Electrolyte for lithium secondary battery and lithium secondary battery

Abstract

The present invention provides an electrolyte which is for a lithium secondary battery and with which high energy density, favorable cycle characteristics, favorable high-temperature storage stability, and excellent charge and discharge characteristics can be all achieved. The present invention pertains to an electrolyte for a lithium secondary battery, the electrolyte comprising: an ether not having a fluorine atom; a hydrofluoroether; a cyclic compound represented by formula (1); an additive that is at least one selected from the group consisting of a sulfurous acid ester, a sulfuric acid ester, a sulfonic acid ester, and an alkyl sulfone; and a lithium bis(fluorosulfonyl)imide.

Description

Electrolyte for lithium secondary battery and lithium secondary battery

[0001] The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery.

[0002] In recent years, technology for converting natural energy such as solar or wind power into electrical energy has been attracting attention. Accordingly, various secondary batteries have been developed as electricity storage devices that are highly safe and capable of storing a large amount of electrical energy.

[0003] Among these, lithium secondary batteries, which are charged and discharged by the movement of lithium ions between a positive electrode and a negative electrode, are known to exhibit high voltage and high energy density. A typical lithium secondary battery is a lithium-ion secondary battery (LIB), which has active materials capable of retaining lithium elements in the positive electrode and the negative electrode, and is charged and discharged by the exchange of lithium ions between the positive electrode active material and the negative electrode active material.

[0004] Furthermore, for the purpose of achieving high energy density and improving productivity, lithium secondary batteries (LMB: Lithium-metal battery) that use lithium metal as the negative electrode active material instead of a material that can insert lithium ions, such as a carbon material, and lithium secondary batteries (AFB: Anode-free battery) that use a negative electrode made of a negative electrode current collector that does not have a negative electrode active material such as a carbon material or lithium metal have been developed.

[0005] On the other hand, lithium secondary batteries that can achieve high voltages have a problem in that the cycle characteristics of the lithium secondary battery deteriorate due to the high reactivity between the battery components and the electrolyte. Specifically, repeated charge and discharge consumes the electrolyte, reducing the battery capacity. Therefore, there is a need for a method to prevent such a decrease in battery capacity.

[0006] For example, Patent Document 1 discloses a positive electrode, a lattice plane (002) spacing (d 002The present invention discloses a lithium secondary battery comprising a negative electrode made of a carbonaceous material having a graphite-type crystal structure in which a particle size distribution is 0.34 nm or less, and a nonaqueous electrolyte solution in which an electrolyte is dissolved in a nonaqueous solvent containing a cyclic carbonate, wherein the nonaqueous electrolyte solution further contains 0.01 to 5 wt % of a dinitrile compound and 0.2 to 3 wt % of an S═O group-containing compound selected from the group consisting of cyclic or chain sulfites, sulfones, sulfonate esters, and sulfate esters.

[0007] Patent Document 2 also describes a battery comprising an electrode assembly in which a positive electrode plate and a negative electrode plate are arranged with a separator interposed therebetween, and an exterior body in which the electrode assembly is housed together with a non-aqueous electrolyte, the non-aqueous electrolyte containing an additive that forms a coating on the surface of the positive electrode active material, and LiPF ２ O ２ (lithium difluorophosphate) is added to the non-aqueous electrolyte secondary battery.

[0008] Furthermore, Patent Document 3 discloses a lithium metal battery including a lithium metal negative electrode, a positive electrode, and a liquid electrolyte disposed between the lithium metal negative electrode and the positive electrode, the liquid electrolyte including a cation represented by Formula 1, an anion, and an organic solvent.

[0009] Japanese Patent No. 4609751 Japanese Patent Publication No. 2014-035953 European Patent No. 3151314

[0010] However, the present inventors have found that it is difficult to provide a lithium secondary battery that has both good cycle characteristics and good high-temperature storage stability using conventional electrolyte solutions for lithium secondary batteries, including those described in the above patent documents.

[0011] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an electrolyte solution that achieves both good cycle characteristics and good high-temperature storage stability in a lithium secondary battery, and a lithium secondary battery using the same.

[0012] An electrolyte solution for a lithium secondary battery according to one embodiment of the present invention contains an ether having no fluorine atoms, a hydrofluoroether, a cyclic compound represented by the following formula (1), an additive which is at least one selected from the group consisting of sulfite esters, sulfate esters, sulfonate esters, and alkyl sulfones, and lithium bis(fluorosulfonyl)imide. In formula (1), m is an integer of 0 to 3, and R ｈ is a monovalent saturated hydrocarbon group having 1 to 10 carbon atoms.

[0013] The present inventors have found that the cycle characteristics of a lithium secondary battery are improved when the electrolyte solution for the lithium secondary battery contains a compound represented by the above formula (1), a hydrofluoroether, and a fluorine-free ether. In addition, they have found that the cycle characteristics are further improved and gas generation during high-temperature storage can be reduced by adding to the electrolyte solution at least one additive selected from the group consisting of sulfite esters, sulfate esters, sulfonate esters, and alkyl sulfones.

[0014] In the electrolyte solution for a lithium secondary battery according to one embodiment of the present invention, the cyclic compound preferably contains at least one selected from the group consisting of cyclohexane, methylcyclohexane, and ethylcyclohexane. According to such an embodiment, the lithium secondary battery tends to have even better energy density, cycle characteristics, and charge / discharge characteristics.

[0015] In the electrolyte solution for a lithium secondary battery according to one embodiment of the present invention, the additive is preferably at least one selected from the group consisting of ethylene sulfite, dimethyl sulfite, 4-methylethylene sulfite, dimethyl sulfone, and 1,3-propane sultone. According to such an embodiment, the lithium secondary battery tends to have even more excellent cycle characteristics and high-temperature storage stability.

[0016] In the electrolyte solution for a lithium secondary battery according to one embodiment of the present invention, the additive is preferably 1,3-propane sultone. According to such an embodiment, the lithium secondary battery tends to have even more excellent cycle characteristics and high-temperature storage stability.

[0017] In the electrolyte solution for a lithium secondary battery according to one embodiment of the present invention, preferably, the product (A×B) of the amount of the electrolyte solution (g) injected into the battery divided by the battery capacity (Ah), A (g / Ah), and the content B (parts by mass) of the additive per 100 parts by mass of the electrolyte solution, is 0.10 or more and 9.5 or less. According to such an embodiment, the lithium secondary battery tends to have even more excellent cycle characteristics and high-temperature storage stability.

[0018] In the electrolyte solution for a lithium secondary battery according to one embodiment of the present invention, the fluorine-free ether preferably includes at least one selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dimethoxypropane. According to such an embodiment, the lithium secondary battery tends to have even more excellent cycle characteristics and charge / discharge characteristics.

[0019] In the electrolyte solution for a lithium secondary battery according to one embodiment of the present invention, the hydrofluoroether preferably contains a chain fluorine compound having at least one of the monovalent groups represented by the following formula (A) or formula (B). According to such an embodiment, the lithium secondary battery tends to have even more excellent cycle characteristics and charge / discharge characteristics. In addition, in formulas (A) and (B), the wavy lines represent bonding sites in the monovalent groups.

[0020] In the electrolyte solution for a lithium secondary battery according to one embodiment of the present invention, the content of the additive is preferably 0.04% by mass or more and 10.00% by mass or less, based on the total mass of the electrolyte solution excluding the additive. According to such an embodiment, the lithium secondary battery tends to have even more excellent cycle characteristics and high-temperature storage stability.

[0021] A lithium secondary battery according to one embodiment of the present invention includes any one of the electrolyte solutions described above, and is charged and discharged by depositing lithium metal on the surface of the negative electrode and dissolving the deposited lithium metal. According to such an embodiment, the lithium secondary battery tends to have even better energy density, cycle characteristics, and high-temperature storage stability.

[0022] The lithium secondary battery according to one embodiment of the present invention preferably contains, as a positive electrode active material, Li ｚ Ni ｘ Co ｙ M １－ｘ－ｙ O ２＋α (wherein 0.5≦x≦1.0, 0≦y≦0.35, 0.9≦z≦1.3, −0.2≦α≦0.15, and M is at least one element selected from Mn, Al, V, Mg, Mo, Nb, Ti, Zr, Fe, Cu, Cr, Zn, F, and B.) According to such an embodiment, it is possible to more effectively suppress decomposition of the electrolyte solution that occurs on the surface of the positive electrode active material, and to simultaneously achieve high energy density, good cycle characteristics, good high-temperature storage stability, and excellent charge-discharge characteristics.

[0023] According to the present invention, it is possible to provide an electrolyte solution that achieves both good cycle characteristics and good high-temperature storage stability in a lithium secondary battery, and a lithium secondary battery using the same.

[0024] 1 is a schematic cross-sectional view of a lithium secondary battery according to one embodiment of the present invention.

[0025] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail with reference to the drawings as necessary. However, the present invention is not limited to this embodiment, and various modifications are possible without departing from the spirit of the present invention. In the drawings, the same elements are given the same reference numerals, and redundant explanations will be omitted. Furthermore, positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings unless otherwise specified. Furthermore, the dimensional ratios of the drawings are not limited to those shown in the drawings.

[0026] 1. Electrolyte Solution In this specification, the term "electrolyte solution" refers to a solution containing an electrolyte and a solvent and having ion conductivity. In a lithium secondary battery, the electrolyte solution acts as a conduction path for lithium ions. In a lithium secondary battery, the electrolyte solution may be impregnated into a separator, or may be enclosed in a sealed container together with a laminate of a positive electrode, a separator, and a negative electrode, or may be used as a material for a member that fills the separator.

[0027] The electrolyte solution for a lithium secondary battery of this embodiment (hereinafter also simply referred to as "electrolyte solution") contains an ether having no fluorine atoms, a hydrofluoroether, a cyclic compound represented by the following formula (1), an additive which is at least one selected from the group consisting of a sulfite ester, a sulfate ester, a sulfonate ester, and an alkyl sulfone, and lithium bis(fluorosulfonyl)imide. In the following formula (1), m is an integer of 0 to 3, and R ｈ is a monovalent saturated hydrocarbon group having 1 to 10 carbon atoms.

[0028] The present inventors have found that the cycle characteristics of a lithium secondary battery are improved when the electrolyte solution of this embodiment contains a compound represented by formula (1), a hydrofluoroether, and a fluorine-free ether. Additionally, they have found that adding at least one additive selected from the group consisting of sulfite esters, sulfate esters, sulfonate esters, and alkyl sulfones to such an electrolyte solution further improves the cycle characteristics and reduces gas generation during high-temperature storage. Furthermore, the electrolyte solution of this embodiment can synergistically improve the energy density and charge / discharge characteristics of a lithium secondary battery, as described below.

