Lithium secondary battery

Abstract

A lithium secondary battery according to the present invention comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode is an electrode at which lithium metal is deposited by charging and from which lithium metal dissolves into the non-aqueous electrolyte by discharging. The non-aqueous electrolyte includes: an oxyacid compound that includes an O-X bond; and a fluorine-containing ether. X is at least one element selected from the group that consists of: elements that can form an alloy with lithium; N; and S.

Description

Lithium secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-024691, filed on February 21, 2024 in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a lithium secondary battery.

[0003] Patent Document 1 describes a compound in which "(a) at least one hydrogen atom of an acid selected from the group consisting of a protonic acid, a sulfonic acid, and a carboxylic acid having a phosphorus atom and / or a boron atom is represented by the following general formula (A1): -SiR ａ１ R ａ２ R ａ３ {In the formula, R ａ１ , R ａ２ , and R ａ３ each independently represents a hydrocarbon group having 1 to 20 carbon atoms which may be substituted; and (b) a silyl group-containing compound (A) substituted with a silyl group represented by the general formula Q ＋ Y － {In the formula, Q ＋ represents a quaternary ammonium group, a quaternary phosphonium group, an alkali metal, or an alkaline earth metal, and Y － represents an alkoxy group or an aryloxy group.}, and / or one or more basic compounds (B) selected from the group consisting of compounds represented by the following general formula (C): X １ -SiR ｃ１ R ｃ２ R ｃ３ {In the formula, R ｃ１ , R ｃ２ , and R ｃ３ each independently represents an optionally substituted hydrocarbon group having 1 to 20 carbon atoms or an optionally substituted alkoxy group having 1 to 20 carbon atoms, and X １ is the general formula OR １ (In the formula, R １ represents a hydrogen atom, an optionally substituted hydrocarbon group having 1 to 20 carbon atoms, a silyl group having 1 to 20 carbon atoms, SO ２ CH ３ , or SO ２ CF ３(A) represents a group represented by the formula (I), or a halogen atom.}; and one or more silicon compounds (C) represented by the formula (I), wherein the composition for adding an electrolyte solution contains 1 ppm by mass or more and 100% by mass or less of the basic compound (B) and / or the silicon compound (C) relative to 100% by mass of the silyl group-containing compound (A).

[0004] Patent Document 2 describes a non-aqueous electrolyte secondary battery including a positive electrode having a positive electrode active material capable of absorbing and releasing metal ions, a negative electrode having a negative electrode active material capable of absorbing and releasing metal ions, and a non-aqueous electrolyte, wherein the positive electrode active material is represented by the following composition formula (I) (Li １＋ｘ M ３ O ２ (I) (In the above composition formula (I), x is -0.1 or more and 0.5 or less, M ３ is a plurality of elements including at least Ni and Co, and Ni / M ３ The molar ratio is 0.55 or more and 1.0 or less.)), and the non-aqueous electrolyte solution contains a lithium transition metal compound represented by the following formula (II) ((R １ ) ａ -X(=O) ｂ -(O-SiR ２ R ３ R ４ ) ｃ (In formula (II), X represents a boron atom, a phosphorus atom, or a sulfur atom, a and b represent integers of 0 to 2, and R １ represents an alkyl group, alkenyl group, or alkynyl group having 1 to 10 carbon atoms, or an aryl group or alkoxy group having 6 to 18 carbon atoms, in which a hydrogen atom bonded to a carbon atom may be substituted with a halogen atom, and may have an ether bond. ２ ~R ４ each independently represent an alkyl group, alkenyl group, or alkynyl group having 1 to 10 carbon atoms, or an aryl group having 6 to 18 carbon atoms, and c represents an integer of 1 to 3.

[0005] International Publication No. 2015 / 098471 Pamphlet Japanese Patent Application Laid-Open No. 2023-063490

[0006] However, even if the electrolyte solutions of Patent Documents 1 and 2 are employed in a lithium secondary battery (hereinafter also referred to as a "lithium metal secondary battery") that has a negative electrode in which lithium metal precipitates upon charging and in which lithium metal dissolves in a non-aqueous electrolyte upon discharging, it is not easy to improve cycle characteristics.

[0007] One aspect of the present disclosure relates to a lithium secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is an electrode on which lithium metal precipitates upon charging and from which the lithium metal dissolves in the non-aqueous electrolyte upon discharging, the non-aqueous electrolyte containing an oxo acid compound containing an O-X bond and a fluorine-containing ether, and the element X is at least one element selected from the group consisting of N, S, and elements capable of forming an alloy with lithium.

[0008] According to the present invention, the cycle characteristics of a lithium metal secondary battery can be improved. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0009] 1 is a longitudinal cross-sectional view schematically illustrating a lithium metal secondary battery according to an embodiment of the present disclosure, and is an enlarged cross-sectional view schematically illustrating an area II in FIG. 1 in a fully discharged state of the lithium metal secondary battery, and is an enlarged cross-sectional view schematically illustrating an area II in FIG. 1 in a charged state of the lithium metal secondary battery.

[0010] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits are exemplified for numerical values ​​of specific physical properties or conditions, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one may be selected from them and used alone, or two or more may be used in combination.

[0011] The present disclosure encompasses any combination of two or more claims arbitrarily selected from the appended claims, i.e., any combination of two or more claims arbitrarily selected from the appended claims may be combined unless a technical contradiction arises.

[0012] The lithium secondary battery (lithium metal secondary battery) according to the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte having lithium ion conductivity. A separator is usually disposed between the positive electrode and the negative electrode.

[0013] The lithium secondary batteries according to the present disclosure include secondary batteries using a liquid nonaqueous electrolyte (electrolytic solution) and solid-state batteries using a gel electrolyte. The gel electrolyte includes an electrolytic solution and a matrix polymer. The matrix polymer may be a fluororesin, an acrylic resin, a polyether resin, or the like.

[0014] A lithium metal secondary battery according to the present disclosure includes a negative electrode in which lithium metal precipitates upon charging and dissolves in a non-aqueous electrolyte upon discharging. The non-aqueous electrolyte includes an oxo acid compound having an O-X bond and a fluorine-containing ether, where the element X is at least one selected from the group consisting of an element capable of forming an alloy with lithium, N, and S. This configuration suppresses capacity degradation of the lithium metal secondary battery during charge / discharge cycles, improving the cycle characteristics of the lithium metal secondary battery. This effect is particularly pronounced at room temperature (e.g., a temperature of 15°C or higher and 35°C or lower). In the presence of an oxo acid compound having an O-X bond and a fluorine-containing ether, a high-quality coating is formed on the surface of the negative electrode containing lithium metal, which is thought to uniformly deposit lithium metal, thereby suppressing capacity degradation.

[0015] Details are explained below.

[0016] (Negative Electrode) The negative electrode provided in the lithium metal secondary battery is an electrode on which lithium metal is deposited upon charging and from which lithium metal dissolves in the non-aqueous electrolyte upon discharging.

