Lithium metal battery electrolyte

Abstract

To provide a lithium metal battery electrolyte that can easily improve the durability of a lithium metal battery and suppress an increase in battery resistance.SOLUTION: An electrolyte salt includes lithium bis-fluorosulfonylimide (LiFSI), an organic solvent includes 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), and the lithium metal battery electrolyte further includes a first compound that is a polyethylene glycol compound having an average molecular weight of 200 to 1200 and is contained in an amount of 0.015 mass% or more and less than 0.3 mass% in the lithium metal battery electrolyte, and a second compound that is at least one selected from the group consisting of diglyme, triglyme, and tetraglyme and is contained in an amount of 0.1 mass% or more and less than 1.0 mass% in the lithium metal battery electrolyte.SELECTED DRAWING: None

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 decline in battery performance. Known techniques for suppressing dendrite formation include applying an organic or inorganic coating to the negative electrode surface or separator surface (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2020-532077 Summary of the Invention [Problem to be solved by the invention]

[0005] The technology disclosed in Patent Document 1 relates to a lithium electrode that includes an olefin-based ion-conducting polymer as a protective layer formed on at least one side of a lithium metal layer. This technology is said to be able to minimize the formation and growth of lithium dendrites. However, it is difficult to form such a protective layer on a thin lithium anode or a separator, which poses challenges such as difficult quality control during the process and increased production costs. Another challenge is that the protective layer increases battery resistance. In addition to polymers, various additives for inorganic Li salts are also known. However, the more additives are added to protect Li, the more multi-component the battery becomes. The additives' solubility decreases, causing them to precipitate and become cloudy. Increasing the Ni ratio in the positive electrode active material improves the battery's energy density, but positive electrode additives are required to suppress positive electrode degradation and gas generation. The more additives are added, the higher the viscosity of the electrolyte, which impairs the diffusion of Li ions and reduces the battery's output and durability. Striking a balance between these two factors is extremely difficult.

[0006] The present invention has been made in view of the above-mentioned problems, and aims to provide an electrolyte for a lithium metal battery that can easily improve the durability of the lithium metal battery and suppress an increase in battery resistance. [Means for solving the problem]

[0007] (1) A lithium metal battery electrolyte solution comprising an electrolyte salt and an organic solvent, wherein the electrolyte salt comprises lithium bis-fluorosulfonylimide (LiFSI), the organic solvent comprises 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), and the lithium metal battery electrolyte solution further comprises a first compound which is a polyethylene glycol compound having an average molecular weight of 250 to 1000, and a second compound which is at least one selected from the group consisting of diglyme, triglyme, and tetraglyme, wherein the first compound is contained in the lithium metal battery electrolyte solution at a concentration of 0.015% by mass or more and less than 0.3% by mass, and the second compound is contained in the lithium metal battery electrolyte solution at a concentration of 0.1% by mass or more and less than 0.3% by mass.

[0008] According to the invention (1), it is possible to provide an electrolyte for a lithium metal battery that can easily improve the durability of the lithium metal battery and can also suppress an increase in battery resistance.

[0009] (2) The lithium metal battery electrolyte solution according to (1), wherein the concentration of lithium bis-fluorosulfonylimide (LiFSI) as the electrolyte salt in the lithium metal battery electrolyte solution is 1.5 mol / L or more and 2.5 mol / L or less.

[0010] According to the invention (2), the durability of the lithium metal battery can be easily improved even in a highly concentrated electrolyte solution, and an increase in the battery resistance can be suppressed.

[0011] (3) The lithium metal battery electrolyte solution according to (1) or (2), wherein the molar ratio of 1,2-dimethoxyethane (DME) as the organic solvent to lithium bis-fluorosulfonylimide (LiFSI) as the electrolyte salt, DME / LiFSI, is 1.5 to 2.3, and the mass proportion of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as the organic solvent relative to the total amount of the lithium bis-fluorosulfonylimide (LiFSI) and the 1,2-dimethoxyethane (DME) is 40 mass% or more and 60 mass% or less.

[0012] According to the invention of (3), even if the amount of TTE, a non-polar solvent, is increased beyond a certain level, i.e., even if the relative amount of DME is reduced, the electrolyte of the present invention can dissolve multiple inorganic additives and multiple organic additives, and therefore, the precipitation of LiFSI and inorganic additives can be suppressed.

[0013] (4) The electrolyte solution for a lithium metal battery according to any one of (1) to (3), wherein the first compound is represented by the following formula (1): [ka] (n in the above formula (1) represents an integer of 5 to 20.)

[0014] According to the invention (4), the durability of the lithium metal battery can be preferably improved by the thin film formed on the surface of the negative electrode by the first compound.

