Electrolyte for lithium metal battery

Abstract

The problem to be solved by the present invention is to provide an electrolyte solution for a lithium metal battery, which can easily improve the durability of the lithium metal battery and can suppress an increase in battery resistance. In order to solve the problem, the present invention provides an electrolyte solution for a lithium metal battery, in which an electrolyte salt contains lithium bis-fluorosulfonyl imide (LiFSI), an organic solvent contains 1, 2-dimethoxyethane (DME), and 1, 1, 2, 2-tetrafluoroethyl-2, 2, 3, 3-tetrafluoropropyl ether (TTE), the electrolyte solution for a lithium metal battery further comprises: a polyethylene glycol compound having an average molecular weight of 200-1200, wherein the electrolyte solution for a lithium metal battery contains 0.015% by mass or more and less than 0.3% by mass of a first compound; and at least one compound selected from the group consisting of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether. The electrolyte solution for a lithium metal battery contains 0.1% by mass or more and less than 1.0% by mass of a second compound.

Description

Technical Field

[0001] The present invention relates to an electrolyte for lithium metal batteries. Background Art

[0002] In recent years, research and development of secondary batteries, which contribute to energy efficiency, has been underway to ensure more people have access to affordable, reliable, sustainable, and advanced energy. Among these secondary batteries, lithium metal batteries, which use lithium metal as the negative electrode, have attracted significant attention.

[0003] As lithium metal batteries charge and discharge, lithium can precipitate from the negative electrode, forming dendrites and increasing porosity, which can degrade battery performance. Known techniques for suppressing these dendrites include applying organic or inorganic coatings to the negative electrode surface or separator surface (e.g., Patent Document 1).

[0004] [Prior Art Literature]

[0005] (Patent Document)

[0006] Patent Document 1: Japanese Patent Application No. 2020-532077 Summary of the Invention

[0007] [Problems to be solved by the invention]

[0008] The technology disclosed in Patent Document 1 relates to a lithium electrode comprising an olefin-based ion-conductive polymer as a protective layer formed on at least one side of a lithium metal layer. It is generally believed that this technology can minimize the formation and growth of lithium dendrites. On the other hand, it is difficult to form the protective layer on a thin lithium negative electrode, or to form a similar protective layer on a separator, resulting in difficulties in process quality management and increased production costs. Furthermore, there is the problem of increased battery resistance due to the protective layer. Various additives for inorganic Li salts are also known, not limited to polymers. However, as several additives are added to protect Li, the solubility of the additives decreases, and the additives precipitate, resulting in turbidity. Furthermore, if the Ni ratio of the positive electrode active material increases, the energy density of the battery can be increased, but a positive electrode additive is required to suppress degradation of the positive electrode and gas generation. As the additives become more diverse, the viscosity of the electrolyte increases, the diffusivity of Li ions deteriorates, and the battery's output and durability decrease. This poses a very difficult balance.

[0009] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is 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.

[0010] [Technical means to solve the problem]

[0011] (1) An electrolyte for a lithium metal battery, comprising an electrolyte salt and an organic solvent, wherein the electrolyte salt comprises lithium bis-fluorosulfonyl imide (LiFSI), the organic solvent comprises 1,2-dimethoxyethane (DME), and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), and the electrolyte for a lithium metal battery 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 diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether; the electrolyte for a lithium metal battery contains 0.015% by mass or more and less than 0.3% by mass of the first compound, and the electrolyte for a lithium metal battery contains 0.1% by mass or more and less than 0.3% by mass of the second compound.

[0012] According to the invention of (1), it is possible to provide an electrolyte for a lithium metal battery that can easily improve the durability of a lithium metal battery and suppress an increase in battery resistance.

[0013] (2) The electrolyte solution for a lithium metal battery according to (1), wherein the concentration of lithium bis-fluorosulfonyl imide (LiFSI) as the electrolyte salt in the electrolyte solution is 1.5 mol / L or more and 2.5 mol / L or less.

[0014] According to the invention of (2), even with a high-concentration electrolyte, the durability of the lithium metal battery can be easily improved, and an increase in battery resistance can be suppressed.