[0017] In lithium metal secondary batteries, for example, 70% or more of the rated capacity is achieved by the deposition and dissolution of lithium metal. The movement of electrons at the negative electrode during charging and discharging is mainly due to the deposition and dissolution of lithium metal at the negative electrode. In lithium metal secondary batteries, for example, 50% or more, even 80% or more, or substantially 100% of the reversible capacity is achieved by the deposition and dissolution of lithium metal. Specifically, 50 to 100% (e.g., 80 to 100%) of the movement of electrons (or current from another perspective) at the negative electrode during charging and discharging is due to the deposition and dissolution of lithium metal.

[0018] In lithium metal secondary batteries, lithium metal is almost always present at the negative electrode. Lithium metal has extremely high reducing power and is prone to side reactions with non-aqueous electrolytes. Furthermore, during charging, a solid electrolyte interphase (SEI) film is formed on the negative electrode due to decomposition and / or reaction of components contained in the non-aqueous electrolyte. In lithium metal secondary batteries, the deposition of lithium metal and the formation of the SEI film proceed in parallel, resulting in a non-uniform thickness of the SEI film and a non-uniform charging reaction. When the charging reaction becomes non-uniform, lithium metal precipitates locally in a dendrite-like shape, potentially isolating portions of the lithium metal. This not only increases the surface area of ​​the lithium metal, but also further increases side reactions with the non-aqueous electrolyte. As a result, the discharge capacity decreases significantly, and cycle characteristics may deteriorate.

[0019] Furthermore, in lithium metal secondary batteries, charging and discharging are performed by the deposition and dissolution of lithium metal in the negative electrode, so the volume change of the negative electrode during charging and discharging is particularly significant. If the negative electrode expands during charging, the electrode assembly including the positive and negative electrodes may expand. If lithium metal deposits unevenly in a dendrite-like form, the electrode assembly expands significantly, and the resulting stress can cause cracks or breakage in the electrodes. Such electrode damage can significantly reduce cycle performance.

[0020] From the above, in order to improve the cycle characteristics of lithium metal secondary batteries, it is desirable to form a good SEI coating to suppress side reactions between lithium metal and the non-aqueous electrolyte and to suppress the deposition of dendritic lithium metal.

[0021] (Negative electrode current collector) The negative electrode includes at least a negative electrode current collector. In a lithium metal secondary battery, lithium metal is deposited on the surface of the negative electrode upon charging. More specifically, lithium ions contained in the non-aqueous electrolyte receive electrons on the negative electrode upon charging, becoming lithium metal, which is then deposited on the surface of the negative electrode. The lithium metal deposited on the surface of the negative electrode dissolves as lithium ions in the non-aqueous electrolyte upon discharging.

[0022] The negative electrode current collector may be any conductive sheet. The conductive sheet may be a metal foil or a resin film having a metal layer. A non-porous conductive sheet (such as a metal foil) or a porous conductive sheet (such as a mesh, net, or punched sheet) may be used as the negative electrode current collector. The negative electrode current collector is preferably made of only a material that does not reversibly react with lithium. Examples of materials that can be used for the negative electrode current collector include copper (Cu), nickel (Ni), iron (Fe), a copper alloy, and stainless steel (SUS).

[0023] From the viewpoint of ensuring a high volumetric energy density, the negative electrode may include only the negative electrode current collector when the lithium metal secondary battery is in a fully discharged state.

[0024] In order to ensure high charge / discharge efficiency, the negative electrode may include a negative electrode mixture disposed on the surface of a negative electrode current collector. The negative electrode mixture includes a negative electrode active material. The negative electrode mixture may further include a binder, a conductive material, etc. The negative electrode mixture is supported on the negative electrode current collector to form a negative electrode mixture layer. The negative electrode mixture layer is formed on the surface of the negative electrode current collector.

[0025] When the negative electrode has a negative electrode composite layer, the open circuit potential of the negative electrode at full charge may be 70 mV or less relative to the dissolution and deposition potential of lithium. When the open circuit potential of the negative electrode at full charge is 70 mV or less relative to the dissolution and deposition potential of lithium, lithium metal is present on the surface of the negative electrode (negative electrode composite layer) at full charge. This means that the negative electrode exhibits capacity due to the deposition and dissolution of lithium metal.

[0026] Here, "fully charged" refers to a state in which the battery is charged to a state of charge (SOC) of, for example, 0.98×C or more, where C is the rated capacity of the battery. The open circuit potential of the negative electrode at full charge can be measured by disassembling a fully charged battery under an argon atmosphere, removing the negative electrode, and assembling a cell with lithium metal as the counter electrode. The nonaqueous electrolyte of the cell may have the same composition as the nonaqueous electrolyte in the disassembled battery.

[0027] Furthermore, the fully discharged state of a lithium metal secondary battery refers to a state in which the battery has been discharged to a state of charge (SOC) of, for example, 0.05×C or less, where C is the rated capacity of the battery. For example, this refers to a state in which the battery has been discharged to a lower limit voltage at a constant current of 0.05 C. The lower limit voltage is, for example, 2.5 V or less.

[0028] The thickness of the negative electrode mixture layer is not particularly limited. When the lithium metal secondary battery is in a fully discharged state, the thickness of the negative electrode active material layer is, for example, 30 μm or more and 300 μm or less.

[0029] When the negative electrode has a negative electrode mixture layer, the negative electrode mixture layer is supported in the form of a film on one or both surfaces of the negative electrode current collector.

[0030] The thickness of the negative electrode current collector is not particularly limited and may be, for example, 5 μm or more and 300 μm or less, or 5 μm or more and 20 μm or less.

[0031] (Negative electrode active material) The negative electrode active material exhibits capacity by absorbing and releasing lithium ions. The negative electrode active material is a material that reversibly absorbs and releases lithium ions. Examples of such materials include carbonaceous materials and alloy-based materials. Examples of carbonaceous materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Examples of alloy-based materials include Si-containing materials (silicon element, silicon alloy, silicon compound (oxide or nitride)), Sn-containing materials (tin element, tin alloy, tin compound (oxide or nitride)), and the like. One type of negative electrode active material may be used alone, or two or more types may be used in combination.

[0032] (Negative Electrode Binder) Examples of the negative electrode binder include fluororesin, polyacrylonitrile, polyimide resin, acrylic resin, polyolefin resin, rubber polymer, etc. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, etc.

[0033] In the negative electrode mixture layer, the amount of the negative electrode binder relative to 100 parts by mass of the negative electrode active material may be 0.1 parts by mass or more, or 0.5 parts by mass or more, and 2.0 parts by mass or less, or 1.2 parts by mass or less.