[0015] (5) The electrolyte solution for a lithium metal battery according to (4), wherein n=6 in the formula (1).

[0016] According to the fifth aspect of the present invention, the durability of the lithium metal battery can be favorably improved by the thin film formed on the surface of the metallic lithium negative electrode by the first compound.

[0017] (6) The electrolyte solution for a lithium metal battery according to any one of (1) to (5), wherein the second compound is diglyme or triglyme.

[0018] According to the sixth aspect of the present invention, the second compound controls the amount of lithium eluted during charging and discharging, thereby favorably improving the durability of the lithium metal battery.

[0019] (7) The electrolyte solution for a lithium metal battery according to any one of (1) to (6), further comprising FOB (difluoro(oxalato)borate) anions and Ca (calcium) cations.

[0020] According to the invention (7), even when the fluorinated ether ratio is about 50%, the solubility of the additive in the high-concentration electrolyte is high, and therefore, a synergistic effect according to the type of additive, such as suppressing a decrease in the capacity retention rate of the battery, can be preferably obtained. DETAILED DESCRIPTION OF THE INVENTION

[0021] <Electrolyte for lithium metal batteries> The lithium metal battery electrolyte solution according to this embodiment contains an electrolyte salt, an organic solvent, a first compound, and a second compound.

[0022] The electrolyte salt essentially contains lithium bis-fluorosulfonylimide (LiFSI). The electrolyte salt may contain a lithium salt other than LiFSI. Examples of lithium salts include LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC(CF3SO2)3, LiN(CF3SO2)2 (LiTFSI), and LiBC4O8. These may be used alone or in combination of two or more. It is preferable that only LiFSI is contained as the electrolyte salt.

[0023] The concentration of LiFSI as the electrolyte salt in the lithium metal battery electrolyte is preferably around 2.0 mol / L, more preferably 1.5 mol / L to 2.5 mol / L, and even more preferably 2.0 mol / L or more. The lithium metal battery electrolyte according to this embodiment can form a thin film on the negative electrode without adding a polymer to the electrolyte, thereby suppressing an increase in the viscosity of the electrolyte. Therefore, even when the concentration of the electrolyte salt or the concentration of the inorganic compound additive is increased, an increase in the viscosity of the electrolyte can be suppressed, thereby suppressing a decrease in the discharge output and charge performance of the lithium metal battery. The LiFSI concentration may be 1.0 mol / L to 2.6 mol / L.

[0024] The viscosity of the lithium metal battery electrolyte solution according to this embodiment is preferably in the range of 5 mPa·s to 12 mPa·s.

[0025] The organic solvent is primarily a chain ether and a chain fluorinated ether. Specifically, 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) are essential. Other organic solvents may also be included. Examples of chain ethers include 1,2-diethoxyethane, diethyl ether, and 1-ethoxy-2-(2-methoxyethoxy)ethane.

[0026] An example of the chain fluorinated ether is a compound represented by the following formula (2). R1 -OR 2 ···(2) In formula (2), R 1 and R 2 R each 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. For example, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether can be mentioned.

[0027] In this embodiment, the chain ethers and chain fluorinated ethers may be used alone or in combination of two or more. Furthermore, organic solvents other than the chain ethers and chain fluorinated ethers may be included. Examples include ethylene carbonate (EC), propylene carbonate (PC), sulfolane (SL), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). These organic solvents other than the chain ethers and chain fluorinated ethers may be used alone or in combination of two or more. These organic solvents other than the chain ethers and chain fluorinated ethers may be included in an amount of 50 mol % or less based on the total amount of the chain ethers and chain fluorinated ethers.

[0028] The organic solvent preferably contains only 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 ratio of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as the organic solvent to the total amount of lithium bis-fluorosulfonylimide (LiFSI) and 1,2-dimethoxyethane (DME) (TTE / (LiFSI+DME)) is preferably 40% by mass or more and 60% by mass or less.

[0029] The first compound is a component that forms a thin film on the negative electrode. By including the first compound in the lithium metal battery electrolyte, a thin film layer with a thickness of about 1 μm is formed between the negative electrode (lithium metal layer) of the lithium metal battery and the lithium metal deposited on the negative electrode. This thin film suppresses the formation of dendrites, thereby improving the durability of the lithium metal battery.

[0030] The first compound is a polyethylene glycol compound having an average molecular weight of 250 to 1000. In this specification and claims, a polyethylene glycol compound is a compound obtained by polymerizing ethylene oxide, and includes those having a linear or branched structure. The polyethylene glycol compound may optionally have a polar substituent. Examples of the substituent include, but are not limited to, a hydroxyl group, a carboxyl group, a thiol group, and a nitrile group. The alkyl group includes a linear alkyl group and a branched alkyl group. Different types of first compounds can be used in combination.

