Lithium metal battery electrolyte, preparation method thereof and lithium metal battery

Abstract

The invention provides a lithium metal battery electrolyte, a preparation method thereof and a lithium metal battery. The lithium metal battery electrolyte comprises an organic solvent, a diluent and a lithium salt, the diluent is selected from at least one of aryl halide and R1-O-R2, and R1 and R2 independently comprise at least one of H, C, N, O, F and P elements. According to the lithium metal battery electrolyte, the preparation method thereof and the lithium metal battery provided by the embodiment of the invention, the high-concentration electrolyte can have lower viscosity and better wettability, and the stability of a solvation structure of an electrolyte system is improved, so that the coulombic efficiency and the number of cycles of the lithium metal battery can be effectively improved.

Description

Technical Field

[0001] This application relates to the technical field of batteries, and particularly to an electrolyte for a lithium metal battery, a preparation method thereof, and a lithium metal battery. Background Art

[0002] As a medium for ion transport, the electrolyte is essential in all rechargeable batteries. In addition, the electrolyte directly interacts with the cathode and anode between the positive and negative electrodes, determining the interfacial stability between the electrode and the electrolyte and the electrochemical performance of the corresponding battery. The stability of the solid electrolyte interphase (EEI, including SEI and CEI) on the cathode and anode is the key to controlling the electrochemical stable potential window (ESPW) and cycle life. On the other hand, the + desolvation process of Li in the electrolyte is also a key factor in the ion transport rate in the electrochemical process, highly affecting the electrochemical kinetics.

[0003] The electrolyte usually consists of salts, solvents, and / or functional additives, and there are cation-solvent, cation-anion, anion-solvent, and solvent-solvent interactions in the electrolyte. Generally speaking, the cation-solvent interaction is stronger than the cation-anion interaction. The higher the solubility of the lithium salt, the greater the cation transference number. More importantly, the relative interaction strength of these two complexes determines the solvation structure of Li+(solvent-separated ion pair); contact ion pair (CIP); or ion aggregates (AGGs). The interaction between the cation and the solvent enhances the oxidative stability of the electrolyte and weakens its reduction stability, while the further incorporation of anions into the cation-solvent complex has the opposite effect, significantly affecting the interfacial stability, electrochemical window, and cycle life. The cation-solvent and cation-anion interactions are the two main factors affecting the solvation chemistry of electrolytes.

[0004] The current electrochemical performance of the electrolyte is relatively limited, specifically manifested as the batteries prepared have the adverse situations of low Coulomb efficiency and low cycle times. Summary of the Invention

[0005] In order to specifically solve the problems of low Coulomb efficiency and low cycle times existing in the above electrolyte, this application provides an electrolyte for a lithium metal battery, a preparation method thereof, and a lithium metal battery, which can effectively reduce the polarization voltage of the battery and improve the Coulomb efficiency of the battery.

[0006] An embodiment of this application provides an electrolyte for a lithium metal battery, including an organic solvent, a diluent, and a lithium salt; the diluent is selected from at least one of halogenated aromatic hydrocarbons and R1-O-R2, where R1 and R2 each independently include at least one of the elements H, C, N, O, F, and P.

[0007] In the composition of the electrolyte described in the present application, the molar ratio of the non-aqueous organic solvent: diluent: lithium salt is (1 to 2.5): (1.6 to 3): (0.7 to 2).

[0008] In the composition of the electrolyte described in the present application, R1 and R2 in R1-O-R2 are each independently selected from one or more of phenyl, haloalkyl, and heterocyclic groups.

[0009] In the composition of the electrolyte described in the present application, the organic solvent is at least one of R3-O-R4, where R3 and R4 each independently include at least one of the elements H, C, N, O, F, and P, and at least one branch is a methyl substituent.

