Lithium metal secondary battery

Abstract

To provide a lithium metal secondary battery excellent in high-temperature storage property, discharge rate characteristics, and cycle characteristics.SOLUTION: A lithium metal secondary battery 100 includes a positive electrode 20 that has a layer 24 including a positive electrode active material on a surface of a positive electrode collector 22, a negative electrode 30 that has a layer 34 including metallic lithium on a surface of a negative electrode collector 32, a separator 10, and an electrolytic solution. The electrolytic solution includes ionic liquid including lithium ions and a specific ether. The specific ether includes any one of dibutyl ether, diamyl ether, diisoamyl ether, hexyl methyl ether, cyclopentyl methyl ether, cyclohexyl methyl ether, and 4-methyl tetrahydropyran. A content ratio of the specific ether in the electrolytic solution is 5 mass% or more and 40 mass% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a lithium metal secondary battery. [Background technology]

[0002] Lithium metal secondary batteries, which can achieve high energy densities, are attracting attention as power sources for mobile phones and laptops, as well as for large-scale power storage and automotive applications. Unlike lithium-ion secondary batteries, which charge and discharge by inserting and extracting lithium ions into the materials that make up the electrodes, lithium metal secondary batteries charge and discharge by depositing and dissolving lithium metal. Because lithium metal is a light metal and has a high theoretical capacity and an extremely base potential, lithium secondary batteries are expected to achieve high energy densities.

[0003] During charging of a lithium metal secondary battery, lithium metal may deposit in a dendritic form (forming dendrites) with the initial point of deposition acting as a root. Dendritic lithium metal has a large specific surface area, which facilitates side reactions with the electrolyte, resulting in the accumulation of large amounts of electrolyte reductive decomposition products on the lithium metal surface. Furthermore, during discharge, the lithium metal deposited during charging dissolves and disappears, but the reductive decomposition products on the surface remain. Therefore, repeated charge-discharge cycles result in the accumulation of reductive decomposition products. This creates the problem of the reductive decomposition products impeding the charge-discharge reaction and reducing cycle characteristics.

[0004] Furthermore, if dendrite deposition progresses further and penetrates the separator, a short circuit occurs between the lithium metal negative electrode and the positive electrode, posing a safety issue.

[0005] For example, in the field of lithium-ion secondary batteries, various studies have been reported on flame-retardant electrolytes as a means of improving safety. Conventionally, electrolytes for lithium-ion secondary batteries have typically been prepared by dissolving lithium salts in volatile, flammable solvents such as cyclic carbonates or chain carbonates. Meanwhile, a technology has been proposed that improves flame retardancy by using solvated ionic liquids, which are organic solvents containing glyme and lithium salts at a molar ratio of 0.70 to 1.25 to form complexes. Furthermore, attempts have been made to use ionic liquids composed of cationic and anionic components as electrolytes. These liquids are liquid at room temperature, nonvolatile, and have a high decomposition temperature.

[0006] However, these solvated ionic liquid-based electrolytes and ionic liquid-based electrolytes have high viscosity, which means they have low permeability to separators, positive electrodes, and negative electrodes, necessitating measures such as pressure treatment during cell fabrication. Furthermore, when the viscosity of an electrolyte is high, differences in the concentration of lithium ions in the electrolyte occur during repeated charge and discharge, which can lead to dendrites and electrolyte decomposition.

[0007] For example, Patent Document 1 discloses a cathode or anode containing a sulfur-based electrode active material containing at least one selected from the group consisting of elemental sulfur, metal sulfides, metal polysulfides, and organic sulfur compounds, and a compound of the following formula R 1 -(OCHR 3 CH2) x -OR 2The present invention discloses an alkali metal-sulfur secondary battery comprising an electrolyte containing an ether compound represented by the formula (I), an alkali metal salt, and a solvent, wherein the ether compound and at least a portion of the alkali metal salt form a complex, and a counter electrode for the positive or negative electrode, the counter electrode containing the alkali metal, an alloy containing the alkali metal, carbon, or an active material that absorbs and desorbs alkali metal ions, the solvent being one or more selected from the group consisting of a fluorine-based solvent, an ionic liquid, and toluene, which is hydrophobic and completely miscible with the complex but does not chemically react with the alkali metal or alkali metal polysulfide. The invention further discloses that the ether compound is tetrahydrofuran (THF), 1,3-dioxolane, 1,4-dioxane, glyme, or a derivative thereof, and the fluorine-based solvent is a hydrofluorocarbon or a hydrofluoroether.

[0008] Patent Document 2 discloses a non-aqueous electrolyte comprising an ionic liquid, a fluorinated solvent, and an alkali metal salt, wherein the ionic liquid comprises a substituted alicyclic quaternary ammonium cation and an anion for the substituted alicyclic quaternary ammonium cation. Furthermore, the patent document discloses that the fluorinated solvent is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] WO2013 / 141195 publication [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-26608 Summary of the Invention [Problem to be solved by the invention]

[0010] The present inventors have extensively investigated electrolyte solutions containing the fluorine-based solvents (hereinafter referred to as hydrofluoroethers) disclosed in Patent Documents 1 and 2 as electrolyte solutions for lithium metal secondary batteries. As a result, they have found that hydrofluoroethers have low coordination properties and maintain the lithium ion complex structure in the electrolyte without disrupting it, making it possible to reduce the viscosity of the electrolyte without adversely affecting the lithium ion transport efficiency in the electrolyte or the battery characteristics. However, the hydrofluoroethers are highly volatile, and have the problem of gasifying during high-temperature storage, causing the battery cell to swell. Another problem is that the battery characteristics include a decrease in discharge rate performance and cycle performance. The cause of this decrease in battery performance is thought to be the volatilization of the hydrofluoroether due to heat generated during charge and discharge reactions, which leads to a decrease in discharge rate performance and cycle performance.