[0031] The first compound is, for example, a compound represented by the following formula (2).

[0032] [ka]

[0033] In the above formula (2), n represents an integer, and R represents an optional substituent.

[0034] The first compound is preferably a compound represented by the following formula (1).

[0035] [ka]

[0036] In the above formula (1), n ​​represents an integer of 5 to 20. In the above formula (1), n=6 is preferred.

[0037] The first compound is contained in the lithium metal battery electrolyte in an amount of 0.015% by mass or more and less than 0.3% by mass, and the content of the first compound is preferably 0.015% by mass or more and less than 0.1% by mass.

[0038] The second compound has the function of adjusting the amount of lithium eluted from the lithium metal negative electrode during charging and discharging, and this function of the second compound can improve the durability of the lithium metal battery.

[0039] The second compound is at least one selected from the group consisting of diglyme (diethylene glycol dimethyl ether), triglyme (triethylene glycol dimethyl ether), and tetraglyme (tetraethylene glycol dimethyl ether). These compounds may be used alone or in combination. The second compound is preferably tetraglyme.

[0040] The second compound is contained in the lithium metal battery electrolyte in an amount of 0.1% by mass or more and less than 1.0% by mass, and the content of the second compound is preferably 0.05% by mass or more and less than 0.25% by mass.

[0041] The lithium metal battery electrolyte solution according to this embodiment may contain substances other than those described above. For example, it is preferable that the electrolyte solution contains an additive. An inorganic oxide of an alkali metal can be used as the additive. The synergistic effect of the inorganic oxide of an alkali metal and the organic solvent allows a lithium metal battery to have a high capacity retention rate.

[0042] The lithium metal battery electrolyte according to this embodiment preferably contains FOB (difluoro(oxalato)borate) anions and Ca (calcium) cations as additives. This allows for the removal of residual moisture and stabilization of Li precipitation. Sources of FOB (difluoro(oxalato)borate) anions include, for example, LiFOB and Ca(FOB)2. The additives are preferably added in an amount of 0.1 wt% to 2 wt% to the electrolyte. Sources of Ca (calcium) cations include, for example, Ca(TFSI)2, Ca(FSI)2, and Ca(BF4)2. When LiFOB (lithium difluoro(oxalato)borate) is used as the additive, its content in the electrolyte is preferably 0.1 wt % to 2 wt %. In addition to the additives described above, the additive may also contain known components used in electrolytes for lithium metal batteries. Examples of such additives include film-forming materials and dispersants. Specific examples of such additives include 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 propane sultone and ethylene sulfide; and nitrile compounds such as acetotrile, adiponitrile, butyronitrile, and diphenyl sulfide.

[0043] <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.

[0044] 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 r Examples of the positive electrode active material include lithium manganate (LiMnO), a hetero-element-substituted Li-Mn spinel represented by LiMnO (p+q+r=1), lithium titanate (an oxide containing Li and Ti), and lithium metal phosphate (LiMPO, M=at least one selected from Fe, Mn, Co, and Ni) represented by LiMnO (x+y=2, M=Al, Mg, Co, Fe, Ni, and Zn). The positive electrode layer may contain a binder, a conductive additive, and the like 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 made of a material that can be used as a positive electrode current collector in a lithium metal battery.

[0045] The negative electrode layer is a layer containing a negative electrode active material. For example, lithium metal or a lithium alloy may be used alone, or a mixture of these may 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. In addition to the above, a composite in which carbon or an organic substance is combined with lithium metal may also be used. A negative electrode current collector may also 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 a lithium metal battery.

[0046] The electrolyte layer contains the electrolyte solution according to the above embodiment. The electrolyte layer may be formed by impregnating a separator that prevents short circuits between the positive electrode and the negative electrode with the electrolyte solution. 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.

[0047] 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]

[0048] The present invention will be described in detail below using examples, but the present invention is not limited to these examples.

[0049] [Preparation of electrolyte for lithium metal batteries] The electrolyte solutions for lithium metal batteries according to the examples and comparative examples were prepared according to the formulations shown in Tables 1 and 2 below.

[0050] [Table 1]

[0051] [Table 2]

[0052] In Tables 1 and 2, "PEG350" means polyethylene glycol with an average molecular weight of 350, and "PEG2000" means polyethylene glycol with an average molecular weight of 2000. Furthermore, "G2" means diglyme, "G3" means triglyme, and "G4" means tetraglyme. The content (mass%) refers to the content in the lithium metal battery electrolyte. The molar ratio of DME and TTE refers to the molar ratio relative to the total amount of DME and TTE.