[0010] In the composition of the electrolyte described in the present application, the diluent molecular structural formula contains at least fluorine; the number of carbon atoms in the main chain where the alkyl chain of the haloalkyl is located does not exceed 10; the haloarene is selected from one or more of fluorobenzene, m-fluorotoluene, and p-fluorotoluene; R1-O-R2 is selected from at least one of trifluoromethoxybenzene, 3-(trifluoromethoxy)fluorobenzene, 3-(trifluoromethoxy)phenol, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 3-(2,2,3,3-tetrafluoropropoxy)-1,2-epoxypropane, bis(2,2,2-trifluoroethyl) ether, 2,2,2-trifluoroethyl formate, tris(2,2,2-trifluoroethyl) orthoformate, 1,1,2,2,-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1-ethoxy-1,1,2,2,2-pentafluoroethane, methoxy nonafluorobutane, and ethoxy nonafluorobutane.

[0011] In the present application, the length of the carbon chain of the main chain where the alkyl chain of the haloalkyl is located should not be too long. Therefore, the number of carbon atoms in the main chain does not exceed 10 here to prevent the carbon chain from being too long, resulting in an increase in steric hindrance of the diluent in the uniformly dispersed solution. Some diluents may participate in the formation of the solvation structure, making it difficult to form a relatively stable solvent structure, which not only prolongs the stirring time but also makes it difficult to achieve a stable state, resulting in the prepared electrolyte not achieving the technical effects of high energy density and high cycle stability.

[0012] In the composition of the electrolyte described in the present application, the lithium salt includes at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium trifluoromethanesulfonate, lithium difluorooxalate borate, lithium bis(oxalate) borate, lithium difluorophosphate, lithium difluorodioxalate phosphate, or lithium tetrafluorooxalate phosphate.

[0013] The lithium salt of the present application needs to be soluble in the corresponding organic solvent and have a small degree of association to ensure high ionic conductivity of the electrolyte; it also needs to be conducive to the formation of a solvation structure, thereby forming a stable low-impedance SEI film.

[0014] Among the components of the electrolyte described in the present application, the organic solvent includes at least one of methyl n-butyl ether, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, ethylene glycol diethyl ether, propylene glycol dimethyl ether, ethylene glycol dibutyl ether, tetrahydrofuran, dimethyltetrahydrofuran, tetrahydropyran, 1,2-dimethoxypropane, 1,3-dimethoxypropane, 1,4-dimethoxybutane, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, fluoroethylene carbonate, ethylene carbonate, vinylene carbonate, methyl butyrate, ethyl propionate, trimethyl phosphate, triethyl phosphate, and γ-butyrolactone.

[0015] The solvent selected for the electrolyte of the present application has an important influence on reducing the polarization voltage of the electrolyte, and its viscosity and dielectric constant also meet the requirements of the movement speed of ions.

[0016] The electrolyte of the present application also has a wide operating temperature range, which is due to the physical properties of the solvent itself and the specific ratio between the components in the electrolyte.

[0017] In the components of the electrolyte described in the present application, the molar ratio of lithium salt: organic solvent: diluent is (1.1~1.5): (2.1~2.5): (0.8~1.8).

[0018] The present application selects weak solvating solvents, medium solvating solvents, and solvents with strong solvating ability, and regulates the size of the interaction between cation-solvent molecules and cation-anions to increase the proportion of CIP and AGG, which is beneficial to the formation of a stable interface layer at the anode and cathode, and reduces the continuous reaction between the electrolyte and lithium, thereby improving the electrochemical performance and safety of lithium metal batteries, and effectively solves the problem of lithium dendrite growth caused by uneven deposition of lithium metal batteries during the cycle process and the continuous reaction of positive and negative electrode materials with the electrolyte, which leads to the dissolution of positive electrode active substances and the formation of dead lithium.

[0019] The lithium metal battery electrolyte provided in the embodiment of the present application can generate different SEI layers when preparing the battery by reasonably adjusting the composition of the electrolyte, thereby effectively reducing the polarization voltage of the battery and improving the coulombic efficiency of the battery.