[0034] (Negative electrode conductive material) The negative electrode conductive material that may be contained as an optional component in the negative electrode mixture is not particularly limited, and known conductive materials may be used. Among them, conductive carbonaceous materials are preferred. Examples of conductive carbonaceous materials include conductive carbon particles such as carbon black (carbon black, acetylene black, ketjen black, etc.), carbon nanotubes (CNTs), and carbon fibers other than CNTs.

[0035] (Positive Electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture. The positive electrode mixture includes a positive electrode active material. The positive electrode mixture may further include a binder, a conductive material, etc. The positive electrode mixture is supported on the positive electrode current collector to form a positive electrode mixture layer. The positive electrode mixture layer is formed on the surface of the positive electrode current collector.

[0036] The thickness of the positive electrode mixture layer is not particularly limited, but may be, for example, 50 μm to 150 μm, or 75 μm to 125 μm. A single positive electrode mixture layer may be formed by a plurality of layers having different morphologies. For example, two or more layers containing positive electrode active materials with different average particle sizes may be stacked, or two or more layers containing positive electrode active materials of different types or compositions may be stacked.

[0037] (Positive Electrode Active Material) A preferred example of the positive electrode active material includes a lithium-containing composite oxide containing at least Ni. From the viewpoint of increasing capacity, the proportion of Ni in the metal elements other than Li in the lithium-containing composite oxide is preferably 80 atomic % or more. Among these, the lithium-containing composite oxide is preferably a lithium nickel oxide (hereinafter also referred to as "composite oxide N") that contains Li and Ni, the proportion of Ni in the metal elements other than Li is 80 atomic % or more, and has a layered rock salt crystal structure. The proportion of Ni in the metal elements other than Li may be 88 atomic % or more, 90 atomic % or more, or 95 atomic % or more. In the composite oxide N, the proportion of Ni in the metal elements other than Li is less than 100 atomic %, may be 99 atomic % or less, or may be 98 atomic % or less.

[0038] The positive electrode active material may contain other materials, but the proportion of the composite oxide N in the positive electrode active material is, for example, 70% by mass or more, 90% by mass or more, 95% by mass or more, or 100%.

[0039] The composite oxide N may further contain at least one selected from the group consisting of Co, Mn, Al, W, Nb, Zr, Zn, Ti, Mg, V, Si, Mo, and Cr. Among these, the composite oxide N preferably contains at least one selected from the group consisting of Co, Mn, and Al. Co, Mn, and Al contribute to stabilizing the crystal structure of the composite oxide N.

[0040] From the viewpoint of reducing costs and increasing capacity, the proportion of Co in the metal elements other than Li contained in the composite oxide N is preferably 0 atomic % or more and 20 atomic % or less, and more preferably more than 0 atomic % and 5 atomic % or less.

[0041] From the viewpoint of cost reduction, the ratio of Mn to the metal elements other than Li contained in the composite oxide N may be 1 atomic % or more and 10 atomic % or less, 2 atomic % or more and 5 atomic % or less, or 3 atomic % or more and 5 atomic % or less.

[0042] The proportion of Al in the metal elements other than Li contained in the composite oxide N may be 0.1 atomic % or more and 5 atomic % or less, 0.2 atomic % or more and 3 atomic % or less, or 0.5 atomic % or more and 1 atomic % or less.

[0043] The composite oxide N is, for example, a compound represented by the formula: Li ｙ Ni ｘ M （１－ｘ） O ２－δ (0.8≦x≦1, 0<y≦1.2 and −0.05≦δ≦0.05). Here, the element M may contain at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, W, and B. x, which indicates the atomic ratio of Ni, may be 0.98 or less, or 0.95 or less. x increases or decreases with charge and discharge of the secondary battery. The element M is represented by the formula: Co １－ｘ－ａ－ｂ Mn ａ Al ｂ (0<a<0.05 and 0<b<0.05).

[0044] (Positive Electrode Binder) As the positive electrode binder, for example, a fluorine-containing polymer, hydrogenated nitrile butadiene rubber, etc. are preferable. A preferred example of a fluorine-containing polymer is a vinylidene fluoride polymer. Examples of vinylidene fluoride polymers include polymers of monomers containing vinylidene fluoride. The fluorine-containing polymer may be a combination of a vinylidene fluoride polymer and another fluorine-containing polymer. The vinylidene fluoride polymer may be a copolymer of vinylidene fluoride and another monomer. Examples of vinylidene fluoride polymers include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, etc. The vinylidene fluoride polymer contains monomer units derived from vinylidene fluoride in a proportion of, for example, 50 mol% or more, and preferably 80 mol% or more.

[0045] In the positive electrode mixture layer, the amount of the positive electrode binder relative to 100 parts by mass of the positive electrode active material may be 0.1 parts by mass or more, or 0.5 parts by mass or more, and 2.0 parts by mass or less, or 1.2 parts by mass or less.

[0046] (Positive electrode conductive material) The positive electrode conductive material that may be contained as an optional component in the positive electrode mixture layer is not particularly limited, and known conductive materials may be used. Among them, conductive carbonaceous materials are preferred. Examples of conductive carbonaceous materials include conductive carbon particles such as carbon black and graphite, carbon nanotubes (CNTs), and carbon fibers other than CNTs.

[0047] (Positive electrode current collector) The positive electrode current collector may be a conductive sheet. The conductive sheet may be a metal foil or a resin film having a metal layer. The positive electrode current collector may be a non-porous conductive sheet (such as a metal foil) or a porous conductive sheet (such as a mesh, net, or punched sheet). The positive electrode composite layer is supported in the form of a film on one or both surfaces of the positive electrode current collector. The material of the positive electrode current collector is not particularly limited, but stainless steel, aluminum, an aluminum alloy, titanium, etc. may be used.

[0048] The thickness of the positive electrode current collector is not particularly limited and may be, for example, 5 μm or more and 300 μm or less, or 5 μm or more and 20 μm or less.

[0049] (Non-aqueous electrolyte) The non-aqueous electrolyte has ion conductivity and contains an oxo acid compound containing an O-X bond and a fluorine-containing ether. The non-aqueous electrolyte may be a liquid non-aqueous electrolyte (i.e., a non-aqueous electrolyte solution) or a gel electrolyte. The non-aqueous electrolyte contains a solvent (nonaqueous solvent) and an electrolyte salt dissolved in the solvent. Examples of the electrolyte salt include lithium salt. The lithium salt generates lithium ions as cations and generates anions that are paired with the cations. The non-aqueous electrolyte containing the lithium salt has lithium ion conductivity. The non-aqueous electrolyte may further contain various additives.