[0029] When a lithium secondary battery is discharged or charged, an oxidation or reduction reaction of lithium occurs, allowing energy to be extracted or stored. Since the decomposition reaction of the electrolyte is generally more likely to occur than the oxidation-reduction reaction of lithium, irreversible decomposition of the electrolyte can cause volume expansion of the battery with each charge / discharge cycle. This can result in electrolyte starvation (a dry-up state), in which the electrolyte no longer fills the electrodes, resulting in a decrease in battery capacity. Such battery volume expansion and electrolyte starvation can be prevented by controlling the properties of the solid electrolyte interface layer (SEI layer) formed on the surface of the negative electrode or other electrode during charge / discharge of the lithium secondary battery. It is believed that the electrolyte solution of this embodiment can form a high-quality SEI layer in the lithium secondary battery. Furthermore, the electrolyte solution of this embodiment can improve the cycle characteristics, high-temperature storage stability, energy density, and charge / discharge characteristics of the lithium secondary battery.

[0030] The cyclic compound of formula (1) has a cyclohexane skeleton and does not have any substituents other than saturated hydrocarbon groups. Such compounds are bulky due to restricted molecular motion, and their low polarity limits intermolecular interactions, resulting in a small number of molecules per unit volume and a low compound density. Therefore, lithium secondary batteries using such electrolytes have improved mass-based energy density. Furthermore, since the cyclic compound of formula (1) does not have reactive groups, it has high oxidation stability. Therefore, it is presumed that electrolytes containing such compounds suppress irreversible decomposition reactions, thereby improving the cycle characteristics and charge / discharge characteristics of batteries.

[0031] Hydrofluoroether is a compound containing fluorine and hydrogen and further containing an ether bond. Because hydrofluoroether contains fluorine and an ether bond, it is likely to undergo a reaction on the negative electrode surface during charge and discharge of a lithium secondary battery. Therefore, in a lithium secondary battery containing the electrolyte solution of this embodiment, it is presumed that the hydrofluoroether is preferentially decomposed during the initial charge, and an SEI layer with a high fluorine content is suitably formed. It is presumed that such an SEI layer suppresses further decomposition of components in the electrolyte solution and the resulting irreversible reduction of lithium ions in the lithium secondary battery.

[0032] Furthermore, in the electrolyte solution of the present embodiment, the fluorine-free ether further improves the solubility of the lithium salt in the electrolyte solution, which is presumably responsible for reducing the internal resistance of a lithium secondary battery containing the electrolyte solution and improving the properties of the formed SEI layer.

[0033] In addition, in the electrolyte solution of this embodiment, lithium bis(fluorosulfonyl)imide is presumed to improve the cycle characteristics and charge / discharge characteristics of the battery, presumably because lithium bis(fluorosulfonyl)imide is a highly stable counter ion of lithium ions.

[0034] Furthermore, by adding at least one additive selected from the group consisting of sulfite esters, sulfate esters, sulfonate esters, and alkyl sulfones to the electrolyte solution of this embodiment, the cycle characteristics can be improved and gas generation during high-temperature storage can be reduced. This is presumably because the additive decomposes near the positive electrode, and the resulting decomposition products exist as a coating near the positive electrode, suppressing the reaction between the positive electrode active material and the electrolyte solution.

[0035] In addition, even when the amount of the additive added is small, the cycle characteristics are sufficiently improved and gas generation during high-temperature storage can be reduced. This is presumably because the cyclic compound and the hydrofluoroether have high oxidation stability and the additive is poorly soluble in the cyclic compound and the hydrofluoroether, so that even a small amount of additive generates sufficient decomposition products to suppress the reaction between the positive electrode active material and the electrolyte near the positive electrode.

[0036] Therefore, it is believed that the synergistic effect of the characteristics of each of the components described above enables a lithium secondary battery containing the electrolyte solution of this embodiment to simultaneously achieve high energy density, good cycle characteristics, good high-temperature storage stability, and excellent charge-discharge characteristics, although the factors are not limited to those described above.

[0037] The specific gravity of the electrolyte solution of this embodiment is preferably 1.10 g / cc or more and 1.40 g / cc or less. When the specific gravity of the electrolyte solution of this embodiment is in the above range, the energy density of a lithium secondary battery using such an electrolyte solution is improved. From the same viewpoint, the specific gravity of the electrolyte solution is preferably 1.15 g / cc or more and 1.35 g / cc or less, or 1.20 g / cc or more and 1.30 g / cc or less.

[0038] The cyclic compound of formula (1) contained in the electrolyte solution, which will be described in detail below, tends to have a higher boiling point than compounds generally used as solvents for electrolyte solutions. The high boiling point of such a cyclic compound of formula (1) further improves the stability of the electrolyte solution against changes in environmental temperature and tends to improve cycle characteristics. From this perspective, the boiling point of the electrolyte solution of this embodiment is preferably 72°C or higher, 80°C or higher, or 85°C or higher under atmospheric pressure.

[0039] Each component contained in the electrolyte solution will be described in detail below.

[0040] 1.1. Solvent 1.1.1. Ether without Fluorine Atoms The electrolyte solution of this embodiment contains an ether without fluorine atoms (hereinafter also referred to as a "non-fluorine ether").

[0041] The number of carbon atoms of the non-fluorinated ether contained in the electrolytic solution of the present embodiment is not particularly limited, and is, for example, from 2 to 20. From the viewpoint of further improving the effects of the electrolytic solution of the present embodiment, the number of carbon atoms of the non-fluorinated ether is preferably from 3 to 15, from 4 to 12, or from 5 to 10.

[0042] The number of ether bonds in the non-fluorinated ether is not particularly limited, and is, for example, from 1 to 10. From the viewpoint of further improving the solubility of the electrolyte in the electrolytic solution, the number of ether bonds in the non-fluorinated ether is preferably 2 or more, 3 or more, and 8 or less, or 5 or less.

[0043] The non-fluorinated ether may be a saturated ether compound (i.e., a compound not containing an unsaturated bond) or an unsaturated ether compound (i.e., a compound containing an unsaturated bond). From the viewpoint of further improving the effects of the electrolytic solution according to the present embodiment, the electrolytic solution preferably contains a saturated non-fluorinated ether.

[0044] The non-fluorinated ether may be a chain ether compound or a cyclic ether compound. From the viewpoint of further improving the effect of the electrolytic solution of the present embodiment, the electrolytic solution preferably contains a chain non-fluorinated ether. The chain non-fluorinated ether may be linear or branched.

[0045] The non-fluorinated ether contained in the electrolytic solution of the present embodiment is not particularly limited, but examples thereof include 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), 1,2-dimethoxypropane (DMP), diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dimethoxypropane, 1,4-dimethoxybutane, 1,1-dimethoxyethane, 2,2-dimethoxypropane, 1,3-dimethoxybutane, 1,2-dimethoxybutane, 2,2-dimethoxybutane, 2,3-dimethoxybutane, 1,2-diethoxypropane, 1,2-diethoxybutane, and 2,3-diethoxybutane. From the viewpoint of further improving the effect of the electrolytic solution of this embodiment, the non-fluorinated ether is preferably 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dimethoxypropane, or a mixture thereof, and more preferably 1,2-dimethoxyethane or 1,2-dimethoxypropane. The non-fluorinated ether may be used alone or in combination of two or more.

[0046] The content of the non-fluorinated ether in the electrolytic solution is not particularly limited, but is, for example, 1.0 vol% to 70.0 vol%, preferably 5.0 vol% to 65.0 vol%, 10.0 vol% to 60.0 vol%, 15.0 vol% to 55.0 vol%, or 20.0 vol% to 50.0 vol%, relative to the total amount of the solvent components in the electrolytic solution. When the content of the non-fluorinated ether is within the above range, a lithium secondary battery having the electrolytic solution of this embodiment tends to have even more excellent cycle characteristics and charge / discharge characteristics.

[0047] 1.1.2. Hydrofluoroether The electrolyte solution of this embodiment contains a hydrofluoroether (hereinafter also referred to as "HFE"). As the HFE, one type may be used alone, or two or more types may be used in combination. Furthermore, in this specification, "hydrofluoroether (HFE)" refers to an ether compound having at least one fluorine atom and a hydrogen atom (preferably an ether compound consisting only of a hydrogen atom, a fluorine atom, an oxygen atom, and a carbon atom).

[0048] The molecular weight of the hydrofluoroether (HFE) contained in the electrolyte solution of the present embodiment is not particularly limited, but is, for example, from 100 to 500. From the viewpoint of making the lithium secondary battery more stable against changes in environmental temperature, the molecular weight of the HFE is preferably from 120 to 450, from 140 to 400, from 160 to 350, or from 180 to 300.

[0049] The number of carbon atoms in the HFE is not particularly limited, but is, for example, 3 or more and 30 or less. From the viewpoint of improving the cycle characteristics, charge / discharge characteristics, and / or stability of the battery, the number of carbon atoms in the HFE is preferably 4 or more, 5 or more, or 6 or more, and from the same viewpoint, it is preferably 25 or less, 20 or less, 15 or less, or 10 or less.

[0050] The HFE may be a chain compound or a cyclic compound. From the viewpoint of improving the cycle characteristics, charge / discharge characteristics, and / or stability of the battery, the electrolyte preferably contains a chain HFE. The chain HFE may be linear or branched.

[0051] The HFE contained in the electrolyte solution is preferably a chain fluorine compound having at least one of the monovalent groups represented by the following formula (A) or formula (B). By using an electrolyte solution containing such an HFE, the cycle characteristics and charge / discharge characteristics of the battery tend to be further improved. From this viewpoint, the HFE contained in the electrolyte solution is preferably one having the following formula (B). In formulas (A) and (B), the wavy lines represent the bonding site in the monovalent group.

[0052] When the HFE contained in the electrolytic solution of the present embodiment has at least one of the structures represented by the above formula (A) or (B), the HFE is more preferably a compound represented by the following formula (A') or (B'). By using an electrolytic solution containing such an HFE, the cycle characteristics and charge / discharge characteristics of the battery tend to be further improved. In addition, in the following formula (A'), R １ is an optionally fluorinated saturated or unsaturated monovalent hydrocarbon group, and in the following formula (B'), R ２is a hydrogen atom or an alkyl group, and R Ｆ is a saturated or unsaturated monovalent hydrocarbon group having a fluorine atom, and m is an integer of 1 or more and 5 or less.

[0053] In the above formula (A') of HFE, R １ is not particularly limited as long as it is a saturated or unsaturated monovalent hydrocarbon group which may be fluorinated, and is, for example, a linear or branched alkyl group, alkenyl group, or alkynyl group having 1 to 5 carbon atoms which may contain a fluorine atom. １ is preferably a linear or branched alkyl group having 1 to 3 carbon atoms and having at least one fluorine atom. １ The number of fluorine atoms therein is not particularly limited, but is, for example, 0 to 10, preferably 1 to 6, and more preferably 2 to 5.

[0054] From the viewpoint of further improving the effect of the electrolyte solution of this embodiment, R １ is preferably a fluorinated methyl group or a fluorinated ethyl group, more preferably a trifluoromethyl group, a trifluoroethyl group, a tetrafluoroethyl group or a pentafluoroethyl group, and even more preferably a 1,1,2,2-tetrafluoroethyl group.

[0055] In the above formula (B') of HFE, R ２ is not particularly limited as long as it is a hydrogen atom or an alkyl group. ２ When R is an alkyl group, the number of carbon atoms therein is not particularly limited, but is, for example, 1 or more and 5 or less, preferably 1 or more and 3 or less, and more preferably 1 or more and 2 or less. ２ is preferably a hydrogen atom.

