Lithium metal battery electrolyte

Abstract

To provide a lithium metal battery electrolytic solution that can reduce the concentration of an electrolyte salt in the electrolytic solution and has excellent safety and durability during charging and discharging.SOLUTION: A lithium metal battery electrolytic solution including an organic solvent and an electrolyte salt, the electrolyte salt includes a lithium salt, and the organic solvent includes a first organic solvent with a vapor pressure of 0.003 MPa or more at 25°C and no flash point, and a second organic solvent with a vapor pressure of 0.007 MPa or less at 25°C.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] In recent years, research and development into secondary batteries that contribute to energy efficiency has been conducted to ensure that more people have access to affordable, reliable, sustainable, and advanced energy. Among secondary batteries, lithium metal batteries, which use lithium metal as the anode, have attracted attention.

[0003] In lithium metal batteries, as the battery is charged and discharged, lithium deposits on the negative electrode, forming dendrites and causing porosity, which can lead to a decrease in battery performance. One known technique for suppressing porosity is to increase the concentration of electrolyte salt (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2018-505538 Summary of the Invention [Problem to be solved by the invention]

[0005] Lithium metal battery electrolytes (hereinafter sometimes simply referred to as "electrolytes") with high electrolyte salt concentrations are expensive and have poor circulation to electrodes, which can lead to reduced uniformity during manufacturing. However, lowering the electrolyte concentration poses the aforementioned porosity problem, as well as safety and durability issues. The technology disclosed in Patent Document 1 claims to provide an optimal balance between efficiency, cycle life, conductivity, cost, and viscosity by using an electrolyte with a predetermined high lithium salt concentration. However, when the electrolyte salt concentration of the electrolyte is within the concentration range disclosed in Patent Document 1 (e.g., 4 to 6 moles of LiFSI per liter of DME), not only is durability at high temperatures insufficient, but safety cannot be improved. Therefore, there has been a need for an electrolyte solution with a low electrolyte concentration that is both safe and has excellent durability during charging and discharging.

[0006] The present invention has been made in view of the above, and aims to provide an electrolyte solution for lithium metal batteries that can reduce the electrolyte salt concentration in the electrolyte solution and is excellent in safety and durability during charging and discharging. [Means for solving the problem]

[0007] (1) The present invention relates to an electrolyte solution for a lithium metal battery, comprising an organic solvent and an electrolyte salt, wherein the electrolyte salt comprises a lithium salt, and the organic solvent comprises a first organic solvent having a vapor pressure of 0.003 MPa or more at 25°C and no flash point, and a second organic solvent having a vapor pressure of 0.007 MPa or less at 25°C.

[0008] According to the invention of (1), when the concentration of the electrolyte salt in the electrolyte solution is low, for example, less than 4 mol / L, the first organic solvent and the second organic solvent are contained in the electrolyte solution, thereby making it possible to provide an electrolyte solution for lithium metal batteries that is safe and has excellent durability during charging and discharging.

[0009] (2) The electrolyte solution for a lithium metal battery according to (1), wherein the electrolyte salt comprises at least one selected from the group consisting of LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC(CF3SO2)3, LiN(CF3SO2)2 (LiTFSI), LiN(FSO2)2 (LiFSI), and LiBC4O8.

[0010] According to the invention (2), the safety of the lithium metal battery and its durability during charging and discharging can be improved.

[0011] (3) The electrolyte solution for a lithium metal battery according to (1) or (2), wherein the concentration of the electrolyte salt is 3.8 mol / L or less.

[0012] According to the invention (3), the solubility of the electrolyte can be improved.

[0013] (4) The electrolyte solution for a lithium metal battery according to any one of (1) to (3), further comprising an additive, the additive being at least one selected from the group consisting of LiNO, lithium nitrite, LiPOF, Cs-PF, PS, ES, DTD, lithium sulfate, and LiFOB.

[0014] According to the invention of (4), since the solubility of the additive in the electrolyte is high, effects according to the type of additive, such as suppressing a decrease in the capacity retention rate of the battery, can be preferably obtained.

