Electrolyte for lithium metal secondary batteries

Abstract

To provide an electrolyte for lithium metal secondary batteries that, when using 1,2-dimethoxyethane (DME) as the solvent, can simultaneously suppress resistance increase, gas generation, and a decrease in the thickness of the negative electrode. [Solution] An electrolyte for a lithium metal secondary battery comprising an electrolyte salt, an organic solvent, and a first additive, wherein the electrolyte salt comprises lithium bis-fluorosulfonylimide (LiFSI), the organic solvent comprises 1,2-dimethoxyethane (DME), and the first additive is at least one of N,N-dimethyltrifluoromethane-sulfonamide (DMTMSA) or N,N-dimethylsulfamoyl fluoride (DMSF), and the first additive is present in the electrolyte for a lithium metal secondary battery in an amount of 0.2 parts by mass or more, when the total mass of the electrolyte salt and the organic solvent is 100 parts by mass.

Description

[Technical Field]

[0001] This invention relates to an electrolyte for lithium metal secondary batteries. [Background technology]

[0002] In recent years, research and development has been conducted on secondary batteries that contribute to energy efficiency, in order to ensure that more people have access to affordable, reliable, sustainable, and advanced energy. Among secondary batteries, lithium metal secondary batteries, which use lithium metal as the negative electrode, are attracting attention.

[0003] To improve the durability of lithium metal secondary batteries, a technique is known to form a protective layer (SEI) on the negative electrode surface by adjusting the electrolyte composition and additives.

[0004] To maintain the specific discharge capacity and Coulomb efficiency of lithium metal secondary batteries above predetermined values ​​after charge-discharge cycles, technologies have been proposed for electrochemical devices containing DMTMSA and / or LiFSI as the main components of the electrolyte (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Special Publication No. 2024-506963 [Overview of the project] [Problems that the invention aims to solve]

[0006] The technology disclosed in Patent Document 1 has problems, for example, in terms of suppressing resistance increase, gas generation, and anode thickness increase in lithium metal secondary batteries for large automotive batteries. Specifically, the increase in lithium anode thickness due to charging and discharging leads to an increase in cell dimensions, which reduces the energy density of the battery. One technology to reduce the increase in lithium anode thickness due to charging and discharging is to increase the battery's constraint force to above a predetermined constraint pressure, but this has the problem that the holding member required to increase the constraint force of the stacked electrode group becomes larger, preventing an increase in the battery's energy density. Therefore, a technology is needed to suppress the volume change in the thickness direction of the anode by means other than increasing the battery's constraint force. Furthermore, it is required to suppress thickness change during charging, for example, from 0.2C to 1.5C. 1,2-dimethoxyethane (DME) is a solvent for the electrolyte that satisfies these requirements, as it offers various advantages such as reduced cell resistance, good liquid flow due to low viscosity, and low cost, and it was desired that the above problems could be solved when DME is used.

[0007] The present invention has been made in view of the above, and aims to provide an electrolyte for lithium metal secondary batteries that can simultaneously suppress the increase in resistance of lithium metal secondary batteries, suppress gas generation, and suppress the increase in the thickness of the negative electrode when 1,2-dimethoxyethane (DME) is used as the solvent. [Means for solving the problem]

[0008] (1) The present invention relates to an electrolyte for a lithium metal secondary battery comprising an electrolyte salt, an organic solvent, and a first additive, wherein the electrolyte salt comprises lithium bis-fluorosulfonylimide (LiFSI), the organic solvent comprises 1,2-dimethoxyethane (DME), and the first additive is at least one of N,N-dimethyltrifluoromethane-sulfonamide (DMTMSA) or N,N-dimethylsulfamoyl fluoride (DMSF), and the first additive is present in an amount of 0.2 parts by mass or more in the electrolyte for a lithium metal secondary battery when the total mass of the electrolyte salt and the organic solvent is 100 parts by mass.

[0009] According to the invention of (1), when 1,2-dimethoxyethane (DME) is used as the solvent, an electrolyte for a lithium metal secondary battery can be provided that can suppress the increase in resistance of the lithium metal secondary battery, suppress gas generation, and suppress the increase in the thickness of the negative electrode.