[0056] In the above formula (B') of the HFE, m is not particularly limited as long as it is an integer of 1 or more and 5 or less. From the viewpoint of further improving the effect of the present embodiment by the electrolytic solution, m is preferably 1 or more and 4 or less, 1 or more and 3 or less, or 1 or more and 2 or less.

[0057] In the above formula (B') of HFE, R Ｆ R is not particularly limited as long as it is a fluorinated saturated or unsaturated monovalent hydrocarbon group, and is, for example, a linear or branched alkyl group, alkenyl group, or alkynyl group having 1 to 5 carbon atoms and having at least one fluorine atom. Ｆ is preferably a linear or branched alkyl group having 1 to 5 carbon atoms and containing at least one fluorine atom, and more preferably a linear or branched alkyl group having 1 to 3 carbon atoms and containing at least one fluorine atom. Ｆ The number of fluorine atoms therein is not particularly limited as long as it is one or more, but is, for example, 1 to 10, preferably 1 to 5, and more preferably 2 to 4.

[0058] The HFE contained in the electrolyte solution of this embodiment is not particularly limited, and examples thereof include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl)ether, ethyl-1,1,2,2-tetrafluoroethyl ether, methyl-1,1,2,2-tetrafluoroethyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, difluoromethyl-2,2,3,3-tetrafluoropropyl ether, methyl perfluorobutyl ether, and ethyl perfluorobutyl ether. From the viewpoint of further improving the cycle characteristics of a lithium secondary battery containing the electrolyte solution of this embodiment, the HFE is preferably 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether or 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. The HFE may be used alone or in combination of two or more.

[0059] The content of HFE contained in the electrolytic solution is, for example, 10.0 vol% or more and 80.0 vol% or less, relative to the total amount of the solvent components of the electrolytic solution. The content of HFE is preferably 20.0 vol% or more and 75.0 vol% or less, 30.0 vol% or more and 72.5 vol% or less, 40.0 vol% or more and 70.0 vol% or less, 45.0 vol% or more and 67.5 vol% or less, or 50.0 vol% or more and 65.0 vol% or less. When the HFE content is within the above range, a lithium secondary battery containing the electrolytic solution of this embodiment tends to have even better cycle characteristics and charge / discharge characteristics.

[0060] 1.1.3. Cyclic Compound of Formula (1) The electrolyte solution of this embodiment contains the cyclic compound of formula (1). In addition, when a plurality of saturated hydrocarbon groups are present in the cyclic compound of formula (1), R ｈ are each independently selected. That is, in this specification, multiple R ｈ The structures may be the same or different from each other.

[0061] In formula (1), m is an integer of 0 to 3. From the viewpoint of improving the oxidation stability of the electrolyte solution and further improving the cycle characteristics and charge / discharge characteristics of a lithium secondary battery containing the electrolyte solution of the present embodiment, m is preferably an integer of 0 to 2, or 0 to 1, and is 1.

[0062] In formula (1), R ｈ may be a chain saturated hydrocarbon group or a cyclic saturated hydrocarbon group. ｈ is preferably a chain-like saturated hydrocarbon group. The chain-like saturated hydrocarbon group may be linear or branched. In this specification, a cyclic saturated hydrocarbon group refers to a saturated hydrocarbon group having at least one cyclic structure.

[0063] In formula (1), R ｈ The number of carbon atoms in R is 1 or more and 10 or less. ｈThe number of carbon atoms is preferably 1 or more and 8 or less, more preferably 1 or more and 5 or less, and even more preferably 1 or more and 2 or less.

[0064] In formula (1), R ｈ According to such an embodiment, the cycle characteristics and charge / discharge characteristics of a lithium secondary battery containing the electrolyte solution of this embodiment tend to be further improved.

[0065] The molecular weight of the cyclic compound of formula (1) of this embodiment is not particularly limited, but is, for example, from 84 to 500. From the viewpoint of further improving the cycle characteristics and charge / discharge characteristics of a lithium secondary battery containing the electrolyte solution of this embodiment, the molecular weight of the cyclic compound of formula (1) is preferably from 84 to 220, from 84 to 180, or from 84 to 150.

[0066] The specific gravity of the cyclic compound of formula (1) contained in the electrolyte at 25° C. is not particularly limited, but is, for example, 0.7 g / cc or more and 1.2 g / cc or less. From the viewpoint of further improving the energy density of a lithium secondary battery containing the electrolyte, the specific gravity of the cyclic compound of formula (1) at 25° C. is preferably 1.1 g / cc or less, 1.0 g / cc or less, or 0.9 g / cc or less.

[0067] The cyclic compound of formula (1) in this embodiment is not particularly limited as long as it is a compound represented by formula (1), and examples thereof include cyclohexane, methylcyclohexane, ethylcyclohexane, propylcyclohexane, butylcyclohexane, tert-butylcyclohexane, isobutylcyclohexane, 1,4-dimethylcyclohexane, and 1,3,5-trimethylcyclohexane. From the viewpoint of further improving the effects of the electrolytic solution of this embodiment, it is preferable to use one type of cyclic compound of formula (1) selected from the group consisting of cyclohexane, methylcyclohexane, and ethylcyclohexane.

[0068] The content of the cyclic compound of formula (1) is preferably 1.0 vol% to 40.0 vol%, 2.5 vol% to 30.0 vol%, 5.0 vol% to 20.0 vol%, 6.0 vol% to 17.5 vol%, or 7.0 vol% to 15.0 vol%, relative to the total amount of the solvent components of the electrolyte solution. When the content of the cyclic compound of formula (1) is within the above range, the lithium secondary battery containing the electrolyte solution of this embodiment tends to be more excellent in energy density, cycle characteristics, and / or charge / discharge characteristics.

[0069] 1.1.4 Other Solvents The electrolytic solution of the present embodiment may further contain a solvent other than the above-described components. That is, the electrolytic solution of the present embodiment may contain, for example, a cyclic compound other than that represented by formula (1) (e.g., a heterocyclic compound), or a chain compound not corresponding to the above-described non-fluorinated ethers and hydrofluoroethers (e.g., a carbonyl compound not having a fluorine atom).

[0070] The content of the other solvent relative to the total amount of the solvent components of the electrolytic solution is not particularly limited, but is, for example, 0.0 vol% to 30.0 vol%, or 1.0 vol% to 25.0 vol%. From the viewpoint of improving the effect of the electrolytic solution of this embodiment, the content of the other solvent is preferably 0.0 vol% to 10.0 vol%, or 0.0 vol% to 5.0 vol%.

[0071] 1.1.5 Solvent Ratio The volume ratio of the content of the cyclic compound of formula (1) to the HFE is not particularly limited, but is, for example, 0.02 to 2.00. From the viewpoint of improving the effect of the electrolytic solution of this embodiment, the volume ratio of the content of the cyclic compound of formula (1) to the HFE is preferably 0.05 to 1.50, 0.07 to 1.00, 0.10 to 0.70, 0.11 to 0.50, 0.12 to 0.40, or 0.13 to 0.30.

[0072] The volume ratio of the content of the cyclic compound of formula (1) to the non-fluorinated ether is not particularly limited, but is, for example, 0.05 to 3.00. From the viewpoint of improving the effect of the electrolytic solution of this embodiment, the volume ratio of the content of the cyclic compound of formula (1) to the non-fluorinated ether is preferably 0.10 to 2.50, 0.15 to 2.00, 0.20 to 1.50, 0.25 to 1.00, 0.27 to 0.75, or 0.30 to 0.50.

[0073] The volume ratio of the content of the HFE to the non-fluorinated ether is not particularly limited, but is, for example, 0.2 to 3.0. From the viewpoint of improving the effect of the electrolytic solution of this embodiment, the volume ratio of the content of the HFE to the non-fluorinated ether is preferably 0.3 to 2.8, 0.4 to 2.6, or 0.5 to 2.5.

[0074] The content of the additives described in detail below is preferably 0.04% by mass or more and 10.00% by mass or less, 0.07% by mass or more and 7.50% by mass or less, 0.10% by mass or more and 5.00% by mass or less, 0.15% by mass or more and 2.50% by mass or less, 0.20% by mass or more and 2.00% by mass or less, 0.25% by mass or more and 1.75% by mass or less, 0.30% by mass or more and 1.50% by mass or less, or 0.33% by mass or more and 1.30% by mass or less, relative to the total mass of the non-fluorinated ether, HFE, and cyclic compound of formula (1). When the content of the additives is within the above ranges, the cycle characteristics and high-temperature storage stability tend to be more excellent.

[0075] 1.2. Electrolyte 1.2.1. Lithium Salt In this specification, the lithium salt includes not only lithium bis(fluorosulfonyl)imide, which will be described in detail below, but also lithium salts other than lithium bis(fluorosulfonyl)imide.

[0076] The concentration of the lithium salt in the electrolyte solution is preferably 0.3 M or more, 0.4 M or more, 0.5 M or more, or 0.8 M or more. The upper limit of the lithium salt concentration is not particularly limited, but may be 10.0 M or less, 5.0 M or less, or 2.0 M or less.

[0077] 1.2.1.1. Lithium bis(fluorosulfonyl)imide The electrolyte solution of this embodiment contains lithium bis(fluorosulfonyl)imide (hereinafter referred to as "specific lithium salt," "LiFSI," or "LiN(SO ２ F) ２ As a result, the lithium secondary battery containing the electrolyte solution of this embodiment tends to be more excellent in energy density, cycle characteristics, and / or charge / discharge characteristics.

[0078] The concentration of the specific lithium salt in the electrolyte is preferably 0.3 M or more, 0.4 M or more, 0.5 M or more, or 0.8 M or more. When the concentration of the specific lithium salt is within the above range, the SEI layer is more easily formed, and the internal resistance tends to be further reduced. In particular, a lithium secondary battery containing a fluorine compound such as hydrofluoroether as a solvent can increase the concentration of the specific lithium salt in the electrolyte, thereby further improving the cycle characteristics and charge / discharge characteristics. The upper limit of the concentration of the specific lithium salt is not particularly limited, but may be 10.0 M or less, 5.0 M or less, or 2.0 M or less.

[0079] 1.2.1.2. Lithium Salts Other Than Lithium Bis(fluorosulfonyl)imide The electrolyte solution of the present embodiment may contain a lithium salt other than lithium bis(fluorosulfonyl)imide (hereinafter also referred to as "other lithium salts"). Examples of other lithium salts include, but are not limited to, inorganic and organic salts of lithium. Specific examples include LiI, LiCl, LiBr, LiF, and LiBF. ４ , LiPF ６ , LiPF ２ O ２ , LiPF ２ (C ２ O ４ ) ２ , LiPF ２ (C ３ O ４ ) ２ , LiAsF ６ , LiSO ３ CF ３, LiN(SO ２ CF ３ ) ２ , LiN(SO ２ CF ３ CF ３ ) ２ , LiBF ２ (C ２ O ４ ), LiB(C ２ O ４ ) ２ , LiB(C ３ O ４ ) ２ , LiB(O ２ C ２ H ４ ) ２ , LiB(O ２ C ２ H ４ ) F ２ , LiB(OCOCF ３ ) ４ , LiNO ３ , and Li ２ SO ４ The above-mentioned other lithium salts may be used alone or in combination of two or more. The electrolytic solution may further contain a salt other than the lithium salt as an electrolyte. Such salts are not particularly limited, but include, for example, salts of Na, K, Ca, and Mg.

