Electrolyte for lithium secondary batteries and lithium secondary batteries

Abstract

The present invention provides an electrolyte for a lithium secondary battery that enables high energy densities and exceptional cycling characteristics. The present invention relates to an electrolyte for a lithium secondary battery that contains a cyclic compound represented by formula (1) or formula (2), a hydrofluoroether, an ether having no fluorine atoms, and a lithium salt.

Description

[Technical Field]

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

[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 energy storage devices that are highly safe and can store large amounts of electrical energy.

[0003] Among these, lithium secondary batteries, which charge and discharge by transferring 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 transferring lithium ions between the positive electrode active material and the negative electrode active material.

[0004] Furthermore, with the aim of achieving higher energy density and improving productivity, lithium secondary batteries (LMB: Lithium-metal battery) that use lithium metal as the negative electrode active material instead of materials that can insert lithium ions, such as carbon materials, 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 carbon material or lithium metal have been developed.

[0005] On the other hand, lithium secondary batteries capable of achieving high voltages have a problem in that the cycle characteristics of the lithium secondary batteries are reduced due to the high reactivity between the battery components and the electrolyte. Therefore, there is a need for a method for preventing the consumption of the electrolyte during repeated charging and discharging and the reduction in battery capacity due to such reactions. For example, Patent Document 1 discloses that an organic electrolyte secondary battery containing an electrolyte containing a nonionic aromatic compound selected from the group consisting of trimellitic acid ester or a derivative thereof, tertiary butyl ester or a derivative thereof, tertiary butyl benzene, isobutyl benzene, and cyclohexyl benzene has a significantly reduced reactivity with the electrolyte at high temperatures, thereby improving the safety of the battery. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-056892 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in conventional lithium secondary batteries, including those described in the above patent documents, the decomposition reaction of the electrolyte occurs before the reduction reaction of lithium, which causes an irreversible decomposition reaction of the electrolyte with each charge / discharge cycle of the battery. It has been found that lithium secondary batteries containing such electrolytes continuously expand with repeated charge / discharge, causing the electrolyte to no longer fill the electrodes (i.e., electrolyte depletion), resulting in a decrease in cell capacity.

[0008] The present invention has been made in view of the above problems, and has an object to provide an electrolyte solution that achieves both high energy density and excellent cycle characteristics in a lithium secondary battery. [Means for solving the problem]

[0009] An electrolyte solution for a lithium secondary battery according to one embodiment of the present invention contains a cyclic compound represented by the following formula (1) or (2), a hydrofluoroether, an ether having no fluorine atom, and a lithium salt. In formula (1), n ​​is an integer of 0 to 3, and R a is a monovalent saturated hydrocarbon group having 1 to 10 carbon atoms, in formula (2), m is an integer of 0 to 3, and R h is a monovalent saturated hydrocarbon group having 1 to 10 carbon atoms. [ka] [ka]

[0010] The present inventors have discovered that by including a compound represented by the above formula (1) or (2) in the electrolyte of a lithium secondary battery, it is possible to achieve both high charge / discharge efficiency during repeated charge / discharge and an increased volume of the electrolyte per unit weight. Furthermore, by including a hydrofluoroether as the solvent of the electrolyte, the reversibility of the lithium oxidation-reduction reaction on the negative electrode surface during repeated charge / discharge of the battery is improved, and compatibility between the solvents and discharge characteristics at low temperatures are also improved. Furthermore, by including an ether without a fluorine atom as the solvent of the electrolyte, the solubility of electrolytes, including lithium salts, is improved. Therefore, it is presumed that the components of the above-mentioned solvents synergistically improve the energy density and cycle characteristics of the lithium secondary battery. However, the factors behind this improvement are not limited to those mentioned above.

[0011] In the electrolytic solution according to one embodiment of the present invention, the cyclic compound is preferably contained in an amount of 5% by volume to 60% by volume based on the total amount of the solvent components of the electrolytic solution. According to such an embodiment, the lithium secondary battery tends to have even better energy density and / or cycle characteristics.

[0012] In the electrolytic solution according to one embodiment of the present invention, the cyclic compound is preferably contained in an amount of 10% by volume to 55% by volume of the total amount of the solvent components of the electrolytic solution. According to such an embodiment, the lithium secondary battery tends to have even better energy density and / or cycle characteristics.

[0013] In the electrolyte solution according to one embodiment of the present invention, preferably, in the above formula (1), n ​​is 1. According to such an embodiment, the lithium secondary battery tends to have even more excellent cycle characteristics.

[0014] In the electrolyte solution according to one embodiment of the present invention, preferably, in the above formula (2), m is 0 or 1. According to such an embodiment, the lithium secondary battery tends to have even more excellent cycle characteristics.

[0015] The electrolyte solution according to one embodiment of the present invention preferably contains the cyclic compound represented by the above formula (1). According to such an embodiment, the lithium secondary battery tends to have even more excellent cycle characteristics.

[0016] In the electrolyte solution according to one embodiment of the present invention, the specific gravity of the electrolyte solution is preferably 1.0 g / cc or more and 1.3 g / cc or less. According to such an embodiment, the lithium secondary battery tends to have a further excellent energy density.

[0017] In the electrolyte solution according to one embodiment of the present invention, the cyclic compound is preferably at least one selected from the group consisting of n-butylbenzene, tert-butylbenzene, isobutylbenzene, sec-butylbenzene, propylbenzene, ethyltoluene, 1,3,5-trimethylbenzene, cyclohexane, methylcyclohexane, and ethylcyclohexane. According to such an embodiment, the lithium secondary battery tends to have even more excellent energy density and / or cycle characteristics.

[0018] In the electrolyte solution according to one embodiment of the present invention, the fluorine-free ether is preferably 1,2-dimethoxyethane, 1,2-dimethoxypropane, or a mixture thereof. According to such an embodiment, the lithium secondary battery tends to have even more excellent cycle characteristics.

[0019] In the electrolyte solution according to one embodiment of the present invention, the lithium salt preferably contains LiN(SO2F)2. According to such an embodiment, the lithium secondary battery tends to have even more excellent cycle characteristics.

[0020] In the electrolytic solution according to one embodiment of the present invention, the hydrofluoroether is preferably 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. In addition, in formulas (A) and (B), the wavy lines represent bonding sites in the monovalent groups. [ka] [ka]

[0021] A lithium secondary battery according to one embodiment of the present invention includes any one of the electrolyte solutions described above. According to such an embodiment, the lithium secondary battery is excellent in both energy density and cycle characteristics.