[0020] Another embodiment of the present application provides a method for preparing a lithium metal battery electrolyte, comprising the following steps: In a glove box filled with protective gas (argon) (water content <10ppm, oxygen content <1ppm), lithium salt is added to an organic solvent and stirred until it is completely dissolved to obtain a lithium metal battery electrolyte.

[0021] In a specific embodiment, a method for preparing an electrolyte for a lithium metal battery includes adding a diluent to an organic solvent in a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), and then adding a lithium salt, and stirring until it is completely dissolved to obtain an electrolyte for a lithium metal battery.

[0022] Another embodiment of the present application provides a lithium metal battery, including the electrolyte for a lithium metal battery described in any one of the above.

[0023] Additional technical solutions and advantages of the present application will be partially given in the following description, partially will become obvious from the following description, or will be understood through the practice of the present application.

[0024] Compared with the prior art, the present invention has at least the following advantages: 1. The electrolyte screened in the present application introduces a solvent with a lower dielectric constant. Compared with traditional electrolytes, the presence of branched chains in the solvent molecules in the present application increases the steric effect of the solvent molecules, thereby reducing the solvation ability, that is, reducing the interaction between Li + -solvent molecules, promoting the interaction between Li + -anions, resulting in the formation of an anion-derived SEI layer, thus achieving a highly reversible deposition / stripping behavior with lithium metal.

[0025] 2. The diluent introduced in the present application can make the high-concentration electrolyte have a lower viscosity and better wettability. When forming a solution, it can increase the ratio of aggregated ion pairs and separated ion pairs. And the weak interaction between the diluent and anions can enhance the stability of the solvation structure of the electrolyte system, and regulate the interaction between lithium ions and the solvation sheath in the local high-concentration electrolyte system by adding the diluent. While improving the stability of the lithium metal negative electrode, the oxidation resistance under high voltage is achieved, and the cycle performance of the lithium metal battery is greatly improved.

[0026] 3. The electrolyte of the present application has good antioxidant stability because the oxidation of the corresponding anion-solvent complex does not involve an intermolecular reaction, and the highest occupied molecular orbital (HOMO) is mainly located on the anion rather than the solvent. This leads to enhanced oxidation stability of the cathode electrolyte interface (CEI) derived from the solvent molecules and anions in the lithium salt, thereby further protecting the electrolyte from decomposition. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those skilled in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0028] Figure 1 It is the cycle number-Coulomb efficiency diagram of the Li-Cu half cell of the battery assembled with the electrolyte in Example 1 of this application; Figure 2 It is the cycle number-Coulomb efficiency diagram of the Li-Cu half cell of the battery assembled with the electrolyte in Example 2 of this application; Figure 3 It is the cycle curve of the 2.3 Ah soft-pack battery provided in Example 1 of this application. The abscissa is the number of cycles, and the ordinate is the capacity and Coulomb efficiency, with the units of mAh and %. Figure 4 It is the cycle curve of the 2.3 Ah soft-pack battery provided in Example 2 of this application. The abscissa is the number of cycles, and the ordinate is the capacity and Coulomb efficiency, with the units of mAh and %. Detailed implementation manners

[0029] The "range" disclosed in this application is defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a specific range. The ranges defined in this way can include the end values or not include the end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of (1~2.5), (1.6~3) and (0.7~2) are listed for a specific parameter, ranges of (1~2) and (1.6~4) are also anticipated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4 and 5 are listed, then the following ranges are all anticipated: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range "a-b" represents the abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been fully listed in this article, and "0-5" is only the abbreviated representation of these numerical combinations. In addition, when it is stated that a certain parameter is an integer ≥2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0030] If there is no special instruction, all implementation manners and optional implementation manners of this application can be combined with each other to form a new technical solution.

[0031] If there is no special instruction, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0032] Unless otherwise specified, all steps of this application can be carried out sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) carried out sequentially, or may also include steps (b) and (a) carried out sequentially. For example, when it is mentioned that the method may further include step (c), it means that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or may also include steps (a), (c), and (b), or may include steps (c), (a), and (b), etc.