[0010] (2) The electrolyte for lithium metal secondary batteries according to (1), wherein the first additive is contained in the electrolyte for lithium metal secondary batteries in an amount of 5.0 parts by mass or less, when the total mass of the electrolyte salt and the organic solvent is 100 parts by mass.

[0011] According to the invention of (2), the effects of suppressing the increase in resistance of the lithium metal secondary battery, suppressing gas generation, and suppressing the increase in the thickness of the negative electrode are more preferably obtained.

[0012] (3) The electrolyte for lithium metal secondary batteries according to (1) or (2), wherein the first additive contains either N,N-dimethyltrifluoromethane-sulfonamide (DMTMSA) or N,N-dimethylsulfamoyl fluoride (DMSF), and the content (mass percentage) of N,N-dimethyltrifluoromethane-sulfonamide (DMTMSA) is less than the content (mass percentage) of N,N-dimethylsulfamoyl fluoride (DMSF).

[0013] According to the invention of (3), it is possible to obtain both the effect of suppressing the increase in the thickness of the negative electrode and the effect of improving the durability of the lithium metal secondary battery.

[0014] (4) The electrolyte for lithium metal secondary batteries according to any one of (1) to (3), further comprising at least one of lithium difluoro(oxalato)borate (LiFOB), lithium difluorophosphate (LiDFP), or LiPF6 as a second additive.

[0015] According to the invention of (4), the effects of suppressing the increase in resistance of the lithium metal secondary battery, suppressing gas generation, and suppressing the increase in the thickness of the negative electrode are more preferably obtained.

[0016] (5) The content of 1,2-dimethoxyethane (DME) in the organic solvent is 20 mol% or more, and the electrolyte solution for a lithium metal secondary battery according to any one of (1) to (4).

[0017] According to the invention of (5), even when 1,2-dimethoxyethane (DME) is used as the main solvent, effects such as suppression of an increase in resistance of the lithium metal secondary battery, suppression of gas generation, and suppression of an increase in the thickness of the negative electrode can be preferably obtained.

[0018] (6) The electrolyte solution for a lithium metal secondary battery according to any one of (1) to (5), further containing a fluorinated ether as the organic solvent.

[0019] According to the invention of (6), effects such as suppression of an increase in resistance of the lithium metal secondary battery, suppression of gas generation, and suppression of an increase in the thickness of the negative electrode can be more preferably obtained.

[0020] (7) The concentration of lithium bis-fluorosulfonylimide (LiFSI) as the electrolyte salt in the electrolyte solution for a lithium metal secondary battery is 1.2 mol / L or more and 2.5 mol / L or less, and the electrolyte solution for a lithium metal secondary battery according to any one of (1) to (6).

[0021] [[ID=2l]] According to the invention of (7), effects such as suppression of an increase in resistance of the lithium metal secondary battery, suppression of gas generation, and suppression of an increase in the thickness of the negative electrode can be more preferably obtained.

Effects of the Invention

[0022] According to the present invention, there is provided an electrolyte solution for a lithium metal secondary battery capable of suppressing an increase in resistance of the lithium metal secondary battery, suppressing gas generation, and suppressing an increase in the thickness of the negative electrode when 1,2-dimethoxyethane (DME) is used as the solvent.

Modes for Carrying Out the Invention

[0023] <Electrolyte Solution for Lithium Metal Secondary Battery> The electrolyte for the lithium metal secondary battery according to this embodiment comprises an electrolyte salt, an organic solvent, and a first additive. Preferably, it also contains a second additive.

[0024] The electrolyte salt must have at least a portion of the structure of FSI (fluorosulfonylimide), and lithium bis-fluorosulfonylimide (LiFSI) is a typical electrolyte salt. The electrolyte salt may also contain lithium salts other than LiFSI. Examples of lithium salts include LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC(CF3SO2)3, LiN(CF3SO2)2 (LiTFSI), and LiBC4O8. These can be used individually or in combination of two or more. It is preferable that the electrolyte salt contains LiFSI as the main component.

