Electrolyte and lithium metal secondary battery

Abstract

To provide an electrolyte that can reduce the direct current resistance of a lithium metal secondary battery and increase the capacity retention rate even after repeated charging and discharging.SOLUTION: An electrolyte includes a lithium salt and an organic solvent, and the concentration of the lithium salt is 2.0 mol / L or more and 3.0 mol / L or less. The lithium salt contains a first lithium salt and a second lithium salt. The first lithium salt is lithium bis(fluorosulfonyl)imide, and the second lithium salt is one or more selected from the group consisting of lithium hexafluorophosphate, lithium difluorooxalatoborate, and lithium difluorophosphate.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In recent years, research and development into secondary batteries that contribute to energy efficiency has been conducted to ensure that many people have access to affordable, reliable, sustainable and advanced energy.

[0003] Known examples of secondary batteries include lithium metal secondary batteries that include a positive electrode having a positive electrode current collector and a positive electrode composite layer containing a lithium composite oxide, a negative electrode having a negative electrode current collector and a lithium metal layer, and a separator impregnated with an electrolyte solution.

[0004] Known examples of electrolyte solutions include nonaqueous electrolyte solutions in which an electrolyte salt is dissolved in a nonaqueous solvent. The electrolyte salt includes at least one first lithium salt selected from LiPF, LiBF, LiN(SOF), LiN(SOCF), and LiN(SOCF) and at least one second lithium salt selected from lithium salts having an oxalic acid skeleton, lithium salts having a phosphate skeleton, and lithium salts having an S=O group. The first and second lithium salts are four or more in total. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2016 / 009994 Summary of the Invention [Problem to be solved by the invention]

[0006] However, repeated charge and discharge of lithium metal secondary batteries increases the DC resistance and reduces the capacity retention rate.

[0007] An object of the present invention is to provide an electrolyte solution that can reduce the DC resistance of a lithium metal secondary battery and increase the capacity retention rate even after repeated charge and discharge. [Means for solving the problem]

[0008] (1) An electrolyte solution comprising a lithium salt and an organic solvent, wherein the concentration of the lithium salt is 2.0 mol / L or more and 3.0 mol / L or less, the lithium salt comprises a first lithium salt and a second lithium salt, the first lithium salt is lithium bis(fluorosulfonyl)imide, and the second lithium salt is one or more selected from the group consisting of lithium hexafluorophosphate, lithium difluorooxalatoborate, and lithium difluorophosphate.

[0009] (2) The electrolyte solution according to (1), wherein the concentration of the lithium bis(fluorosulfonyl)imide is 1.98 mol / L or more and 2.6 mol / L or less.

[0010] (3) The electrolyte solution according to (1) or (2), wherein the concentration of the lithium hexafluorophosphate is 1.0 mol / L or less.

[0011] (4) The electrolyte solution according to (1) or (2), wherein the concentration of the lithium difluorooxalatoborate is 1.0 mol / L or less.

[0012] (5) The electrolyte solution according to (1) or (2), wherein the concentration of the lithium difluorophosphate is 0.02 mol / L or more.

[0013] (6) The electrolyte solution according to (1) or (2), wherein the concentration of the lithium hexafluorophosphate is 1.0 mol / L or less and the concentration of the lithium difluorophosphate is 0.02 mol / L or more.

[0014] (7) The concentration of the lithium difluorooxalatoborate is 1.0 mol / L or less; The electrolyte solution according to (1) or (2), wherein the concentration of the lithium difluorophosphate is 0.02 mol / L or more.

[0015] (8) The concentration of the lithium hexafluorophosphate is 1.0 mol / L or less; The electrolyte solution according to (1) or (2), wherein the concentration of the lithium difluorooxalatoborate is 1.0 mol / L or less.

[0016] (9) The electrolytic solution according to any one of (1) to (8), wherein the organic solvent comprises 1,2-dimethoxyethane and a hydrofluoroether.

[0017] (10) A lithium metal secondary battery comprising the electrolyte solution according to any one of (1) to (9). [Effects of the Invention]

[0018] According to the present invention, it is possible to provide an electrolyte solution that can reduce the DC resistance of a lithium metal secondary battery and increase the capacity retention rate even after repeated charge and discharge. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention will be described.