[0011] In view of the above problems, an object of the present invention is to provide a lithium metal secondary battery that is excellent in high-temperature storage stability, discharge rate characteristics, and cycle characteristics. [Means for solving the problem]

[0012] The present inventors have found that a lithium metal secondary battery using an ionic liquid containing lithium ions and a secondary battery electrolyte containing a specific ether can achieve good high-temperature storage stability, discharge rate characteristics, and cycle characteristics.

[0013] That is, in order to solve the above problems, the following means are provided.

[0014] [1] Provided is a lithium metal secondary battery comprising: a positive electrode having a positive electrode active material layer containing a positive electrode active material on the surface of a positive electrode current collector; a negative electrode having a lithium metal layer containing metallic lithium on the surface of a negative electrode current collector; a separator; and an electrolyte solution, wherein the electrolyte solution contains an ionic liquid containing lithium ions and a specific ether, the specific ether being any of dibutyl ether, diamyl ether, diisoamyl ether, hexyl methyl ether, cyclopentyl methyl ether, cyclohexyl methyl ether, and 4-methyltetrahydropyran, and the specific ether is contained in the electrolyte solution in an amount of 5% by mass to 40% by mass.

[0015] [2] The ionic liquid containing lithium ions contains at least one cation selected from the group consisting of pyrrolidinium cations and imidazolium cations, and a lithium metal secondary battery is provided in which the cations and lithium ions are contained in a molar ratio of 2:1 to 1:1.

[0016] [3] The lithium ion-containing ionic liquid provides a lithium metal secondary battery containing monoglyme and lithium ions in a molar ratio of 3:1 to 1:1. [Effects of the Invention]

[0017] According to this configuration, the use of an electrolyte solution containing an ionic liquid containing lithium ions and a specific ether reduces the viscosity of the electrolyte solution without destroying the lithium ion complex structure in the electrolyte solution. Furthermore, since the specific ether has low volatility, a lithium metal secondary battery with excellent high-temperature storage stability, discharge rate characteristics, and cycle characteristics can be realized. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a cross-sectional view of a lithium metal secondary battery according to an embodiment of the present invention; [Figure 2] FIG. 1 is a diagram showing a complex formed from an anion and a lithium cation in which only one specific ether of the present embodiment is coordinated. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present embodiment will be described in detail below with reference to the accompanying drawings. The drawings used in the following description may show characteristic portions enlarged for the sake of clarity, and the dimensional proportions of each component may differ from the actual proportions. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate modifications may be made within the scope of the present invention.

[0020] [Lithium metal secondary battery] Fig. 1 is a cross-sectional schematic diagram of a lithium metal secondary battery according to this embodiment. The lithium metal secondary battery 100 shown in Fig. 1 mainly comprises a laminate 40, a case 50 that houses the laminate 40 in a sealed state, and a pair of leads 60, 62 connected to the laminate 40. Although not shown, an electrolyte is also housed in the case 50 together with the laminate 40.

[0021] The laminate 40 is formed by disposing a positive electrode 20 and a negative electrode 30 facing each other with a separator 10 sandwiched therebetween. The positive electrode 20 is formed by providing a positive electrode active material layer 24 on a plate-shaped (film-shaped) positive electrode current collector 22. The negative electrode 30 is formed by providing a lithium metal layer 34 on a plate-shaped (film-shaped) negative electrode current collector 32.

[0022] The positive electrode active material layer 24 and the lithium metal layer 34 are in contact with both sides of the separator 10. Leads 62 and 60 are connected to the ends of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the ends of the leads 62 and 60 extend to the outside of the case 50. Although FIG. 1 illustrates a case where one laminate 40 is provided inside the case 50, multiple laminates may be provided.

[0023] [Electrolyte for secondary batteries] The electrolyte solution for a secondary battery according to this embodiment (hereinafter simply referred to as "electrolyte solution") contains an ionic liquid containing lithium ions and a specific ether, and the specific ether is contained in the electrolyte solution in an amount of 5% by mass to 40% by mass. This reduces the viscosity of the electrolyte solution without destroying the lithium ion complex structure in the electrolyte solution. Furthermore, since the specific ether has low volatility, a lithium metal secondary battery with excellent high-temperature storage stability, discharge rate characteristics, and cycle characteristics can be realized.

[0024] [Ionic liquids containing lithium ions] The ionic liquid containing lithium ions according to this embodiment refers to a mixture of an ionic liquid composed of a cation component and an anion component and a lithium salt, or a solvated ionic liquid in which a high concentration of lithium salt is added to glyme, an organic solvent, to form a complex. These are flame-retardant but tend to have high viscosity.

[0025] [Ionic liquid] Ionic liquids are ionic compounds that are liquid at room temperature and consist of cationic and anionic components. Examples of cationic components that can be used include quaternary ammonium compounds made from nitrogen-containing compound cations and quaternary phosphonium compounds made from phosphorus-containing compound cations.

[0026] Examples of quaternary ammonium cations consisting of nitrogen-containing compound cations include, but are not limited to, tetraalkylammonium cations, pyrrolidinium cations, piperidinium cations, pyrazolium cations, pyrrolinium cations, pyrrolium cations, pyridinium cations, thiazolium cations, and imidazolium cations.

[0027] The tetraalkylammonium cation is represented by the chemical formula (1), and R, R 2、R3 and R4 each independently represent a linear or branched alkyl group, alkenyl group, alkynyl group, or alkylenealkoxy group having 1 to 5 carbon atoms, and may be the same or different. Examples of the cation include, but are not limited to, diethylmethylmethoxyethylammonium cation, trimethylethylammonium cation, trimethylpropylammonium cation, trimethylhexylammonium cation, tetrapentylammonium cation, and diethylmethyl(2-methoxyethyl)ammonium cation.