[0025] The concentration of LiFSI as an electrolyte salt in the electrolyte solution for lithium metal secondary batteries is preferably between 1.0 mol / L and 3 mol / L, and more preferably between 1.2 mol / L and 2.5 mol / L. If the concentration is below 1.0 mol / L, it becomes difficult to suppress the increase in the thickness of the lithium in the negative electrode, and if it is above 3 mol / L, the viscosity of the electrolyte becomes high, making it difficult to support rapid charging. In particular, when applied to BEVs (Battery Electric Vehicles), considering the usage, 1.2 to 2.5 mol / L is appropriate.

[0026] The viscosity of the electrolyte for the lithium metal secondary battery according to this embodiment is preferably in the range of 5 mPa·s to 12 mPa·s.

[0027] The organic solvent must contain 1,2-dimethoxyethane (DME). The inclusion of DME in the organic solvent improves the solubility of the electrolyte salt (LiFSI, etc.), thereby ensuring a favorable conduction path for lithium ions in the electrolyte. Furthermore, from a cost perspective, it is preferable to use DME as the main solvent. From the viewpoint of LiFSI solubility, it is preferable that the content of DME as a solvent component in the electrolyte be at least 20 mol%. More preferably, the above content is 40 mol% or more, even more preferably 45 mol% or more, and most preferably 60 mol% or more, as this provides a sufficient resistance reduction effect.

[0028] To achieve localized high concentrations, it is preferable to include fluorinated ethers in addition to 1,2-dimethoxyethane (DME) as the organic solvent. These have low coordination properties, which allow the solvation of DME to remain stable while reducing the viscosity of the electrolyte, thereby further reducing internal resistance. Consequently, high-rate charging and discharging are possible. Specific examples of fluorinated ethers include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) and compounds represented by the following formula (1). 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane is also an example. Multiple chain esters may be mixed to improve degradation resistance and adjust viscosity.

[0029] R 1 -OR 2 ...(1) In equation (1), R 1 and R 2 Each of these independently represents a fluorinated hydrocarbon group such as a fluorinated alkyl group. 1 and R 2 The number of carbon atoms is not particularly limited, but may be, for example, 1 to 8. Examples of compounds represented by formula (1) include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether.

[0030] The organic solvent may include organic solvents other than those listed above. For example, it may include chain ethers such as 1,2-diethoxyethane, diethyl ether, and 1-ethoxy-2-(2-methoxyethoxy)ethane. Alternatively, it may include organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), sulfolane (SL), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), as long as they do not impair the effects of the present invention.

[0031] The organic solvent preferably includes 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). In this case, the molar ratio of 1,2-dimethoxyethane (DME) as the organic solvent to lithium bis-fluorosulfonylimide (LiFSI) as the electrolyte salt (DME / LiFSI) is preferably 1.5 to 2.3. Furthermore, the mass percentage of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE / (LiFSI+DME)) as the organic solvent to the total amount of lithium bis-fluorosulfonylimide (LiFSI) and 1,2-dimethoxyethane (DME) (LiFSI+DME) is preferably 20% by mass or more and 60% by mass or less.

[0032] The first additive is a component that coordinates with the lithium metal, stabilizes it, and forms a thin film on the negative electrode. By including the first additive in the electrolyte for the lithium metal secondary battery according to this embodiment, the additive slowly decomposes and acts gradually on the negative electrode (lithium metal layer) and the lithium metal deposited on the negative electrode of the lithium metal secondary battery. This reduces porosity during lithium deposition over a long period, continuously suppresses dendrite formation, and improves the durability of the lithium metal secondary battery.