[0020] [Electrolyte] The electrolyte solution of this embodiment contains a lithium salt and an organic solvent. The concentration of the lithium salt in the electrolyte solution of this embodiment is 2.0 mol / L or more and 3.0 mol / L or less, and preferably 2.2 mol / L or more and 2.8 mol / L or less. If the concentration of the lithium salt in the electrolyte solution is less than 2.0 mol / L or more than 3.0 mol / L, the direct current resistance of the lithium metal secondary battery increases and the capacity retention rate decreases.

[0021] The lithium salt includes a first lithium salt and a second lithium salt, wherein the first lithium salt is lithium bis(fluorosulfonyl)imide, and the second lithium salt is one or more selected from the group consisting of lithium hexafluorophosphate, lithium difluorooxalatoborate, and lithium difluorophosphate.

[0022] The concentration of lithium bis(fluorosulfonyl)imide in the electrolyte solution of this embodiment is preferably 1.98 mol / L or more and 2.6 mol / L or less, and more preferably 2.0 mol / L or more and 2.4 mol / L or less. When the concentration of lithium bis(fluorosulfonyl)imide in the electrolyte solution of this embodiment is 1.98 mol / L or more and 2.6 mol / L or less, the direct current resistance of the lithium metal secondary battery is likely to be low and the capacity retention rate is likely to be high even after repeated charge and discharge.

[0023] The concentration of lithium hexafluorophosphate in the electrolyte solution of this embodiment is preferably 1.0 mol / L or less, and more preferably 0.01 mol / L or more and 0.8 mol / L or less. When the concentration of lithium hexafluorophosphate in the electrolyte solution of this embodiment is 1.0 mol / L or less, the direct current resistance of the lithium metal secondary battery is likely to be low and the capacity retention rate is likely to be high even after repeated charge and discharge.

[0024] The concentration of lithium difluorooxalatoborate in the electrolyte solution of this embodiment is preferably 1.0 mol / L or less, and more preferably 0.01 mol / L or more and 0.8 mol / L or less. When the concentration of lithium difluorooxalatoborate in the electrolyte solution of this embodiment is 1.0 mol / L or less, the direct current resistance of the lithium metal secondary battery is likely to be low and the capacity retention rate is likely to be high even after repeated charge and discharge.

[0025] The concentration of lithium difluorophosphate in the electrolyte solution of this embodiment is preferably 0.02 mol / L or more, and more preferably 0.04 mol / L or more and 0.2 mol / L or less. When the concentration of lithium difluorophosphate in the electrolyte solution of this embodiment is 0.02 mol / L or more, the direct current resistance of the lithium metal secondary battery is likely to be low and the capacity retention rate is likely to be high even after repeated charge and discharge.

[0026] The organic solvent is not particularly limited as long as it can dissolve the lithium salt, but examples thereof include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, fluoroethylene carbonate, vinylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, hydrofluoroether, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propionitrile, anisole, acetate ester, butyrate ester, propionate ester, etc., and two or more of them may be used in combination. Among these, a mixed solvent of 1,2-dimethoxyethane and hydrofluoroether is preferred.

[0027] The volume ratio of hydrofluoroether to 1,2-dimethoxyethane in the mixed solvent is preferably 0.02 or more and 0.80 or less, and more preferably 0.05 or more and 0.70 or less. When the volume ratio of hydrofluoroether to 1,2-dimethoxyethane in the mixed solvent is 0.02 or more, the direct current resistance of the lithium metal secondary battery is likely to be low and the capacity retention rate is likely to be high even after repeated charge and discharge. On the other hand, when the volume ratio of hydrofluoroether to 1,2-dimethoxyethane in the mixed solvent is 0.80 or less, lithium bis(fluorosulfonyl)imide is likely to dissolve.

[0028] The hydrofluoroether is not particularly limited, but 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, and 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane.

[0029] [Lithium metal secondary battery] The lithium metal secondary battery of this embodiment has a separator impregnated with an electrolyte disposed between a positive electrode and a negative electrode. The positive electrode has a positive electrode current collector and a positive electrode mixture layer containing a lithium composite oxide. The negative electrode has a negative electrode current collector and a lithium metal layer.