[0080] The total concentration of the other lithium salts in the electrolyte solution is, for example, 0.0 M or more, 0.3 M or more, 0.4 M or more, 0.5 M or more, or 0.8 M or more. The upper limit of the concentration of the other lithium salts is not particularly limited, but may be 10.0 M or less, 5.0 M or less, or 2.0 M or less.

[0081] 1.3. Additive The electrolyte solution of this embodiment contains at least one additive selected from the group consisting of sulfite esters, sulfate esters, sulfonate esters, and alkyl sulfones.

[0082] The molecular weight of the sulfite ester is not particularly limited, but is, for example, from 50 to 200. From the viewpoint of further improving the effect of the electrolyte solution of the present embodiment, the molecular weight of the sulfite ester is preferably from 60 to 175, from 70 to 150, or from 80 to 140.

[0083] The sulfite ester may be a cyclic sulfite ester or a chain sulfite ester. Examples of cyclic sulfate esters include compounds represented by the following formula (2). In the following formula (2), R are each independently a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, and n is an integer of 1 to 6. R are each independently preferably a hydrogen atom, a methyl group, or an ethyl group. n is preferably 1 to 3, or 1 to 2. Of the multiple Rs, the number of Rs that are alkyl groups having 1 to 3 carbon atoms is preferably 3 or less, 2 or less, 1 or less, or 0 per molecule. That is, the alkyl group having 1 to 3 carbon atoms in the following formula is preferably 3 or less, 2 or less, 1 or less, or 0. (2)

[0084] Examples of the chain sulfite ester include (RO) ２ S(=O) is exemplified by compounds represented by formula (RO) ２ In S(═O), each R is independently a monovalent hydrocarbon group having 1 to 8 carbon atoms. The R may be a cyclic hydrocarbon group or a chain hydrocarbon group, preferably a chain hydrocarbon group. The chain hydrocarbon group may be linear or branched, preferably linear. The hydrocarbon group may be a saturated hydrocarbon group or an unsaturated hydrocarbon group, preferably a saturated hydrocarbon group (i.e., an alkyl group). The number of carbon atoms in the R is preferably 1 or more and 5 or less, and 1 or more and 3 or less. Each R is independently preferably a methyl group, an ethyl group, or a propyl group, and is preferably a methyl group or an ethyl group.

[0085] Examples of cyclic sulfites include, but are not limited to, ethylene sulfite (CAS No. 3741-38-6) and 4-methylethylene sulfite (CAS No. 1469-73-4). Examples of chain sulfites include, but are not limited to, dimethyl sulfite (CAS No. 616-42-2).

[0086] The molecular weight of the sulfate ester is not particularly limited, but is, for example, from 50 to 200. From the viewpoint of further improving the effect of the electrolyte solution of the present embodiment, the molecular weight of the sulfate ester is preferably from 60 to 175, from 70 to 150, or from 80 to 140.

[0087] The sulfate ester may be a cyclic sulfate ester or a chain sulfate ester. Examples of cyclic sulfate esters include compounds represented by the following formula (3). The definitions and preferred embodiments of R and n in the following formula (3) are the same as the definitions and preferred embodiments of R and n in the above formula (2). R and n in the following formula (3) may be the same as or different from R and n in the above formula (2). (3)

[0088] Examples of the chain sulfate ester include (RO) ２ S (= O) ２ Examples include compounds represented by formula (RO): ２ S (= O) ２ The definition and preferred embodiment of R in the formula (RO) ２ The definition and preferred embodiments of R in S(=O) are the same as those in Formula (RO). ２ S (= O) ２ In the formula (RO), R is ２ It may be the same as or different from R in S(=O).

[0089] The cyclic sulfate ester is not particularly limited, but for example, ethylene sulfate (CAS No. 1072-53-3) can be used. The chain sulfate ester is not particularly limited, but for example, dimethyl sulfite (CAS No. 77-78-1) and diethyl sulfite (CAS No. 64-67-5) can be used.

[0090] The molecular weight of the sulfonate ester is not particularly limited, but is, for example, from 50 to 200. From the viewpoint of further improving the effect of the electrolytic solution of the present embodiment, the molecular weight of the sulfate ester is preferably from 60 to 175, from 70 to 150, or from 80 to 140.

[0091] The sulfonate ester may be a cyclic sulfonate ester or a chain sulfonate ester. Examples of cyclic sulfonate esters include compounds represented by the following formula (4). The definition and preferred embodiments of R in the following formula (4) are the same as the definition and preferred embodiments of R in the above formula (2). R in the following formula (4) may be the same as or different from R in the above formula (2). In the following formula (4), n is an integer of 1 or more and 7 or less, preferably 1 or more and 4 or less, and 2 or more and 3 or less. (4)

[0092] Examples of the chain sulfonic acid ester include (RO)S(R)(=O) ２ Examples of the compound represented by the formula (RO)S(R)(=O) ２ The definition and preferred embodiment of R in the formula (RO) ２ The definition and preferred embodiments of R in S(=O) are the same as those in the formula (RO)S(R)(=O). ２ In the formula (RO), R is ２ It may be the same as or different from R in S(=O).

[0093] The cyclic sulfonate ester is not particularly limited, but examples thereof include 1,3-propane sultone, 1,4-butane sultone, and 2,4-butane sultone. The chain sulfonate ester is not particularly limited, but examples thereof include methyl methanesulfonate, ethyl methanesulfonate, and propyl ethanesulfonate.

[0094] The molecular weight of the alkyl sulfone is not particularly limited, but is, for example, from 50 to 200. From the viewpoint of further improving the effect of the electrolyte solution of the present embodiment, the molecular weight of the sulfate ester is preferably from 60 to 175, from 70 to 150, or from 80 to 140.

[0095] The alkyl sulfone may be a cyclic alkyl sulfone or a chain alkyl sulfone. Examples of cyclic alkyl sulfones include compounds represented by the following formula (5). The definitions and preferred embodiments of R and n in the following formula (5) are the same as the definitions and preferred embodiments of R and n in the above formula (4). R and n in the following formula (5) may be the same as or different from R and n in the above formula (4). (5)

[0096] Examples of the chain alkyl sulfone include R ２ S (= O) ２ Examples of the compound represented by formula R ２ S (= O) ２ The definition and preferred embodiment of R in the formula (RO) ２ The definition and preferred embodiments of R in S(=O) are the same as those of the formula R ２ S (= O) ２ In the formula (RO), R is ２ It may be the same as or different from R in S(=O).

[0097] Examples of cyclic alkyl sulfones include, but are not limited to, cyclopropanesulfonylmethane and benzenesulfonylmethane. Examples of chain alkyl sulfones include, but are not limited to, dimethylsulfone, methylsulfonylmethane, diethylsulfone, and ethylsulfonylpropane.

[0098] The sulfite esters, sulfate esters, sulfonate esters, and alkyl sulfones are not particularly limited, but for example, hydrogen atoms may be substituted with other functional groups such as halogen groups or hydroxyl groups. That is, the additive may be a compound having a substituent in the above-mentioned sulfite esters, sulfate esters, sulfonate esters, and alkyl sulfones. Examples of the substituent include halogen groups and hydroxyl groups. Use of the above-mentioned additives tends to further improve the cycle characteristics and high-temperature storage stability of the battery. The above-mentioned additives may be used alone or in combination of two or more.

[0099] The content of the additive selected from the group consisting of sulfite esters, sulfate esters, sulfonate esters, and alkyl sulfones is preferably 0.04% by mass to 10.00% by mass, 0.07% by mass to 7.50% by mass, 0.10% by mass to 5.00% by mass, 0.15% by mass to 2.50% by mass, 0.20% by mass to 2.00% by mass, 0.25% by mass to 1.75% by mass, 0.30% by mass to 1.50% by mass, or 0.33% by mass to 1.30% by mass, relative to the total mass of the electrolyte solution excluding the additives. By having the additive content within the above ranges, the cycle characteristics and high-temperature storage stability tend to be more excellent.

[0100] Since the electrolyte solution of this embodiment contains an HFE as described above, even if the amount of the additive in the electrolyte solution is small, the electrolyte solution tends to have sufficiently excellent cycle characteristics and high-temperature storage stability.

[0101] The product (A × B) of the amount of electrolyte (g) injected into a lithium secondary battery of this embodiment containing the electrolyte solution of this embodiment divided by the battery capacity (Ah) of the lithium secondary battery, A (g / Ah), and the content B (parts by mass) of the additive per 100 parts by mass of the electrolyte solution, is preferably 0.050 to 10, 0.10 to 9.5, 0.15 to 8.0, 0.20 to 5.00, 0.50 to 4.5, 0.70 to 4.0, 0.80 to 3.5, 0.90 to 3.0, or 1.0 to 2.8. When the product (A × B) is within the above range, the cycle characteristics and high-temperature storage stability tend to be more excellent. In other words, the electrolyte solution of this embodiment is preferably used by adjusting the amount of electrolyte solution injected into the lithium secondary battery and the battery capacity (Ah) of the lithium secondary battery so that the product (A × B) falls within the above range with respect to the content B defined above. Note that the content B is the parts by mass of the additive when the total mass of the electrolyte solution excluding the additive is taken as 100 parts by mass.

[0102] The value A (g / Ah) is preferably 1.0 to 5.0, 1.5 to 4.0, 1.6 to 3.3, 2.0 to 3.0, 2.1 to 2.9, 2.2 to 2.8, 2.3 to 2.7, or 2.3 to 2.6. When the value A (g / Ah) is within the above range in addition to the product (A×B), the cycle characteristics and high-temperature storage stability tend to be even more excellent.

[0103] The presence of the cyclic compound of formula (1), HFE, non-fluorinated ether, lithium salt, additive, etc. in the electrolyte solution can be confirmed by estimating the molecular structure by measuring or analyzing using a known method. Such methods are not particularly limited, but include, for example, methods using NMR, mass spectrometry, elemental analysis, infrared spectroscopy, etc. In addition, the molecular structure of the solvent can also be estimated by theoretical calculation using molecular dynamics, molecular orbital method, etc.

[0104] 2. Method for Preparing Electrolyte Solution In preparing the electrolyte solution of this embodiment, a solution obtained by mixing the cyclic compound of formula (1), HFE, a non-fluorinated ether, and, if necessary, a solvent other than the above is used as a solvent, and a specific lithium salt, and if necessary, other lithium salts and / or salts other than lithium salts, and an additive are dissolved in the solution as an electrolyte to prepare the electrolyte solution. The mixing ratio of the additive, solvent, and lithium salt may be appropriately adjusted so that the content, concentration, etc. of the additive, each solvent, and each electrolyte falls within the above-mentioned ranges.

[0105] 3. Lithium Secondary Battery The lithium secondary battery of this embodiment includes the above-described electrolyte solution. By including the above-described electrolyte solution in the lithium secondary battery, the battery has good cycle characteristics, good high-temperature storage stability, and excellent charge / discharge characteristics.