[0050] The gel electrolyte contains an electrolyte salt and a matrix polymer, or contains an electrolyte salt, a solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

[0051] The concentration of the electrolyte salt in the nonaqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0052] (Oxo Acid Compound Containing an O—X Bond) In an oxo acid compound containing an O—X bond (hereinafter also referred to as "oxo acid compound OX"), the element X is at least one selected from the group consisting of elements capable of forming an alloy with lithium, N, and S. The element X contributes to the formation of a good SEI (Solid Electrolyte Interphase) coating at a negative electrode where lithium metal is almost always present. In particular, some of the elements capable of forming an alloy with lithium are thought to form alloy components on the surface of the lithium metal. An SEI coating containing an alloy component has low reaction resistance, making it easier for lithium metal to deposit throughout the negative electrode. As a result, non-uniform or localized deposition of lithium metal is suppressed, thereby suppressing deterioration of the discharge capacity of a lithium metal secondary battery during charge / discharge cycles.

[0053] The element X forming the O-X bond may be, for example, at least one element selected from the group consisting of Si, Mg, Zn, Y, Cu, Al, Se, In, Ag, As, Bi, Ca, Cd, Ga, Ge, N, Pb, Pd, Pt, Rh, Ru, S, Sn, Sr, Te, and Ti. Multiple oxo acid compounds OX containing different elements X may be used. Furthermore, when one oxo acid compound OX has multiple —O-X- bonds, the elements X contained in all of the —O-X- bonds may be the same or different.

[0054] Among the elements X, elements from Group 14 of the periodic table are particularly preferred, since they are highly stable and can form oxo acid compounds that have appropriate reactivity with metallic lithium.

[0055] Among the elements X, at least one of Si and Sn is preferred, and Si is most preferred, since it is particularly stable and can form an oxo acid compound having a suitable reactivity with metallic lithium.

[0056] The O-X bond may be a part of an M-O-X bond. The element M is a central element of an oxo acid that bonds to the oxo group (=O), and may be an element such as B, C, S, P, or N. Among these, an S-O-Si bond, an S-O-Sn bond, a P-O-Si bond, and a P-O-Sn bond are preferred in that they have an appropriate reactivity with metallic lithium.

[0057] Examples of oxoacids that can form the oxoacid compound OX include sulfuric acid, sulfonic acid, phosphoric acid, phosphonic acid, phosphinic acid, and boric acid. An example of the oxoacid compound OX is a compound represented by the formula (1):

[0058]

[0059] In formula (1), R １ is an alkyl group, an alkenyl group, or an alkynyl group, and R ２ are each independently an alkyl group, an alkenyl group, or an alkynyl group, and a is a number that is 1 smaller than the valence of the element X. However, from the viewpoint of controlling reactivity, R １ At least one hydrogen atom of R is substituted with a fluorine atom. ２ At least one of the hydrogen atoms in R may be substituted with a fluorine atom. ２ may be the same or different from each other.

[0060] R １ The number of carbon atoms in R is, for example, 1 to 6, and preferably 1 to 3. １ Specific examples include a trifluoromethyl group, a tetrafluoroethyl group, and a difluoromethyl group.

[0061] R ２ The number of carbon atoms in R is, for example, 1 to 6, and preferably 1 to 3. ２ Specific examples of the alkyl group include a methyl group, an ethyl group, and a propyl group.

[0062] Specific examples of the oxo acid compound OX include trialkylsilyl trifluoromethanesulfonate, trialkyltitanyl trifluoromethanesulfonate, and more specifically, triethylsilyl trifluoromethanesulfonate. (CF ３ SO ２ OSi(C ２ H ５ ) ３ ), trimethylsilyl trifluoromethanesulfonate (CF ３ SO ２ OSi(CH3) ３ ), tripropylsilyl trifluoromethanesulfonate (CF ３ SO ２ OSi(C ３ H ７ ) ３ ), triethylrutitanyl trifluoromethanesulfonate (CF ３ SO ２ OTi (C ２ H ５ ) ３ ) etc.

[0063] The oxo acid compound OX may be a salt (oxoacid salt) of an oxo acid and an element X (e.g., Sn, Ti, Mg, Al, Zn). More specifically, tin di(trifluoromethanesulfonate) (Sn(II) (OSO ２ -CF ３ ) ２ ), Mg(OSO ２ -CF ３ ) ２ ), Al(OSO ２ -CF ３ ) ３ ), Zn(OSO ２ -CF ３ ) ２ ) and other salts.

[0064] The content of the oxo acid compound OX in the non-aqueous electrolyte may be, for example, 0.001% by mass or more and 10% by mass or less. When the content of the oxo acid compound is equal to or more than the above-mentioned lower limit, a good SEI coating is formed, and good cycle characteristics are likely to be obtained. Furthermore, when the content of the oxo acid compound is equal to or less than the above-mentioned upper limit, side reactions are reduced, and the excellent ionic conductivity of the non-aqueous electrolyte is easily maintained.

[0065] (Fluorine-Containing Ether) The fluorine-containing ether is contained at least in the non-aqueous electrolyte. The fluorine-containing ether may be contained as a component of the non-aqueous solvent in the non-aqueous electrolyte, or may be contained as an additive in the non-aqueous electrolyte. In particular, the fluorine-containing ether is preferably contained as a main component of the non-aqueous solvent. The fluorine-containing ether is a hydrocarbon compound in which at least one hydrogen atom is substituted with a fluorine atom and which has an ether bond. The content of the fluorine-containing ether in the non-aqueous electrolyte is, for example, preferably 20% by mass or more and 80% by mass or less, and more preferably 40% by mass or more and 70% by mass or less. The non-aqueous electrolyte may further contain a non-fluorine-containing ether that does not contain fluorine.

[0066] The lowest unoccupied molecular orbital (LUMO) of the non-fluorine-containing ether (hereinafter also referred to as the "first ether compound") exists at a high energy level. Therefore, the ether is not easily reductively decomposed even when it comes into contact with lithium metal, which has strong reducing power. Furthermore, the ether oxygen strongly interacts with lithium ions, so that the lithium salt contained as the electrolyte salt in the non-aqueous electrolyte can be easily dissolved.

[0067] The first ether compound may be, for example, a compound represented by the general formula (2): １ -(OCH ２ CH ２ ) ｎ -OR ２ (In formula (2), R １ and R ２ are each independently an alkyl group having 1 to 5 carbon atoms, and n is 1 to 3.

[0068] In formula (2), R １and R ２ are each independently preferably an alkyl group having 1 to 2 carbon atoms. Also, n is preferably 1 or 2. １ and R ２ When n is in this range, the solubility of the electrolyte salt in the non-aqueous electrolyte is increased due to the interaction between oxygen in the first ether compound and lithium ions, while ensuring high fluidity and high lithium ion conductivity of the non-aqueous electrolyte.

[0069] Specific examples of the first ether compound include 1,2-dimethoxyethane, 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, etc. One type of first ether compound may be used alone, or two or more types may be used in combination.

[0070] The first ether compound is suitable as a non-aqueous solvent for use in a lithium metal secondary battery because it is less likely to undergo a side reaction with lithium metal and it increases the solubility of a lithium salt in the non-aqueous solvent. However, the first ether compound has a strong interaction with lithium ions, and therefore the desolvation energy of the ether for the lithium ions tends to be large.