[0028] [ka]

[0029] The pyrrolidinium cation is represented by chemical formula (2), where R1 and R2 each independently represent a linear or branched alkyl group, alkenyl group, alkynyl group, or alkylenealkoxy group having 1 to 5 carbon atoms, and may be the same or different. Examples include, but are not limited to, 1,1-dimethylpyrrolidinium cation, 1-ethyl-1-methylpyrrolidinium cation, 1-methyl-1-propylpyrrolidinium cation, and 1-butyl-1-methylpyrrolidinium cation.

[0030] [ka]

[0031] The piperidinium cation is represented by chemical formula (3), where R1 and R2 each independently represent a linear or branched alkyl group, alkenyl group, alkynyl group, or alkylenealkoxy group having 1 to 5 carbon atoms, and may be the same or different. Examples include, but are not limited to, 1,1-dimethylpiperidinium cation, 1-ethyl-1-methylpiperidinium cation, 1-methyl-1-propylpiperidinium cation, and 1-butyl-1-methylpiperidinium cation.

[0032] [ka]

[0033] Examples of pyrazolium cations include, but are not limited to, 1,2-dimethylpyrazolium cation, 1-ethyl-2-methylpyrazolium cation, 1-propyl-2-methylpyrazolium cation, 1-butyl-2-methylpyrazolium cation, and the like.

[0034] Examples of pyrrolinium cations include, but are not limited to, 1,2-dimethylpyrrolinium cation, 1-ethyl-2-methylpyrrolinium cation, 1-propyl-2-methylpyrrolinium cation, 1-butyl-2-methylpyrrolinium cation, and the like.

[0035] Examples of pyrrolium cations include, but are not limited to, 1,2-dimethylpyrrolium cation, 1-ethyl-2-methylpyrrolium cation, 1-propyl-2-methylpyrrolium cation, 1-butyl-2-methylpyrrolium cation, and the like.

[0036] Examples of pyridinium cations include, but are not limited to, N-methylpyridinium cation, N-ethylpyridinium cation, and N-butylpyridinium cation.

[0037] Examples of thiazolium cations include, but are not limited to, ethyldimethylthiazolium cation, butyldimethylthiazolium cation, hexadimethylthiazolium cation, and methoxyethylthiazolium cation.

[0038] The imidazolium cation is represented by chemical formula (4), where R1 and R2 each independently represent a linear or branched alkyl group, alkenyl group, alkynyl group, alkoxymethyl group, or alkoxyethyl group having 1 to 5 carbon atoms, and may be the same or different. Examples include 1-ethyl-3-methylimidazolium cation, 1-propyl-3-methylimidazolium cation, and 1-butyl-3-methylimidazolium cation, but are not limited to these.

[0039] [ka]

[0040] An example of a quaternary phosphonium system consisting of a phosphorus-containing compound cation is a phosphonium cation having a skeleton represented by chemical formula (5). In chemical formula (5), R1, R2, R3, and R4 each independently represent a linear or branched alkyl group, alkenyl group, alkynyl group, or alkylenealkoxy group having 1 to 5 carbon atoms, and may be the same or different. They may also have a cyclic structure such as a five-membered ring or a six-membered ring.

[0041] [ka]

[0042] Specific examples include, but are not limited to, tetraethylphosphonium cation, tetramethylphosphonium cation, tetrapropylphosphonium cation, tetrabutylphosphonium cation, triethylmethylphosphonium cation, trimethylethylphosphonium cation, dimethyldiethylphosphonium cation, trimethylpropylphosphonium cation, trimethylbutylphosphonium cation, dimethylethylpropylphosphonium cation, methylethylpropylbutylphosphonium cation, and the like.

[0043] The cation is preferably a pyrrolidinium cation or an imidazolium cation. Ionic liquids containing these cations have high ionic conductivity and can achieve even better cycle characteristics.

[0044] The anion of the ionic liquid is ClO4 - , PF6 - , BF4 - , AsF6 - , B(C2O4)2 - , CF3SO3 - , Cl - , Br - , I - Among them are BF4 - BF3(CF3) in which at least one fluorine atom is substituted with a fluoroalkyl group - , BF3(C2F5) - , BF3(C3F7) - , BF2(CF3)2 - , BF2(CF3)(C2F5) - Or PF6 - PF5(CF3) in which at least one fluorine atom is substituted with a fluoroalkyl group - , PF5(C2F5) - , PF5(C3F7) - , PF4(CF3)2 - , PF4(CF3)(C2F5) - , PF3(CF3)3 - etc. may also be used.

[0045] Other examples include anions containing the chemical structural formula represented by chemical formula (6). In chemical formula (6), R1 and R2 are selected from the group consisting of halogen and alkyl fluoride. R1 and R2 may be the same or different. Specific examples include: - N(FSO2)2, - N(CF3SO2)2, - N(C2F5SO2)2, - N(CF3SO2)(C4F9SO2).

[0046] [ka]

[0047] Examples of anions include anions having a chemical structural formula represented by chemical formula (7). In chemical formula (8), R1, R2, and R3 are selected from the group consisting of halogen and alkyl fluoride. R1, R2, and R3 may be the same or different. Specific examples include: - C(CF3SO2)3, - C(C2F5SO2)3, etc.

[0048] [ka]

[0049] In this embodiment, an ionic liquid containing these cations and anions as its constituent elements can be used. However, the anion is hydrophilic, and BF4 - and PF6 - From the viewpoint of improving the solubility of lithium salts, it is more preferable to use an ionic liquid using an imide anion such as that of chemical formula (6) than an anion such as those mentioned above.

[0050] [Grime] An example of the glyme in this embodiment is a compound represented by the following chemical formula (8).