[0015] (5) The electrolyte solution for lithium metal batteries according to any one of (1) to (4), wherein the viscosity of the electrolyte solution for lithium metal batteries is 30 mPa·s or less.

[0016] According to the invention (5), the resistance of the battery can be reduced.

[0017] (6) The electrolyte solution for a lithium metal battery according to any one of (1) to (5), wherein the first organic solvent is a fluorine-substituted chain hydrocarbon.

[0018] According to the invention (6), the safety of the lithium metal battery can be improved.

[0019] (7) The lithium metal battery electrolyte solution according to any one of (1) to (6), wherein the second organic solvent contains at least one selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1-ethoxy-2-(2-methoxyethoxy)ethane, and the content of the second organic solvent in the lithium metal battery electrolyte solution is 10 mass % or more.

[0020] According to the seventh aspect of the present invention, the durability of the lithium metal battery can be improved.

[0021] (8) The electrolyte solution for a lithium metal battery according to any one of (1) to (7), wherein the first organic solvent is at least one selected from the group consisting of 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane, methyl nonafluoroisobutyl ether, ethyl nonafluorobutyl ether, 1,1,1,2,3,4,4,5,5,5-decafluoro-3-methoxy-2-(trifluoromethyl)pentane, CF3CHFCHFCF2CF3, CF3CH2CF2CH3, CF3CF2CF2CF2CF2CHF2, and CF3CF2CF2CF2CH2CH3.

[0022] According to the invention (8), the safety of the lithium metal battery can be improved.

[0023] (9) The electrolyte solution for a lithium metal battery according to any one of (1) to (8), wherein the second organic solvent contains at least one selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1-ethoxy-2-(2-methoxyethoxy)ethane, and also contains at least one selected from the group consisting of ethylene carbonate, propylene carbonate, sulfolane, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0024] According to the invention (9), the durability and oxidation resistance of the lithium metal battery can be improved.

[0025] (10) The electrolyte solution for a lithium metal battery according to any one of (1) to (9), wherein the flash point of the electrolyte solution for a lithium metal battery is 50° C. or higher.

[0026] According to the invention (10), the safety of the lithium metal battery can be improved. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a graph showing the relationship between the electrolyte salt concentration and the viscosity of the electrolyte solution for each type of organic solvent. [Figure 2] 1 is a graph showing the relationship between the electrolyte salt concentration and the capacity retention rate for each type of organic solvent. [Figure 3] 1 is a graph showing the relationship between the negative electrode deposition Coulomb efficiency (%) and the oxidation resistance and reduction resistance when the mixing ratio of the organic solvent is changed. [Figure 4] 1 is a graph showing the relationship between the content of additives and the negative electrode deposition Coulomb efficiency (%), as well as oxidation resistance and reduction resistance. [Figure 5] 1 is a chart showing a reduction wave when the electrode potential is swept from +3 V to 0 V with and without an additive. [Figure 6] 1 is a graph showing the relationship between the number of cycles and the capacity retention rate. [Figure 7] 1 is a micrograph showing the state of the negative electrode of a battery using a predetermined electrolyte after a cycle test. [Figure 8] 1 is a micrograph showing the state of the negative electrode of a battery using a predetermined electrolyte after a cycle test. [Figure 9] FIG. 1 is a diagram showing the results of solubility tests according to Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0028] <Electrolyte for lithium metal batteries> The lithium metal battery electrolyte solution according to this embodiment contains an organic solvent and an electrolyte salt. The organic solvent includes at least a first organic solvent and a second organic solvent, which will be described later. By including the first organic solvent and the second organic solvent in the electrolyte solution, the flash point and combustion point of the electrolyte solution are increased, thereby improving the safety of the electrolyte solution. Furthermore, durability during charge and discharge can be improved. The electrolyte solution may contain additives other than those mentioned above.