[0022] In the lithium secondary battery according to one embodiment of the present invention, preferably, lithium metal is deposited on the surface of the negative electrode and the deposited lithium metal is dissolved, thereby performing charging and discharging. According to such an embodiment, the lithium secondary battery tends to further improve the effect of the electrolyte.

[0023] A lithium secondary battery according to one embodiment of the present invention preferably includes a negative electrode made of a negative electrode current collector that does not have a negative electrode active material. According to such an embodiment, the lithium secondary battery tends to further improve the effects of the electrolyte. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide an electrolyte solution that achieves both high energy density and excellent cycle characteristics in a lithium secondary battery. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a schematic cross-sectional view of a lithium secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] 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. Note that in the drawings, the same elements will be given the same reference numerals, and duplicate explanations will be omitted. Furthermore, unless otherwise specified, positional relationships such as up, down, left, and right will be based on the positional relationships shown in the drawings. Furthermore, the dimensional ratios of the drawings are not limited to those shown in the drawings.

[0027] [Electrolyte] The electrolytic solution is an ionically conductive solution containing an electrolyte and a solvent, and serves as a conduction path for lithium ions in a lithium secondary battery. The electrolytic solution may be impregnated into the 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.

[0028] The electrolyte solution according to this embodiment contains, as a solvent, a cyclic compound represented by the following formula (1) or (2) (hereinafter also referred to as the cyclic compound of formula (1) and the cyclic compound of formula (2)), a hydrofluoroether, and an ether having no fluorine atoms. In the following formula (1), n ​​is an integer of 0 to 3, and R ais a monovalent saturated hydrocarbon group having 1 to 10 carbon atoms, in formula (2), m is an integer of 0 to 3, and R h is a monovalent saturated hydrocarbon group having 1 to 10 carbon atoms. [ka] [ka]

[0029] When a lithium secondary battery is charged or discharged, an oxidation or reduction reaction of lithium occurs, allowing it to extract or store energy. Since the decomposition reaction of the electrolyte is generally more likely to occur than the oxidation or reduction reaction of lithium, the volume of the battery expands with each charge / discharge cycle due to irreversible decomposition of the electrolyte. This results in the electrolyte drying up, where the electrolyte no longer fills the electrodes, and the battery capacity also decreases. As a result of extensive research, the present inventors have found that by using at least one of the cyclic compounds of the above formula (1) and the cyclic compounds of the formula (2) as a solvent in a lithium secondary battery, the volume of the electrolyte solution per unit weight can be increased and the irreversible decomposition reaction of the electrolyte solution can be suppressed during charge-discharge cycles. The reason for this is presumed to be, but is not limited to, the following.

[0030] The cyclic compounds of formula (1) and formula (2) have a benzene ring or cyclohexane skeleton and no 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 a high energy per unit weight, improving the mass-based energy density of lithium secondary batteries. Furthermore, the cyclic compounds of formula (1) and formula (2) have high oxidation stability because they do not have reactive groups. Therefore, it is presumed that electrolytes containing such compounds suppress irreversible decomposition reactions, thereby improving the cycle characteristics of batteries.

[0031] On the other hand, hydrofluoroethers are compounds containing fluorine and hydrogen, and are prone to reactions on the surface of the negative electrode during charging and discharging of lithium secondary batteries. When a lithium secondary battery having an electrolyte containing such a compound is charged and discharged, the hydrofluoroether and other compounds in the electrolyte are decomposed, resulting in the formation of a solid electrolyte interface layer (SEI layer) on the surface of the negative electrode, etc. In particular, when the electrolyte contains hydrofluoroether, it is presumed that an SEI layer with a high fluorine content is preferably formed during charging of the lithium secondary battery. It is presumed that such an SEI layer inhibits further decomposition of components in the electrolyte and the resulting irreversible reduction of lithium ions in the lithium secondary battery.

[0032] Furthermore, it is presumed that the electrolyte solution according to the present embodiment contains an ether having no fluorine atoms, thereby further improving the solubility of lithium salt in the electrolyte solution, reducing the internal resistance of a lithium secondary battery containing the electrolyte solution, and providing favorable properties to the SEI layer that is formed. Therefore, it is believed that the synergistic effect of the characteristics of each of the components described above allows the lithium secondary battery containing the electrolyte solution of this embodiment to achieve both high energy density and excellent cycle characteristics, although the factors are not limited to those described above.

[0033] Each component contained in the electrolyte solution will be described in detail below.

[0034] (Cyclic Compound of Formula (1) and Cyclic Compound of Formula (2)) The electrolyte solution according to this embodiment contains the cyclic compound of the above formula (1) or (2). The electrolyte solution may contain at least one of the cyclic compound of formula (1) and the cyclic compound of formula (2), and may contain only the cyclic compound of formula (1), only the cyclic compound of formula (2), or both the cyclic compound of formula (1) and the cyclic compound of formula (2). In addition, when a plurality of saturated hydrocarbon groups are present in the cyclic compounds of formula (1) and formula (2), R a or R h are each independently selected. That is, as used herein, multiple R a , or multiple R h The structures may be the same or different from each other.

[0035] The electrolyte solution according to this embodiment preferably contains a cyclic compound represented by formula (1). When the electrolyte solution contains the cyclic compound of formula (1), the cycle characteristics of the lithium secondary battery tend to be further improved.

[0036] In formula (1), n ​​is an integer of 0 or more and 3 or less. From the viewpoint of improving the oxidation stability of the electrolyte solution and further improving the cycle characteristics of a lithium secondary battery containing the electrolyte solution, n is preferably 1 or 2, and more preferably 1.

[0037] In formula (1), R a may be a chain saturated hydrocarbon group or a cyclic saturated hydrocarbon group. a 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.

[0038] In formula (1), when n is 1, R a The number of carbon atoms in R is preferably 2 or more and 10 or less. a When the number of carbon atoms in R is within the above range, the effect of the electrolyte solution of the present embodiment tends to be further improved. a The number of carbon atoms in the group is preferably 2 or more and 8 or less, more preferably 3 or more and 6 or less, and even more preferably 4 or 5. Also, when n is 2 or 3, R a From the same viewpoint as above, the number of carbon atoms in each of the groups is preferably 1 or more and 8 or less, more preferably 1 or more and 5 or less, even more preferably 1 or more and 3 or less, and even more preferably 1 or 2.