[0051] [ka]

[0052] In the chemical formula (8), R1 and R2 are each independently an alkyl group having 1 to 3 carbon atoms.

[0053] x is represented by 1 to 3, and examples thereof include monoglyme (G1, x=1), diglyme (G2, x=2), and triglyme (G3, x=3). Among these, monoglyme is more preferable in terms of reducing the viscosity of the electrolyte.

[0054] One type of glyme may be used, or multiple types may be used.

[0055] [Lithium salt] Examples of lithium salts in this embodiment include LiPF, LiBF, LiAsF, LiClO, and LiB. 10 Cl 10 , Li2B 12 Cl 12 , LiB(C2O4)2, LiCF3SO3, LiCl, LiBr, LiI, etc., and it is also possible to use LiBF3(CF3), LiBF3(C2F5), LiBF3(C3F7), LiBF2(CF3)2, LiBF2(CF3)(C2F5) in which at least one fluorine atom of LiBF4 has been substituted with a fluoroalkyl group, or LiPF5(CF3), LiPF5(C2F5), LiPF5(C3F7), LiPF4(CF3)2, LiPF4(CF3)(C2F5), LiPF3(CF3)3 in which at least one fluorine atom of LiPF6 has been substituted with a fluoroalkyl group.

[0056] Lithium salts also include salts made of compounds containing the chemical structural formula shown in chemical formula (9). In chemical formula (9), R1 and R2 are selected from the group consisting of halogens and alkyl fluorides. R1 and R2 may be different. Specific examples include LiN(FSO2)2, LiN(CF3SO2)2, LiN(C2F5SO2)2, and LiN(CF3SO2)(C4F9SO2).

[0057] [ka]

[0058] Lithium salts also include salts made of compounds containing the chemical structural formula represented by chemical formula (10). In chemical formula (10), R1, R2, and R3 are selected from the group consisting of halogens and alkyl fluorides. R1, R2, and R3 may be different. Specific examples include LiC(CF3SO2)3 and LiC(C2F5SO2)3. Among these, the lithium salt represented by chemical formula (9) is more preferred because it can increase solubility.

[0059] [ka]

[0060] [Specific Ether] More preferred specific examples of the specific ether in this embodiment include dibutyl ether, diamyl ether, diisoamyl ether, hexyl methyl ether, cyclopentyl methyl ether, cyclohexyl methyl ether, and 4-methyltetrahydropyran.

[0061] The reason for this effect is speculated as follows, using Figure 2. The specific ether has one ether oxygen site capable of coordinating to the lithium ion. Compared to the counter anion that is thought to form a complex with the lithium ion in the electrolyte, there is only one coordination site, and the ether oxygen has a sterically bulky structure. This prevents multiple ether solvents from coordinating to the lithium ion, and does not significantly disrupt the complex structure. Therefore, there is no effect on lithium ion transport. In addition, the molecular weight of the specific ether is not excessively large, so it can be easily dissolved in the electrolyte. Note that the specific ether in Figure 2 is explained as an example of 4-methyltetrahydropyran.

[0062] On the other hand, in the case of hydrofluoroether, due to the electron-withdrawing property of fluorine atoms, the electron density of the ether oxygen of hydrofluoroether is thought to be lower than that of ordinary ethers. Therefore, the coordination property of hydrofluoroether to lithium ions is extremely low, and volatility is high. In contrast, the specific ether that does not contain fluorine atoms is thought to be coordinated to lithium ions to an extent that does not hinder the transport of lithium ions, and as a result, volatility is suppressed.

[0063] Furthermore, the proportion of the specific ether in the electrolyte is preferably 5% by mass or more and 40% by mass or less. When the amount of ether added to the electrolyte is 5% by mass or more and 40% by mass or less, the viscosity of the electrolyte becomes favorable without reducing lithium ion transport, and the impregnation into the separator is improved. Therefore, better battery characteristics are obtained. If the amount is less than 5% by mass, the viscosity of the electrolyte cannot be sufficiently reduced. If the amount is more than 40% by mass, the proportion of ether coordinated to lithium ions increases, affecting lithium ion transport in the electrolyte.

[0064] The electrolyte solution may also contain other components as needed to improve its functionality, such as other conventionally known solvents, overcharge inhibitors, dehydrating agents, deoxidizing agents, and characteristic improving aids.

[0065] [Other solvents] The electrolyte solution of the present invention may contain other solvents in addition to the ionic liquid, the specific ether, and the glyme represented by the chemical formula (4), as long as the electrolyte solution does not undergo phase separation. Examples of other solvents include dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, di-n-propyl carbonate, methyl-n-propyl carbonate, ethyl-n-propyl carbonate, methyl isopropyl carbonate, ethyl-n-propyl carbonate, ethyl isopropyl carbonate, di-n-propyl carbonate, diisopropyl carbonate, 3-fluoropropylmethyl carbonate, propylene carbonate, ethylene carbonate, butylene carbonate, 4-chloro- Examples of such solvents include 1,3-dioxolan-2-one, 4-fluoro-1,3-dioxolan-2-one, 4-trifluoromethyl-1,3-dioxolan-2-one, vinylene carbonate, dimethylvinylene carbonate, vinylene carbonate, and fluoroethylene carbonate, as well as other carbonate esters such as alkyl propionates, dialkyl malonates, and alkyl acetates, as well as other carboxylic acid esters such as γ-butyrolactone, cyclic sulfonates such as propane sultone, and alkyl sulfonates. The content of such other solvents is preferably 10% by volume or less, and more preferably 5% by volume or less, when the total amount of solvents used in the electrolytic solution is taken as 100% by volume.

[0066] [Other ingredients] The electrolyte solution of the present invention may contain other components as needed to improve its functionality, such as conventionally known overcharge inhibitors, dehydrating agents, deoxidizing agents, and characteristic improving aids for improving capacity retention after high-temperature storage.