[0029] The viscosity of the electrolyte is preferably 30 mPa·s or less, which allows the resistance of the battery to be reduced. The viscosity of the electrolyte is more preferably 20 mPa·s or less.

[0030] (First organic solvent) The first organic solvent has a vapor pressure of 0.003 MPa or more at 25°C and no flash point. The first organic solvent is preferably a fluorine-substituted chain hydrocarbon. Specifically, the first organic solvent is preferably at least one selected from the group consisting of 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane, methyl nonafluoroisobutyl ether, ethyl nonafluorobutyl ether, 1,1,1,2,3,4,4,5,5,5-decafluoro-3-methoxy-2-(trifluoromethyl)pentane, CF3CHFCHFCF2CF3 (HFC-43-10mee), CF3CH2CF2CH3 (HFC-365mfc), CF3CF2CF2CF2CF2CHF2 (HFC-52-13p), and CF3CF2CF2CF2CH2CH3 (HFC-569sf). By including the first organic solvent in the electrolyte solution, the flash point and combustion point of the electrolyte solution can be increased, thereby improving safety. By including the first organic solvent in the electrolyte solution, the flash point of the electrolyte solution can be increased to 50°C or higher, for example. The content of the first organic solvent in the organic solvent is preferably 20% by mass or more, and may be 30% by mass or more.

[0031] (Second organic solvent) The second organic solvent has a vapor pressure of 0.007 MPa or less at 25°C. The second organic solvent preferably contains at least one selected from the group consisting of 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, and 1-ethoxy-2-(2-methoxyethoxy)ethane. When 1,2-dimethoxyethane is contained as the second organic solvent, the content of 1,2-dimethoxyethane in the electrolytic solution is preferably 10% by mass or more, and more preferably 11% by mass or more and 50% by mass or less.

[0032] The second organic solvent preferably contains 1,2-dimethoxyethane (DME) as an essential component and at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), sulfolane (SL), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). That is, the second organic solvent may be a mixed solvent containing two or more solvents.

[0033] (electrolyte salt) The electrolyte salt is a source of lithium ions, which are a charge transfer medium, and includes a lithium salt, which preferably includes at least one selected from the group consisting of LiPF, LiBF, LiClO, LiAsF, LiCFSO, LiC(CFSO), LiN(CFSO) (LiTFSI), LiN(FSO) (LiFSI), and LiBCO.

[0034] The concentration of the electrolyte salt in the electrolytic solution is preferably 3.8 mol / L or less, which can improve the durability of the battery and the liquid circulation to the electrodes.

[0035] (additives) The electrolyte solution according to this embodiment may contain an additive. Known components used in electrolyte solutions for lithium metal batteries can be used as the additive, such as a film-forming material and a dispersant. Specific examples of the additive include LiNO3, lithium nitrite, LiPO2F2, Cs-PF6, PS, ES, DTD, lithium sulfate, and LiFOB. The electrolyte solution according to this embodiment has a low concentration of the electrolyte salt in the electrolyte solution, for example, 3.8 mol / L or less, which can improve the solubility of additives such as LiNO3. The concentration of the additive in the electrolyte solution is preferably 0.01 to 5.0 mol / L.

[0036] <Lithium metal battery> A lithium metal battery is constructed using the electrolyte solution according to the present embodiment. The specific configuration of the lithium metal battery is not particularly limited except for the electrolyte solution, and any configuration used in known lithium metal batteries can be used without limitation. In a typical embodiment, the lithium metal battery has 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 contains the electrolyte solution according to the present embodiment.

[0037] (positive electrode layer) 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 battery. Examples of the positive electrode active material include layered active materials containing lithium, spinel-type active materials, and olivine-type active materials. Specific examples of the positive electrode active material include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and LiNi p Mn q Co r O2(p+q+r=1), LiNi p Al q Co rExamples of such an element-substituted Li-Mn spinel include lithium manganate (LiMnO), LiMnO (p+q+r=1), lithium manganate (LiMnO), LiMnMyO (x+y=2, M=at least one selected from Al, Mg, Co, Fe, Ni, and Zn), lithium titanate (oxide containing Li and Ti), and lithium metal phosphate (LiMPO, M=at least one selected from Fe, Mn, Co, and Ni).