[0033] Unless otherwise specified, the terms "comprising" and "including" mentioned in this application mean open-ended, and can also be closed-ended. For example, the "comprising" and "including" may mean that other components not listed can also be included, or may only include the components listed.

[0034] Unless otherwise specified, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) while B is true (or exists); or both A and B are true (or exist).

[0035] An embodiment of this application provides an electrolyte for a lithium metal battery, which includes an organic solvent, a diluent, and a lithium salt; wherein, the diluent is selected from at least one of halogenated aromatic hydrocarbons and R1-O-R2, and wherein, R1 and R2 each independently include at least one of the elements H, C, N, O, F, and P.

[0036] The electrolyte for a lithium metal battery provided by the embodiment of this application can generate different SEI layers when preparing the battery by reasonably adjusting the composition of the electrolyte, thereby effectively reducing the polarization voltage of the battery and improving the Coulomb efficiency of the battery.

[0037] In some embodiments, the molar ratio of the non-aqueous organic solvent: diluent: lithium salt is (1 to 2.5): (1.6 to 3): (0.7 to 2).

[0038] In the components of the electrolyte described in this application, R1 and R2 in R1-O-R2 are each independently selected from one or several of phenyl, halogenated alkyl, and heterocyclic groups.

[0039] In the components of the electrolyte described in this application, the organic solvent is at least one of R3-O-R4, and wherein, R3 and R4 each independently include at least one of the elements H, C, N, O, F, and P, and at least one of the branches is a methyl substituent.

[0040] Among the components of the electrolyte described in this application, the molecular structural formula of the diluent contains at least fluorine element; the number of carbon atoms in the main chain where the alkyl chain of the haloalkyl is located does not exceed 10; the haloarene is selected from one or more of fluorobenzene, m-fluorotoluene, and p-fluorotoluene; R1-O-R2 is selected from at least one of trifluoromethoxybenzene, 3-(trifluoromethoxy)fluorobenzene, 3-(trifluoromethoxy)phenol, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 3-(2,2,3,3-tetrafluoropropoxy)-1,2-epoxypropane, bis(2,2,2-trifluoroethyl) ether, 2,2,2-trifluoroethyl formate, tris(2,2,2-trifluoroethyl) orthoformate, 1,1,2,2,-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1-ethoxy-1,1,2,2,2-pentafluoroethane, methoxy nonafluorobutane, and ethoxy nonafluorobutane.

[0041] The length of the carbon chain of the main chain where the alkyl chain of the haloalkyl in this application is located should not be too long. Therefore, the number of carbon atoms in the main chain here does not exceed 10 to prevent the carbon chain from being too long, resulting in an increase in steric hindrance of the diluent in the homogeneous dispersion solution. Some diluents may participate in the formation of the solvation structure, making it difficult to form a relatively stable solvent structure, which not only prolongs the stirring time but also makes it difficult to achieve a stable state, resulting in the prepared electrolyte not achieving the technical effects of high energy density and high cycle stability.

[0042] Among the components of the electrolyte described in this application, the lithium salt includes at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium trifluoromethanesulfonate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium difluorophosphate, lithium difluoro(dioxalato)phosphate, or lithium tetrafluoro(oxalato)phosphate.

[0043] The lithium salt of this application needs to be soluble in the corresponding organic solvent and have a small degree of association to ensure high ionic conductivity of the electrolyte; it also needs to be convenient for forming a solvation structure and then forming a stable low-impedance SEI film.

[0044] Among the components of the electrolyte described in this application, the organic solvent includes at least one of methyl n-butyl ether, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, ethylene glycol diethyl ether, propylene glycol dimethyl ether, ethylene glycol dibutyl ether, tetrahydrofuran, dimethyltetrahydrofuran, tetrahydropyran, 1,2-dimethoxypropane, 1,3-dimethoxypropane, 1,4-dimethoxybutane, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, fluoroethylene carbonate, ethylene carbonate, vinylene carbonate, methyl butyrate, ethyl propionate, trimethyl phosphate, triethyl phosphate, and γ-butyrolactone.