[0067] Examples of overcharge inhibitors include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partially fluorinated compounds of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; and fluorine-containing anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, and 2,6-difluoroaniol. One or more overcharge inhibitors may be used alone or in combination. When the electrolyte contains an overcharge inhibitor, the content of the overcharge inhibitor in the electrolyte is preferably 0.1 to 5% by mass. By incorporating 0.1% or more by mass of the overcharge inhibitor in the electrolyte, it becomes easier to prevent secondary battery explosions and fires due to overcharge, thereby enabling the secondary battery to be used more stably.

[0068] Examples of dehydrating agents include molecular sieves, sodium sulfate, magnesium sulfate, calcium hydride, potassium hydride, sodium hydride, lithium aluminum hydride, calcium chloride, and metallic sodium. The solvent used in the electrolytic solution of the present invention is preferably one that has been dehydrated with the dehydrating agent and then rectified. Alternatively, a solvent that has been dehydrated with the dehydrating agent alone without rectification may be used.

[0069] Examples of the property improving aids include carbonate compounds such as fluoroethylene carbonate, trifluoropropylene carbonate, phenylethylene carbonate, erythrityl carbonate, and spiro-bis-dimethylene carbonate; carboxylic acid anhydrides such as succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, and phenylsuccinic anhydride; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, and dicarboxylic acid anhydrides. Examples of suitable auxiliary property improving agents include sulfur-containing compounds such as methyl sulfone, diphenyl sulfone, methyl phenyl sulfone, dibutyl disulfide, dicyclohexyl disulfide, tetramethylthiuram monosulfide, N,N-dimethylmethanesulfonamide, and N,N-diethylmethanesulfonamide; nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide; hydrocarbon compounds such as heptane, octane, and cycloheptane; and fluorine-containing aromatic compounds such as fluorobenzene, difluorobenzene, hexafluorobenzene, and benzotrifluoride. These auxiliary property improving agents may be used alone or in combination. When the electrolytic solution contains the auxiliary property improving agent, the content of the auxiliary property improving agent in the electrolytic solution is preferably 0.1 to 5% by mass.

[0070] "Positive electrode" [Cathode active material layer] The positive electrode active material layer 24 is mainly composed of a positive electrode active material, a positive electrode binder, and a necessary amount of a positive electrode conductive additive.

[0071] [Cathode active material] The positive electrode active material used in the positive electrode active material layer 24 is a material that can absorb and release lithium ions, desorb and insert lithium ions (intercalation), or convert lithium ions into counter anions of lithium ions (e.g., PF6 -An electrode active material that can reversibly dope and undope with

[0072] For example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese spinel (LiMn2O4), and the general formula: LiNi x Co y Mn z M a O2 (x + y + z + a = 1, 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, 0 ≤ a ≤ 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), lithium vanadium compound (LiV2O5), olivine-type LiMPO4 (where M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li4Ti5O 12 )、LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1), etc. Composite metal oxides can be mentioned.

[0073] The composition ratio of the positive electrode active material in the positive electrode active material layer 24 is preferably 80% by mass or more and 90% by mass or less. Also, the composition ratio of the conductive assistant in the positive electrode active material layer 24 is preferably 0.5% by mass or more and 10% by mass or less, and the composition ratio of the positive electrode binder in the positive electrode active material layer 24 is preferably 0.5% by mass or more and 10% by mass or less.

[0074] [Positive electrode binder] The positive electrode binder binds the active materials together and also binds the positive electrode active materials to the positive electrode current collector 22. Any positive electrode binder may be used as long as it is capable of the above-mentioned bonding, and examples thereof include fluororesins such as polyvinylidene fluoride (PVDF), polyethersulfone (PESU), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF).

[0075] In addition to the above, vinylidene fluoride-based fluororubbers such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFPTFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber) may also be used as the positive electrode binder.

[0076] Furthermore, an electron-conductive conductive polymer or an ion-conductive conductive polymer may be used as the positive electrode binder. Examples of electron-conductive conductive polymers include polyacetylene. In this case, the positive electrode binder also functions as a conductive additive, so there is no need to add a conductive additive. Examples of ion-conductive conductive polymers include composites of polymer compounds such as polyethylene oxide and polypropylene oxide with lithium salts or lithium-based alkali metal salts.

[0077] [Positive electrode current collector] The positive electrode current collector 22 may be any conductive plate material, such as a thin metal plate made of aluminum or nickel foil.

[0078] [Conductive additive for positive electrode] The positive electrode conductive additive is not particularly limited, and any known conductive additive can be used as long as it improves the conductivity of the positive electrode active material layer 24. Examples of the conductive additive include carbon-based materials such as graphite and carbon black, fine metal powders such as copper, nickel, stainless steel, and iron, and conductive oxides such as ITO.

[0079] "Negative electrode" The negative electrode 30 includes a negative electrode current collector 32 and a lithium metal layer 34 .

[0080] [Negative electrode current collector] The negative electrode current collector 32 may be any conductive plate material, such as a thin metal plate made of copper or nickel foil.

[0081] [Lithium metal layer] The lithium metal layer 34 may be a foil-like lithium metal or particulate lithium metal having a thickness of about 10 to 100 μm that is previously formed on the negative electrode current collector 32. Alternatively, a lithium metal layer 34 may be used in which lithium ions from the positive electrode are deposited as the lithium metal layer 34 on the negative electrode current collector 32 during charging, utilizing a charge / discharge reaction.

[0082] [Separator] The separator 10 may be formed from an electrically insulating porous structure, and examples thereof include a monolayer or laminate of a film made of polyethylene, polypropylene, or polyolefin, or a stretched film of a mixture of the above resins, or a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, and polypropylene.