[0038] The positive electrode layer may contain a binder, a conductive additive, etc. in addition to the positive electrode active material. A positive electrode current collector may be disposed 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 lithium metal batteries.

[0039] (negative electrode layer) The negative electrode layer is a layer containing a negative electrode active material. For example, lithium metal, a lithium alloy, or a mixture thereof can be used as the negative electrode active material. Examples of elements that can form an alloy with lithium metal include Al, Mg, K, Na, Ca, Sr, Ba, Si, Ge, Sb, Pb, Sn, In, and Zn. A negative electrode current collector may be disposed 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 for lithium metal batteries.

[0040] (electrolyte layer) The electrolyte layer contains the electrolyte solution according to the above embodiment. A separator for preventing short-circuiting between the positive electrode and the negative electrode may be provided as the electrolyte layer. The separator may be made of a material known as a separator for lithium metal batteries, such as a nonwoven fabric or a microporous film. The electrolyte layer may be formed by impregnating the separator with the electrolyte solution.

[0041] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention. [Example]

[0042] The present invention will be described in detail below using examples, but the present invention is not limited to these examples.

[0043] Figure 1 shows the relationship between the concentration (mol / L) of the lithium salt (LiFSI) in each type of organic solvent in the electrolyte and the viscosity (mPa·s) of the electrolyte. In Figure 1, "liquid LIB electrolyte" refers to the electrolyte for lithium-ion secondary batteries. Only the viscosity is shown because the viscosity of the electrolyte for lithium-ion secondary batteries is assumed to be the target value. In Figure 1, "HFE" refers to the first organic solvent, 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane. As shown in Figure 1, by mixing the first organic solvent with the second organic solvent, it is possible to obtain a viscosity equivalent to that of the electrolyte for lithium-ion secondary batteries, even when the electrolyte salt concentration is, for example, 3.8 mol / L or less. Furthermore, the use of the first organic solvent can increase the combustion point and flash point of the electrolyte.

[0044] Figure 2 is a graph showing the relationship between electrolyte salt concentration and battery capacity retention (C rate 0.3C, 25°C, 0.05 MPa, 8 cycles) when the second organic solvents, DME and DMC, are used alone as organic solvents in lithium metal batteries. LiFSI was used as the electrolyte salt. The capacity retention was measured using a simple cell with an electrode area of ​​15φ. As shown in Figure 2, it is clear that the capacity retention of DME is less affected by electrolyte salt concentration than that of DMC.

[0045] Figure 3 shows the negative electrode Li coulombic efficiency (%) and 4.3 V current value (mA / cm) when a lithium metal battery was formed using the second organic solvent, DME, and DMC, at a mass ratio of 100:0, 50:50, and 0:100, respectively, as the organic solvent. 2) relationship. The battery cell was Li / Cu AF (25°C), and the negative electrode Li Coulombic efficiency (%) was calculated as the ratio of the discharge capacity after 10 cycles to the initial discharge capacity. LiFSI was used as the electrolyte salt, with a concentration of 6 mol / L. The 4.3 V current value was the current value measured by LSV. In Figure 3, the graph indicated by black circles shows the negative electrode Li Coulombic efficiency, and a high negative electrode Li Coulombic efficiency suggests that the battery has excellent reduction resistance. The graph indicated by white circles shows the 4.3 V current value. A high 4.3 V current value suggests that the battery has excellent oxidation resistance. As Figure 3 shows, it is clear that when the DMC blend ratio is 50 mass% or higher, the oxidation resistance of the battery can be increased to the same level as when DMC is used alone as an organic solvent.