[0071] On the other hand, the ether oxygen of the fluorine-containing ether (hereinafter also referred to as the "second ether compound") has a moderately small interaction with lithium ions, and the desolvation energy from the second ether compound is moderately small. Due to its strong electronegativity, the fluorine atom has the function of attracting electrons from the entire molecule of the second ether compound toward the inner core. The introduction of fluorine lowers the orbital level of the lone electron pair of the ether oxygen of the second ether compound. As a result, it is thought that the overlap between the orbitals of the lithium ion and the ether oxygen is alleviated, making it easier for the lithium ion to be desolvated.

[0072] As described above, the second ether compound improves the balance between the capture and desolvation of lithium ions in the nonaqueous electrolyte. This facilitates the reduction of lithium ions to lithium metal on the negative electrode surface, making it difficult for lithium metal to deposit locally on the negative electrode surface. This reduces the variation in the thickness of the SEI coating, making it easier for the charging reaction to proceed uniformly across the entire negative electrode.

[0073] The second ether compound may be, for example, a compound represented by the general formula (3): ａ１ H ｂ１ F ｃ１ O ｄ１ (CF ２ OCH ２ ) C ａ２ H ｂ２ F ｃ２ O ｄ２ (wherein a1≧1, a2≧0, b1≦2a1, b2≦2a2, c1=(2a1+1)−b1, c2=(2a2+1)−b2, d1≧0, and d2≧0) may be a fluorinated ether compound having a fluorination rate of 60% or more.

[0074] The fluorination rate of the second ether compound is a value expressed as a percentage (%) of the number of fluorine atoms relative to the total number of fluorine atoms and hydrogen atoms contained in the second ether compound. The fluorination rate of the second ether compound may be 65% or more.

[0075] When a1, a2, b1, b2, c1, c2, d1, and d2 in formula (3) and the fluorination rate satisfy the above ranges, the interaction between oxygen in the second ether compound and lithium ions is likely to be alleviated, a more uniform SEI coating film can be formed, and high fluidity and high lithium ion conductivity of the nonaqueous electrolyte can be easily ensured.

[0076] Furthermore, the oxidative decomposition reaction of the first ether compound, which may occur at the interface between the positive electrode and the non-aqueous electrolyte, is suppressed, thereby protecting the positive electrode. Regarding oxidative decomposition reactions, a first ether compound whose LUMO is at a high energy level has low oxidation resistance and is prone to oxidative decomposition at the interface between the positive electrode and the non-aqueous electrolyte. Meanwhile, the fluorinated moiety of the second ether compound is prone to interact with the transition metal on the positive electrode surface. It is believed that this interaction suppresses the oxidative decomposition reaction of the first ether compound.

[0077] Examples of the second ether compound include 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, etc. One type of second ether compound may be used alone, or two or more types may be used in combination.

[0078] The effect of the oxo acid compound OX in improving the SEI film is exhibited when used in combination with a second ether compound. It is believed that the second ether compound uniformly deposits lithium metal on the negative electrode surface, thereby uniformly promoting the reaction of the oxo acid compound OX on the negative electrode surface. In the absence of the second ether compound, the oxo acid compound OX reacts non-uniformly with the non-uniform deposition of lithium metal, making it difficult to form a good SEI film. In this case, the oxo acid compound OX may actually cause side reactions, resulting in a deterioration in cycle performance.

[0079] The total proportion of the first ether compound and the second ether compound in the nonaqueous solvent is, for example, 80% by volume or more, or may be 90% by volume or more, or may be 95% by volume or more. When the total proportion is 80% by volume or more, the above-mentioned effects are easily obtained, and the cycle characteristics of the lithium metal secondary battery are easily significantly improved.

[0080] The volume ratio V1 / V2 of the volume V1 of the first ether compound to the volume V2 of the second ether compound in the non-aqueous solvent is preferably 1 / 0.5 to 1 / 6, and more preferably 1 / 0.5 to 1 / 2. When the volume ratio V1 / V2 is within this range, the oxo acid compound OX is more likely to react uniformly, and side reactions between lithium metal and the non-aqueous electrolyte are more likely to be suppressed. The solubility of the electrolyte salt in the non-aqueous solvent is also increased. The volume ratio V1 / V2 is appropriately adjusted depending on the fluorination rate of the second ether compound, etc. In the present disclosure, the proportion of each solvent in the total non-aqueous solvent is expressed as a volume-based proportion (vol %) at 25°C.

[0081] The non-aqueous solvent may contain a third solvent other than the first ether compound and the second ether compound. The non-aqueous solvent may contain, for example, an ester, a nitrile, an amide, or a halogen-substituted product thereof. The non-aqueous electrolyte may contain one or more third solvents. The halogen-substituted product has a structure in which at least one hydrogen atom is substituted with a halogen atom. Examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and / or an iodine atom.

[0082] Examples of esters include carbonate esters and carboxylic acid esters. Examples of cyclic carbonate esters include ethylene carbonate, propylene carbonate, butylene carbonate, and fluoroethylene carbonate. Examples of chain carbonate esters include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate. Examples of cyclic carboxylic acid esters include γ-butyrolactone and γ-valerolactone. Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and methyl fluoropropionate.

[0083] Examples of nitriles include acetonitrile, propionitrile, and benzonitrile.

[0084] Examples of the amide include dimethylformamide and dimethylacetamide.

[0085] Ethers other than the ethers already described may also be used as the non-aqueous solvent. Among such ethers, examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, and crown ether. Examples of chain ethers that can be used include chain ethers other than the first ether compound. Examples include diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,1-dimethoxymethane, and 1,1-diethoxyethane.

[0086] However, the non-aqueous solvent used in the non-aqueous electrolyte is not limited to these.

[0087] (Electrolyte Salt) The electrolyte salt contained in the non-aqueous electrolyte may be, for example, a lithium salt, which is a salt of lithium ions and anions.

[0088] As the lithium salt, a known lithium salt used in a non-aqueous electrolyte of a lithium metal secondary battery can be used. The anion of the lithium salt is BF. ４ － , ClO ４ － , P.F. ６ － , AsF ６ － , SbF ６ － , AlCl ４ － , SCN － , C.F. ３ SO ３－ , C.F. ３ CO ２ － Examples of the anion include anions of imides, anions of oxalates, etc. The non-aqueous electrolyte may contain one or more of these anions.

[0089] Examples of anions of imides include N(SO ２ C ｍ F ２ｍ＋１ ) (SO ２ C ｎ F ２ｎ＋１ ) － (m and n are each independently an integer of 0 or more.) Each of m and n may be 0 to 3, or may be 0, 1, or 2. The anion of the imide is N(SO ２ CF ３ ) ２ － , N(SO ２ C ２ F ５ ) ２ － , N(SO ２ F) ２ － Hereinafter, N(SO ２ F) ２ － is FSI － Lithium ion and FSI － The salt lithium bis(fluorosulfonyl)imide is sometimes referred to as LiFSI.