[0083] [case] The case 50 seals the laminate 40 and the electrolyte solution inside. The case 50 is not particularly limited as long as it can prevent leakage of the electrolyte solution to the outside and intrusion of moisture and the like into the lithium secondary battery 100 from the outside.

[0084] For example, as shown in Figure 1, a metal laminate film in which metal foil 52 is coated on both sides with polymer films 54 can be used as case 50. Hereinafter, a battery using a laminate form will be called a laminate cell. For example, aluminum foil can be used as metal foil 52, and a film such as polypropylene can be used as polymer film 54. For example, a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide, is preferred as the material for outer polymer film 54, and polyethylene (PE), polypropylene (PP), etc. is preferred as the material for inner polymer film 54.

[0085] [Lead] The leads 60, 62 are made of a conductive material such as nickel, aluminum, etc. The leads 60, 62 are welded to the negative electrode current collector 32 and the positive electrode current collector 22, respectively, and the separator 10 is sandwiched between the positive electrode active material layer 24 of the positive electrode 20 and the lithium metal layer 34 of the negative electrode 30. The battery is then inserted into the case 50 together with the electrolyte, and the entrance of the case 50 is sealed.

[0086] [Secondary battery manufacturing method] A method for manufacturing the lithium metal secondary battery 100 according to this embodiment will be described.

[0087] The positive electrode 20 can be produced by forming a positive electrode active material layer 24 on a positive electrode current collector 22. First, a positive electrode active material, a positive electrode binder, and a solvent are mixed to produce a positive electrode coating material. If necessary, a conductive additive may be further added. Examples of solvents that can be used include water and N-methyl-2-pyrrolidone. The composition ratio of the positive electrode active material, conductive additive, and positive electrode binder is preferably 80% to 98% by mass: 0.1% to 10% by mass: 0.1% to 10% by mass. These are adjusted so that the total is 100% by mass.

[0088] The method of mixing the components constituting the positive electrode paint is not particularly limited, and the mixing order is also not particularly limited. The positive electrode paint is applied to the positive electrode current collector 22 to form the positive electrode current collector layer 24. The application method is not particularly limited, and any method commonly used in producing electrodes can be used. Examples include slit die coating and doctor blade methods.

[0089] Next, the solvent in the positive electrode current collector layer 24 applied to the positive electrode current collector 22 is removed. The removal method is not particularly limited. For example, the solvent can be dried in an air atmosphere, a reducing atmosphere, or an inert atmosphere at 80°C to 150°C.

[0090] The positive electrode 20 having the positive electrode active material layer 24 formed thereon is then subjected to a pressing process using a roll press or the like, as necessary. The linear pressure of the roll press varies depending on the material used, but is adjusted so that the density of the positive electrode active material layer 24 becomes a predetermined value. The relationship between the density of the positive electrode active material layer 24 and the linear pressure can be determined in advance by considering the relationship between the density and the linear pressure and the ratio of the materials constituting the positive electrode active material layer 24.

[0091] The negative electrode 30 is fabricated by forming a lithium metal foil or particulate lithium metal with a thickness of approximately 10 to 100 μm as a lithium metal layer 34 on the negative electrode current collector 32. Alternatively, the negative electrode 30 can be fabricated by utilizing a charge / discharge reaction to deposit lithium ions from the positive electrode as the lithium metal layer 34 on the negative electrode current collector 32 during charging.

[0092] When particulate lithium metal is formed in advance as the lithium metal layer 34, for example, lithium metal particles whose surfaces have been protected with lithium carbonate or the like are dispersed in an organic solvent (e.g., hexane) to prepare a coating liquid. Next, the coating liquid is formed on the negative electrode current collector 32 in an inert atmosphere and dried to form the lithium metal layer 34. The coating method is not particularly limited. Possible coating methods include, for example, a knife coater method and a gravure coater method.

[0093] Next, the positive electrode 20 having the positive electrode active material layer 24, the negative electrode 30 having the lithium metal layer 34, the separator 10 interposed between the positive electrode and the negative electrode, and the electrolyte are sealed in the case 50.

[0094] For example, the positive electrode 20, the negative electrode 30, and the separator 10 are stacked together, and the stack 40 is placed in a bag-shaped case 50 that has been prepared in advance.

[0095] Finally, the electrolyte solution of this embodiment is poured into the case 50, and the case 50 is vacuum-sealed to produce the lithium metal secondary battery 100. Note that instead of pouring the electrolyte solution into the case, the laminate 40 may be impregnated with the electrolyte solution. Since the positive electrode active material layer 24 has been adjusted to a predetermined density, the electrolyte solution sufficiently penetrates into the voids formed in the positive electrode active material layer 24.

[0096] Although the embodiments of the present invention have been described above in detail with reference to the drawings, the configurations and combinations thereof in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope of the present invention. [Example]

[0097] The lithium metal secondary battery of the present disclosure will be specifically described below using examples. As Examples 1 to 30, lithium metal secondary batteries were fabricated by changing the ionic liquid, glyme, and specific ether species described in the embodiments, as well as by changing the mixed molar ratio with the lithium salt, as described below, and the high-temperature storage characteristics, cycle characteristics, and discharge rate characteristics were evaluated. For comparison, Comparative Examples 1 to 9 were also fabricated and similarly evaluated. It should be noted that Examples 1, 2, and 22 to 30 should be read as Reference Examples 1, 2, and 22 to 30.

[0098] Example 1 <Preparation of electrolyte> Lithium bis(fluorosulfonyl)imide (hereinafter abbreviated as LiFSA; molecular weight 187.07) was prepared as the lithium salt, which is the source of lithium ions, and diethylmethylmethoxyethylbis(fluorosulfonyl)imide (hereinafter abbreviated as DEMEFSA) was added as the ionic liquid at a molar ratio of 1:1. 4-Methyltetrahydropyran, a specific ether, was added to the solution at a molar ratio of 1:0.6 to LiFSA to prepare the electrolyte.