[0046] Figure 4 shows the relationship between the amount of LiNO3 added and the negative electrode Li coulombic efficiency (%) and the 4.3 V current value (mA / cm) when a 50:50 mass ratio mixture of DME and DMC, the second organic solvent, was used as the organic solvent for a lithium metal battery, and LiNO3 was further added as an additive to form a lithium metal battery. 2 ) is a graph showing the relationship between the negative electrode Li coulomb efficiency (%) and the 4.3 V current value (mA / cm 2 The measurement and calculation conditions for ) were the same as those in Figure 3. In Figure 4, the graph indicated by black circles shows the negative electrode Li coulomb efficiency, and the graph indicated by white circles shows the 4.3 V current value. As shown in Figure 4, it is clear that by increasing the amount of LiNO3 added, the reduction resistance of the battery can be significantly improved while maintaining or improving the oxidation resistance.

[0047] Figure 5 shows the LSV measurement results for lithium metal batteries fabricated using a 50:50 mixture of DME and DMC (second organic solvent) as the organic solvent for the lithium metal battery, with 2.0 mass% LiNO3 added as an additive, and with no LiNO3 added as an additive. LiFSI was used as the electrolyte salt, with a concentration of 6 mol / L. Battery cells were fabricated under the same conditions as in Figure 3 and elsewhere. LSV measurements were performed by sweeping the voltage from 3.0 V to 0 V. In Figure 5, the horizontal axis represents potential (V), and the vertical axis represents current (mA). As shown in Figure 5, adding LiNO3 as an additive resulted in a high current value, suggesting that the decomposition of LiFSI was promoted. This is expected to result in the formation of a LiNO3 film on the anode, which is expected to prevent atomization and protect against hot spots.

[0048] Figure 6 compares the capacity retention of lithium metal batteries fabricated using electrolytes containing DME and DMC alone as second organic solvents with those fabricated using a 50:50 DME / DMC mixture with 1.0 wt% LiNO3 and 0.3 wt% LiPO2F2 as additives. The lithium metal batteries were fabricated as shown in Figure 3, except for the use of laminated cells with an electrode area of ​​3 x 4 cm. The cycle test conditions were a C-rate of 0.3 C, 45°C, and 1 MPa. As shown in Figure 6, the electrolyte fabricated using a 50:50 DME / DMC mixture with 1 wt% LiNO3 and 0.3 wt% LiPO2F2 as additives significantly reduced the decrease in battery capacity retention with increasing cycle count compared to the electrolytes fabricated using DME and DMC alone as organic solvents.

[0049] Figure 7 shows SEM images (Scanning Electron Microscope S-4800, Hitachi High-Tech Corporation) of the anode of a lithium metal battery formed using an electrolyte containing DMC alone as the organic solvent after the cycle test (26 cycles, SOC 100%) shown in Figure 6. Figure 8 shows SEM images of the anode of a lithium metal battery formed using an electrolyte containing a 50:50 mixture of DME and DMC by mass, with 1 mass% LiNO3 and 0.3 mass% LiPO2F2 added as additives after the cycle test (18 cycles, SOC 100%) shown in Figure 6. As shown in Figures 7 and 8, the anode of Figure 8 clearly shows less porosity than the anode of Figure 7.

[0050] Figure 9 shows the solubility (good solubility: OK, poor solubility: NG) of the additive (LiNO3: 0.75%, LiPO2F2: 0.2%) when the mixing ratio of 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane and the concentration of lithium salt (LiFSI) are varied using 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane as the first organic solvent and DME as the second organic solvent. As shown in Figure 9, it is clear that as long as the lithium salt concentration is less than 4 mol / L, the solubility of the additive can be maintained even when the amount of 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane added is increased to about 20 mass%.

[0051] Next, the electrolyte solutions according to the examples and comparative examples of the present invention will be described.

[0052] [Preparation of electrolyte] Example 1 As shown in Table 1, 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane was used as the first organic solvent, DME was used as the second organic solvent, LiFSI was used as the lithium salt, and LiNO3 and LiPO2F2 were used as additives, and the electrolyte solution according to Example 1 was prepared by mixing these components in the amounts shown in Table 1.