[0033] The first additive is at least one of N,N-dimethyltrifluoromethane-sulfonamide (DMTMSA) or N,N-dimethylsulfamoyl fluoride (DMSF). The electrolyte for lithium metal secondary batteries may contain either DMTMSA or DMSF as the first additive, but it is preferable to include both DMTMSA and DMSF. In particular, it is preferable that the amounts added are equal or that DMSF > DMTMSA. DMSF acts relatively quickly on the lithium negative electrode, while DMTMSA is less likely to decompose on the lithium negative electrode and decomposes more slowly, thus acting over a longer period. Therefore, it is preferable to include more DMSF than DMTMSA, as DMSF has a faster initial consumption rate. This allows for both the suppression of the increase in negative electrode thickness and the improvement of the durability of the lithium metal secondary battery. Furthermore, the inclusion of both DMSF and DMTMSA provides the effect of sustained action from the initial to the later stages of durability testing.

[0034] The content of the first additive in the electrolyte for lithium metal secondary batteries is 0.2 parts by mass or more, when the total mass of the electrolyte salt and organic solvent is 100 parts by mass. Preferably, the above content is 5.0 parts by mass or less. This preferably provides the effects of the present invention. More preferably, the above content is 0.5 parts by mass or more and 2.0 parts by mass or less. When both DMTMSA and DMSF are included as the first additive, this refers to the total content of DMTMSA and DMSF.

[0035] The content of DMTMSA as the first additive is preferably 2.0 parts by mass or less. This can reduce the initial resistance of the lithium metal secondary battery and improve its durability.

[0036] The content of DMSF as the first additive is preferably 5.0 parts by mass or less. This reduces the increase in resistance when the lithium metal secondary battery undergoes repeated charge-discharge cycles.

[0037] The second additive is at least one of lithium difluoro(oxalato)borate (LiFOB), lithium difluorophosphate (LiDFP), or LiPF6. These may be used individually or in combination of two or more. When LiFOB is included as the second additive, it removes residual moisture and suppresses the porosity of the Li precipitate. When LiDFP is included as the second additive, it can suppress the degradation of the positive electrode active material and improve durability. When LiPF6 is included as the second additive, it can also suppress the degradation of the positive electrode and improve durability.

[0038] The content of LiFOB as the second additive in the electrolyte for lithium metal secondary batteries is preferably 0.1 parts by mass or more and 5 parts by mass or less. The content of LiDFP in the electrolyte for lithium metal secondary batteries is preferably 0.1 parts by mass or more and 2 parts by mass or less. Similarly, the content of LiPF6 in the electrolyte for lithium metal secondary batteries is preferably 0.1 parts by mass or more and 2 parts by mass or less. If the content of LiDFP and LiPF6 is 0.1 parts by mass or less, sufficient effects cannot be obtained, and if it is 2 parts by mass or more, the solubility in the electrolyte is insufficient, and even if the amount of additive is increased, sufficient effects cannot be obtained, and there is a possibility that the electrolyte will decompose due to a side reaction and the amount of gas generated will increase. Note that the definition of the content of the second additive is the same as the definition of the content of the first additive, meaning parts by mass when the total mass of the electrolyte salt and organic solvent is 100 parts by mass.

[0039] The electrolyte for lithium metal secondary batteries according to this embodiment may contain components other than those listed above. For example, it may contain calcium cations that remove residual moisture and stabilize Li deposition. Examples of sources for calcium cations include Ca(TFSI)2, Ca(FOB)2, Ca(FSI)2, and Ca(BF4)2. In addition to the above, the electrolyte for lithium metal secondary batteries may contain known components used in electrolytes for lithium metal secondary batteries. Examples include film-forming agents and dispersants. Specifically, it may contain alkali metal salts such as Mg(FSI)2, Mg(TFSI)2, Ba(FSI)2, Ba(TFSI)2, Zn(FSI)2, Zn(TFSI)2, LiTFSI, LiNO3, lithium nitrite, LiPO2F2, CsPF6, and lithium sulfate; organic additives such as propanesultone and ethylene sulfite; and nitrile compounds such as acetolyl, adiponitrile, butyronitrile, and diphenyl sulfide.

[0040] <Lithium metal secondary battery> A lithium metal secondary battery is constructed using the electrolyte according to this embodiment. The specific configuration of the lithium metal secondary battery is not particularly limited except for the electrolyte, and configurations used in known lithium metal secondary batteries can be used without restriction. In a typical embodiment, the lithium metal secondary battery has a laminate including a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer, and the electrolyte layer may contain the electrolyte according to this embodiment. The laminate is housed in an outer casing such as a laminate film.