[0039] In formula (1), R a R when there are multiple a The total number of carbon atoms in R is not particularly limited, and is, for example, 2 or more and 30 or less. a The total number of carbon atoms is preferably 2 or more and 10 or less, and more preferably 3 or more and 6 or less.

[0040] In formula (1), R a R may have a branched chain. a The branched chain structure of the cyclic compound of formula (1) preferably has at least one secondary or tertiary carbon atom, and more preferably has at least one tertiary carbon atom.

[0041] The molecular weight of the cyclic compound of formula (1) of this embodiment is not particularly limited and is, for example, 10 or more and 500 or less. From the viewpoint of further improving the cycle characteristics of a lithium secondary battery containing an electrolyte solution, the molecular weight of the cyclic compound of formula (1) is preferably 10 or more and 220 or less, more preferably 10 or more and 180 or less, even more preferably 10 or more and 150 or less, and even more preferably 115 or more and 140 or less.

[0042] The specific gravity of the cyclic compound of formula (1) contained in the electrolyte at 25°C is not particularly limited and 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 more preferably 1.1 g / cc or less, more preferably 1.0 g / cc or less, and even more preferably 0.9 g / cc or less.

[0043] 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 butylbenzene, isobutylbenzene, tert-butylbenzene, propylbenzene, 2-ethyltoluene, 1,3,5-trimethylbenzene (mesitylene), ethylbenzene, xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 3-ethyltoluene, 4-ethyltoluene, cumene, 1,3-diethylbenzene, 1,4-diethylbenzene, sec-butylbenzene, o-cymene, p-cymene, 1,2-diethylbenzene, pentylbenzene, neopentylbenzene, tert-amylbenzene, isopentylbenzene, etc. From the viewpoint of further improving the effects of the electrolytic solution of this embodiment, it is preferable to use butylbenzene, isobutylbenzene, tert-butylbenzene, propylbenzene, ethyltoluene, or 1,3,5-trimethylbenzene as the cyclic compound of formula (1). The cyclic compounds of formula (1) may be used singly or in combination of two or more.

[0044] In formula (2), m is an integer of 0 or more and 3 or less. From the viewpoint of improving the oxidation stability of the electrolyte solution and further improving the cycle characteristics of a lithium secondary battery containing the electrolyte solution, m is preferably 0 or more and 2 or less, more preferably 0 or 1, and even more preferably 1.

[0045] In equation (2), 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.

[0046] In equation (2), R h The number of carbon atoms in R is 1 or more and 10 or less. h The number of carbon atoms in the group is preferably 1 or more and 8 or less, more preferably 1 or more and 5 or less, and even more preferably 1 or 2.

[0047] In equation (2), R h According to such an embodiment, the cycle characteristics of a lithium secondary battery having the electrolyte solution tend to be further improved.

[0048] The molecular weight of the cyclic compound of formula (2) of this embodiment is not particularly limited and is, for example, from 84 to 500. From the viewpoint of further improving the cycle characteristics of a lithium secondary battery containing an electrolyte solution, the molecular weight of the cyclic compound of formula (2) is preferably from 84 to 220, more preferably from 84 to 180, and even more preferably from 84 to 150.

[0049] The specific gravity of the cyclic compound of formula (2) contained in the electrolyte at 25°C is not particularly limited and 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 (2) at 25°C is more preferably 1.1 g / cc or less, more preferably 1.0 g / cc or less, and even more preferably 0.9 g / cc or less.

[0050] The cyclic compound of formula (2) in this embodiment is not particularly limited as long as it is a compound represented by formula (2), and examples thereof include cyclohexane, methylcyclohexane, ethylcyclohexane, propylcyclohexane, butylcyclohexane, tert-butylcyclohexane, isobutylcyclohexane, 1,4-dimethylcyclohexane, 1,3,5-trimethylcyclohexane, etc. From the viewpoint of further improving the effects of the electrolytic solution of this embodiment, it is preferable to use one type selected from the group consisting of cyclohexane, methylcyclohexane, and ethylcyclohexane as the cyclic compound of formula (2).

[0051] Furthermore, the cyclic compounds of formula (1) and formula (2) in this embodiment are not particularly limited as long as they are compounds represented by formula (1) and formula (2). However, from the same viewpoint as above, the cyclic compounds are preferably at least one selected from the group consisting of n-butylbenzene, tert-butylbenzene, isobutylbenzene, sec-butylbenzene, propylbenzene, ethyltoluene, 1,3,5-trimethylbenzene, cyclohexane, methylcyclohexane, and ethylcyclohexane.

[0052] In the electrolyte, the total content of the cyclic compound of formula (1) and the cyclic compound of formula (2) is preferably 5% by volume or more and 60% by volume or less, more preferably 10% by volume or more and 55% by volume or less, even more preferably 15% by volume or more and 50% by volume or less, and even more preferably 20% by volume or more and 45% by volume or less. When the total content of the cyclic compound of formula (1) and the cyclic compound of formula (2) is within the above range, a lithium secondary battery containing the electrolyte tends to have even better energy density and / or cycle characteristics.

[0053] Furthermore, in the electrolytic solution, the content of the cyclic compound of formula (1) is, from the same viewpoint as above, preferably 5 vol% to 55 vol%, more preferably 10 vol% to 55 vol%, even more preferably 15 vol% to 50 vol%, still more preferably 20 vol% to 45 vol%, and particularly preferably 25 vol% to 40 vol%. Furthermore, in the electrolytic solution, the content of the cyclic compound of formula (2) is preferably from 1 volume % to 50 volume %, more preferably from 3 volume % to 40 volume %, even more preferably from 5 volume % to 30 volume %, and still more preferably from 8 volume % to 20 volume %, from the same viewpoint as above.

[0054] (hydrofluoroether) The electrolyte solution according to 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. 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).

[0055] The molecular weight of the hydrofluoroether (HFE) contained in the electrolyte solution of the present embodiment is not particularly limited and 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, more preferably from 140 to 400, even more preferably from 160 to 350, and still more preferably from 180 to 300.

[0056] The number of carbon atoms in HFE is not particularly limited and is, for example, 3 or more and 30 or less. From the viewpoint of improving the cycle characteristics and / or stability of the battery, the number of carbon atoms in 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.