[0106] The type of lithium secondary battery is not particularly limited as long as it is charged and discharged by a redox reaction of lithium and has an electrolyte solution, and examples thereof include lithium ion batteries, lithium metal batteries, anode-free lithium secondary batteries, lithium sulfur batteries, lithium oxygen batteries, lithium-air batteries, etc. From the viewpoint of further improving the effects of the electrolyte solution of this embodiment, the lithium secondary battery is preferably an anode-free lithium secondary battery or a lithium metal battery, and more preferably an anode-free lithium secondary battery.

[0107] In a preferred embodiment of the present invention, the lithium secondary battery is one in which lithium metal is deposited on the surface of the negative electrode and the deposited lithium metal is dissolved to charge and discharge the battery. Examples of such a lithium secondary battery include an anode-free lithium secondary battery and a lithium metal battery. In this embodiment, the lithium secondary battery tends to further enhance the effects of the electrolyte.

[0108] Furthermore, when a positive electrode containing a high-nickel active material is used as the positive electrode of a lithium secondary battery, problems such as a deterioration in cycle life due to decomposition of the electrolyte solution occurring on the surface of the active material and generation of gas during high-temperature storage become prominent. Here, by using the electrolyte solution of this embodiment, even when a positive electrode containing a high-nickel active material is used for the positive electrode, it is possible to more effectively suppress the decomposition of the electrolyte solution occurring on the surface of the active material, and to simultaneously achieve high energy density, good cycle characteristics, good high-temperature storage stability, and excellent charge / discharge characteristics.

[0109] Therefore, the electrolyte solution of this embodiment contains Li as the positive electrode active material. ｚ Ni ｘ Co ｙ M １－ｘ－ｙ O ２＋α (wherein 0.5≦x≦1.0, 0≦y≦0.35, 0.9≦z≦1.3, −0.2≦α≦0.15, and M is at least one element selected from Mn, Al, V, Mg, Mo, Nb, Ti, Zr, Fe, Cu, Cr, Zn, F, and B). In the high-nickel active material, x is preferably 0.6 or more, 0.7 or more, or 0.8 or more.

[0110] Furthermore, the lithium secondary battery of this embodiment, which includes the above-described electrolyte solution, tends to have excellent discharge capacity not only in normal charging at room temperature (25°C), but also in charging in a low-temperature environment, charging in a high-temperature environment, and high-speed charging.

[0111] Various types of lithium secondary batteries will be described in detail below.

[0112] 3.1. Anode-Free Battery Figure 1 is a schematic cross-sectional view of an anode-free battery of this embodiment. As shown in Figure 1, the anode-free battery 100 of this embodiment includes a positive electrode 120, a negative electrode 140 that does not have a negative electrode active material, a separator 130 disposed between the positive electrode 120 and the negative electrode 140, and an electrolyte solution not shown in Figure 1. The positive electrode 120 has a positive electrode current collector 110 on the surface opposite to the surface facing the separator 130. Each component of the anode-free battery 100 will be described below.

[0113] The anode-free lithium secondary battery of this embodiment (hereinafter also referred to as "anode-free battery" or "AFB") has a negative electrode consisting of a negative electrode current collector without a negative electrode active material, and uses the above-mentioned electrolyte. The negative electrode active material is a material that causes an electrode reaction, i.e., an oxidation reaction and a reduction reaction, at the negative electrode. Specifically, the negative electrode active material of this embodiment includes lithium metal and a host material of lithium element (lithium ion or lithium metal). The host material of lithium element refers to a material provided to hold lithium ion or lithium metal at the negative electrode. Examples of such holding mechanisms include intercalation, alloying, and metal cluster occlusion, and a typical example is intercalation.

[0114] In an anode-free battery, the negative electrode does not contain any negative electrode active material before the initial charge of the battery, and consists only of a negative electrode current collector. Therefore, after the initial charge, lithium metal is deposited on the negative electrode, and the deposited lithium metal is electrolytically dissolved, thereby performing charging and discharging. Therefore, an anode-free battery has the advantage that the volume and mass occupied by the negative electrode active material are reduced, resulting in a smaller overall volume and mass of the battery, and therefore, in principle, a higher energy density. An anode-free battery has a negative electrode consisting of a negative electrode current collector without any negative electrode active material.

[0115] In the anode-free battery of this specification, the anode "has no anode active material" means that the anode has no or substantially no anode active material. The anode substantially having no anode active material means that the content of the anode active material in the anode is 10.0 mass% or less relative to the entire anode. The content of the anode active material in the anode of the anode-free battery is preferably 5.0 mass% or less, 1.0 mass% or less, 0.1 mass% or less, or 0.0 mass% relative to the entire anode. When the anode has no anode active material or the content of the anode active material in the anode is within the above range, the energy density of the lithium secondary battery is high.

[0116] In the anode-free battery of this specification, the term "before initial charge" refers to the state of the battery from assembly to the first charge, and the term "at the end of discharge" refers to the state of the battery in which the voltage is preferably 1.0 V or more and 3.8 V or less, and more preferably 1.0 V or more and 3.0 V or less.

[0117] In the anode-free battery of this embodiment, when the battery voltage is 1.0 V or more and 3.5 V or less, the lithium metal content may be 10.0 mass % or less (preferably 5.0 mass % or less, or 1.0 mass % or less) with respect to the entire negative electrode; when the battery voltage is 1.0 V or more and 3.0 V or less, the lithium metal content may be 10.0 mass % or less (preferably 5.0 mass % or less, or 1.0 mass % or less) with respect to the entire negative electrode; or when the battery voltage is 1.0 V or more and 2.5 V or less, the lithium metal content may be 10.0 mass % or less (preferably 5.0 mass % or less, or 1.0 mass % or less) with respect to the entire negative electrode.

[0118] In the anode-free battery of this embodiment, the mass M of lithium metal deposited on the negative electrode when the battery voltage is 4.2 V is ４．２ The mass M of lithium metal deposited on the negative electrode when the battery voltage is 3.0 V ３．０ The ratio M ３．０ / M ４．２ is preferably 40.0% or less, 38.0% or less, or 35.0% or less. ３．０ / M ４．２ may be 1.0% or more, 2.0% or more, 3.0% or more, or 4.0% or more.

[0119] Examples of negative electrode active materials include lithium metal and alloys containing lithium metal, carbon-based materials, metal oxides, and metals that can be alloyed with lithium and alloys containing such metals. Examples of carbon-based materials include, but are not limited to, graphene, graphite, hard carbon, and carbon nanotubes. Examples of metal oxides include, but are not limited to, silicon oxide compounds, titanium oxide compounds, and cobalt oxide compounds. Examples of metals that can be alloyed with lithium include silicon, germanium, tin, lead, aluminum, and gallium.

[0120] The negative electrode of an anode-free battery is not particularly limited as long as it does not have a negative electrode active material and can be used as a current collector. For example, it can be made of at least one material selected from the group consisting of Cu, Ni, Ti, Fe, and other metals that do not react with Li, their alloys, and stainless steel (SUS). Preferably, it can be made of at least one material selected from the group consisting of Cu, Ni, their alloys, and stainless steel (SUS). Use of such a negative electrode tends to further improve the energy density and productivity of the battery. The above-mentioned negative electrode materials can be used alone or in combination of two or more. In this specification, the term "metal that does not react with Li" refers to a metal that does not react with lithium ions or lithium metal to form an alloy under the operating conditions of a lithium secondary battery.

[0121] The average thickness of the negative electrode of the anode-free battery is not particularly limited, but is, for example, 3.0 μm to 30.0 μm. From the viewpoint of reducing the volume occupied by the negative electrode in the anode-free battery and improving the energy density, the average thickness of the negative electrode is preferably 4.0 μm to 20.0 μm, 5.0 μm to 18.0 μm, or 6.0 μm to 15 μm.

[0122] The separator for an anode-free battery is not particularly limited as long as it has the function of physically and / or electrically isolating the positive electrode and the negative electrode and the function of ensuring ionic conductivity of lithium ions. Examples of such separators include insulating porous materials, polymer electrolytes, gel electrolytes, and inorganic solid electrolytes, and typically include at least one selected from the group consisting of insulating porous materials, polymer electrolytes, and gel electrolytes. Furthermore, as the separator, one type of material may be used alone, or two or more types of materials may be used in combination.

[0123] As the separator of the anode-free battery, an insulating porous material, a polymer electrolyte, or a gel electrolyte is preferably used alone or in combination of two or more thereof. When an insulating porous material is used alone as the separator, the lithium secondary battery must further include an electrolytic solution.

[0124] When the separator includes an insulating porous member, the pores of the member are filled with an ion-conductive substance, which allows the member to exhibit ion conductivity. Thus, in this embodiment, the pores are filled with, for example, the electrolyte solution of this embodiment or a gel electrolyte containing the electrolyte solution of this embodiment.

[0125] The material constituting the insulating porous member is not particularly limited, and examples thereof include insulating polymer materials, specifically polyethylene (PE) and polypropylene (PP). That is, the separator may be a porous polyethylene (PE) film, a porous polypropylene (PP) film, or a laminate structure thereof.

[0126] The separator may be coated with a separator coating layer. The separator coating layer may cover both sides of the separator, or may cover only one side. From the viewpoint of improving the cycle characteristics of the lithium secondary battery of this embodiment, it is preferable to coat both sides of the separator. Note that the separator coating layer of this embodiment is a uniformly continuous film-like coating layer, for example, a film-like coating layer that is uniformly continuous over an area of ​​50% or more of the separator surface.

[0127] The separator coating layer is not particularly limited, and is preferably made of a binder such as polyvinylidene fluoride (PVDF), a mixture of styrene butadiene rubber and carboxymethyl cellulose (SBR-CMC), or polyacrylic acid (PAA). The separator coating layer may contain inorganic particles such as silica, alumina, titania, zirconia, or magnesium hydroxide added to the binder.

[0128] The average thickness of the separator including the separator coating layer is not particularly limited, but is, for example, 3.0 μm to 40.0 μm. In a lithium secondary battery, from the viewpoint of reliably isolating the positive electrode and the negative electrode while reducing the volume occupied by the separator in the battery, the average thickness of the separator is preferably 5.0 μm to 30.0 μm, 7.0 μm to 10.0 μm, or 10.0 μm to 20.0 μm.

[0129] The positive electrode of the anode-free battery is not particularly limited as long as it is generally used in lithium secondary batteries, and known materials can be appropriately selected depending on the application of the lithium secondary battery. From the viewpoint of improving the stability and output voltage of the battery, it is preferable that the positive electrode has a positive electrode active material. When the positive electrode has a positive electrode active material, lithium ions are typically charged and desorbed from the positive electrode active material by charging and discharging the battery. In this specification, the term "positive electrode active material" refers to a material that causes an electrode reaction, i.e., an oxidation reaction and a reduction reaction, at the positive electrode. Specifically, examples of the positive electrode active material include a host material for lithium element (typically, lithium ions).