[0030] That is, in the lithium metal secondary battery of this embodiment, lithium metal is deposited on the negative electrode during charging, and lithium ions are eluted from the negative electrode during discharging. Therefore, in the lithium metal secondary battery of this embodiment, the negative electrode does not need to have a lithium metal layer in the initial state. In this case, by charging the lithium metal secondary battery before use, lithium metal is deposited on the negative electrode current collector and a lithium metal layer is formed.

[0031] The positive electrode current collector is not particularly limited, but may be, for example, aluminum foil.

[0032] The positive electrode mixture layer contains a lithium composite oxide, and may further contain other components.

[0033] The lithium composite oxide is not particularly limited, but examples thereof include LiCoO2, Li(Ni 5 / 10 Co 2 / 10 Mn 3 / 10 )O 2、 Li(Ni) 6 / 10 Co 2 / 10 Mn 2 / 10 )O 2、 Li(Ni) 8 / 10 Co 1 / 10 Mn 1 / 10)O 2、 Li(Ni) 0.8 Co 0.15 Al 0.05 )O 2、 Li(Ni) 1 / 6 Co 4 / 6 Mn 1 / 6 )O 2、 Li(Ni) 1 / 3 Co 1 / 3 Mn 1 / 3 )O 2、 Examples include LiCoO4, LiMn2O4, LiNiO2, and LiFePO4, and two or more of these may be used in combination.

[0034] Examples of other components include a positive electrode active material other than the lithium composite oxide, a conductive additive, a binder, and the like.

[0035] The negative electrode current collector is not particularly limited, but examples thereof include copper foil.

[0036] The material for forming the separator is not particularly limited, but examples thereof include polyolefins such as polyethylene and polypropylene, aramid, polyimide, fluororesin, glass fiber, and cellulose fiber.

[0037] The lithium metal secondary battery of this embodiment can be manufactured using a known method.

[0038] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and the above-described embodiments may be modified as appropriate within the scope of the spirit of the present invention. [Example]

[0039] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0040] [Example 1] The lithium salts, lithium bis(fluorosulfonyl)imide (LiFSI) and lithium difluorophosphate (LiPO2F2), were dissolved in a mixed solvent (volume ratio 0.81:0.19) of 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (HFE). The concentrations of LiFSI and LiPO2F2 in the electrolyte were 2.46 mol / L and 0.04 mol / L, respectively.

[0041] [Example 2] An electrolyte solution was obtained in the same manner as in Example 1, except that lithium difluorooxalatoborate (LiFOB) was used instead of LiPO2F2, and the concentrations of LiFSI and LiFOB in the electrolyte solution were 2.35 mol / L and 0.1 mol / L, respectively.

[0042] [Example 3] An electrolyte solution was obtained in the same manner as in Example 1, except that lithium hexafluorophosphate (LiPF6) was used instead of LiPO2F2, and the concentrations of LiFSI and LiPF6 in the electrolyte solution were 2.40 mol / L and 0.1 mol / L, respectively.

[0043] [Example 4] An electrolyte solution was obtained in the same manner as in Example 1, except that LiPF6 and LiFOB were used instead of LiPO2F2, and the concentrations of LiFSI, LiPF6, and LiFOB in the electrolyte solution were 2.25 mol / L, 0.1 mol / L, and 0.1 mol / L, respectively.

[0044] [Example 5] An electrolyte solution was obtained in the same manner as in Example 1, except that LiFSI, LiPF6, and LiPO2F2 were used as lithium salts and the concentrations of LiFSI, LiPF6, and LiPO2F2 in the electrolyte solution were 2.36 mol / L, 0.1 mol / L, and 0.04 mol / L, respectively.

[0045] [Example 6] An electrolyte solution was obtained in the same manner as in Example 3, except that the concentrations of LiFSI and LiPF6 in the electrolyte solution were changed to 2.26 mol / L and 0.2 mol / L, respectively.

[0046] [Example 7] An electrolyte solution was obtained in the same manner as in Example 3, except that the concentrations of LiFSI, LiPF6, and LiPO2F2 in the electrolyte solution were changed to 2.15 mol / L, 0.2 mol / L, and 0.15 mol / L, respectively.