[0090] The anion of the oxalates may contain boron and / or phosphorus. Examples of the anion of the oxalates include bisoxalate borate anion, BF ２ (C ２ O ４ ) － , P.F. ４ (C ２ O ４ ) － , P.F. ２ (C ２ O ４ ) ２ － etc.

[0091] In order to prevent lithium metal from being deposited in a dendrite-like form, the non-aqueous electrolyte contains anions of imides, PF ６ － The non-aqueous electrolyte may contain at least one selected from the group consisting of oxalate anions and oxalate anions. When a non-aqueous electrolyte containing oxalate anions is used, lithium metal is more likely to be uniformly precipitated in the form of fine particles due to the interaction between the oxalate anions and lithium. This makes it possible to suppress the uneven progress of charge / discharge reactions due to localized precipitation of lithium metal. Since the effect of uniformly precipitating lithium metal in the form of fine particles is enhanced, the use of bisoxalate borate anions and / or BF ２ (C ２ O ４ ) － Alternatively, an anion of an oxalate may be combined with another anion. The other anion may be PF ６ － , and / or anions of imides.

[0092] Among these, LiFSI is preferably used because it can form a uniform SEI film on the negative electrode and effectively suppress the deposition of dendritic lithium metal. Furthermore, from the viewpoint of reducing the viscosity of the non-aqueous electrolyte and reducing costs, a mixture of lithium ions and PF is also used together with LiFSI. ６ － Lithium hexafluorophosphate (LiPF ６ ) is preferably included.

[0093] The electrolyte salts are LiFSI and LiPF ６ When the non-aqueous electrolyte contains LiFSI, the molar concentration M1 of LiFSI and LiPF ６ The ratio of the molar concentration M1 to the molar concentration M2: M1 / M2 is preferably 1 / 0.5 to 1 / 9, and more preferably 1 / 2 to 1 / 5. At this time, a more uniform SEI coating is formed, and uniform charge / discharge reactions are more likely to occur.

[0094] The electrolyte salt is lithium ion and BF ２ (C ２ O ４ ) －It is preferable that the battery contains lithium difluorobisoxalatoborate (LiFOB), which is a salt of lithium metal. This facilitates uniform precipitation of lithium metal in the form of fine particles, which is thought to be able to suppress the progress of non-uniform charge-discharge reactions associated with local precipitation of lithium metal.

[0095] The concentration of the electrolyte salt in the non-aqueous electrolyte is preferably 0.8 mol / L to 3 mol / L, and more preferably 0.8 mol / L to 1.8 mol / L. When the concentration of the electrolyte salt is in this range, high ionic conductivity (e.g., lithium ion conductivity) of the non-aqueous electrolyte can be ensured.

[0096] Here, the concentration of the electrolyte salt is the sum of the concentration of the dissociated lithium salt and the concentration of the undissociated lithium salt. The concentration of the anion in the non-aqueous electrolyte may be set within the above-mentioned range of the concentration of the lithium salt.

[0097] (Additives) The non-aqueous electrolyte may contain an additive. The additive may form a coating on the negative electrode. The formation of a coating derived from the additive on the negative electrode facilitates more uniform charge-discharge reactions and inhibits the generation of dendritic lithium metal. This further enhances the effect of inhibiting volume changes in the negative electrode during charge-discharge, thereby further inhibiting deterioration in cycle characteristics. Examples of such additives include vinylene carbonate, fluoroethylene carbonate, and vinyl ethyl carbonate. One type of additive may be used alone, or two or more types may be used in combination.

[0098] An example of the configuration of a lithium metal secondary battery will be described below with reference to the drawings.

[0099] Fig. 1 is a longitudinal cross-sectional view schematically illustrating a lithium metal secondary battery according to an embodiment of the present disclosure, Fig. 2 and Fig. 3 are enlarged cross-sectional views schematically illustrating an area II in Fig. 1 .

[0100] The lithium metal secondary battery 10 is a cylindrical battery including a cylindrical battery case, a wound electrode group 14 housed in the battery case, and a nonaqueous electrolyte (not shown). The battery case is composed of a case body 15, which is a cylindrical metal container with a bottom, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is disposed between the case body 15 and the sealing body 16, thereby ensuring the airtightness of the battery case. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the winding axis direction.

[0101] Case body 15 has a step 21 formed, for example, by pressing a portion of the side wall of case body 15 from the outside. Step 21 may be formed in an annular shape along the circumferential direction of case body 15 on the side wall of case body 15. In this case, sealing body 16 is supported by the surface of step 21 on the opening side.

[0102] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. These components are stacked in this order in the sealing body 16. The sealing body 16 is attached to the opening of the case body 15 so that the cap 26 is located on the outside of the case body 15 and the filter 22 is located on the inside of the case body 15. Each of the above components constituting the sealing body 16 is, for example, disk-shaped or ring-shaped. Each component except for the insulating member 24 is electrically connected to each other.

[0103] The electrode group 14 has a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all strip-shaped. The positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed between them so that the width directions of the strip-shaped positive electrode 11 and the negative electrode 12 are parallel to the winding axis. In a cross section perpendicular to the winding axis of the electrode group 14, the positive electrodes 11 and the negative electrodes 12 are alternately stacked in the radial direction of the electrode group 14 with the separator 13 interposed between them.

[0104] The positive electrode 11 is electrically connected to a cap 26, which also serves as a positive electrode terminal, via a positive electrode lead 19. One end of the positive electrode lead 19 is connected, for example, near the center in the longitudinal direction of the positive electrode 11. The positive electrode lead 19 extends from the positive electrode 11 through a through-hole (not shown) formed in the insulating plate 17 and extends to the filter 22. The other end of the positive electrode lead 19 is welded to the surface of the filter 22 on the electrode group 14 side.

[0105] The negative electrode 12 is electrically connected to the case body 15, which also serves as a negative electrode terminal, via a negative electrode lead 20. One end of the negative electrode lead 20 is connected to, for example, an end of the negative electrode 12 in the longitudinal direction, and the other end is welded to the inner bottom surface of the case body 15.

[0106] As shown in Fig. 2, the positive electrode 11 includes a positive electrode current collector 110 and a positive electrode composite layer 111 disposed on both surfaces of the positive electrode current collector 110. The negative electrode 12 includes a negative electrode current collector 120. Fig. 2 shows a cross section in a fully discharged state, and Fig. 3 shows a cross section in a charged state. In the negative electrode 12 of the lithium metal secondary battery 10, lithium metal 121 is precipitated upon charging, and the precipitated lithium metal 121 dissolves in the non-aqueous electrolyte upon discharging.