[0053] <Examples 2 to 4, Comparative Examples 1 to 3> The electrolyte solutions of the Examples and Comparative Examples were prepared in the same manner as in Example 1, except that the materials constituting the electrolyte solutions were those shown in Table 1.

[0054] [Table 1]

[0055] The electrolyte solutions according to the examples and comparative examples were evaluated and measured according to the following evaluation and measurement items. The results are shown in Table 2.

[0056] [Solubility test] The electrolyte solutions according to the Examples and Comparative Examples were weighed into plastic containers and stirred for 30 minutes using a magnetic stirrer. The appearance was observed to check for dissolution. As a result of observing the appearance, those with good solubility were rated as "OK," and those with poor solubility from the outside were rated as "NG." The evaluation results are shown in Table 2.

[0057] [Viscosity measurement] The viscosity of the electrolyte solution according to each of the examples and comparative examples was measured using a viscosity measuring device (digital viscometer, manufactured by Brookfield). The results are shown in Table 2.

[0058] [Flash point measurement] The flash points of the electrolytes of each example and comparative example were measured using a Seta closed-cup flash point tester. The sample was placed in a sealed sample cup and kept at a constant temperature. After a specified time had passed, a test flame was poked into the sample cup to check for ignition. This procedure was repeated at different temperatures to determine the lowest temperature at which ignition was confirmed. The results are shown in Table 2.

[0059] [Fire point measurement] The flash points of the electrolytes according to each example and comparative example were measured using a tag open-type flash point tester. The sample was placed in a sample cup, and a test flame was passed over the sample. The temperature at which combustion continued for 5 seconds after the test flame had passed was determined. The results are shown in Table 2.

[0060] [Fabrication of lithium metal batteries] Using the electrolyte solutions according to the respective Examples and Comparative Examples, lithium metal batteries according to the respective Examples and Comparative Examples were fabricated according to the following procedures.

[0061] (Preparation of positive electrode) Acetylene black (AB) as an electron conductive material, polyvinylidene fluoride (PVDF) as a binder, and polyvinylpyrrolidone (PVP) as a dispersant were premixed with N-methyl-2-pyrrolidone (NMP) as a dispersion solvent, and the mixture was wet mixed in a planetary mixer to obtain a premixed slurry. 0.8 Co 0.1 Mn 0.1 The resulting premixed slurry was mixed with O2 (NCM811) and a pre-dope material, and dispersed using a planetary mixer to obtain a positive electrode paste. The NCM811 particles had a median diameter of 12 μm. The resulting positive electrode paste was then applied to an aluminum positive electrode collector without a primer layer, dried, pressed with a roll press, and then dried in a vacuum at 120°C to form a positive electrode plate with a positive electrode composite layer. The resulting positive electrode plate was punched out to a size of 30 mm x 40 mm to form a positive electrode.

[0062] (Preparing the negative electrode) The negative electrode was made of a clad material consisting of a 10 μm thick copper foil and a 20 μm thick lithium foil, which was punched out to have an electrode area of ​​34 mm × 44 mm.

[0063] (Preparing the separator) An alumina-coated polyethylene microporous membrane was used as the separator. The electrolyte solutions shown in Table 1 were used.

[0064] (Fabrication of lithium metal batteries) A positive electrode, separator, and negative electrode were placed in a container made by heat-sealing aluminum laminate for secondary batteries (manufactured by Dai Nippon Printing) into a bag-shaped container, and then an electrolyte was poured in to prepare a lithium metal battery.

[0065] [Charge / discharge test] Charge-discharge tests were conducted on the lithium metal batteries according to the above examples and comparative examples. The initial charge-discharge test consisted of charging to 4.3 V at a C rate of 0.1 C in a thermostatic chamber at 25°C and discharging to 2.65 V at 0.1 C. After two cycles, the third cycle involved CC-CV charging in the same manner as the first and second cycles, followed by constant-current discharging at a voltage of 50% SOC. The voltage value after 10 seconds was measured, and the direct current resistance (DCR) was calculated from the slope of the current value versus the voltage value after 10 seconds. The results are shown in Table 2.