[0045] The electrolyte of the present application has a wide operating temperature range, which benefits from the physical properties of the solvent itself and the specific ratio between the components in the electrolyte.

[0046] In the composition of the electrolyte described in the present application, the molar ratio of lithium salt: organic solvent: diluent in the composition of the electrolyte is (1.1~1.5):(2.1~2.5):(0.8~1.8).

[0047] Another embodiment of the present application provides a lithium metal battery, including the lithium metal battery electrolyte described in any one of the above.

[0048] Another embodiment of the present application provides a preparation method of a lithium metal battery electrolyte, including the following steps: In a glove box filled with a protective gas (argon) (moisture < 10 ppm, oxygen content < 1 ppm), add a lithium salt to an organic solvent and stir until it is completely dissolved to obtain a lithium metal battery electrolyte.

[0049] In a specific embodiment, the preparation method of the lithium metal battery electrolyte includes adding a diluent to an organic solvent in a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), then adding a lithium salt, and stirring until it is completely dissolved to obtain a lithium metal battery electrolyte.

[0050] Another embodiment of the present application provides a lithium metal battery, including the lithium metal battery electrolyte described in any one of the above.

[0051] The following embodiments more specifically describe the content disclosed in the present application. These embodiments are only for illustrative purposes, because various modifications and changes within the scope of the content disclosed in the present application are obvious to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on the amount of substance, and all reagents and raw materials used in the embodiments are commercially available or can be synthesized according to conventional methods, and the instruments used in the embodiments are all commercially available.

[0052] Example 1 This embodiment provides a preparation method of a lithium metal battery electrolyte, including the following steps: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), take 1.864 g of 1,2-dimethoxypropane (DMP) and 8.168 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) and mix them, then slowly add 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI), that is, mix them in a molar ratio of 1.2:2.35:1, and stir until it is completely dissolved to obtain a lithium metal battery electrolyte, denoted as Example 1.

[0053] Example 2 This embodiment provides a preparation method of a lithium metal battery electrolyte. The difference from Embodiment 1 is that no diluent is added in this embodiment. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), 1.864 g of 1,2-dimethoxypropane (DMP) electrolyte is taken out, and then 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI) is slowly added, that is, mixed at a molar ratio of 1.2:1, and stirred until it is completely dissolved to obtain a lithium metal battery electrolyte, denoted as Embodiment 2.

[0054] Embodiment 3 This embodiment provides a preparation method of a lithium metal battery electrolyte. The difference from Embodiment 1 is the type of solvent. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), 2.118 g of 1,4-dimethoxybutane (DMB) is taken, and then 8.168 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) is added and mixed with it. Then 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI) is slowly added, that is, mixed at a molar ratio of 1.2:2.35:1, and stirred until it is completely dissolved to obtain a lithium metal battery electrolyte, denoted as Embodiment 3.

[0055] Embodiment 4 This embodiment provides a preparation method of a lithium metal battery electrolyte. The difference from Embodiment 3 is that no diluent is added in this embodiment. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), 2.118 g of 1,4-dimethoxybutane (DMB) electrolyte is taken out, and then 8.168 g of lithium bis(fluorosulfonyl)imide (LiFSI) is slowly added, that is, mixed at a molar ratio of 1.2:1, and stirred until it is completely dissolved to obtain a lithium metal battery electrolyte, denoted as Embodiment 4.

[0056] Embodiment 5 This embodiment provides a preparation method of a lithium metal battery electrolyte. The difference from Embodiment 1 is the type of solvent. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), 3.195 g of triethylene glycol dimethyl ether (TGME) is taken, and then 8.168 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) is added and mixed with it. Then 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI) is slowly added, that is, mixed at a molar ratio of 1.2:2.35:1, and stirred until it is completely dissolved to obtain a lithium metal battery electrolyte, denoted as Embodiment 5.