[0053] [Preparation of test cell] Test cells were prepared using the electrolyte solutions according to the above examples and comparative examples according to the following procedure.

[0054] (Preparation of positive electrode) 2 wt% of acetylene black (AB) as an electron conductive material, 1.5 wt% of 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 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 had a median diameter of 4 μm. The resulting positive electrode paste was then applied to an aluminum positive electrode current collector without a primer layer, dried, and pressed with a roll press to obtain a 64 μm thick electrode composite layer with a density of 3.3 g / cm. 3 The cathode was then dried in a vacuum at 120°C to form a cathode plate having a cathode composite layer. The resulting cathode plate was punched out to a size of 30 mm x 40 mm to form a cathode.

[0055] (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.

[0056] (Preparing the separator) An alumina-coated polyethylene microporous membrane was used as the separator. The electrolyte solutions shown in Table 1 were used.

[0057] (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 350 μl of electrolyte was then poured into it.The container was then left at 45°C for 5 hours and subjected to two 0.1 C charge-discharge cycles (4.3 V to 2.65 V) to produce a lithium metal battery.

[0058] [Capacity retention rate measurement] The capacity retention rate was measured using test cells prepared using the electrolyte solutions of the above examples and comparative examples. CCCV charging was performed at 25°C at a charge rate of 0.33C (1 / 3C) to 4.3V, followed by CV charging for 20 minutes. After leaving the cells for 10 minutes, they were discharged at 0.33C to 2.65V. This was taken as 100% battery capacity.

[0059] [BOL resistivity measurement] The BOL resistivity was measured using test cells prepared using the electrolyte solutions according to the above examples and comparative examples. Specifically, the cells were charged to 50% of the discharge capacity at the time of initial capacity measurement, to obtain an SOC of 50%. From this voltage, a 4.5 C discharge was performed for 10 seconds, and the resistance value was calculated. This resistance value was measured on a positive electrode with an electrode area of ​​12 cm. 2 Divide by the resistivity Ω cm 2 The results are shown in Tables 1 and 2.

[0060] [Capacity retention rate evaluation] The battery was subjected to 99 charge-discharge cycles from 4.3 V to 2.65 V in a thermostatic chamber at 25°C, with an upper limit voltage of 4.3 V, charging at 0.33 C, a lower limit voltage of 2.65 V, and discharging at 0.33 C. After discharge, the battery was left for 6 hours, and the 100th rated capacity measurement was performed to calculate the capacity retention rate.

[0061] As shown in Tables 1 and 2, the test cells prepared using the electrolyte solutions according to the examples exhibited preferable capacity retention rates compared to the comparative examples. It is also clear that the battery resistance can be reduced by using the first compound within a predetermined molecular weight range.

Claims

1. An electrolyte solution for a lithium metal battery comprising an electrolyte salt and an organic solvent, the electrolyte salt comprises lithium bis-fluorosulfonylimide (LiFSI); the organic solvent comprises 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE); Further, the composition comprises a first compound which is a polyethylene glycol compound having an average molecular weight of 250 to 1200, and a second compound which is at least one selected from the group consisting of diglyme, triglyme, and tetraglyme, the first compound is contained in the lithium metal battery electrolyte solution in an amount of 0.015 mass % or more and less than 0.3 mass %, The lithium metal battery electrolyte solution contains the second compound in an amount of 0.05 mass % or more and less than 0.3 mass %.

2. 2. The lithium metal battery electrolyte solution according to claim 1, wherein the concentration of lithium bis-fluorosulfonylimide (LiFSI) as the electrolyte salt in the lithium metal battery electrolyte solution is 1.5 mol / L or more and 2.5 mol / L or less.

3. the molar ratio of lithium bis-fluorosulfonylimide (LiFSI) as the electrolyte salt to 1,2-dimethoxyethane (DME) as the organic solvent: DME / LiFSI is 1.5 to 2.3; 3. The lithium metal battery electrolyte solution according to claim 1, wherein the mass ratio of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as the organic solvent to the total amount of the lithium bis-fluorosulfonylimide (LiFSI) and the 1,2-dimethoxyethane (DME) is 40 mass% or more and 60 mass% or less.

4. 3. The electrolyte solution for a lithium metal battery according to claim 1, wherein the first compound is represented by the following formula (1): 【Chemical 1】 (n in the above formula (1) represents an integer of 5 to 20.)

5. 5. The electrolyte for a lithium metal battery according to claim 4, wherein n=6 in the formula (1).

6. 3. The electrolyte for a lithium metal battery according to claim 1, wherein the second compound is diglyme or triglyme.

7. 3. The electrolyte for a lithium metal battery according to claim 1, further comprising FOB (difluoro(oxalato)borate) anions and Ca (calcium) cations.

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