Electrolyte and lithium-ion secondary battery

Abstract

To provide an electrolytic solution capable of improving the initial capacity and capacity retention rate of a lithium ion secondary battery during high-rate charging and high-rate discharging.SOLUTION: An electrolyte solution includes magnesium fluoride, a lithium salt, a first solvent, and a second solvent. The first solvent has a dielectric constant less than 10 and the second solvent has a dielectric constant greater than 50.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In light of climate-related disasters, there is growing interest in electric vehicles as a way to reduce CO2 emissions, and the use of lithium-ion secondary batteries is being considered for in-vehicle use. Furthermore, by making the entire system of an electric vehicle smaller and lighter, energy efficiency can be improved.

[0003] One example of a lithium-ion secondary battery is a lithium-ion secondary battery in which a positive electrode current collector, a positive electrode composite layer, a separator, a negative electrode composite layer, and a negative electrode current collector are sequentially stacked, with the separator filled with an electrolyte. During charging and discharging, a lithium-ion secondary battery forms an SEI film at the interface between the electrode and the electrolyte. Therefore, although the redox potential of lithium is low, continuous decomposition of the electrolyte is suppressed.

[0004] Patent Document 1 describes an electrolyte solution in which lithium hexafluorophosphate (LiPF6) is dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-149211 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the initial capacity and capacity retention rate during high-rate charging and high-rate discharging decrease, making it impossible to realize a high-rate battery.

[0007] An object of the present invention is to provide an electrolyte solution that can improve the initial capacity and capacity retention rate of a lithium ion secondary battery during high-rate charging and high-rate discharging. [Means for solving the problem]

[0008] In one aspect of the present invention, an electrolyte solution includes magnesium fluoride, a lithium salt, a first solvent, and a second solvent, wherein the first solvent has a dielectric constant of less than 10, and the second solvent has a dielectric constant of more than 50.

[0009] In the electrolytic solution, the volume ratio of the second solvent to the total amount of the first solvent and the second solvent may be 6% or more and 12% or less.

[0010] The electrolyte may have a magnesium fluoride content of 0.3% by mass or more and 1.0% by mass or less.

[0011] The first solvent may be one or more selected from the group consisting of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0012] The second solvent may be one or more selected from the group consisting of ethylene carbonate, propylene carbonate, and vinylene carbonate.

[0013] Another aspect of the present invention is a lithium ion secondary battery comprising the above-described electrolyte solution. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide an electrolyte solution that can improve the initial capacity and capacity retention rate of a lithium ion secondary battery during high-rate charging and high-rate discharging. [Brief explanation of the drawings]

[0015] [Figure 1]FIG. 2 is a diagram showing the initial charge-discharge curves during high-rate discharge of a lithium ion secondary battery comprising the electrolyte solution of Example 1. [Figure 2] 1 is a diagram showing the initial charge-discharge curves during high-rate discharge of a lithium ion secondary battery including the electrolyte solution of Comparative Example 1. FIG. [Figure 3] FIG. 2 is a graph showing the cycle characteristics of a lithium ion secondary battery containing the electrolyte solution of Example 1 during high-rate discharge. [Figure 4] FIG. 10 is a graph showing the cycle characteristics of a lithium ion secondary battery containing the electrolyte solution of Example 2 during high-rate discharge. [Figure 5] FIG. 1 is a graph showing the cycle characteristics of a lithium ion secondary battery including the electrolytic solution of Comparative Example 1 during high-rate discharge. [Figure 6] 10 is a graph showing the cycle characteristics of a lithium ion secondary battery including the electrolytic solution of Comparative Example 2 during high-rate discharge. FIG. [Figure 7] FIG. 2 is a diagram showing charge / discharge curves during high-rate charging of a lithium ion secondary battery including the electrolytic solution of Example 1. [Figure 8] 1 is a diagram showing charge / discharge curves during high-rate charging of a lithium ion secondary battery including the electrolytic solution of Comparative Example 1. FIG. [Figure 9] FIG. 2 is a graph showing the cycle characteristics of a lithium ion secondary battery containing the electrolytic solution of Example 1 during high-rate charging. [Figure 10] FIG. 10 is a graph showing the cycle characteristics of a lithium ion secondary battery containing the electrolytic solution of Example 2 during high-rate charging. [Figure 11] 1 is a graph showing the cycle characteristics of a lithium ion secondary battery including the electrolytic solution of Comparative Example 1 during high-rate charging. [Figure 12] 10 is a graph showing the cycle characteristics of a lithium ion secondary battery including the electrolytic solution of Comparative Example 2 during high-rate charging. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of the present invention will be described.