[0015] (3) An electrolyte for a lithium metal battery according to (1) or (2), wherein the molar ratio of 1,2-dimethoxyethane (DME) as the aforementioned organic solvent to lithium bis-fluorosulfonyl imide (LiFSI) as the aforementioned electrolyte salt: DME / LiFSI is 1.5 to 2.3, and the mass ratio of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as the aforementioned organic solvent to the total amount of the aforementioned lithium bis-fluorosulfonyl imide (LiFSI) and the aforementioned 1,2-dimethoxyethane (DME) is 40% by mass or more and 60% by mass or less.

[0016] According to the invention of (3), even if the amount of TTE as a non-polar solvent is increased to a certain amount or more, that is, even if the relative amount of DME is reduced, the electrolyte of the present invention can dissolve a variety of inorganic additives and a variety of organic additives, thereby suppressing the precipitation of LiFSI and inorganic additives.

[0017] (4) The electrolyte for a lithium metal battery according to any one of (1) to (3), wherein the first compound is represented by the following formula (1):

[0018]

[0019] (n in the above formula (1) represents an integer of 5 to 20).

[0020] According to the invention of (4), the durability of the lithium metal battery can be preferably improved by forming a thin film of the first compound on the surface of the negative electrode.

[0021] (5) The electrolyte solution for lithium metal batteries according to (4), wherein in the above formula (1), n=6.

[0022] According to the invention of (5), the durability of the lithium metal battery can be preferably improved by forming a thin film of the first compound on the surface of the metal lithium negative electrode.

[0023] (6) The electrolyte solution for a lithium metal battery according to any one of (1) to (5), wherein the second compound is diethylene glycol dimethyl ether or triethylene glycol dimethyl ether.

[0024] According to the invention of (6), the amount of lithium elution during charge and discharge is controlled by the second compound, thereby preferably improving the durability of the lithium metal battery.

[0025] (7) The electrolyte solution for a lithium metal battery according to any one of (1) to (6), further comprising a difluorooxalatoborate (FOB) anion and a Ca (calcium) cation.

[0026] According to the invention of (7), the electrolyte of the present invention has a high solubility of the additive in the high-concentration electrolyte even when the fluorinated ether ratio is about 50%, so a synergistic effect corresponding to the type of additive, such as suppressing the decrease in the battery capacity retention rate, can be preferably obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] none DETAILED DESCRIPTION

[0028] <Electrolyte for lithium metal batteries>

[0029] The electrolyte solution for a lithium metal battery according to this embodiment includes an electrolyte salt, an organic solvent, a first compound, and a second compound.

[0030] As an electrolyte salt, lithium bis-fluorosulfonyl imide (LiFSI) must be included. As an electrolyte salt, lithium salts other than LiFSI may also be included. Examples of lithium salts include LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC(CF3SO2)3, LiN(CF3SO2)2 (LiTFSI), and LiBC4O8. These can be used alone or in combination of two or more. As an electrolyte salt, it is preferred to include only LiFSI.

[0031] The concentration of LiFSI as an electrolyte salt in the lithium metal battery electrolyte is preferably around 2.0 mol / L, more preferably 1.5 mol / L or more and 2.5 mol / L or less and 2.0 mol / L or more. The lithium metal battery electrolyte of this embodiment can form a thin film on the negative electrode without adding a polymer to the electrolyte, so the increase in the viscosity of the electrolyte can be suppressed. Thus, even when the concentration of the electrolyte salt and the concentration of the inorganic compound additive are increased, the increase in the viscosity of the electrolyte can be suppressed, thereby reducing the discharge output and charging performance of the lithium metal battery. The above-mentioned concentration of LiFSI can also be 1.0 mol / L or more and 2.6 mol / L or more.

[0032] The viscosity of the electrolyte solution for lithium metal batteries according to the present embodiment is preferably in the range of 5 mPa·s to 12 mPa·s.

[0033] The organic solvent is primarily composed of chain ethers and chain fluorinated ethers. 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. For example, examples of chain ethers include 1,2-diethoxyethane, diethyl ether, and 1-ethoxy-2-(2-methoxyethoxy)ethane.

[0034] Examples of the chain fluorinated ether include compounds represented by the following formula (2).

[0035] R 1 -OR 2 ···(2)

[0036] In formula (2), R 1 and R 2 Each independently represents a fluorinated hydrocarbon group such as a fluorinated alkyl group. 1 and R 2 The number of carbon atoms in is not particularly limited, and may be, for example, 1 to 8. For example, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether can be mentioned.