[0130] Such positive electrode active materials are not particularly limited, and examples thereof include metal oxides and metal phosphates. The metal oxides are not particularly limited, and examples thereof include cobalt oxide-based compounds, manganese oxide-based compounds, and nickel oxide-based compounds. The metal phosphates are not particularly limited, and examples thereof include iron phosphate-based compounds and cobalt phosphate-based compounds. Typical positive electrode active materials include LiCoO ２ , Li ｚ Ni ｘ Co ｙM １－ｘ－ｙ O ２＋α (wherein 0.5≦x≦1.0, 0≦y≦0.35, 0.9≦z≦1.3, −0.2≦α≦0.15, and M is at least one element selected from Mn, Al, V, Mg, Mo, Nb, Ti, Zr, Fe, Cu, Cr, Zn, F, and B. This compound will also be referred to as “LiNCM” hereinafter.), LiNi ｘ Mn ｙ O(x+y=1), LiNiO ２ , LiMn ２ O ４ , LiFePO, LiCoPO, LiFeOF, LiNiOF, and LiTiS ２ The above positive electrode active materials may be used singly or in combination of two or more.

[0131] Among the above positive electrode active materials, LiNCM, LiNi ｘ Mn ｙ O(x+y=1), LiNiO ２ and LiNiOF are preferred, and LiNCM is more preferred. Furthermore, LiNCM preferably satisfies 0.6≦x≦1.0, 0.7≦x≦1.0, or 0.8≦x≦1.0.

[0132] The positive electrode may contain components other than the above-described positive electrode active material. Such components are not particularly limited and include, for example, a sacrificial positive electrode material, a conductive additive, a binder, a gel electrolyte, and a polymer electrolyte. Here, the sacrificial positive electrode material is a lithium-containing compound that undergoes an oxidation reaction and substantially no reduction reaction in the charge / discharge potential range of the positive electrode active material. In particular, in an anode-free battery, the positive electrode may include a sacrificial positive electrode. In particular, the positive electrode may include a gel electrolyte. In this embodiment, the gel electrolyte functions to improve the adhesion between the positive electrode and the positive electrode current collector, allowing for a thinner positive electrode current collector to be attached, thereby further improving the energy density of the battery. When attaching the positive electrode current collector to the surface of the positive electrode, a positive electrode current collector formed on a release paper may be used.

[0133] The conductive additive in the positive electrode is not particularly limited, and examples thereof include carbon black, single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), carbon nanofibers (CF), etc. The binder is not particularly limited, and examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, styrene butadiene rubber, acrylic resin, polyimide resin, etc.

[0134] The gel electrolyte is not particularly limited, but examples thereof include those containing a polymer, an organic solvent, and a lithium salt. The polymer in the gel electrolyte is not particularly limited, but examples thereof include a copolymer of polyethylene and / or polyethylene oxide, polyvinylidene fluoride, and a copolymer of polyvinylidene fluoride and hexafluoropropylene. The polymer electrolyte is not particularly limited, but examples thereof include a solid polymer electrolyte mainly containing a polymer and an electrolyte, and a semi-solid polymer electrolyte mainly containing a polymer, an electrolyte, and a plasticizer.

[0135] The average thickness of the positive electrode is preferably 20.0 μm or more and 100.0 μm or less, 30.0 μm or more and 80.0 μm or less, or 40.0 μm or more and 70.0 μm or less, although the average thickness of the positive electrode can be adjusted appropriately depending on the desired battery capacity.

[0136] A positive electrode current collector is disposed on one side of the positive electrode of an anode-free battery. The positive electrode current collector is not particularly limited as long as it is a conductor that does not react with lithium ions in the battery. Examples of such positive electrode current collectors include aluminum. Note that a positive electrode current collector may not be provided, in which case the positive electrode itself functions as a current collector. Note that the positive electrode current collector acts to donate and accept electrons to the positive electrode (particularly the positive electrode active material) and is in physical and / or electrical contact with the positive electrode.

[0137] In the anode-free battery, the average thickness of the positive electrode current collector is preferably 1.0 μm or more and 15.0 μm or less, 2.0 μm or more and 10.0 μm or less, or 3.0 μm or more and 6.0 μm or less. According to such an embodiment, the volume occupied by the positive electrode current collector in the anode-free battery is reduced, thereby further improving the energy density of the anode-free battery.

[0138] One use mode of a lithium secondary battery, including an anode-free battery, will be described. In one use mode of a lithium secondary battery, a positive electrode terminal and a negative electrode terminal for connecting the battery to an external circuit are joined to a positive electrode current collector and a negative electrode, respectively. The lithium secondary battery is charged and discharged by connecting the negative electrode terminal to one end of the external circuit and the positive electrode terminal to the other end of the external circuit. The lithium secondary battery is charged by applying a voltage to the positive electrode terminal and the negative electrode terminal such that a current flows from the negative electrode terminal (negative electrode) through the external circuit to the positive electrode terminal (positive electrode). After charging, the lithium secondary battery is discharged by connecting the positive electrode terminal and the negative electrode terminal via a desired external circuit.

[0139] In anode-free batteries, it is presumed that a solid electrolyte interface layer (SEI layer) is formed on the surface of the negative electrode (the interface between the negative electrode and the separator) upon initial charging, but the battery does not necessarily have to have an SEI layer. Charging an anode-free battery causes lithium metal deposition at the interface between the negative electrode and the SEI layer, the interface between the negative electrode and the separator, and / or the interface between the SEI layer and the separator. Furthermore, in anode-free batteries, the lithium metal deposition on the negative electrode undergoes electrolytic dissolution upon discharge. When an SEI layer is formed in the battery, the lithium metal deposition at at least one of the interface between the negative electrode and the SEI layer and / or the interface between the SEI layer and the separator undergoes electrolytic dissolution.

[0140] The method for producing an anode-free battery is not particularly limited as long as it is a method that can produce a lithium secondary battery having the above-described configuration, and examples thereof include the following methods.

[0141] The positive electrode current collector and positive electrode of an anode-free battery are manufactured, for example, as follows. The above-described positive electrode active material, conductive additive, and binder are mixed to obtain a positive electrode mixture. The compounding ratios may be, for example, 50.0% by mass to 99.0% by mass of the positive electrode active material, 0.5% by mass to 30.0% by mass of the conductive additive, and 0.5% by mass to 30.0% by mass of the binder, relative to the total positive electrode mixture. The obtained positive electrode mixture is applied to one side of a metal foil (e.g., Al foil) having a predetermined thickness (e.g., 5.0 μm to 1.0 mm) as a positive electrode current collector, and then press-molded. The resulting molded body is punched to a predetermined size to obtain a positive electrode current collector and a positive electrode.

[0142] Next, the negative electrode material for the above-described anode-free battery, for example, a metal foil (e.g., electrolytic Cu foil) having a thickness of 1.0 μm or more and 1.0 mm or less, is washed with a solvent containing sulfamic acid, and then punched out to a predetermined size. The punched piece is then subjected to ultrasonic cleaning with ethanol and then dried to obtain a negative electrode.

[0143] Next, a separator having the above-described configuration is prepared. The separator may be manufactured by a conventionally known method, or a commercially available separator may be used. A fibrous or porous functional buffer layer may be provided between the separator and the negative electrode to mitigate volume expansion and contraction associated with the dissolution and precipitation of lithium metal. The functional buffer layer preferably has ionic or electrical conductivity, but does not necessarily have to have such conductivity.

[0144] Next, the electrolyte solution of this embodiment is prepared by the above-mentioned preparation method.

[0145] The positive electrode current collector with the positive electrode formed thereon, the separator, and the negative electrode obtained as described above are stacked in this order so that the positive electrode and the separator face each other to obtain a laminate. The resulting laminate is sealed together with an electrolyte in a sealed container to obtain an anode-free battery. The sealed container is not particularly limited, and examples thereof include a laminate film.

[0146] The amount of electrolyte to be injected may be adjusted appropriately depending on the size of the battery. The amount of electrolyte to be injected is not particularly limited, but is, for example, 10.0 g to 30.0 g, 12.5 g to 25.0 g, 15.0 g to 20.0 g, or 10.0 g to 20.0 g.

[0147] The value A (g / Ah) obtained by dividing the amount of electrolyte (g) injected into the battery by the battery capacity (Ah) is not particularly limited, but is preferably set to, for example, the range exemplified in the section on electrolyte.

[0148] 3.2. Lithium Metal Battery A lithium metal battery (hereinafter also referred to as "LMB") is manufactured using an electrode having lithium metal or a lithium metal alloy on its surface, or using lithium metal as the negative electrode. The LMB of this embodiment contains the electrolyte of this embodiment. As with anode-free batteries, the LMB is charged and discharged by deposition of lithium metal on the surface of the negative electrode and electrolytic dissolution of the deposited lithium.

[0149] As mentioned above, LMBs differ from anode-free batteries in that the negative electrode has lithium metal as the negative electrode active material before the initial charge of the battery.

[0150] The lithium metal battery of this embodiment includes a positive electrode current collector, a positive electrode having a positive electrode active material disposed on the positive electrode current collector, a negative electrode having lithium metal facing the positive electrode, and a separator disposed between the positive electrode and the negative electrode. The configurations and preferred aspects of the positive electrode current collector, positive electrode, and separator are the same as those of an anode-free battery, except for the points described below.

[0151] The negative electrode of the LMB is not particularly limited as long as it contains lithium metal or a lithium metal alloy. Because the LMB uses a negative electrode containing lithium metal or a lithium metal alloy with a large specific capacity and a low redox potential, the battery generally has a higher energy density than a lithium-ion battery. Examples of such negative electrodes include a lithium metal electrode, an electrode in which rolled lithium metal foil is laminated onto the surface of a conductive metal foil such as copper to form a clad material, an electrode in which lithium metal is electrochemically deposited on the surface of a metal foil such as copper, and an electrode in which metallic lithium is vacuum-deposited. From the viewpoint of further improving the effects of this embodiment, an electrode in which lithium metal foil is laminated onto the surface of a conductive metal such as copper, or an electrode in which lithium metal is electrochemically deposited, is preferred, and an electrode in which lithium metal foil is laminated onto the surface of a conductive metal such as copper is more preferred.

[0152] The average thickness of the LMB negative electrode is not particularly limited, but is, for example, 5.0 μm to 100.0 μm. From the viewpoint of improving the capacity and / or energy density of the battery, it is preferably 8.0 μm to 50.0 μm, 10.0 μm to 40.0 μm, or 10.0 μm to 20.0 μm.

[0153] The LMB may be fabricated using known materials and known fabrication methods, and may be fabricated in the same manner as the anode-free battery described above, except that lithium metal or a lithium metal alloy is used for the negative electrode.

[0154] The lithium metal battery of this embodiment may be a lithium-air battery as one aspect thereof. In a lithium-air battery, during discharge, lithium ions in the electrolyte react with oxygen to form lithium peroxide, and during charge, the lithium peroxide decomposes again into lithium ions and oxygen, thereby achieving charge and discharge. A lithium-air battery can be fabricated using a conventional configuration, for example, by configuring the positive electrode and positive electrode current collector in the configuration of the lithium metal battery described above to contain an oxygen-containing gas such as air, allowing oxygen to be used as the positive electrode active material. It should be noted that various components of the lithium-air battery, such as the positive electrode, positive electrode current collector, and conductive agent, may be made of known components that constitute lithium-air batteries.