[0057] 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). Use of an electrolyte solution containing such an HFE tends to further improve the cycle characteristics of the battery. From this perspective, the HFE contained in the electrolyte solution is preferably one having the following formula (A). In formulas (A) and (B), the wavy lines represent the bonding sites in the monovalent groups. [ka] [ka]

[0058] When the HFE contained in the electrolytic solution of this 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 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 2 is a hydrogen atom or an alkyl group, and RF 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. [ka] [ka]

[0059] In the above formula (A') of HFE, R 1 is not particularly limited as long as it is an optionally 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, 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, and is, for example, 0 or more and 10 or less, preferably 1 or more and 6 or less, and more preferably 2 or more and 5 or less. 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.

[0060] In the above formula (B') of HFE, R 2 There is no particular limitation on R 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, and is, for example, 1 or more and 5 or less, preferably 1 or more and 3 or less, and more preferably 1 or 2. From the viewpoint of further improving the effect of the electrolytic solution according to this embodiment, R 2 is preferably a hydrogen atom.

[0061] In the above formula (B') of 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 electrolytic solution of this embodiment, m is preferably 1 or more and 4 or less, more preferably 1 or more and 3 or less, and even more preferably 1 or more and 2 or less.

[0062] 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, and is, for example, 1 or more and 10 or less, preferably 1 or more and 5 or less, and more preferably 2 or more and 4 or less.

[0063] The HFE contained in the electrolytic solution of the present 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, 1,1,2,2-tetrafluoroethyl-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 having 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.

[0064] The content of HFE contained in the electrolytic solution is, for example, 5.0% by volume or more and 80% by volume or less, based on the total amount of the solvent components of the electrolytic solution. The content of HFE is preferably 5.0% by volume or more and 70% by volume or less, more preferably 10% by volume or more and 65% by volume or less, even more preferably 15% by volume or more and 60% by volume or less, still more preferably 20% by volume or more and 55% by volume or less, and particularly preferably 25% by volume or more and 50% by volume or less. When the HFE content is within the above range, lithium secondary batteries containing the electrolytic solution tend to have even more excellent cycle characteristics.

[0065] (Ether without fluorine atoms) The electrolyte solution according to this embodiment contains an ether that does not have a fluorine atom (hereinafter, also referred to as a "non-fluorine ether").

[0066] 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 in the non-fluorinated ether is preferably from 3 to 15, more preferably from 4 to 12, and even more preferably from 5 to 10.

[0067] The number of ether bonds in the non-fluorinated ether is not particularly limited and is, for example, 1 or more and 10 or less. 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, or 3 or more. Moreover, the number of ether bonds in the non-fluorinated ether is preferably 8 or less, or 5 or less.

[0068] The non-fluorinated ether may be a saturated ether compound or an unsaturated ether compound. 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.

[0069] The non-fluorinated ether contained in the electrolyte solution of this embodiment is not particularly limited, and examples thereof include 1,2-dimethoxyethane (DME), 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, 2,3-diethoxybutane, and diethoxyethane. From the viewpoint of further improving the effects of the electrolyte solution of this embodiment, the non-fluorinated ether is preferably 1,2-dimethoxyethane, 1,2-dimethoxypropane, or a mixture thereof, and more preferably 1,2-dimethoxyethane or 1,2-dimethoxypropane. The non-fluorinated ethers may be used alone or in combination of two or more.

[0070] The content of the non-fluorinated ether contained in the electrolyte is not particularly limited, and is, for example, 1.0 volume % or more and 70 volume % or less relative to the total amount of the solvent components of the electrolyte. The content of the non-fluorinated ether is preferably 5.0 volume % or more and 65 volume % or less, more preferably 10 volume % or more and 60 volume % or less, even more preferably 15 volume % or more and 55 volume % or less, and still more preferably 20 volume % or more and 50 volume % or less. When the content of the non-fluorinated ether is within the above range, a lithium secondary battery having the electrolyte tends to have even more excellent cycle characteristics.

[0071] (Other solvents) The electrolyte solution of this embodiment may further contain a solvent other than the cyclic compound of formula (1) or (2), HFE, and non-fluorinated ether. That is, the electrolyte solution of this embodiment may contain, for example, a heterocyclic compound other than the cyclic compound represented by formula (1) or (2), or may contain, for example, a carbonyl compound not having a fluorine atom other than the above-mentioned ether compound.

[0072] The content of the solvent other than the cyclic compound of formula (1) or formula (2), HFE, and non-fluorinated ether contained in the electrolytic solution is not particularly limited, and is, for example, 0.0 vol% to 30 vol%, or 1.0 vol% to 25 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 vol%, and more preferably 0 vol% to 5.0 vol%.

[0073] The volume ratio of the content of the cyclic compound of formula (1) or formula (2) to HFE is not particularly limited and is, for example, 0.2 or more and 2.0 or less. 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) or formula (2) to HFE is preferably 0.3 or more and 1.8 or less, more preferably 0.4 or more and 1.7 or less, even more preferably 0.5 or more and 1.6 or less, and even more preferably 0.6 or more and 1.4 or less. The volume ratio of the content of the cyclic compound of formula (1) or formula (2) to the non-fluorinated ether is not particularly limited and 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 cyclic compound of formula (1) or formula (2) to the non-fluorinated ether is preferably 0.3 to 2.7, more preferably 0.4 to 2.5, and even more preferably 0.5 to 2.0. Furthermore, the volume ratio of the content of HFE to the non-fluorinated ether is not particularly limited and 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 HFE to the non-fluorinated ether is preferably 0.3 to 2.8, more preferably 0.4 to 2.6, and even more preferably 0.5 to 2.5.

[0074] (lithium salts) The lithium salt contained in the electrolyte solution is not particularly limited and may include inorganic and organic salts of lithium. Specific examples include LiI, LiCl, LiBr, LiF, LiBF, LiPF, LiPF, LiPF(C), LiPF(C), LiAsF, LiSOCF, LiN(SO), LiN(SOCF), LiN(SOCF), LiN(SOCFCF), LiBF(C), LiB(C), LiB(C), LiB(C), LiB(O), LiB(OCH), LiB(OCH)F, LiB(OCOCF), LiNO, and LiSO. To further improve the energy density and / or cycle characteristics of a lithium secondary battery using the electrolyte solution of this embodiment, it is preferable that the lithium salt contain at least LiN(SOF). The above lithium salts may be used alone or in combination of two or more. The electrolytic solution may further contain a salt other than a lithium salt as an electrolyte, such as salts of Na, K, Ca, and Mg.

