

[0070] (Comparative Example 2) The non-aqueous electrolyte did not contain CF2=CH-O-CF2-CF2H, which was used in the non-aqueous electrolyte of Example 5. A cell B2 was fabricated in the same manner as in Example 5 except for this.

[0071] (Comparative Example 3) A cell B3 was prepared in the same manner as in Example 7 except that the non-aqueous electrolyte did not contain CF2=CH-O-CF2-CF2H, which was used in the non-aqueous electrolyte in Example 7.

[0072] (Battery evaluation) (Charge / discharge cycle characteristics) The evaluation cell was charged at a constant current of 0.3 It at a temperature of 25°C until the voltage reached 4.1 V, and then charged at a constant voltage of 4.1 V until the current reached 0.05 It. Next, the cell was discharged at a constant current of 0.3 It until the voltage reached 2.5 V. This charge-discharge cycle was repeated 20 times. The ratio of the discharge capacity to the charge capacity at the second and 20th cycles was defined as the charge-discharge efficiency (R 2cyc , R 20cyc The results are shown in Table 1.

[0073] [Table 1]

[0074] Comparing Examples 1 and 7 with Comparative Examples 1, 2, and 3 in Table 1, it can be seen that when the nonaqueous electrolyte contained fluorinated vinylene ether compound A, the charge-discharge efficiency at the second and twentieth cycles was significantly improved. Furthermore, comparing Example 1 with Example 8, it can be seen that the improvement in charge-discharge efficiency is more pronounced when the lithium transition metal oxide of the positive electrode contains 80 mol % or more of Ni relative to the total number of moles of metal elements excluding Li. Furthermore, Examples 1 and 5 show that the charge-discharge efficiency is significantly improved when the ether compound X contains saturated hydrofluoroether compound Z. [Industrial Applicability]

[0075] The present disclosure can be used in lithium secondary batteries.

Claims

1. a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte; The non-aqueous electrolyte contains CF as the fluorinated vinylene ether compound A having a —CF═CH—O— bond. ２ =CH-O-CF ２ -CF ２ H, and a non-aqueous solvent and an oxalate complex lithium salt, Lithium secondary battery.

2. The positive electrode is A lithium transition metal composite oxide A in which the ratio of Ni to the total number of moles of metal elements excluding Li is 80 mol % or more, The lithium secondary battery according to claim 1 .

3. the non-aqueous solvent contains an ether compound X different from the fluorinated vinylene ether compound A, 3. The lithium secondary battery according to claim 1 or 2.

4. The ether compound X is R １ -(OCH ２ CH ２ ) ｎ -OR ２ (In the formula, R １ and R ２ each represents an alkyl group having 1 to 5 carbon atoms, and n is an integer of 1 or more, The lithium secondary battery according to claim 3 .

5. The ether compound X is (C ｘ１ H ｙ１ F ｚ１ )—O—(C ｘ２ H ｙ２ F ｚ２ ) (wherein x1, x2, and z1 are integers of 1 or more, y1, y2, and z2 are integers of 0 or more, and x1+x2≦10, 1≦y1+y2), 5. The lithium secondary battery according to claim 3 or 4.

6. the proportion of the ether compound X in the non-aqueous electrolyte is 50% by volume or more; The lithium secondary battery according to any one of claims 3 to 5.

7. The negative electrode is Lithium metal precipitates during charging, and lithium metal dissolves in the non-aqueous electrolyte during discharging. The lithium secondary battery according to any one of claims 1 to 6.

8. The oxalate complex lithium salt includes lithium difluorooxalatoborate. The lithium secondary battery according to any one of claims 1 to 7.

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