[0041] The positive electrode layer is a layer containing a positive electrode active material. The positive electrode active material is not particularly limited as long as it is a material that can be used as a positive electrode active material for a lithium metal secondary battery. However, the suitable amounts and combinations of the first additive differ between polycrystalline and single-particle positive electrode active materials. In the case of a polycrystalline positive electrode, new surfaces are formed due to cracking of the active material during charge and discharge, but the consumption rate of DMTMSA and DMSF on the positive electrode becomes faster, the addition concentration decreases, and it becomes difficult to obtain the effect of increasing the thickness of the negative electrode. Therefore, when the addition amounts of DMTMSA and DMSF are increased, as a result, a large amount of surplus DMTMSA and DMSF remains, and there is a problem that the resistance of the cell increases significantly during the durability test. When a single-particle positive electrode is used, for example, the addition amount of DMTMSA can be reduced to about 0.2% to 2% by mass, which is the same addition amount as that of LiFOB and LiDFP, which are the second additives. Thereby, B and P, which are components of the elements of the SEI film, also act effectively, and there is an advantage that the synergistic effects with the components of DMSF and DMTMSA are likely to occur. Therefore, the electrolyte of the present invention exhibits particularly effective effects especially when a single-particle positive electrode active material is used.

[0042] Examples of the type of the positive electrode active material include, for example, a layered active material containing lithium, a spinel-type active material, an olivine-type active material, and the like. Specific examples of the positive electrode active material include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), LiNi p Mn q Co r O2 (p + q + r = 1), LiNi p Al q Co rExamples include O2 (p+q+r=1), lithium manganese oxide (LiMn2O4), heteroatomically substituted Li-Mn spinel represented as Li1+xMn2-x-yMyO4 (x+y=2, M=at least one selected from Al, Mg, Co, Fe, Ni, and Zn), lithium titanate (oxide containing Li and Ti), and metallic lithium phosphate (LiMPO4, M=at least one selected from Fe, Mn, Co, and Ni). The positive electrode layer may contain binders, conductive additives, etc., in addition to the positive electrode active material. A positive electrode current collector may also be placed adjacent to the positive electrode layer. The positive electrode current collector is not particularly limited as long as it is a material that can be used as a positive electrode current collector for a lithium metal secondary battery. Examples of positive electrode current collectors include aluminum, aluminum alloys, stainless steel, nickel, iron, and titanium.

[0043] The negative electrode layer is a layer containing a negative electrode active material. As the negative electrode active material, for example, lithium metal or lithium alloy can be used alone, or a mixture thereof. Examples of elements that can form alloys with lithium metal include Al, Mg, K, Na, Ca, Sr, Ba, Si, Ge, Sb, Pb, Sn, In, and Zn. In addition, composites in which carbon or organic materials are compounded within lithium metal may be used. Furthermore, a negative electrode current collector may be placed adjacent to the negative electrode layer. The negative electrode current collector is not particularly limited as long as it is a material that can be used as a negative electrode current collector in a lithium metal secondary battery. Examples of negative electrode current collectors include copper, copper alloys, nickel, and stainless steel.

[0044] The electrolyte layer contains the electrolyte solution according to the above embodiment. The electrolyte layer may be constructed by impregnating a separator, which prevents short circuits between the positive and negative electrodes, with the electrolyte solution. As the separator, materials known as separators for lithium metal secondary batteries, such as nonwoven fabrics and microporous films, can be used.

[0045] Although preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and any modifications and improvements that can achieve the objectives of the present invention are included in the present invention. [Examples]

[0046] The present invention will be described in detail below using examples. However, the present invention is not limited to these examples.