[0075] The total concentration of lithium salts in the electrolyte is not particularly limited, and is preferably 0.30M or more, more preferably 0.40M or more, even more preferably 0.50M or more, and even more preferably 0.80M or more. When the lithium salt concentration is within the above range, the SEI layer tends to be formed more easily, and the internal resistance tends to be further reduced. In particular, a lithium secondary battery containing a fluorine compound as a solvent can have a higher concentration of lithium salt in the electrolyte, thereby further improving cycle characteristics and rate performance. The upper limit of the lithium salt concentration is not particularly limited, and the lithium salt concentration may be 10.0M or less, 5.0M or less, or 2.0M or less.

[0076] The specific gravity of the electrolyte solution of this embodiment is preferably 1.0 g / cc or more and 1.3 g / cc or less. When the specific gravity of the electrolyte solution 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 more preferably 1.05 g / cc or more and 1.25 g / cc or less, and even more preferably 1.1 g / cc or more and 1.2 g / cc or less.

[0077] The cyclic compound of formula (1) or (2) contained in the electrolyte solution tends to have a higher boiling point than compounds generally used as a solvent for the electrolyte solution. The high boiling point of the cyclic compound of formula (1) or (2) tends to further improve the stability of the electrolyte solution against changes in environmental temperature and improve cycle characteristics. From this perspective, the boiling point of the electrolyte solution of this embodiment is preferably 72°C or higher, more preferably 80°C or higher, and even more preferably 85°C or higher under atmospheric pressure.

[0078] The presence of the cyclic compound of formula (1) or formula (2), HFE, non-fluorinated ether, etc. in the electrolyte solution can be confirmed by estimating the molecular structure through measurement or analysis using a known method. Examples of such methods include NMR, mass spectrometry, elemental analysis, and infrared spectroscopy. The molecular structure of the solvent can also be estimated through theoretical calculations using molecular dynamics, molecular orbital theory, etc.

[0079] (Method for preparing electrolyte solution) In preparing the electrolyte solution of this embodiment, a solution obtained by mixing the cyclic compound of formula (1) or (2), HFE, a non-fluorinated ether, and optionally a solvent other than those mentioned above is used as a solvent, and at least one of the lithium salts is dissolved in the solution, thereby preparing the electrolyte solution. The mixing ratio of the solvent and the lithium salt may be appropriately adjusted so that the type, content, concentration, etc. of each solvent and lithium salt are within the above-mentioned ranges.

[0080] [Lithium secondary battery] The lithium secondary battery of this embodiment includes the above-described electrolyte solution, which provides excellent energy density and cycle characteristics.

[0081] 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.

[0082] In a lithium secondary battery according to one embodiment of the present invention, lithium metal is preferably deposited on the surface of the negative electrode, and charging and discharging are performed by dissolving the deposited lithium metal. Examples of such lithium secondary batteries include anode-free lithium secondary batteries and lithium metal batteries. In this embodiment, the lithium secondary battery tends to further improve the effects of the electrolyte.

[0083] The following describes in detail various types of lithium secondary batteries.

[0084] [Anode-free battery] Fig. 1 is a schematic cross-sectional view of an anode-free battery according to this embodiment. As shown in Fig. 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 Fig. 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.

[0085] The anode-free lithium secondary battery (hereinafter also referred to as "anode-free battery" or "AFB") of this embodiment 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 in the negative electrode. Examples of such holding mechanisms include intercalation, alloying, and metal cluster occlusion, and intercalation is a typical example.

[0086] In an anode-free battery, the negative electrode does not contain a 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, in an anode-free battery, the volume and mass occupied by the negative electrode active material are reduced, and the volume and mass of the entire battery are reduced, which has the advantage of, in principle, having a high energy density.

[0087] A lithium secondary battery according to one embodiment of the present invention preferably includes a negative electrode made of a negative electrode current collector that does not have a negative electrode active material. An example of such a lithium secondary battery is the anode-free battery of this embodiment. According to the above aspect, the lithium secondary battery acts synergistically with the electrolyte solution of this embodiment, which tends to further improve the effects of this embodiment.

[0088] In the anode-free battery of this specification, the anode "does not have a negative electrode active material" means that the negative electrode does not have or substantially does not have a negative electrode active material. The anode "substantially does not have a negative electrode active material" means that the content of the negative electrode active material in the negative electrode is 10% by mass or less relative to the entire negative electrode. The content of the negative electrode active material in the negative electrode of the anode-free battery is preferably 5.0% by mass or less relative to the entire negative electrode, and may be 1.0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less. When the negative electrode does not have a negative electrode active material or the content of the negative electrode active material in the negative electrode is within the above range, the energy density of the lithium secondary battery is high. 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 when the voltage is 1.0 V or more and 3.8 V or less, preferably 1.0 V or more and 3.0 V or less.

[0089] 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 mass % or less (preferably 5.0 mass % or less, and may be 1.0 mass % or less) relative 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 mass % or less (preferably 5.0 mass % or less, and may be 1.0 mass % or less) relative 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 mass % or less (preferably 5.0 mass % or less, and may be 1.0 mass % or less) relative to the entire negative electrode.

[0090] 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 Ratio of M 3.0 / M 4.2 is preferably 40% or less, more preferably 38% or less, and even more preferably 35% 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.

[0091] 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, titanium oxide compounds and cobalt oxide compounds. Examples of metals that can be alloyed with lithium include silicon, germanium, tin, lead, aluminum, and gallium.

[0092] 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. Examples include at least one 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, the negative electrode is at least one 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 are 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.

[0093] The average thickness of the negative electrode of the anode-free battery is not particularly limited and is, for example, 3.0 μm to 30 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 μm, more preferably 5.0 μm to 18 μm, and even more preferably 6.0 μm to 15 μm.

[0094] The negative electrode of an anode-free battery may have at least a portion of its surface facing the positive electrode coated with a compound (hereinafter also referred to as "negative electrode coating agent") containing an aromatic ring to which two or more elements selected from the group consisting of N, S, and O are independently bonded. It is believed that the negative electrode coating agent can be retained on the negative electrode by at least one element selected from the group consisting of N, S, and O being coordinately bonded to a metal atom constituting the negative electrode. Furthermore, the use of a negative electrode coated with the negative electrode coating agent is expected to suppress non-uniform deposition of lithium metal on the surface, thereby suppressing the dendritic growth of lithium metal deposited on the negative electrode.