[0037] In the present embodiment, above-mentioned chain ether and chain fluorinated ether can use one or be used in combination with two or more.In addition, the organic solvent beyond chain ether and chain fluorinated ether can also be included.For example, ethylene carbonate (EC), propylene carbonate (PC), sulfolane (SL), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) etc. can be enumerated.The organic solvent beyond these chain ether and chain fluorinated ether can use one or be used in combination with two or more.Also can be relative to the total amount of chain ether and chain fluorinated ether, for the ratio below 50 mole % containing the organic solvent beyond these chain ether and chain fluorinated ether.

[0038] As an organic solvent, it is preferred that only 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) be included. In this case, the molar ratio of 1,2-dimethoxyethane (DME) as an organic solvent to lithium bis-fluorosulfonyl imide (LiFSI) as an electrolyte salt: DME / LiFSI is preferably 1.5 to 2.3. In addition, the mass ratio (TTE / (LiFSI+DME)) of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as an organic solvent to the total amount of lithium bis-fluorosulfonyl imide (LiFSI) and 1,2-dimethoxyethane (DME) is preferably 40% by mass or more and 60% by mass or less.

[0039] 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 approximately 1 μm thick 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 dendrite formation and improves the durability of the lithium metal battery.

[0040] The first compound is a polyethylene glycol compound having an average molecular weight of 250 to 1000. In this specification and claims, the so-called polyethylene glycol compound is a compound obtained by polymerizing ethylene oxide, including compounds having a linear or branched structure. The polyethylene glycol compound may also have a polar substituent. There are no particular limitations on the substituents, and examples thereof include hydroxyl groups, carboxyl groups, thiol groups, and nitrile groups. The alkyl groups mentioned above include linear alkyl groups and branched alkyl groups. As the first compound, different types of compounds may be used in combination.

[0041] The first compound is, for example, a compound represented by the following formula (2).

[0042]

[0043] In the above formula (2), n represents an integer, and R represents an arbitrary substituent.

[0044] The first compound is preferably a compound represented by the following formula (1).

[0045]

[0046] In the above formula (1), n ​​represents an integer of 5 to 20. In the above formula (1), n=6 is preferred.

[0047] The lithium metal battery electrolyte contains 0.015% by mass or more and less than 0.3% by mass of the first compound. The content of the first compound is preferably 0.015% by mass or more and less than 0.1% by mass.

[0048] The second compound has the function of adjusting the amount of lithium eluted from the lithium metal negative electrode during charge and discharge. This function of the second compound can improve the durability of the lithium metal battery.

[0049] The second compound is at least one selected from the group consisting of diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), and tetraethylene glycol dimethyl ether (tetraglyme). These compounds may be used alone or in combination of two or more. The second compound is preferably tetraethylene glycol dimethyl ether.

[0050] The lithium metal battery electrolyte contains 0.1% by mass or more and less than 1.0% by mass of the second compound, preferably 0.05% by mass or more and less than 0.25% by mass.

[0051] The lithium metal battery electrolyte of this embodiment may also contain substances other than those listed above. For example, it is preferable to include an additive. Alkali metal inorganic oxides can be used as additives. The synergistic effect of the alkali metal inorganic oxide and the organic solvent described above can produce a lithium metal battery with a high capacity retention rate.

[0052] The lithium metal battery electrolyte of this embodiment preferably contains FOB (difluorooxalatoborate) anions and Ca (calcium) cations as additives. This removes residual moisture and stabilizes lithium precipitation. Examples of sources of FOB (difluorooxalatoborate) anions include LiFOB and Ca(FOB)2. The amount of these additives added to the electrolyte is preferably 0.1 wt% to 2 wt%. Examples of sources of Ca (calcium) cations include Ca(TFSI)2, Ca(FSI)2, and Ca(BF4)2. When LiFOB (lithium difluorooxalatoborate) is used as an additive, its content in the electrolyte is preferably 0.1 wt% to 2 wt%. In addition to the above, known components used in lithium metal battery electrolytes may also be included as additives. Examples include film-forming materials and dispersants. Specifically, it may also include: Mg(FSI)2, Mg(TFSI)2, Ba(FSI)2, Ba(TFSI)2, Zn(FSI)2, Zn(TFSI)2, LiTFSI, LiNO3, lithium nitrite, LiPO2F2, CsPF6, lithium sulfate as alkali metal salts; propane sultone, ethylene sulfide as organic additives; acetonitrile, adiponitrile, butyronitrile, diphenyl sulfide, etc. as nitrile compounds.