[0047] [Example 8] An electrolyte solution was obtained in the same manner as in Example 3, except that LiFOB was used instead of LiPF6.

[0048] [Example 9] An electrolyte solution was obtained in the same manner as in Example 6, except that the concentration of LiPO2F2 in the electrolyte solution was changed to 0.08 mol / L.

[0049] [Comparative Example 1] An electrolyte solution was obtained in the same manner as in Example 1, except that LiFSI was used as the lithium salt and the concentration of LiFSI in the electrolyte solution was 2.50 mol / L.

[0050] Comparative Example 2 An electrolyte solution was obtained in the same manner as in Example 3, except that the concentrations of LiFSI and LiPF6 in the electrolyte solution were changed to 2.50 mol / L and 1.0 mol / L, respectively.

[0051] [Fabrication of lithium metal secondary batteries] A coating liquid for a positive electrode mixture layer was obtained by mixing a lithium-nickel-cobalt-manganese composite oxide as a lithium composite oxide, acetylene black as a conductive additive, and polyvinylidene fluoride as a binder.

[0052] As a positive electrode current collector, the area is 12 cm 2 The coating solution for the positive electrode composite layer was applied to an Al foil having a thickness of 15 μm, dried, and then coated to a thickness of 20 mg / cm 2 After forming the positive electrode mixture layer, the layer was rolled to obtain a positive electrode.

[0053] The negative electrode current collector and separator each had an area of ​​12 cm 2 , a 12 μm thick Cu foil and a 20 μm thick porous polyolefin film were used.

[0054] The positive electrode (positive electrode composite layer, positive electrode current collector), separator, and negative electrode current collector were laminated in this order, and the separator was impregnated with an electrolyte solution, and then sealed with a laminate film to obtain a lithium metal secondary battery.

[0055] [Charge and discharge test of lithium metal secondary batteries] The discharge capacity per unit area of ​​lithium metal is 3mAh / cm 2 The charge-discharge test was carried out under the following conditions: The lithium metal secondary battery was assembled in a jig, restrained at a restraining pressure of 0.05 MPa, and then left at the measurement temperature (25°C) for 1 hour. Next, constant current charging was carried out at 0.2 C. The termination condition for the constant current charging was the specified capacity (3 mAh / cm 2 ), or 5 hours after the start of constant-current charging, or the voltage reached 0.8 V. At this time, lithium metal precipitated on the Cu foil, forming a lithium metal layer with a thickness of approximately 15 μm. After this, a 5-minute pause was allowed. Next, constant-current discharge was performed at 0.2 C. The termination condition for constant-current discharge was when the specified capacity (3 mAh / cm2) was reached. 2 ), 5 hours had elapsed since the start of constant-current discharge, or the voltage had reached -0.8 V. After this, a 5-minute break was allowed. Next, the above constant-current discharge and constant-current charge were repeated 50 times.

[0056] The current value at which discharge can be completed in one hour is defined as 1C, relative to the discharge capacity specified per unit area of ​​lithium metal.

[0057] formula (Volume at 50th cycle) / (Volume at 1st cycle) x 100 Next, a constant current discharge was performed at a charge rate of 50%, and the voltage was measured after 10 seconds. (Voltage after 10 seconds) / (Current) / (Collector area) The direct current resistance (DCR) was calculated using the following equation.

[0058] Table 1 shows the evaluation results of the DCR and capacity retention rate of the lithium metal secondary battery.

[0059] [Table 1]

[0060] From Table 1, it can be seen that when the electrolyte solutions of Examples 1 to 9 were used, the DCR after charge-discharge testing of the lithium metal secondary battery was low and the capacity retention rate was high. In contrast, the electrolyte solution of Comparative Example 1 did not contain LiPF6, LiFOB, or LiPO2F2, and therefore the DCR after charge-discharge testing of the lithium metal secondary battery was high and the capacity retention rate was low. Furthermore, the electrolyte solution of Comparative Example 2 had a total lithium salt concentration of 3.54 mol / L, and therefore the DCR after charge-discharge testing of the lithium metal secondary battery was high and the capacity retention rate was low.