[0095] The negative electrode coating agent is not particularly limited as long as it is a compound containing an aromatic ring to which two or more elements selected from the group consisting of N, S, and O are independently bonded, i.e., a compound having a structure in which two or more N, S, or O are independently bonded to an aromatic ring. Examples of aromatic rings include aromatic hydrocarbons such as benzene, naphthalene, azulene, anthracene, and pyrene, and heteroaromatic compounds such as furan, thiophene, pyrrole, imidazole, pyrazole, pyridine, pyridazine, pyrimidine, and pyrazine. Among these, aromatic hydrocarbons are preferred, with benzene and naphthalene being more preferred, and benzene being even more preferred. In addition, the negative electrode coating agent preferably has one or more nitrogen atoms bonded to the aromatic ring. Furthermore, the negative electrode coating agent is more preferably a compound having a structure in which a nitrogen atom is bonded to the aromatic ring and, in addition to the nitrogen atom, one or more elements selected from the group consisting of N, S, and O are each independently bonded. When such a compound in which a nitrogen atom is bonded to an aromatic ring is used as the negative electrode coating agent, the cycle characteristics of the battery tend to be further improved. Specific examples of the negative electrode coating agent include at least one selected from the group consisting of benzotriazole, benzimidazole, benzimidazole thiol, benzoxazole, benzoxazole thiol, benzothiazole, mercaptobenzothiazole, and derivatives thereof. Among these, at least one selected from the group consisting of benzotriazole, benzimidazole, benzoxazole, mercaptobenzothiazole, and derivatives thereof is preferred as the negative electrode coating agent. According to such an embodiment, the electrical connection between the negative electrode and the lithium ions coordinated by the negative electrode coating agent is further improved, which tends to further improve the cycle characteristics of the battery.

[0096] The anode coating agent may be applied to at least a portion of the surface of the anode facing the positive electrode, i.e., 10% or more of the surface area of ​​the anode is coated with the anode coating agent, preferably 20% or more, more preferably 40% or more, even more preferably 60% or more, and even more preferably 80% or more.

[0097] 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.

[0098] As the separator for the anode-free battery, an insulating porous material, a polymer electrolyte, or a gel electrolyte can be used alone or in combination of two or more. When an insulating porous material is used alone as the separator, the lithium secondary battery must further include an electrolytic solution.

[0099] 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. 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 laminated structure thereof.

[0100] The separator may be coated with a separator coating layer. The coating layer may cover both sides of the separator or 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. 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.

[0101] The average thickness of the separator including the separator coating layer is not particularly limited and is, for example, 3.0 μm to 40 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 μm, more preferably 7.0 μm to 10 μm, and even more preferably 10 μm to 20 μm.

[0102] 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, the positive electrode active material can be a host material for lithium element (typically, lithium ion).

[0103] 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 LiCoO2, LiNi x Co y Mn z O(x+y+z=1), LiNi x Co y Al z O(x+y+z=1), LiNi x Mn y O(x+y=1), LiNiO2, LiMn2O4, LiFePO4, LiCoPO4, LiFeOF, LiNiOF, and LiTiS2. The above-mentioned positive electrode active materials may be used singly or in combination of two or more.

[0104] The positive electrode may contain components other than the above-mentioned positive electrode active material. Such components are not particularly limited, and examples thereof include 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 does not substantially undergo a reduction reaction in the charge / discharge potential range of the positive electrode active material, and particularly in an anode-free battery, the positive electrode may include a sacrificial positive electrode. In particular, the positive electrode may contain a gel electrolyte. According to such an embodiment, the gel electrolyte functions to improve the adhesive strength between the positive electrode and the positive electrode current collector, making it possible to attach a thinner positive electrode current collector, 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.

[0105] 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.

[0106] 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.

[0107] The average thickness of the positive electrode is preferably 20 μm or more and 100 μm or less, more preferably 30 μm or more and 80 μm or less, and even more preferably 40 μm or more and 70 μm or less, although the average thickness of the positive electrode can be adjusted appropriately depending on the desired battery capacity.

[0108] 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. An example of such a positive electrode current collector is 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 receive electrons to the positive electrode (particularly the positive electrode active material) and is in physical and / or electrical contact with the positive electrode.

[0109] In the anode-free battery, the average thickness of the positive electrode current collector is preferably 1.0 μm or more and 15 μm or less, more preferably 2.0 μm or more and 10 μm or less, and even more preferably 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.

[0110] 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 the positive electrode current collector and the 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. Lithium secondary batteries are charged by applying a voltage to the positive and negative terminals such that current flows from the negative terminal (negative electrode) through an external circuit to the positive terminal (positive electrode). After charging, the lithium secondary battery is discharged by connecting the positive and negative terminals via the desired external circuit.

[0111] In an anode-free battery, 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 an anode-free battery, the lithium metal deposition formed on the negative electrode undergoes electrolytic dissolution upon discharge. If an SEI layer is formed in the battery, the lithium metal deposition formed at at least one of the interface between the negative electrode and the SEI layer and the interface between the SEI layer and the separator undergoes electrolytic dissolution.

[0112] 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. 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% by mass to 99% by mass of the positive electrode active material, 0.5% by mass to 30% by mass of the conductive additive, and 0.5% by mass to 30% 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 obtained molded body is punched to a predetermined size by a punching process to obtain a positive electrode current collector and a positive electrode. Next, the negative electrode material of the above-mentioned 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 dried to obtain a negative electrode. 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 deposition of lithium metal. The functional buffer layer preferably has ionic or electrical conductivity, but does not necessarily have to. Next, the electrolyte solution of this embodiment is prepared by the above-mentioned preparation method. 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.

[0113] [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 elemental 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 depositing lithium metal on the surface of the negative electrode and electrolytically dissolving the deposited lithium. From the viewpoint of further improving the effects of this embodiment using the electrolyte, it is preferable that the lithium secondary battery is charged and discharged by depositing lithium metal on the surface of the negative electrode and dissolving the deposited lithium metal. On the other hand, as mentioned above, LMB differs 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.

[0114] 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.

[0115] 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, it generally has a higher energy density than lithium-ion batteries. 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.

[0116] The average thickness of the LMB negative electrode is not particularly limited, but is, for example, 5.0 μm to 100 μm. From the viewpoint of improving the capacity and / or energy density of the battery, it is preferably 8.0 μm to 50 μm, more preferably 10 μm to 40 μm, and even more preferably 10 μm to 20 μm.

[0117] The negative electrode of the LMB may be coated with a coating agent to suppress dendritic growth of lithium metal, similar to that used in the anode-free battery described above.

[0118] The LMB may be produced using known materials and known production methods, and may be produced in the same manner as the above-mentioned anode-free battery, except that lithium metal or a lithium metal alloy is used for the negative electrode.

[0119] 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.