[0047] [Preparation of electrolyte for lithium metal secondary batteries] The electrolytes for lithium metal secondary batteries according to each example and comparative example were prepared using the formulations shown in Tables 1 and 2 below. The abbreviations shown in Tables 1 and 2 are the same as those described in the above embodiments. In Tables 1 and 2, "parts by mass" refers to parts by mass when the total mass of the electrolyte salt and the organic solvent is 100 parts by mass. The water content of the electrolyte was measured by the Karl Fischer method and was 30 ppm or less in all cases.

[0048] [Table 1]

[0049] [Table 2]

[0050] [Preparation of test cells] Using the electrolytes described in each of the above examples and comparative examples, test cells were prepared according to the following procedure.

[0051] (Fabrication of the positive electrode) As an electronically conductive material, 2 wt% acetylene black (AB), 1.5 wt% polyvinylidene fluoride (PVDF) as a binder, and polyvinylpyrrolidone (PVP) as a dispersant were pre-mixed with N-methyl-2-pyrrolidone (NMP) as a dispersion solvent, and wet-mixed in a rotary-orbit mixer to obtain a pre-mixed slurry. Subsequently, LiNi was used as the positive electrode active material. 0.8 Co 0.1 Mn 0.1O2 (NCM811) and the resulting pre-mixed slurry were mixed and dispersed using a planetary mixer to obtain a positive electrode paste. NCM811 consists of single particles with a median diameter of 4 μm. The capacity per unit weight was 197-205 mAh / g. Next, the obtained positive electrode paste was applied to an aluminum positive electrode current collector without a primer layer, dried, and pressed with a roll press. The electrode composite layer had a thickness of 64 μm and a density of 3.3-3.5 g / cm³. 3 A positive electrode was obtained. Subsequently, it was dried in a vacuum at 120°C to form a positive electrode plate with a positive electrode composite layer. The obtained positive electrode plate was punched out to a size of 30 mm x 40 mm to serve as the positive electrode.

[0052] (Preparing the negative electrode) For the negative electrode, a cladding material consisting of a 10 μm thick copper foil and a 20 μm thick lithium foil was used. The negative electrode was punched out to an electrode area of ​​34 mm x 44 mm.

[0053] (Preparing the separator) As a separator, an alumina-coated polyethylene microporous membrane was used with the alumina-coated side facing the positive electrode. The electrolytes used were those shown in Tables 1 and 2, respectively.

[0054] (Manufacturing of lithium metal secondary batteries) A container made by heat-sealing aluminum laminate for secondary batteries (manufactured by Dai Nippon Printing Co., Ltd.) into a bag-like shape was prepared. After introducing the positive electrode, separator, and negative electrode into the container, 350 μl of electrolyte was poured in and the container was sealed under reduced pressure. Then, it was left at 45°C for 5 hours, and the cell was created by restraining it with a cushioned metal plate to achieve a holding pressure of 1 MPa. By performing two charge-discharge cycles at 0.1C (4.3V to 2.65V), a lithium metal secondary battery with a capacity of approximately 50 mAh was fabricated.

[0055] [Li thickness measurement] Using the electrolytes described in each of the above examples and comparative examples, test cells were prepared and, after completing a 50 cycy test, CCCV charging was performed at 4.3V to bring the State of Charge (SOC) to 100%. At this time, the element was removed from the laminate cell, the positive electrode was peeled off, and the thickness of the separator and negative electrode was measured with a micrometer. The thickness of the negative electrode lithium metal layer in the charged state was determined by subtracting the thickness of the negative electrode current collector foil and separator from this value. The results are shown in Tables 1 and 2.

[0056] [Capacity retention rate measurement] The capacity retention rate was measured using test cells prepared with the electrolytes described in each of the above examples and comparative examples. CCCV charging was performed at 25°C with a charge rate of 0.33C (1 / 3C) to 4.3V, followed by CV charging for 20 minutes. After standing for 10 minutes, the battery was discharged at 0.33C to 2.65V. This was considered the 100% battery capacity. Subsequently, 149 charge-discharge cycles between 4.3V and 2.65V were performed in a constant temperature bath at 25°C with an upper voltage limit of 4.3V, a charge rate of 0.33C, a lower voltage limit of 2.65V, and a discharge rate of 0.33C. After the discharge was completed and the battery was left to stand for 6 hours, the rated capacity at 1 / 3C was measured at the 150th cycle, and the capacity retention rate was calculated. The results are shown in Tables 1 and 2.