[0053] Lithium Metal Batteries

[0054] The electrolyte of this embodiment forms a lithium metal battery. The specific structure of the lithium metal battery is not particularly limited except for the electrolyte, and any structure used in known lithium metal batteries can be used without restriction. In a typical embodiment, the lithium metal battery comprises a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer. The electrolyte layer can contain the electrolyte of this embodiment.

[0055] 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 can be used as a positive electrode active material for a lithium metal battery. Examples of positive electrode active materials include layered active materials containing lithium, spinel-type active materials, olivine-type active materials, and the like. Specific examples of positive electrode active materials include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), LiNiO2, and the like. p Mn q Co r O2 (p+q+r=1), LiNi p Al q Co rO2 (p+q+r=1), lithium manganate (LiMn2O4), heterogeneous element substituted Li-Mn spinel represented by Li1+xMn2-x-yMyO4 (x+y=2, M is selected from at least one of Al, Mg, Co, Fe, Ni and Zn), lithium titanate (an oxide containing Li and Ti), lithium metal phosphate (LiMPO4, M is selected from at least one of Fe, Mn, Co and Ni), etc. The positive electrode layer may also contain a binder, a conductive additive, etc. in addition to the positive electrode active material. In addition, a positive electrode collector may be arranged adjacent to the positive electrode layer. As the positive electrode collector, there is no particular limitation as long as it is a material that can be used as a positive electrode collector for a lithium metal battery.

[0056] The negative electrode layer is a layer containing a negative electrode active material. As the negative electrode active material, for example, a substance using lithium metal or a lithium alloy alone or a mixture of these can be used. 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 material in which carbon or an organic matter is compounded in lithium metal can also be used. In addition, a negative electrode collector can also be arranged adjacent to the negative electrode layer. As the negative electrode collector, there is no particular limitation as long as it is a material that can be used as a negative electrode collector for a lithium metal battery.

[0057] The electrolyte layer comprises the electrolyte solution of the above embodiment. As the electrolyte layer, a separator that prevents short circuits between the positive and negative electrodes may be impregnated with the electrolyte solution to form the electrolyte layer. As the separator, materials known as separators for lithium metal batteries, such as nonwoven fabrics and microporous membranes, may be used.

[0058] While 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 that can achieve the object of the present invention are encompassed by the present invention.

[0059] [Example]

[0060] Hereinafter, the present invention will be described in detail using examples, but the present invention is not limited to these examples.

[0061] [Preparation of electrolyte for lithium metal batteries]

[0062] According to the formulations shown in Tables 1 and 2 below, electrolytes for lithium metal batteries of the respective Examples and Comparative Examples were prepared.

[0063] [Table 1]

[0064]

[0065] [Table 2]

[0066]

[0067] In Tables 1 and 2, "PEG350" refers to polyethylene glycol with an average molecular weight of 350, and "PEG2000" refers to polyethylene glycol with an average molecular weight of 2000. Furthermore, "G2" refers to diethylene glycol dimethyl ether, "G3" refers to triethylene glycol dimethyl ether, and "G4" refers to tetraethylene glycol dimethyl ether. The content (mass %) refers to the content in the lithium metal battery electrolyte. The molar ratio of DME to TTE refers to the molar ratio relative to the total amount of DME and TTE.

[0068] [Preparation of test monomers]

[0069] Using the electrolyte solutions of the above-described Examples and Comparative Examples, test cells were produced according to the following procedures.

[0070] (Production of positive electrode)

[0071] 2 wt% of acetylene black (AB) as an electronic conductive material, 1.5 wt% of polyvinylidene fluoride (PVDF) as a binder (adhesive), and polyvinyl pyrrolidone (PVP) as a dispersant were pre-mixed in N-methyl-2-pyrrolidone (NMP) as a dispersion solvent and wet-mixed using a rotary mixer to obtain a pre-mixed slurry. 0.8 Co 0.1 Mn 0.1 O2 (NCM811) and pre-doped materials were mixed with the obtained pre-mixed slurry and dispersed using a planetary mixer to obtain a positive electrode paste. The median particle size of NCM811 was 4 μm. Next, the obtained positive electrode paste was applied to an aluminum positive electrode collector without a primer layer, dried, and pressurized by roller pressing to obtain an electrode mixture layer with a thickness of 64 μm and a density of 3.3 g / cm 3 The positive electrode was then dried in a vacuum at 120°C to form a positive electrode plate with a positive electrode mixture layer. The resulting positive electrode plate was punched into a size of 30 mm x 40 mm to form a positive electrode.