[0120] [Lithium-ion battery] A lithium-ion battery (hereinafter also referred to as "LIB") has a host material of lithium element (lithium ion or lithium metal) in its negative electrode, and 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.

[0121] 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.

[0122] 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. [Example]

[0123] The present embodiment will be described in more detail below using examples and comparative examples, but the present embodiment is not limited to the following examples.

[0124] [Example 1] An anode-free lithium secondary battery (AFB) of Example 1 was fabricated as follows.

[0125] (Preparing the 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.

[0126] (Preparation of positive electrode) Next, a positive electrode was fabricated. 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 with a positive electrode current collector. The basis weight of the resulting positive electrode was 21 mg / cm. 2 It was.

[0127] (Preparing the 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).

[0128] (Preparation of Electrolyte) The electrolyte solution was prepared as follows: Three solvents were mixed so that the concentrations were 40% by volume of butylbenzene, 40% by volume of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and 20% by volume of 1,2-dimethoxyethane. LiN(SO2F2)2 was dissolved in the resulting mixture to a molar concentration of 1.0 M, thereby obtaining an electrolyte solution.

[0129] (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 14.0 g, and the cells were vacuum-sealed under a reduced pressure of -50 kPa.

[0130] [Examples 2 to 13] A lithium secondary battery was obtained in the same manner as in Example 1, except that the electrolyte solution was prepared using the solvents and electrolytes listed in Table 1.

[0131] [Comparative Examples 1 to 11] A lithium secondary battery was obtained in the same manner as in Example 1, except that the electrolyte solution was prepared using the solvents and electrolytes listed in Table 2. The comparative example does not contain at least one of the cyclic compound represented by the above formula (1) or formula (2), hydrofluoroether (HFE), and ether not having a fluorine atom.

[0132] In Table 1, "AC1" represents butylbenzene represented by the following formula (3), "AC2" represents isobutylbenzene represented by the following formula (4), "AC3" represents tert-butylbenzene represented by the following formula (5), "AC4" represents propylbenzene represented by the following formula (6), "AC5" represents ethyltoluene represented by the following formula (7), "AC6" represents 1,3,5-trimethylbenzene represented by the following formula (8), "AC7" represents toluene represented by the following formula (9), "HC1" represents cyclohexane represented by the following formula (10), and "HC2" represents toluene represented by the following formula (11). "HC3" represents methylcyclohexane represented by the formula (11), "HC3" represents ethylcyclohexane represented by the formula (12), "HFE1" represents 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether represented by the formula (C), "HFE2" represents 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether represented by the formula (D), "DME" represents 1,2-dimethoxyethane, "DMP" represents 1,2-dimethoxypropane, and "THF" represents tetrahydrofuran. Furthermore, regarding the lithium salt used as the electrolyte, "LiFSI" represents LiN(SO2F)2.

[0133] [ka] [ka] [ka] [ka] [ka] [ka] [ka] [ka] [ka] [ka] [ka] [ka]

[0134] In Tables 1 and 2, each solvent is classified as either a cyclic compound of formula (1) or formula (2), an HFE, or a non-fluorinated ether, as defined above. Furthermore, in Tables 1 and 2, the numerical value to the right of each solvent indicates the content in volume percent relative to the total amount of solvent. For example, in Table 1, Example 1 means that the mixture contains 40 volume percent of AC1, 40 volume percent of HFE1, and 20 volume percent of DME, and contains 1.0 M LiFSI as an electrolyte.

[0135] In addition, in each example in Tables 1 and 2, when the electrolyte solution contains a cyclic compound of formula (1) or formula (2), the number of substituents bonded to the benzene ring of the cyclic compound of formula (1) or formula (2) is shown in the column "Number of substituents (compound of formula (1) or formula (2))". Furthermore, the specific gravity of each electrolyte solution is shown in the column "Specific gravity of electrolyte solution".

[0136] [phase Soluble Presence or absence of 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 Tables 1 and 2, those judged to have phase separation are indicated as "phase separation."

[0137] [Discharge capacity measurement] The fabricated lithium secondary battery was CC charged at 0.8 A until the voltage reached 4.2 V (initial charge), and then CC discharged at 0.8 A until the voltage reached 3.0 V (hereinafter referred to as "initial discharge"). Next, in an environment at a temperature of 25°C or 0°C, the battery was CC charged at 2.4 A until the voltage reached 4.2 V, and then CC discharged at 2.4 A until the voltage reached 3.0 V. The discharge capacities at a temperature of 25°C are shown in Tables 1 and 2.

[0138] [Evaluation of cycle characteristics] The cycle characteristics of the lithium secondary batteries fabricated in each of the Examples and Comparative Examples were evaluated as follows.

[0139] The fabricated lithium secondary batteries were subjected to initial charging and initial discharging. Subsequently, they were subjected to CC charging at 2.4 A until the voltage reached 4.2 V, followed by CC discharging at 2.4 A until the voltage reached 3.0 V. This cycle was repeated in an environment at a temperature of 25°C. For each example, the capacity was determined from the initial discharge (hereinafter referred to as "initial capacity"), and the number of cycles at which the discharge capacity reached 80% of the initial capacity (referred to as "cycle number" in the tables) is shown in Tables 1 and 2.

[0140] [Table 1]

[0141] [Table 2]

[0142] In Tables 1 and 2, "-" in the electrolyte component indicates that the corresponding component is not present. Soluble In the presence or absence of 〇" indicates that no phase separation occurred, and "-" for the electrolyte specific gravity, discharge capacity, and number of cycles indicates that the data was not measured.

[0143] From Tables 1 and 2, it can be seen that Examples 1 to 13, which are lithium secondary batteries using an electrolyte solution containing a cyclic compound represented by the above formula (1) or formula (2), a hydrofluoroether, and an ether having no fluorine atoms as a solvent, have excellent cycle characteristics and do not undergo phase separation, compared to the comparative examples which do not use such an electrolyte solution.

[0144] Next, we considered lithium metal batteries.

[0145] [Example 14] A lithium metal battery (LMB) of Example 14 was fabricated as follows.

[0146] (Preparing the negative electrode) First, a clad material in which a 20.0 μm thick Li foil and an 8.0 μm thick electrolytic Cu foil were bonded together was prepared and punched out to a predetermined size (43 mm×43 mm).

[0147] (Preparation of positive electrode) Next, a positive electrode was fabricated. 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 one side of a 12 μm Al foil and press-molded. The resulting molded body was punched out to a predetermined size (40 mm × 40 mm) to obtain a positive electrode with a positive electrode current collector. The basis weight of the resulting positive electrode was 21 mg / cm. 2 It was.