[0057] [Measuring Resistance Increase Rate] The resistance increase rate was measured using test cells prepared with the electrolytes described in each of the above examples and comparative examples. Specifically, the resistance value was determined after the initial resistance measurement and the rated capacity measurement at 150 cycles, and the ratio of the resistivity at 150 cycles to the initial resistance measurement was calculated. The results are shown in Tables 1 and 2.

[0058] [BOL resistivity measurement] The BOL resistivity was measured using test cells prepared with the electrolytes described in each of the above examples and comparative examples. Specifically, the cells were charged to 50% of the discharge capacity at the time of initial capacity measurement, resulting in a state of charge (SOC) of 50%. A 4.5C discharge was performed for 10 seconds using this voltage, and the resistance value was calculated. This resistance value was measured over a positive electrode area of ​​12 cm². 2 Divide by Ω·cm to obtain resistivity. 2 The following was calculated. The results are shown in Tables 1 and 2.

[0059] [Measuring the amount of gas inside the cell] Using test cells prepared with the electrolytes described in each of the above examples and comparative examples, the amount of gas inside the cell after 150 charge-discharge cycles was measured. Specifically, the procedure was as follows: First, the laminate cell after the durability test was adjusted to SOC0, and small cuts were made in the laminate cell to create holes. Next, argon gas was passed through this cell, and the mixed gas containing argon expelled from inside the cell was measured by gas chromatography. Each component was identified, and the amount of gas for each component was derived from the calibration curve for each component, and these were summed up to obtain the total generated gas amount. The results are shown in Tables 1 and 2.

Claims

1. An electrolyte for a lithium metal secondary battery comprising an electrolyte salt, an organic solvent, and a first additive, The electrolyte salt comprises lithium bis-fluorosulfonyliimide (LiFSI), The organic solvent comprises 1,2-dimethoxyethane (DME), The first additive is at least one of N,N-dimethyltrifluoromethane-sulfonamide (DMTMSA) or N,N-dimethylsulfamoyl fluoride (DMSF), An electrolyte for a lithium metal secondary battery, wherein the first additive is contained in the electrolyte for a lithium metal secondary battery in an amount of 0.2 parts by mass or more, when the total mass of the electrolyte salt and the organic solvent is 100 parts by mass.

2. The electrolyte for a lithium metal secondary battery according to claim 1, wherein the first additive is contained in the electrolyte for a lithium metal secondary battery in an amount of 5.0 parts by mass or less, when the total mass of the electrolyte salt and the organic solvent is 100 parts by mass.

3. The first additive includes both N,N-dimethyltrifluoromethane-sulfonamide (DMTMSA) and N,N-dimethylsulfamoyl fluoride (DMSF), The electrolyte for lithium metal secondary batteries according to claim 1 or 2, wherein the content (mass percentage) of N,N-dimethyltrifluoromethane-sulfonamide (DMTMSA) is less than the content (mass percentage) of N,N-dimethylsulfamoyl fluoride (DMSF).

4. As a second additive, lithium difluoro(oxalato)borate (LiFOB), lithium difluorophosphate (LiDFP), or LiPF ６ The electrolyte for a lithium metal secondary battery according to claim 1 or 2, further comprising at least one of the following.

5. The electrolyte for lithium metal secondary batteries according to claim 1 or 2, wherein the content of 1,2-dimethoxyethane (DME) in the organic solvent is 20 mol% or more.

6. The electrolyte for a lithium metal secondary battery according to claim 1 or 2, further comprising a fluorinated ether as the organic solvent.

7. The electrolyte for a lithium metal secondary battery according to claim 1 or 2, wherein the concentration of lithium bis-fluorosulfonylimide (LiFSI) as the electrolyte salt in the electrolyte for the lithium metal secondary battery is 1.2 mol / L or more and 2.5 mol / L or less.

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