[0099] <Positive electrode> As the positive electrode 20, first, a composite metal oxide Li 1.0 Ni 0.78 Co 0.19 Al 0.03 A coating solution was prepared by dispersing 90% by mass of O2, 6% by mass of Ketjen Black, and 4% by mass of polyvinylidene fluoride in N-methyl-2-pyrrolidone. The coating solution was then applied to an aluminum foil positive electrode current collector and dried to form a positive electrode active material layer 24. The positive electrode active material layer 24 was then pressed using a press. A 30 x 40 mm sample was cut out, and an aluminum foil lead was attached by ultrasonic welding to produce a positive electrode 20.

[0100] <Negative electrode> First, a 20 μm thick lithium foil was attached to a 10 μm thick copper foil to prepare the negative electrode 30. The foil was cut into a size of 30×40 mm, and a nickel foil lead was attached by ultrasonic welding to prepare the negative electrode 30.

[0101] <Fabrication of lithium metal secondary batteries> A 13 μm thick polypropylene separator was prepared, and a 32 × 42 mm separator 10 was fabricated. The positive electrode 20, separator 10, and negative electrode 30 were stacked in this order and placed in a 50 × 60 mm bag-shaped aluminum laminate exterior. At this time, the positive electrode active material layer 24 of the positive electrode 20 and the lithium metal layer 31 of the negative electrode 30 were each positioned adjacent to the separator 10. After the electrolyte prepared above was poured into the battery, the battery was vacuum-sealed, and a lithium metal secondary battery 100 for evaluation was fabricated.

[0102] "High-temperature storage stability evaluation" The high-temperature storage stability evaluation was performed according to the following procedure. The fabricated lithium metal secondary battery of Example 1 was charged to an end voltage of 4.4 V at a constant current equivalent to 0.1 C in a thermostatic chamber at 25°C. Note that 1 C represents the current value required to charge or discharge the battery's reference capacity in 1 hour, and 0.1 C represents 1 / 10 of that current value. The charged lithium metal secondary battery was stored in a thermostatic chamber at 45°C for 24 hours, and the expansion of the lithium metal secondary battery's thickness was observed. The high-temperature storage stability evaluation was performed by assigning a "good" to a secondary battery whose thickness after storage was less than 120 mm, and an "x" to a battery whose thickness was 120 mm or greater, assuming that the thickness of the lithium metal secondary battery before storage was 100 mm. The thickness of the lithium metal secondary battery was measured in the stacking direction of the positive electrode, negative electrode, and separator.

[0103] "Cycle characteristic evaluation" The cycle characteristics were evaluated according to the following procedure. In a thermostatic chamber at 25°C, the lithium metal secondary battery of Example 1 was charged to 4.4 V at a constant current equivalent to 0.1 C, and then discharged to 3.0 V at a constant current equivalent to 0.1 C to measure the discharge capacity. This constitutes one cycle, and 100 cycles were performed. The ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle was calculated, and the cycle characteristics were evaluated as the discharge capacity retention rate (%) after 100 cycles.

[0104] "Discharge rate characteristic evaluation" The discharge rate characteristic evaluation was performed according to the following procedure. At 25°C, the lithium metal secondary battery of Example 1 was charged at a constant current equivalent to 0.1 C to an end voltage of 4.4 V, and then discharged at a constant current equivalent to 0.1 C to 3.0 V (first cycle). Next, it was charged at a constant current equivalent to 0.5 C to an end voltage of 4.4 V, and then discharged at a constant current equivalent to 0.5 C to 3.0 V (second cycle). The ratio of the discharge capacity in the second cycle to the discharge capacity in the first cycle was calculated, and this was taken as the "discharge rate characteristic (%)."

[0105] <Evaluation of high-temperature storage stability of lithium metal secondary batteries> The high-temperature storage stability was evaluated using the evaluation method described above. The evaluation results are shown in Table 1.

[0106] <Evaluation of cycle characteristics of lithium metal secondary batteries> The cycle characteristics were evaluated using the evaluation method described above, and the evaluation results are shown in Table 1.

[0107] <Evaluation of discharge rate characteristics of lithium metal secondary batteries> The discharge rate characteristics were evaluated using the evaluation method described above. The evaluation results are shown in Table 1.