[0017] [Electrolyte] The electrolyte solution of this embodiment contains magnesium fluoride (MgF), a lithium salt, a first solvent, and a second solvent, the first solvent having a dielectric constant less than 10, and the second solvent having a dielectric constant greater than 50. Because magnesium fluoride has a high solubility in the first solvent, the use of the electrolyte solution of this embodiment improves the initial capacity and capacity retention of a lithium ion secondary battery during high-rate charging and high-rate discharging. This is presumably due to the formation of a uniform SEI film at the interface between the electrode and the electrolyte during charging and discharging. The magnesium fluoride is presumably solvated without being ionized.

[0018] In contrast, metal fluorides other than magnesium fluoride (e.g., sodium fluoride (NaF), calcium fluoride (CaF), etc.) have low solubility in the first solvent, causing the electrolyte to become cloudy. Although the initial capacity and capacity retention rate of the lithium-ion secondary battery may be maintained during high-rate charging and high-rate discharging, the reproducibility is low. This is presumably because the SEI film formed during charging and discharging changes depending on the suspension state.

[0019] The volume ratio of the second solvent to the total amount of the first solvent and the second solvent in the electrolyte solution of this embodiment is preferably 6% or more and 12% or less. When the volume ratio of the second solvent to the total amount of the first solvent and the second solvent in the electrolyte solution of this embodiment is 6% or more, it is presumed that the SEI (the organic coating portion formed by the second solvent) is less likely to be lost, and the capacity retention rate is improved. On the other hand, when the volume ratio of the second solvent to the total amount of the first solvent and the second solvent in the electrolyte solution of this embodiment is 12% or less, it is presumed that a thin organic coating is formed, and the resistance during high-rate operation is reduced.

[0020] The content of magnesium fluoride in the electrolyte solution of this embodiment is preferably 0.3% by mass or more and 1.0% by mass or less, and more preferably 0.3% by mass or more and 0.6% by mass or less. When the content of magnesium fluoride in the electrolyte solution of this embodiment is 0.3% by mass or more, it is possible to reduce the amount of the second solvent required to form a defect-free SEI, thereby reducing resistance during high-rate operation. On the other hand, when the content of magnesium fluoride in the electrolyte solution of this embodiment is 1.0% by mass or less, the resistance of the magnesium fluoride itself can be suppressed, improving the initial capacity of the lithium ion secondary battery. Taking into account the resistance of the magnesium fluoride itself and the reduction in resistance due to the effect of reducing the amount of the second solvent by adding magnesium fluoride, it is estimated that the resistance of the SEI can be reduced comprehensively when the amount of magnesium fluoride added is 0.3% by mass or more and 0.6% by mass or less.

[0021] The first solvent is not particularly limited as long as it has a dielectric constant of less than 10, and for example, a chain carbonate can be used. Examples of the chain carbonate include dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), and two or more of them may be used in combination.

[0022] The second solvent is not particularly limited as long as it has a dielectric constant greater than 50, and for example, a cyclic carbonate can be used. Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), and vinylene carbonate (VC), and two or more of them may be used in combination.

[0023] The lithium salt is not particularly limited, but examples thereof include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), and lithium bis(fluorosulfonyl)imide (LiFSI), and two or more of these may be used in combination.

[0024] The electrolyte solution of this embodiment may further contain an additive such as a thickener.

[0025] The thickener is not particularly limited, but examples thereof include carboxymethyl cellulose (CMC), polyethylene oxide (PEO), polyvinylidene fluoride (PVdF), etc., and two or more of these may be used in combination.

[0026] The viscosity of the electrolyte solution of this embodiment may be 5000 mPa·s or more, which improves the handling properties of the electrolyte solution during production.

[0027] [Lithium-ion secondary battery] The lithium ion secondary battery of this embodiment includes the electrolyte solution of this embodiment. The lithium ion secondary battery of this embodiment is not particularly limited, but may, for example, include a positive electrode current collector, a positive electrode composite layer, a separator, a negative electrode composite layer, and a negative electrode current collector, which are stacked in this order, and the separator is filled with the electrolyte solution.

[0028] The positive electrode current collector is not particularly limited, but examples thereof include metal foils such as aluminum foils.

[0029] The positive electrode mixture layer contains a positive electrode active material, and may further contain a solid electrolyte, a conductive additive, a binder, and the like, as necessary.

[0030] The positive electrode active material is not particularly limited as long as it can absorb and release lithium ions. For example, 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 / 3Co 1 / 3 Mn 1 / 3 )O 2、 Examples of the positive electrode active material include LiCoO4, LiMn2O4, LiNiO2, LiFePO4, lithium sulfide, sulfur, etc. These positive electrode active materials may be used alone or in combination.

[0031] The negative electrode active material is not particularly limited as long as it is capable of absorbing and releasing lithium ions, and examples thereof include metallic lithium, lithium alloys, metal oxides, metal sulfides, metal nitrides, Si, SiO, carbon materials, etc. These negative electrode active materials may be used alone or in combination.

[0032] Examples of carbon materials include artificial graphite, natural graphite, hard carbon, and soft carbon.

[0033] The separator is preferably a high-rate separator so that the separator itself does not have a large resistance. The structure is not particularly limited, but examples thereof include a microporous sheet separator. The material is not particularly limited, but examples thereof include polypropylene.