[0107] (Separator) The separator 13 is made of a porous sheet having ion permeability and insulating properties. Examples of porous sheets include microporous films, woven fabrics, and nonwoven fabrics. The material of the separator is not particularly limited, but may be a polymeric material. Examples of polymeric materials include olefin resins, polyamide resins, and cellulose. Examples of olefin resins include polyethylene, polypropylene, and olefin copolymers containing at least one of ethylene and propylene as monomer units. The separator 13 may contain additives as needed. Examples of additives include inorganic fillers.

[0108] The separator 13 may include multiple layers with different shapes and / or compositions. Such a separator 13 may be, for example, a laminate of a polyethylene microporous film and a polypropylene microporous film, or a laminate of a nonwoven fabric containing cellulose fibers and a nonwoven fabric containing thermoplastic resin fibers. A microporous film, woven fabric, nonwoven fabric, or the like, having a polyamide resin coating formed on its surface, may also be used as the separator 13. Such a separator 13 has high durability, so damage is suppressed even when pressure is applied while in contact with multiple protrusions. Furthermore, from the viewpoint of ensuring heat resistance and / or strength, the separator 13 may include a layer containing an inorganic filler on the surface facing the positive electrode 11 and / or the surface facing the negative electrode 12.

[0109] (Other) A spacer may be provided between the negative electrode 12 and the separator 13 to form a space for accommodating the lithium metal 121. As described above, in the lithium metal secondary battery 10, the volume change of the negative electrode 12 associated with charging and discharging is particularly significant. If the negative electrode 12 expands during charging, the electrode group 14 including the positive electrode 11 and the negative electrode 12 may expand. The stress generated by the expansion may cause cracks or breakage in the electrodes. By providing a spacer, such electrode damage can be more easily suppressed. Note that the spacer may be provided not only between the negative electrode 12 and the separator 13, but also between the positive electrode 11 and the separator 13 at the same time.

[0110] Any known spacer can be used without any particular limitation. For example, a spacer can be provided between the negative electrode 12 and the separator 13 by using a negative electrode current collector 120 having a first surface and a second surface opposite to the first surface, with a plurality of protrusions protruding from each surface.

[0111] In the illustrated example, a cylindrical lithium metal secondary battery having a cylindrical battery case has been described, but the lithium metal secondary battery according to the present disclosure is not limited to this case. The lithium metal secondary battery according to the present disclosure can also be applied to, for example, a prismatic battery having a prismatic battery case, a laminated battery having a resin outer casing such as an aluminum laminate sheet, etc. Furthermore, the electrode group is not limited to a wound type, and may be, for example, a stacked type electrode group in which multiple positive electrodes and multiple negative electrodes are alternately stacked with a separator interposed between the positive electrodes and the negative electrodes.

[0112] (Additional Notes) The above description discloses the following technologies. (Technology 1) A lithium secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is an electrode from which lithium metal precipitates upon charging and from which the lithium metal dissolves into the non-aqueous electrolyte upon discharging, and the non-aqueous electrolyte contains an oxoacid compound having an O-X bond and a fluorine-containing ether, and wherein the element X is at least one element selected from the group consisting of elements capable of forming an alloy with lithium, N, and S. (Technology 2) The lithium secondary battery according to Technology 1, wherein the element X is at least one element selected from the group consisting of Si, Mg, Zn, Y, Cu, Al, Se, In, Ag, As, Bi, Ca, Cd, Ga, Ge, N, Pb, Pd, Pt, Rh, Ru, S, Sn, Sr, Te, and Ti. (Technology 3) The lithium secondary battery according to Technology 1 or 2, wherein the element X is an element from Group 14 of the periodic table. (Technology 4) The lithium secondary battery according to any one of Technologies 1 to 3, wherein the element X is at least one of Si and Sn. (Technology 5) The oxo acid compound is represented by the formula (1): The sulfonic acid compound is represented by the formula: １ is an alkyl group, an alkenyl group, or an alkynyl group, and R ２ are each independently an alkyl group, an alkenyl group, or an alkynyl group, a is a number that is smaller by 1 than the valence of the element X, and at least R １wherein at least one hydrogen atom is substituted with a fluorine atom. (Technology 6) The lithium secondary battery according to any one of Technologies 1 to 4, wherein the sulfonic acid compound includes triethylsilyl trifluoromethanesulfonate. (Technology 7) The lithium secondary battery according to any one of Technologies 1 to 4, wherein the oxo acid compound includes tin(II) trifluoromethanesulfonate. (Technology 8) The lithium secondary battery according to any one of Technologies 1 to 7, wherein the content of the oxo acid compound in the non-aqueous electrolyte is 0.001% by mass or more and 10% by mass or less. (Technology 9) The lithium secondary battery according to any one of Technologies 1 to 8, wherein the content of the fluorine-containing ether in the non-aqueous electrolyte is 20% by mass or more and 80% by mass or less.

[0113] [Examples] The lithium metal secondary battery according to the present disclosure will be specifically described below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0114] Lithium metal secondary batteries having the structure shown in FIG. 1 were fabricated according to the following procedure. Examples 1 to 3 (1) Fabrication of Positive Electrode A positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride as a binder were mixed in a mass ratio of 95:2.5:2.5. A suitable amount of N-methyl-2-pyrrolidone as a dispersion medium was added to the mixture and stirred to prepare a positive electrode composite slurry. A lithium-containing transition metal oxide containing Ni, Co, and Al and having a crystal structure belonging to the space group R-3m was used as the positive electrode active material.

[0115] The positive electrode composite slurry was applied to both sides of an aluminum foil serving as a positive electrode current collector and dried. The dried product was compressed in the thickness direction using a roller. The resulting laminate was cut to a predetermined electrode size to produce a positive electrode having a positive electrode composite layer on both sides of the positive electrode current collector. In addition, an exposed portion of the positive electrode current collector not having a positive electrode composite layer was formed in a partial region of the positive electrode. One end of an aluminum positive electrode lead was attached to the exposed portion of the positive electrode current collector by welding.

[0116] (2) Preparation of Negative Electrode A negative electrode current collector was formed by cutting a 10 μm thick electrolytic copper foil to a predetermined electrode size. This negative electrode current collector was used as the negative electrode for the preparation of a battery. One end of a nickel negative electrode lead was attached to the negative electrode current collector by welding.

[0117] (3) Preparation of Non-Aqueous Electrolyte An electrolyte solution was prepared by dissolving a predetermined oxo acid compound OX and a lithium salt (LiFSI) shown in Table 1 in a non-aqueous solvent shown in Table 1. The content (mass%) of the oxo acid compound OX and the concentration (mol / L) of LiFSI in the electrolyte solution are shown in Table 1.

[0118] For example, in the case of the lithium metal secondary battery A1 of Example 1, the first ether compound is 1,2-dimethoxyethane (DME) and the second ether compound is 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (CHF) having a fluorination rate of 70%. ２ CF ２ OCH ２ CF ３ ) were mixed so that the volume ratio V1 / V2 of the respective volumes V1 and V2 was 9 / 40 to prepare a non-aqueous solvent. Lithium bissulfonylimide (LiFSI) was dissolved in the obtained non-aqueous solvent to a concentration of 1 mol / L, and further, triethylsilyl trifluoromethanesulfonate (CF ３ SO ２ OSi(C ２ H ５ ) ３ ) was dissolved in the electrolyte solution so that the content thereof was 0.30 mass % to prepare an electrolyte solution.