[0072] (Preparation of negative electrode)

[0073] A cladding material of 10 μm thick copper foil and 20 μm thick lithium foil was used as the negative electrode. This was punched out to a size of 34 mm × 44 mm to produce the negative electrode.

[0074] (Preparation of diaphragm)

[0075] A polyethylene microporous membrane coated with aluminum oxide was used as the separator, and the electrolytes listed in Table 1 were used.

[0076] (Manufacturing of lithium metal batteries)

[0077] A lithium metal battery was prepared by heat-sealing a secondary battery aluminum laminate (manufactured by Dai Nippon Printing) into a bag-shaped container. The positive electrode, separator, and negative electrode were then introduced, and 350 μl / well of electrolyte was injected. The container was then left at 45°C for 5 hours and charged and discharged twice at 0.1 C (4.3 V to 2.65 V).

[0078] [Capacity retention rate measurement]

[0079] The capacity retention rate was measured using test cells produced using the electrolyte solutions of the aforementioned Examples and Comparative Examples. The cells were charged to 4.3 V using constant-current constant-voltage (CCCV) charging at 0.33 C (1 / 3 C) at 25°C, followed by CV charging for 20 minutes. After standing for 10 minutes, the cells were discharged at 0.33 C to 2.65 V. This was considered 100% battery capacity.

[0080] [BOL specific resistance measurement]

[0081] The BOL specific resistance was measured using test cells prepared using the electrolyte solutions of the above-mentioned Examples and Comparative Examples. Specifically, the cells were charged to 50% of the discharge capacity during the initial capacity measurement, resulting in a SOC of 50%. At this voltage, a 4.5 C discharge was performed for 10 seconds, and the resistance value was calculated. This resistance value was divided by the positive electrode area of ​​12 cm 2 , calculate the specific resistance Ω･cm 2 The results are shown in Tables 1 and 2.

[0082] [Capacity maintenance rate evaluation]

[0083] In a thermostatic chamber at 25°C, 99 charge-discharge cycles were performed from 4.3 V to 2.65 V, with a maximum voltage of 4.3 V, charge at 0.33 C, a minimum voltage of 2.65 V, and discharge at 0.33 C. After the discharge cycle was completed, the battery was left for 6 hours. The rated capacity was measured at the 100th cycle, and the capacity retention rate was calculated.

[0084] As shown in Tables 1 and 2, the test cells produced using the electrolyte solutions of the Examples achieved better capacity retention than the Comparative Examples. Furthermore, it was found that the battery resistance value can be reduced by setting the first compound to a predetermined molecular weight.

Claims

1. An electrolyte for a lithium metal battery, comprising an electrolyte salt and an organic solvent, The electrolyte salt includes lithium bis-fluorosulfonyl imide (LiFSI), The organic solvent includes 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). The lithium metal battery electrolyte further comprises: a first compound being a polyethylene glycol compound having an average molecular weight of 250 to 1200; and a second compound being at least one selected from the group consisting of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether; The lithium metal battery electrolyte contains 0.015% by mass or more and less than 0.3% by mass of the first compound, The lithium metal battery electrolyte contains 0.05% by mass or more and less than 0.3% by mass of the second compound.

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

3. The electrolyte for lithium metal batteries according to claim 1 or 2, wherein The molar ratio of 1,2-dimethoxyethane (DME) as the organic solvent to lithium bis-fluorosulfonyl imide (LiFSI) as the electrolyte salt is DME / LiFSI, which is 1.5 to 2.

3. 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-fluorosulfonyl imide (LiFSI) and 1,2-dimethoxyethane (DME) is 40 mass % or more and 60 mass % or less.

4. The electrolyte for lithium metal batteries according to claim 1 or 2, wherein The first compound is represented by the following formula (1): (n in the above formula (1) represents an integer of 5 to 20).

5. The electrolyte for lithium metal batteries according to claim 4, wherein In the aforementioned formula (1), n=6.

6. The electrolyte for lithium metal batteries according to claim 1 or 2, wherein The second compound is diethylene glycol dimethyl ether or triethylene glycol dimethyl ether.

7. The electrolyte for lithium metal batteries according to claim 1 or 2, wherein It also contains FOB (difluorooxalatoboric acid) anions and Ca (calcium) cations.

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