[0108] Example 2 Except for changing the ionic liquid to trimethylpropylammonium bis(fluorosulfonyl)imide (hereinafter abbreviated as TMPAFSA), an electrolyte solution was prepared in the same manner as in Example 1, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0109] Example 3 An electrolyte solution was prepared in the same manner as in Example 1, except that the ionic liquid was changed to 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (hereinafter abbreviated as P13FSA) and the specific ether was changed to diamyl ether at a ratio of 1:0.4 to LiFSA, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0110] Example 4 Except for changing the ionic liquid to P13FSA and the specific ether to dibutyl ether at a ratio of 1:0.8 to LiFSA, an electrolyte solution was prepared in the same manner as in Example 1, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0111] Example 5 Except for changing the ionic liquid to P13FSA and the specific ether to diisoamyl ether at a ratio of 1:0.3 to LiFSA, an electrolyte solution was prepared in the same manner as in Example 1, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0112] (Examples 6 to 8) Except for changing the ionic liquid to P13FSA and the specific ethers to hexyl methyl ether, cyclohexyl methyl ether, and cyclopentyl methyl ether, respectively, an electrolyte solution was prepared in the same manner as in Example 1, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0113] Examples 9 to 12 Except for changing the ratio of P13FSA as the ionic liquid and 4-methyltetrahydropyran as the specific ether to LiFSA at 1:0.3, 1:0.6, 1:1, and 1:3, respectively, an electrolyte solution was prepared in the same manner as in Example 1, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0114] Example 13 An electrolyte solution was prepared in the same manner as in Example 1, except that the ionic liquid was 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (hereinafter abbreviated as EMImFSA) and the specific ether was 4-methyltetrahydropyran in a 1:1 ratio relative to LiFSA, and a lithium metal secondary battery 100 for evaluation was fabricated. The resulting lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0115] Example 14 An electrolyte solution was prepared in the same manner as in Example 1, except that the ionic liquid was changed to EMImFSA and the specific ether was changed to dibutyl ether at a ratio of 1:0.8 to LiFSA, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0116] (Examples 15 to 17) Except for changing the ionic liquid to EMImFSA and the specific ether to hexyl methyl ether, cyclohexyl methyl ether, or cyclopentyl methyl ether, an electrolyte solution was prepared in the same manner as in Example 1, and a lithium metal secondary battery 100 for evaluation was fabricated. The resulting lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0117] (Examples 18 to 19) Except for changing the ratio of EMImFSA as the ionic liquid and 4-methyltetrahydropyran as the specific ether to LiFSA at 1:3 and 1:0.3, respectively, an electrolyte solution was prepared in the same manner as in Example 1, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0118] (Examples 20 to 21) An electrolyte solution was prepared in the same manner as in Example 1, except that the ionic liquid was changed to EMImFSA and LiFSA at a ratio of 1:1.5 and 1:2, and the specific ether was changed to 4-methyltetrahydropyran and LiFSA at a ratio of 1:0.3, respectively, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0119] Example 22 LiFSA was prepared as a lithium salt, and diglyme (hereinafter abbreviated as G2) was added as a glyme at a molar ratio of 1:1.7. 4-Methyltetrahydropyran was added as a specific ether at a ratio of 1:0.4 relative to the LiFSA concentration to prepare an electrolyte solution, and a lithium metal secondary battery 100 for evaluation was fabricated. The resulting lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0120] Example 23 Except for changing the glyme to triglyme (hereinafter abbreviated as G3), an electrolyte solution was prepared in the same manner as in Example 22, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0121] (Examples 24 to 27) Except for changing the ratio of LiFSA to monoglyme (hereinafter abbreviated as G1) as the glyme to 1:1, 1:1.7, 1:2.5, and 1:3, the electrolyte solution was prepared in the same manner as in Example 22, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0122] Example 28 Except for changing the glyme to G1 and the specific ether to 4-methyltetrahydropyran at a ratio of 1.3:1 relative to LiFSA, an electrolyte solution was prepared in the same manner as in Example 22, and a lithium metal secondary battery 100 for evaluation was fabricated. The resulting lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0123] (Examples 29 to 30) Except for changing the ratio of G1 as the glyme and hexyl methyl ether as the specific ether to LiFSA at 0.4:1 and 1:1, respectively, an electrolyte solution was prepared in the same manner as in Example 22, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0124] (Comparative Example 1) An electrolyte solution was prepared in the same manner as in Example 1, except that the ionic liquid was changed to P13FSA and the specific ether was changed to 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (hereinafter abbreviated as HFE), and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0125] (Comparative Example 2) Except for changing the ionic liquid to EMImFSA and the specific ether to HFE, an electrolyte solution was prepared in the same manner as in Example 1, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0126] (Comparative Example 3) Except for changing the specific ether to HFE, an electrolyte solution was prepared in the same manner as in Example 25, and a lithium metal secondary battery 100 for evaluation was fabricated. The resulting lithium metal secondary battery 100 was fabricated in the same manner as in Example 1. The results are shown in Table 1.

[0127] Comparative Example 4 Except for changing the ratio of HFE to LiFSA as the specific ether to 1:0.6, an electrolyte solution was prepared in the same manner as in Example 25, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0128] (Comparative Examples 5 to 6) Except for changing the ratio of 4-methyltetrahydropyran to LiFSA as the specific ether to 1.5:1 and 0.1:1, an electrolyte solution was prepared in the same manner as in Example 25, and a lithium metal secondary battery 100 for evaluation was fabricated. The obtained lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0129] (Comparative Examples 7 to 9) Except for changing the specific ether to diethyl ether, dipropyl ether, or dioctyl ether, an electrolyte solution was prepared in the same manner as in Example 25, and a lithium metal secondary battery 100 for evaluation was fabricated. The resulting lithium metal secondary battery 100 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0130] As shown in Table 1, the lithium metal secondary batteries of Examples 1 to 30 are superior to the lithium metal secondary batteries of Comparative Examples 1 to 9 in high-temperature storage stability, cycle characteristics, and discharge rate characteristics. [Table 1] [Explanation of symbols]

[0131] 10... separator, 20... positive electrode, 22... positive electrode current collector, 24... positive electrode active material layer, 30... negative electrode, 32... negative electrode current collector, 34... lithium metal layer, 40... laminate, 50... case, 60, 62... leads, 100... lithium metal secondary battery.

Claims

[Claim 1] a negative electrode having a layer containing metallic lithium on the surface of a negative electrode current collector; a separator; and an electrolyte solution, wherein the electrolyte solution contains an ionic liquid containing lithium ions and a specific ether, the ionic liquid containing lithium ions containing at least one cation selected from the group consisting of pyrrolidinium cations and imidazolium cations, and the cations and lithium ions are contained in a molar ratio of 2:1 to 1:1, and the specific ether is any of dibutyl ether, diamyl ether, diisoamyl ether, hexyl methyl ether, cyclopentyl methyl ether, cyclohexyl methyl ether, and 4-methyltetrahydropyran, and the specific ether accounts for 5% by mass or more and 40% by mass or less of the electrolyte solution.

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