[0155] 3.3 Lithium-Ion Batteries Lithium-ion batteries (LIBs) have a host material of lithium element (lithium ions or lithium metal) at their negative electrode. When the battery is charged, the material is filled with lithium element, and when the host material releases lithium element, the battery is discharged. LIBs differ from anode-free batteries, particularly in that the negative electrode has a host material of lithium element.

[0156] The lithium ion battery can be manufactured using known materials and manufacturing methods. The electrolyte solution of the present embodiment may be used to exhibit ionic conductivity inside the lithium ion battery, and there are no particular limitations on the manufacturing stage at which it is injected or the names of the components contained therein.

[0157] The shape of the battery of the lithium secondary battery according to this embodiment is not particularly limited, and may be, for example, a sheet type, a laminated sheet type, a thin type, a cylindrical type with a bottom, a prismatic type with a bottom, etc. From the viewpoint of more effectively and reliably achieving the effects of this embodiment, the sheet type, the laminated sheet type, or the thin type is preferred.

[0158] <Additional Notes> Embodiments of the present disclosure include the following aspects: [1] An electrolyte solution for a lithium secondary battery, comprising: an ether having no fluorine atoms, a hydrofluoroether, a cyclic compound represented by the following formula (1), an additive being at least one selected from the group consisting of a sulfite ester, a sulfate ester, a sulfonate ester, and an alkyl sulfone, and lithium bis(fluorosulfonyl)imide: (In formula (1), m is an integer of 0 to 3, and R ｈ is a monovalent saturated hydrocarbon group having 1 to 10 carbon atoms.) [2] The electrolyte solution according to [1], wherein the cyclic compound includes at least one selected from the group consisting of cyclohexane, methylcyclohexane, and ethylcyclohexane. [3] The electrolyte solution according to [1] or [2], wherein the additive is at least one selected from the group consisting of ethylene sulfite, dimethyl sulfite, 4-methylethylene sulfite, dimethyl sulfone, and 1,3-propane sultone. [4] The electrolyte solution according to [3], wherein the additive is 1,3-propane sultone. [5] The electrolyte solution according to any one of [1] to [4], wherein the product (A×B) of the value A (g / Ah) obtained by dividing the amount of electrolyte (g) injected into the battery by the battery capacity (Ah) and the content B (parts by mass) of the additive relative to 100 parts by mass of the electrolyte solution is 0.10 to 9.5. [6] The electrolyte solution according to any one of [1] to [5], wherein the fluorine atom-free ether comprises at least one selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dimethoxypropane. [7] The electrolyte solution according to any one of [1] to [6], wherein the hydrofluoroether comprises a chain fluorine compound having at least one of monovalent groups represented by the following formula (A) or formula (B): (In formulas (A) and (B), the wavy line represents a bonding site in a monovalent group.) [8] The electrolyte solution according to any one of [1] to [7], wherein the content of the additive is 0.04 mass % or more and 10.00 mass % or less with respect to the total mass of the electrolyte solution excluding the additive. [9] A lithium secondary battery comprising the electrolyte solution according to any one of [1] to [8], wherein lithium metal is deposited on the surface of a negative electrode and the deposited lithium metal dissolves, thereby carrying out charging and discharging.

[10] A lithium secondary battery comprising, as a positive electrode active material, Li ｚ Ni ｘ Co ｙ M １－ｘ－ｙ O ２＋α (wherein 0.5≦x≦1.0, 0≦y≦0.35, 0.9≦z≦1.3, −0.2≦α≦0.15, and M is at least one element selected from Mn, Al, V, Mg, Mo, Nb, Ti, Zr, Fe, Cu, Cr, Zn, F, and B).

[0159] The present embodiment will be described in more detail below using examples and comparative examples. The present embodiment is not limited by the following examples. ５ It was conducted in Pa.

[0160] 1. Fabrication of Anode-Free Lithium Secondary Battery An anode-free lithium secondary battery (AFB) of Example 1 was fabricated as follows.

[0161] 1.1. Preparation of Negative Electrode First, an electrolytic Cu foil having a thickness of 4.0 μm was prepared and punched out to a predetermined size (105 mm×55 mm) by a punching process.

[0162] 1.2. Preparation of Positive Electrode Next, a positive electrode was prepared. LiNi was used as the positive electrode active material. 0.85 Co 0.12 Al 0.03A mixture of 96 parts by mass of O2, 2.0 parts by mass of carbon black as a conductive additive, and 2.0 parts by mass of polyvinylidene fluoride (PVDF) as a binder was applied to both sides of a 12 μm Al foil and press-molded. The resulting molded body was punched out to a predetermined size (100 mm × 50 mm) to obtain a positive electrode having a positive electrode current collector. The basis weight of the resulting positive electrode was 18 mg / cm. ２ The capacity of the positive electrode active material was 200 mAh / g.

[0163] 1.3. Preparation of Separator A separator having a predetermined size (108 mm x 58 mm) was prepared, in which both sides of a 12 μm polyethylene microporous membrane were coated with 2.0 μm polyvinylidene fluoride (PVDF).

[0164] 1.4. Preparation of Electrolyte Solution An electrolyte solution was prepared as follows. Three solvents were mixed so that the concentration was 10% by volume of cyclohexane, 60% by volume of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and 30% by volume of 1,2-dimethoxyethane. LiN(SO 4 ) was added to the resulting mixture to a molar concentration of 1.2 M. ２ F) ２ Furthermore, 1,3-propane sultone was dissolved in the electrolyte so that the content was 0.50 parts by mass when the total mass of the electrolyte excluding the additives was 100 parts by mass. In this way, an electrolyte was obtained.

[0165] 1.5. Battery Assembly The punched separators, positive electrodes, and negative electrodes were stacked in the order separator / negative electrode / separator / positive electrode / separator / ... / positive electrode / separator / negative electrode / separator, resulting in 20, 21, and 42 positive electrode, negative electrode current collector foil, and separator sheets, respectively. A 0.2 mm-thick aluminum tab was then ultrasonically welded to the uncoated portion of the positive electrode active material of the stacked positive electrodes, and a copper / nickel tab was ultrasonically welded to the negative electrodes. Then, while measuring the weight with an electronic balance, electrolyte was poured in until the amount reached the value listed in Table 1, and the battery was vacuum-sealed under a reduced pressure of -50 kPa to obtain a lithium secondary battery.

[0166] Lithium secondary batteries of Examples 2 to 13 were obtained in the same manner as in Example 1, except that the electrolyte solution was prepared using the solvent and electrolyte shown in Table 1.

[0167] Lithium secondary batteries of Comparative Examples 1 to 5 were obtained in the same manner as in Example 1, except that the electrolytic solutions were prepared using the solvents and electrolytes listed in Table 1. The comparative examples did not contain at least one of the cyclic compound represented by formula (1) above, the hydrofluoroether (HFE), the non-fluorinated ether, the specific lithium salt, and the additive.

[0168] The abbreviations in Table 1 correspond to the following compounds: LiFSI: LiN(SO ２ F) ２ (Lithium bis(fluorosulfonyl)imide) LiPF ６ : Lithium hexafluorophosphate DME: 1,2-dimethoxyethane DEE: 1,2-diethoxyethane DMP: 1,2-dimethoxypropane HFE1: 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether HFE2: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether HC1: Cyclohexane HC2: Methylcyclohexane HC3: Ethylcyclohexane DMC: Dimethyl carbonate EC: Ethylene carbonate

[0169] Dimethyl carbonate and ethylene carbonate are solvents that do not correspond to any of the non-fluorinated ethers, HFEs, and the compounds of formula (1).

[0170] In Table 1, the numbers in the columns for non-fluorinated ether, HFE, and compound of formula (1) indicate the content relative to the total amount of solvent in vol%. The numbers to the right of the specific lithium salt and other lithium salt indicate the concentration in the electrolyte in the unit M (molar). The additive amount B indicates the content of the additive in parts by mass when the total mass of the electrolyte excluding additives is 100 parts by mass. For example, Example 1 indicates that the electrolyte contains 30% by volume of DME, 60% by volume of HFE2, and 10% by volume of HC2, 1.2 M LiFSI, and 0.5 parts by mass of 1,3-propane sultone relative to 100 parts by mass of the total mass of the electrolyte excluding additives.

[0171] In Table 1, in each example and comparative example, when the electrolyte solution contains a cyclic compound of formula (1), the number of substituents bonded to the benzene ring of the cyclic compound of formula (1) is shown in the column "Number of substituents (compound of formula (1))." The specific gravity of each electrolyte solution is shown in the column "Electrolyte specific gravity (g / cc)."

[0172] In Table 1, in each example and comparative example, "injection amount" refers to the amount (g) of electrolyte injected into the assembled battery. Furthermore, "injection amount / battery capacity A" refers to the value obtained by dividing the injection amount by the battery capacity of the assembled battery. Furthermore, "B x A" refers to the value obtained by multiplying the addition amount B by the injection amount / battery capacity A. Furthermore, battery capacity A refers to the ideal capacity (6.9 Ah) of the battery calculated from the total amount of positive electrode active material.

[0173] 2. Evaluation Hereinafter, the initial CC charging of the fabricated lithium secondary battery is also referred to as "initial charging," and the initial CC discharging is also referred to as "initial discharging." Furthermore, in an environment at a temperature of 25°C, the battery was initially charged at a current of 0.7 A until the voltage reached 4.2 V, and then initially discharged at a current of 0.7 A until the voltage reached 3.0 V. The capacity determined from this initial discharge is also referred to as "initial capacity." CC charging refers to charging at a constant current value, and CC discharging refers to discharging at a constant current value.

[0174] 2.1. Compatibility After preparing the above electrolyte solution, it was left to stand for 1 hour under an argon atmosphere and then visually observed for the presence or absence of phase separation. In Table 1, "-" indicates that phase separation was not possible because only one type of solvent was used. "X" indicates that phase separation occurred and it was judged to be incompatible. "〇" indicates that there was no phase separation and it was judged to be compatible. In addition, cases in which LiFSI remained undissolved were indicated as "salt insoluble." In the cases indicated as "-", "X", and "〇", LiFSI was completely dissolved in the solvent.

[0175] 2.2. Discharge Capacity (Charge / Discharge Characteristics) The fabricated lithium secondary battery was initially charged at a current of 0.7 A in an environment at a temperature of 25°C until the voltage reached 4.2 V, and then initially discharged at a current of 0.7 A until the voltage reached 3.0 V. Next, in an environment at a temperature of 25°C, the battery was CC charged at a current of 0.7 A until the voltage reached 4.2 V, and then CC discharged at a current of 0.7 A until the voltage reached 3.0 V, and the discharge capacity was determined. This value is shown as "Discharge Capacity (Ah), 25°C (0.1 C)."