[0034] The lithium ion secondary battery of this embodiment can be manufactured by a known method.

[0035] 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]

[0036] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0037] [Example 1] LiPF6 was dissolved in an EC-DMC mixed solvent (EC: 9% by volume) to a concentration of 1 M, and then MgF2 was dissolved therein to a concentration of 0.6% by mass to obtain an electrolyte solution.

[0038] [Example 2] LiPF6 was dissolved in an EC-DMC mixed solvent (EC: 12% by volume) to a concentration of 1 M, and then MgF2 was dissolved therein to a concentration of 0.3% by mass to obtain an electrolyte solution.

[0039] [Comparative Example 1] LiPF6 was dissolved in an EC-DMC mixed solvent (EC: 33% by volume) to a concentration of 1 M to obtain an electrolyte solution.

[0040] Comparative Example 2 LiPF6 was dissolved in an EC-DMC mixed solvent (EC: 12% by volume) to a concentration of 1 M to obtain an electrolyte solution.

[0041] [Fabrication of lithium-ion secondary batteries] The electrolyte solution of each of the examples and comparative examples was filled into a separator to prepare a lithium ion secondary battery having the following configuration.

[0042] Positive electrode: 16mmφ Positive electrode current collector: 20 μm thick Al foil Positive electrode active material: Lithium cobalt oxide (3mAh) Negative electrode: 16mmφ Negative electrode current collector: 20 μm thick Cu foil Negative electrode active material: graphite (3.6mAh) Separator: Microporous polypropylene sheet The battery was constructed in an argon atmosphere by impregnating a separator with an electrolyte, sandwiching it between positive and negative electrodes, and arranging it in a coin-shaped cell.

[0043] [High rate discharge] A constant current charge / discharge test of the lithium ion secondary battery was carried out under the following conditions. Charging rate: 1C Discharge rate: 20C Temperature: 35℃ Number of cycles: 100 cycles

[0044] 1 and 2 show the initial charge-discharge curves during high-rate discharge of the lithium ion secondary batteries containing the electrolyte solutions of Example 1 and Comparative Example 1, respectively.

[0045] 1 and 2 show that the initial capacity of the lithium ion secondary battery during high-rate discharge increases when the electrolyte solution of Example 1 is used. In contrast, the electrolyte solution of Comparative Example 1 does not contain MgF2, and therefore the initial capacity of the lithium ion secondary battery during high-rate discharge decreases.

[0046] 3 to 6 show the cycle characteristics during high-rate discharge of lithium ion secondary batteries containing the electrolyte solutions of Examples 1 and 2 and Comparative Examples 1 and 2, respectively.

[0047] 3 to 6 show that the initial capacity and capacity retention rate of the lithium ion secondary battery during high-rate discharge are increased when the electrolyte solutions of Examples 1 and 2 are used. In contrast, the electrolyte solutions of Comparative Examples 1 and 2 do not contain MgF2, and therefore the initial capacity and capacity retention rate of the lithium ion secondary battery during high-rate discharge are decreased.

[0048] [High rate charging] A constant current charge / discharge test of the lithium ion secondary battery was carried out under the following conditions. Charging rate: 10C Discharge rate: 1C Temperature: 35℃ Number of cycles: 100 cycles

[0049] 7 and 8 show the initial charge / discharge curves during high-rate charging of the lithium ion secondary batteries containing the electrolyte solutions of Example 1 and Comparative Example 1, respectively.

[0050] 7 and 8 show that the initial capacity of the lithium ion secondary battery during high-rate charging increases when the electrolyte solution of Example 1 is used. In contrast, the electrolyte solution of Comparative Example 1 does not contain MgF2, and therefore the initial capacity of the lithium ion secondary battery during high-rate charging decreases.

[0051] 9 to 12 show the cycle characteristics during high-rate charging of lithium ion secondary batteries containing the electrolyte solutions of Examples 1 and 2 and Comparative Examples 1 and 2, respectively.

[0052] 9 to 12 show that the initial capacity and capacity retention rate of the lithium ion secondary battery during high-rate charging are increased when the electrolyte solutions of Examples 1 and 2 are used. In contrast, the electrolyte solutions of Comparative Examples 1 and 2 do not contain MgF2, and therefore the initial capacity and capacity retention rate of the lithium ion secondary battery during high-rate charging are decreased.

Claims

1. Magnesium fluoride, A lithium salt; A first solvent; a second solvent; a volume ratio of the second solvent to the total amount of the first solvent and the second solvent is 6% or more and 12% or less; the first solvent is dimethyl carbonate; The second solvent is ethylene carbonate.

2. The electrolyte according to claim 1, wherein the magnesium fluoride content is 0.3% by mass or more and 1.0% by mass or less.

3. A lithium ion secondary battery comprising the electrolyte solution according to claim 1 or 2.

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