[0148] (Preparing the separator) A separator of a predetermined size (50 mm x 50 mm) was prepared, in which both sides of a 12 μm polyethylene microporous membrane were coated with 2.0 μm polyvinylidene fluoride (PVDF).

[0149] (Preparation of Electrolyte) The electrolyte solution was prepared as follows: Three solvents were mixed so that the concentrations were 40% by volume of butylbenzene, 40% by volume of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and 20% by volume of 1,2-dimethoxyethane. LiN(SO2F2)2 was dissolved in the resulting mixture to a molar concentration of 1.0 M, thereby obtaining an electrolyte solution.

[0150] (Battery assembly) The positive electrode current collector with the positive electrode formed thereon, the separator, and the negative electrode were stacked in this order, with the positive electrode facing the separator, to obtain a laminate. Furthermore, a 100 μm Al terminal and a 100 μm Ni terminal were joined to the positive electrode current collector and the negative electrode, respectively, by ultrasonic welding, and then inserted into a laminate outer casing. The electrolyte solution obtained as described above was then poured into the outer casing. The outer casing was sealed to obtain a lithium secondary battery.

[0151] [Comparative Example 12] A lithium secondary battery was obtained in the same manner as in Example 14, except that the electrolyte solution was prepared using the solvents shown in Table 3.

[0152] In Table 3, the columns "Number of substituents (cyclic compound of formula (1))", "Presence or absence of phase separation", and "Specific gravity of electrolyte" indicate the same items as in Tables 1 and 2.

[0153] [Discharge capacity measurement] The fabricated lithium secondary battery was CC charged at 0.8 A until the voltage reached 4.2 V (initial charge), and then CC discharged at 0.8 A until the voltage reached 3.0 V (initial discharge). Next, in an environment at a temperature of 25°C or 0°C, the battery was CC charged at 2.4 A until the voltage reached 4.2 V, and then CC discharged at 2.4 A until the voltage reached 3.0 V. The discharge capacity at a temperature of 25°C is shown in Tables 1 and 2.

[0154] [Evaluation of cycle characteristics] The cycle characteristics of the lithium secondary batteries fabricated in Example 14 and Comparative Example 12 were evaluated based on the capacity retention rate as follows.

[0155] The fabricated lithium secondary battery was subjected to initial charging and initial discharging (first cycle). From the second cycle onward, the battery was CC charged at 2.4 A until the voltage reached 4.2 V, and then CC discharged at 2.4 A until the voltage reached 3.0 V. This cycle was repeated until the 100th cycle in an environment at 25°C. The ratio of the capacity determined from the discharge of the 100th cycle (100th cycle capacity) to the capacity determined from the discharge of the second cycle (second cycle capacity) (100th cycle capacity / second cycle capacity) was calculated, and this ratio is shown in Table 3 as the capacity retention rate (%).

[0156] [Table 3]

[0157] In Table 3, "-" in the electrolyte component or the number of its substituents means that the corresponding component is not present, and "-" in the presence or absence of phase separation means that phase separation is not present.

[0158] From Table 3, it can be seen that Example 14, which is a lithium metal battery using an electrolyte solution containing the cyclic compound represented by the above formula (1), a hydrofluoroether, and an ether having no fluorine atoms as a solvent, has a higher energy density and excellent cycle characteristics than Comparative Example 12, which does not.

[0159] 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. [Explanation of symbols]

[0160] 100...lithium secondary battery, 110...positive electrode current collector, 120...positive electrode, 130...separator, 140...negative electrode.

Claims

1. A cyclic compound represented by the following formula (1) or formula (2), A hydrofluoroether, an ether having no fluorine atom; and a lithium salt, When the cyclic compound represented by the formula (1) is contained, the content thereof is 5% by volume or more and 55% by volume or less with respect to the total amount of the solvent components of the electrolytic solution, In the case where the cyclic compound represented by the formula (2) is contained, the content thereof is 1% by volume or more and 50% by volume or less with respect to the total amount of the solvent components of the electrolytic solution, the content of the hydrofluoroether is 5.0% by volume or more and 80% by volume or less with respect to the total amount of solvent components of the electrolytic solution, the content of the fluorine-free ether is 1.0% by volume or more and 70% by volume or less with respect to the total amount of solvent components of the electrolyte solution, The electrolyte for a lithium secondary battery, wherein the total concentration of the lithium salts is 0.30 M or more and 5.0 M or less. 【Chemical 1】 (In formula (1), n ​​is an integer of 0 to 3, and R a is a monovalent saturated hydrocarbon group having 1 to 10 carbon atoms. 【Chemistry 2】 (In formula (2), m is an integer of 0 to 3, and R h is a monovalent saturated hydrocarbon group having 1 to 10 carbon atoms.

2. An electrolyte solution as described in claim 1, wherein n is 1 in formula (1).

3. The electrolyte solution according to claim 1 or 2, wherein m is 0 or 1 in the formula (2).

4. The electrolyte solution according to any one of claims 1 to 3, comprising a cyclic compound represented by formula (1).

5. 5. The electrolyte solution according to claim 1, wherein the specific gravity of the electrolyte solution is 1.0 g / cc or more and 1.3 g / cc or less.

6. The electrolytic solution according to any one of claims 1 to 5, wherein the cyclic compound is at least one selected from the group consisting of n-butylbenzene, tert-butylbenzene, isobutylbenzene, sec-butylbenzene, propylbenzene, ethyltoluene, 1,3,5-trimethylbenzene, cyclohexane, methylcyclohexane, and ethylcyclohexane.

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

8. The lithium salt is LiN(SO 2 F) 2 The electrolyte solution according to any one of claims 1 to 7, comprising:

9. The electrolytic solution according to any one of claims 1 to 8, wherein the hydrofluoroether is a chain fluorine compound having at least one of monovalent groups represented by the following formula (A) or formula (B): 【Chemistry 3】 【Chemistry 4】 (In formulas (A) and (B), the wavy lines represent bonding sites in monovalent groups.)

10. A lithium secondary battery comprising the electrolyte solution according to any one of claims 1 to 9.

11. 11. The lithium secondary battery according to claim 10, wherein charging and discharging are performed by depositing lithium metal on the surface of the negative electrode and dissolving the deposited lithium metal.

12. The lithium secondary battery according to claim 11, comprising a negative electrode made of a negative electrode current collector having no negative electrode active material.

Images