[0119] In Example 3, tin(II) trifluoromethanesulfonate (Sn(II)(OSO ２ -CF ３ ) ２ ) was used.

[0120] (4) Battery Fabrication In an inert gas atmosphere, the positive electrode obtained in (1) above and the negative electrode obtained in (2) above were stacked with a polyethylene microporous film interposed therebetween as a separator. The resulting stack was spirally wound to produce an electrode group. The resulting electrode group was housed in a bag-shaped exterior body formed of a laminate sheet with an aluminum layer, and after injecting an electrolyte, the exterior body was sealed. In this manner, lithium metal secondary batteries A1 to A3 of Examples 1 to 3 were fabricated.

[0121] (5) Evaluation The resulting lithium metal secondary battery was subjected to a charge / discharge test according to the following procedure to evaluate the cycle characteristics.

[0122] First, the lithium metal secondary battery was charged in a thermostatic chamber at 25° C. under the following conditions, then rested for 20 minutes, and discharged under the following conditions.

[0123] (Charging) The battery was charged at a constant current of 0.1 It until the battery voltage reached 4.3 V, and then charged at a constant voltage of 4.3 V until the current value reached 0.01 It.

[0124] (Discharge) The battery was discharged at a constant current of 0.1 It until the battery voltage reached 2.5 V.

[0125] The above charge and discharge cycles were counted as one cycle, and a charge-discharge test was performed for 380 cycles. The discharge capacity at the first cycle was measured and defined as the initial discharge capacity. The ratio of the discharge capacity at the 380th cycle to the initial discharge capacity was calculated as the capacity retention rate (%), which was used as an index of cycle characteristics.

[0126] Comparative Example 1 An electrolyte solution was prepared in the same manner as in Example 1, except that the oxo acid compound OX was not used, and a lithium metal secondary battery was fabricated and evaluated.

[0127] Comparative Example 2 Instead of the oxo acid compound OX, 3,3,3-trifluoropropylmethyldimethoxysilane (CH ３ (CH ３ O) ２ SiCH ２ CH ２ CF ３) was used in the content shown in Table 1, an electrolyte solution was prepared in the same manner as in Example 1, and a lithium metal secondary battery was fabricated and evaluated.

[0128] Comparative Example 3 An electrolyte solution was prepared in the same manner as in Example 1, except that only DME was used as the solvent without using HFE, and the oxo acid compound OX was used at the content shown in Table 1, and a lithium metal secondary battery was fabricated and evaluated.

[0129] The results of Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1. The following symbols are used in Table 1: OX1: triethylsilyl trifluoromethanesulfonate (CF ３ SO ２ OSi(C ２ H ５ ) ３ OX2: tin(II) trifluoromethanesulfonate (Sn(II)(OSO ２ -CF ３ ) ２ ) NOX: 3,3,3-trifluoropropylmethyldimethoxysilane (CH ３ (CH ３ O) ２ SiCH ２ CH ２ CF ３ ) HFE: 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (CHF ２ CF ２ OCH ２ CF ３ ) DME: 1,2-dimethoxyethane (CH ３ OCH ２ CH ２ OCH ３ ) LiFSI: Lithium bis(fluorosulfonyl)imide

[0130]

[0131] As shown in Table 1, the lithium metal secondary batteries of Examples 1 to 3, which used the oxo acid compound OX and the fluorine-containing ether in the electrolyte, were able to achieve high capacity retention even after 380 cycles. The capacity retention of the lithium metal secondary battery of Comparative Example 3, which used the oxo acid compound OX without the fluorine-containing ether, was actually lower than that of Comparative Example 1.

[0132] The lithium metal secondary battery according to the present disclosure has excellent cycle characteristics. Therefore, the lithium metal secondary battery according to the present disclosure is useful in a variety of applications, such as electronic devices such as mobile phones, smartphones, and tablet devices, electric vehicles including hybrids and plug-in hybrids, and home storage batteries combined with solar cells. While the present invention has been described with reference to currently preferred embodiments, such disclosure should not be construed as limiting. Various modifications and variations will undoubtedly become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Accordingly, the appended claims should be construed to include all modifications and variations that do not depart from the true spirit and scope of the present invention.

[0133] REFERENCE SIGNS LIST 10 Lithium metal secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode group 15 Case body 16 Sealing body 17, 18 Insulating plate 19 Positive electrode lead 20 Negative electrode lead 21 Step 22 Filter 23 Lower valve body 24 Insulating member 25 Upper valve body 26 Cap 27 Gasket 110 Positive electrode current collector 111 Positive electrode composite layer 120 Negative electrode current collector 121 Lithium metal

Claims

1. A lithium secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is an electrode on which lithium metal precipitates upon charging and from which the lithium metal dissolves in the non-aqueous electrolyte upon discharging, the non-aqueous electrolyte containing an oxo acid compound having an O-X bond and a fluorine-containing ether, and the element X is at least one element selected from the group consisting of N, S, and elements capable of forming an alloy with lithium.

2. The lithium secondary battery according to claim 1, wherein the element X is at least one selected from the group consisting of Si, Mg, Zn, Y, Cu, Al, Se, In, Ag, As, Bi, Ca, Cd, Ga, Ge, N, Pb, Pd, Pt, Rh, Ru, S, Sn, Sr, Te, and Ti.

3. The lithium secondary battery according to claim 1, wherein the element X is an element in Group 14 of the periodic table.

4. The lithium secondary battery according to claim 1, wherein the element X is at least one of Si and Sn.

5. The oxoacid compound has the formula (1): The sulfonic acid compound is represented by the formula: １ is an alkyl group, an alkenyl group, or an alkynyl group, and R ２ are each independently an alkyl group, an alkenyl group, or an alkynyl group, a is a number that is smaller by 1 than the valence of the element X, and at least R １ 2. The lithium secondary battery according to claim 1, wherein at least one of the hydrogen atoms is substituted with a fluorine atom.

6. The lithium secondary battery according to claim 5, wherein the sulfonic acid compound comprises trialkylsilyl trifluoromethanesulfonate.

7. The lithium secondary battery according to claim 1, wherein the oxo acid compound comprises a trifluoromethanesulfonic acid X salt.

8. The lithium secondary battery according to any one of claims 1 to 7, wherein the content of the oxo acid compound in the non-aqueous electrolyte is 0.001 mass % or more and 10 mass % or less.

9. The lithium secondary battery according to any one of claims 1 to 7, wherein the content of the fluorine-containing ether in the non-aqueous electrolyte is 20 mass % or more and 80 mass % or less.

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