Claims

1. comprising a lithium salt and an organic solvent, the concentration of the lithium salt is 2.0 mol / L or more and 3.0 mol / L or less; the lithium salts include a first lithium salt and a second lithium salt; the first lithium salt is lithium bis(fluorosulfonyl)imide; the second lithium salt is at least one selected from the group consisting of lithium hexafluorophosphate, lithium difluorooxalatoborate, and lithium difluorophosphate; the organic solvent comprises 1,2-dimethoxyethane and a hydrofluoroether; The electrolyte solution has a concentration of the lithium bis(fluorosulfonyl)imide of 1.98 mol / L or more and 2.6 mol / L or less.

2. A method for producing a liquid crystal display device, comprising: the concentration of the lithium salt is 2.0 mol / L or more and 3.0 mol / L or less; the lithium salts include a first lithium salt and a second lithium salt; the first lithium salt is lithium bis(fluorosulfonyl)imide; the second lithium salt is at least one selected from the group consisting of lithium hexafluorophosphate, lithium difluorooxalatoborate, and lithium difluorophosphate; the organic solvent comprises 1,2-dimethoxyethane and a hydrofluoroether; The electrolyte solution has a concentration of the lithium difluorooxalatoborate of 1.0 mol / L or less.

3. A method for producing a liquid crystal display device, comprising: a lithium salt and an organic solvent; the concentration of the lithium salt is 2.0 mol / L or more and 3.0 mol / L or less; the lithium salts include a first lithium salt and a second lithium salt; the first lithium salt is lithium bis(fluorosulfonyl)imide; the second lithium salt is at least one selected from the group consisting of lithium hexafluorophosphate, lithium difluorooxalatoborate, and lithium difluorophosphate; the organic solvent comprises 1,2-dimethoxyethane and a hydrofluoroether; The electrolyte solution has a concentration of the lithium difluorophosphate of 0.02 mol / L or more.

4. A method for producing a liquid crystal display device, comprising: the concentration of the lithium salt is 2.0 mol / L or more and 3.0 mol / L or less; the lithium salts include a first lithium salt and a second lithium salt; the first lithium salt is lithium bis(fluorosulfonyl)imide; the second lithium salt is at least one selected from the group consisting of lithium hexafluorophosphate, lithium difluorooxalatoborate, and lithium difluorophosphate; the organic solvent comprises 1,2-dimethoxyethane and a hydrofluoroether; The concentration of the lithium hexafluorophosphate is 1.0 mol / L or less, The electrolyte solution has a concentration of the lithium difluorophosphate of 0.02 mol / L or more.

5. A method for producing a liquid crystal display device, comprising: a lithium salt and an organic solvent; the concentration of the lithium salt is 2.0 mol / L or more and 3.0 mol / L or less; the lithium salts include a first lithium salt and a second lithium salt; the first lithium salt is lithium bis(fluorosulfonyl)imide; the second lithium salt is at least one selected from the group consisting of lithium hexafluorophosphate, lithium difluorooxalatoborate, and lithium difluorophosphate; the organic solvent comprises 1,2-dimethoxyethane and a hydrofluoroether; The concentration of the lithium difluorooxalatoborate is 1.0 mol / L or less, The electrolyte solution has a concentration of the lithium difluorophosphate of 0.02 mol / L or more.

6. A method for producing a liquid crystal display device, comprising: the concentration of the lithium salt is 2.0 mol / L or more and 3.0 mol / L or less; the lithium salts include a first lithium salt and a second lithium salt; the first lithium salt is lithium bis(fluorosulfonyl)imide; the second lithium salt is at least one selected from the group consisting of lithium hexafluorophosphate, lithium difluorooxalatoborate, and lithium difluorophosphate; the organic solvent comprises 1,2-dimethoxyethane and a hydrofluoroether; The concentration of the lithium hexafluorophosphate is 1.0 mol / L or less, The electrolyte solution has a concentration of the lithium difluorooxalatoborate of 1.0 mol / L or less.

7. A lithium metal secondary battery comprising the electrolyte solution according to any one of claims 1 to 6.

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