[0176] The fabricated lithium secondary battery was initially charged at a current of 0.7 A in an environment at a temperature of 25° C. until the voltage reached 4.2 V, and then initially discharged at a current of 0.7 A until the voltage reached 3.0 V. Next, in an environment at a temperature of 25° C., the battery was CC charged at a current of 0.7 A until the voltage reached 4.2 V, and then CC discharged at a current of 34.5 A until the voltage reached 3.0 V, and the discharge capacity was determined. This value is shown in "Discharge Capacity (Ah), 25° C. (5C)."

[0177] The fabricated lithium secondary battery was initially charged at a current of 0.7 A in an environment at a temperature of 25°C until the voltage reached 4.2 V, and then initially discharged at a current of 0.7 A until the voltage reached 3.0 V. Next, in an environment at a temperature of -30°C, the battery was CC charged at a current of 0.7 A until the voltage reached 4.2 V, and then CC discharged at a current of 0.7 A until the voltage reached 3.0 V, and the discharge capacity was determined. This value is shown in "Discharge capacity (Ah), -30°C (0.1C)."

[0178] 2.3 Number of Cycles (Cycle Characteristics) The cycle characteristics of the lithium secondary batteries fabricated in each of the Examples and Comparative Examples were evaluated as follows.

[0179] The fabricated lithium secondary batteries were initially charged at a current of 0.7 A in an environment at a temperature of 25°C until the voltage reached 4.2 V, and then initially discharged at a current of 0.7 A until the voltage reached 3.0 V. Next, in an environment at a temperature of 25°C, the batteries were CC charged at a current of 2.8 A until the voltage reached 4.2 V, and then CC discharged at a current of 4.8 A until the voltage reached 3.0 V. The discharge capacity was determined from CC discharge for each example and comparative example. Table 1 shows the number of cycles at which the discharge capacity reached 80% of the initial capacity (referred to as "cycle number" in Table 1).

[0180] 2.4 Volume Increase (High-Temperature Storage Stability) Using the lithium secondary batteries of each Example and Comparative Example, the batteries were initially charged at a current of 0.7 A in an environment at a temperature of 25°C until the voltage reached 4.2 V, and then initially discharged at a current of 0.7 A until the voltage reached 3.0 V. Next, the batteries were CC charged at a current of 0.7 A in an environment at a temperature of 25°C until the voltage reached 4.2 V, and the volumes of the lithium secondary batteries were measured. Thereafter, the lithium secondary batteries were left in a thermostatic chamber at 60°C for 7 days, and the volumes of the lithium secondary batteries were measured again. The change in volume is shown in Table 1 as "Volume Increase."

[0181]

[0182] In Table 1, "-" in the electrolyte composition indicates that the corresponding component is not contained, "-" in the number of substituents indicates that the compound of formula (1) is not used, and "-" in the electrolyte specific gravity, injected amount, injected amount / battery capacity, B×A, discharge capacity, number of cycles, and volume increase indicates that measurement was not performed.

[0183] 2.6. Amount of Electrolyte Added An experiment was conducted to confirm the relationship between the amount of electrolyte added and battery performance. Lithium secondary batteries of Experimental Examples 1 and 2 were obtained in the same manner as in Examples 7 and 6, except that the amount of electrolyte added was changed to the value shown in Table 2. The obtained lithium secondary batteries of Experimental Examples 1 and 2 were evaluated for battery performance in the same manner as above.

[0184]

[0185] 3. Evaluation Results Table 1 shows that Examples 1 to 13, which are lithium secondary batteries using an electrolyte solution containing an ether having no fluorine atoms, a hydrofluoroether, a cyclic compound represented by the above formula (1), at least one additive selected from the group consisting of a sulfite ester, a sulfate ester, a sulfonate ester, and an alkyl sulfone, and lithium bis(fluorosulfonyl)imide, exhibit good cycle characteristics, good high-temperature storage stability, and excellent charge-discharge characteristics, compared to the comparative example, which is a lithium secondary battery using a different electrolyte solution.

[0186] Furthermore, when Experimental Examples 1 and 2 in Table 2 are compared with Examples 1, 6, and 7 in Table 1, it is found that the electrolyte solution of this embodiment can further improve the performance of the battery when the amount of electrolyte injected is adjusted so that the product B×A falls within a predetermined range.

[0187] 4. Lithium metal battery Next, we examined lithium metal batteries.

[0188] A lithium metal battery (LMB) of Example 14 was fabricated as follows.

[0189] 4.1. Preparation of Negative Electrode First, a clad material was prepared in which a 20.0 μm thick Li foil was bonded to both sides of an 8.0 μm thick electrolytic Cu foil, and the clad material was punched out to a predetermined size (105 mm×55 mm).

[0190] 4.2. Preparation of Positive Electrode Next, a positive electrode was prepared. LiNi was used as the positive electrode active material. 0.85 Co 0.12 Al 0.03 A mixture of 96 parts by mass of O2, 2.0 parts by mass of carbon black as a conductive additive, and 2.0 parts by mass of polyvinylidene fluoride (PVDF) as a binder was applied to both sides of a 12 μm Al foil and press-molded. The resulting molded body was punched out to a predetermined size (100 mm × 50 mm) to obtain a positive electrode having a positive electrode current collector. The basis weight of the resulting positive electrode was 18 mg / cm. ２ It was.

[0191] 4.3. Preparation of Separator A separator of a predetermined size (108 mm x 58 mm) was prepared, in which both sides of a 12 μm polyethylene microporous membrane were coated with 2.0 μm polyvinylidene fluoride (PVDF).

[0192] 4.4. Preparation of Electrolyte Solution An electrolyte solution was prepared as follows. Three solvents were mixed so that the concentration was 10% by volume of methylcyclohexane, 60% by volume of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and 30% by volume of 1,2-dimethoxyethane. LiN(SO 4 ) was added to the resulting mixture to a molar concentration of 1.2 M. ２ F) ２ Furthermore, 1,3-propane sultone was dissolved in the electrolyte so that the content was 0.50 parts by mass when the total mass of the electrolyte excluding the additives was 100 parts by mass. In this way, an electrolyte was obtained.

[0193] 4.5. Battery Assembly The punched separators, positive electrodes, and negative electrodes were stacked in the order separator / negative electrode / separator / positive electrode / separator / ... / positive electrode / separator / negative electrode / separator, resulting in 20, 21, and 42 positive electrode, negative electrode current collector foil, and separator sheets, respectively. A 0.2 mm-thick aluminum tab was then ultrasonically welded to the uncoated portion of the positive electrode active material of the stacked positive electrodes, and a copper / nickel tab was ultrasonically welded to the negative electrodes. Then, while measuring the weight with an electronic balance, electrolyte was poured in until the amount reached the value listed in Table 3. The battery was then vacuum-sealed under a reduced pressure of -50 kPa to obtain a lithium secondary battery.

[0194] A lithium metal battery of Comparative Example 6 was obtained in the same manner as in Example 14, except that the electrolyte solution was prepared using the solvents shown in Table 3.

[0195] In Table 3, each column represents the same items as in Table 1.

[0196] 5. Evaluation The compatibility, 25°C (0.1C) discharge capacity, -30°C (0.1C) discharge capacity, and volume increase were evaluated in the same manner as in Table 1. The capacity retention rate at the 100th cycle was measured as follows. Using the lithium secondary batteries of the Examples and Comparative Examples, the batteries were initially charged at a current of 0.7 A in an environment at a temperature of 25°C until the voltage reached 4.2 V, and then initially discharged at a current of 0.7 A until the voltage reached 3.0 V. Next, in an environment at a temperature of 25°C, the batteries were CC charged at a current of 1.4 A until the voltage reached 4.2 V, and then CC discharged at a current of 3.5 A until the voltage reached 3.0 V (first cycle). This cycle was then repeated 99 times, for a total of 100 cycles. For the Examples and Comparative Examples, the capacity retention rate was calculated by dividing the capacity determined from the CC discharge at the 100th cycle by the capacity determined from the CC discharge at the 1st cycle, and this value was used as the capacity retention rate at the 100th cycle.

[0197]

[0198] 6. Evaluation Results Table 3 shows that Example 14, which is a lithium metal battery using an electrolyte solution containing an ether having no fluorine atoms, a hydrofluoroether, a cyclic compound represented by the above formula (1), at least one additive selected from the group consisting of a sulfite ester, a sulfate ester, a sulfonate ester, and an alkyl sulfone, and lithium bis(fluorosulfonyl)imide, exhibits good cycle characteristics, good high-temperature storage stability, and excellent charge-discharge characteristics, compared to Comparative Example 6, which is a lithium metal battery using a different electrolyte solution. Industrial Applicability

[0199] A lithium secondary battery prepared using the electrolyte solution of the present invention has excellent cycle characteristics and is therefore industrially applicable as an electrolyte solution for electricity storage devices used in a variety of applications.

[0200] 100... Lithium secondary battery, 110... Positive electrode current collector, 120... Positive electrode, 130... Separator, 140... Negative electrode

Claims

1. An electrolyte for a lithium secondary battery comprising: an ether having no fluorine atoms; a hydrofluoroether; a cyclic compound represented by the following formula (1); an additive which is at least one selected from the group consisting of a sulfite ester, a sulfate ester, a sulfonate ester, and an alkylsulfone; and lithium bis(fluorosulfonyl)imide. (In formula (1), m is an integer of 0 to 3, and R ｈ is a monovalent saturated hydrocarbon group having 1 to 10 carbon atoms.

2. The electrolyte according to claim 1, wherein the cyclic compound comprises at least one selected from the group consisting of cyclohexane, methylcyclohexane, and ethylcyclohexane.

3. The electrolyte according to claim 1, wherein the additive is at least one selected from the group consisting of ethylene sulfite, dimethyl sulfite, 4-methylethylene sulfite, dimethyl sulfone, and 1,3-propane sultone.

4. The electrolyte according to claim 3, wherein the additive is 1,3-propane sultone.

5. The electrolyte solution according to claim 1, wherein the product (A x B) of the amount of electrolyte solution (g) injected into the battery divided by the battery capacity (Ah), A (g / Ah), and the content B (parts by mass) of the additive per 100 parts by mass of the electrolyte solution, is 0.10 or more and 9.5 or less.

6. The electrolyte solution according to claim 1, wherein the ether having no fluorine atoms includes at least one selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dimethoxypropane.

7. The electrolyte solution according to claim 1, wherein the hydrofluoroether contains a chain fluorine compound having at least one of monovalent groups represented by the following formula (A) or formula (B): (In formulas (A) and (B), the wavy lines represent binding sites in monovalent groups.) 8. The electrolyte solution according to claim 1, wherein the content of the additive is 0.04 mass % or more and 10.00 mass % or less with respect to the total mass of the electrolyte solution excluding the additive.

9. A lithium secondary battery comprising the electrolyte solution according to any one of claims 1 to 8, in which lithium metal is deposited on the surface of the negative electrode and the deposited lithium metal is dissolved, thereby carrying out charging and discharging.

10. Li as a positive electrode active material ｚ Ni ｘ Co ｙ M １－ｘ－ｙ O ２＋α (wherein 0.5≦x≦1.0, 0≦y≦0.35, 0.9≦z≦1.3, −0.2≦α≦0.15, and M is at least one element selected from Mn, Al, V, Mg, Mo, Nb, Ti, Zr, Fe, Cu, Cr, Zn, F, and B).

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