[0066] [Cycle durability test] A cycle durability test was carried out using the lithium metal batteries according to the above examples and comparative examples according to the following procedure. The lithium metal batteries were charged to 4.3 V at 0.3 C in a 45°C thermostatic chamber and then discharged to 2.65 V at 0.3 C. This was repeated 50 times, and the post-durability capacity retention rate was calculated using the following formula (1), with the discharge current value at 0.3 C set as 100. Capacity retention rate after endurance (%) = (cell capacity after 50 cycles) / (initial cell capacity) × 100 (1)

[0067] [Table 2]

[0068] The results shown in Table 2 clearly show that the lithium metal batteries fabricated using the electrolyte solutions according to each Example have superior electrolyte characteristics and battery cell characteristics compared to the lithium metal batteries fabricated using the electrolyte solutions according to each Comparative Example.

Claims

1. An electrolyte solution for a lithium metal battery, comprising an organic solvent and an electrolyte salt, the electrolyte salt includes a lithium salt; the organic solvent includes a first organic solvent having a vapor pressure of 0.003 MPa or more at 25°C and no flash point, and a second organic solvent having a vapor pressure of 0.007 MPa or less at 25°C; the content of the first organic solvent in the organic solvent is 10% by mass or more and 30% by mass or less, An electrolyte for a lithium metal battery further comprising an additive, the additive being at least one selected from the group consisting of LiNO 3 , lithium nitrite, LiPO 2 F 2 , Cs-PF6, PS, ES, DTD, lithium sulfate, and LiFOB.

2. The electrolyte salt is LiPF ６ , LiBF ４ , LiClO ４ , LiAsF ６ , LiCF ３ SO ３ , LiC(CF ３ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ (LiTFSI), LiN(FSO ２ ) ２ (LiFSI), and LiBC ４ O ８ 2. The electrolyte solution for a lithium metal battery according to claim 1, comprising at least one selected from the group consisting of:

3. 3. The electrolyte solution for a lithium metal battery according to claim 1, wherein the concentration of the electrolyte salt is 3.8 mol / L or less.

4. 4. The lithium metal battery electrolyte according to claim 1, wherein the viscosity of the lithium metal battery electrolyte is 30 mPa·s or less.

5. 5. The electrolyte solution for a lithium metal battery according to claim 1, wherein the first organic solvent is a fluorine-substituted chain hydrocarbon.

6. 6. The lithium metal battery electrolyte solution according to claim 1, wherein the second organic solvent comprises at least one selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1-ethoxy-2-(2-methoxyethoxy)ethane, and the content of the second organic solvent in the lithium metal battery electrolyte solution is 10 mass% or more.

7. The first organic solvent may be 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane, methyl nonafluoroisobutyl ether, ethyl nonafluorobutyl ether, 1,1,1,2,3,4,4,5,5,5-decafluoro-3-methoxy-2-(trifluoromethyl)pentane, CF ３ CHFCHFCF ２ CF ３ , C.F. ３ CH ２ CF ２ CH ３ , C.F. ３ CF ２ CF ２ CF ２ CF ２ CHF ２ , and CF ３ CF ２ CF ２ CF ２ CH ２ CH ３ The electrolyte solution for lithium metal batteries according to any one of claims 1 to 6, which is at least one selected from the group consisting of:

8. The lithium metal battery electrolyte solution according to any one of claims 1 to 7, wherein the second organic solvent contains at least one selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1-ethoxy-2-(2-methoxyethoxy)ethane, and also contains at least one selected from the group consisting of ethylene carbonate, propylene carbonate, sulfolane, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

9. The lithium metal battery electrolyte according to any one of claims 1 to 8, wherein the flash point of the lithium metal battery electrolyte is 50°C or higher.

Images