[0057] Example 6 This example provides a method for preparing an electrolyte for a lithium metal battery. The difference from Example 5 is that no diluent is added in this example. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), take 3.195 g of triethylene glycol dimethyl ether (TGME), and then slowly add 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI), that is, mix them in a molar ratio of 1.2:1, and stir until it is completely dissolved to obtain an electrolyte for a lithium metal battery, denoted as Example 6.

[0058] Example 7 This example provides a method for preparing an electrolyte for a lithium metal battery. The difference from Example 1 is the type of solvent. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), take 1.293 g of tetrahydrofuran (THF), then add 8.168 g of 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether (TTE), and then slowly add 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI), that is, mix them in a molar ratio of 1.2:2.35:1, and stir until it is completely dissolved to obtain an electrolyte for a lithium metal battery, denoted as Example 7.

[0059] Example 8 This example provides a method for preparing an electrolyte for a lithium metal battery. The difference from Example 7 is that no diluent is added in this example. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), take 1.293 g of tetrahydrofuran (THF), and then slowly add 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI), that is, mix them in a molar ratio of 1.2:1, and stir until it is completely dissolved to obtain an electrolyte for a lithium metal battery, denoted as Example 8.

[0060] Example 9 This example provides a method for preparing an electrolyte for a lithium metal battery. The difference from Example 1 is the type of solvent. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen < 1 ppm), 1.544 g of 2-methyltetrahydrofuran (2-ME-THF) was taken, then 8.168 g of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) was added, and then 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI) was slowly added, and they were mixed at a molar ratio of 1.2:2.35:1. After stirring until it was completely dissolved, a lithium metal battery electrolyte was obtained, denoted as Example 9.

[0061] Example 10 This example provides a preparation method of a lithium metal battery electrolyte, which is different from Example 1 in the type of solvent. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen < 1 ppm), 1.544 g of tetrahydropyran (THP) was taken, then 8.168 g of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) was added, and then 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI) was slowly added, and they were mixed at a molar ratio of 1.2:2.35:1. After stirring until it was completely dissolved, a lithium metal battery electrolyte was obtained, denoted as Example 10.

[0062] Example 11 This example provides a preparation method of a lithium metal battery electrolyte, which is different from Example 1 in the type of diluent. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen < 1 ppm), 1.864 g of 1,2-dimethoxypropane (DMP) and 9.677 g of ethoxy(pentafluoro)cyclotriphosphazene (PFPN) were mixed, and then 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI) was slowly added, and they were mixed at a molar ratio of 1.2:2.35:1. After stirring until it was completely dissolved, a lithium metal battery electrolyte was obtained, denoted as Example 11.

[0063] Example 12 This example provides a preparation method of a lithium metal battery electrolyte, which is different from Example 1 in the type of diluent. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen < 1 ppm), 1.864 g of 1,2-dimethoxypropane (DMP) and 10.913 g of tris(trifluoroethoxy)methane (TFEO) were mixed, and then 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI) was slowly added, and they were mixed at a molar ratio of 1.2:2.35:1. After stirring until it was completely dissolved, a lithium metal battery electrolyte was obtained, denoted as Example 12.

[0064] Example 13 This example provides a method for preparing an electrolyte for a lithium metal battery. The difference from Example 1 lies in the type of diluent. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), take 1.864 g of 1,2 - dimethoxypropane (DMP) and 7.034 g of 1,1,2,2 - tetrafluoroethyl - 2,2,2 - trifluoroethyl ether (TFTFE) and mix them. Then slowly add 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI), that is, mix them in a molar ratio of 1.2:2.35:1, and stir until it is completely dissolved to obtain an electrolyte for a lithium metal battery, denoted as Example 13.

[0065] Example 14 This example provides a method for preparing an electrolyte for a lithium metal battery. The difference from Example 1 lies in the type of lithium salt added. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), take 1.864 g of 1,2 - dimethoxypropane (DMP) and 8.168 g of 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether (TTE) and mix them. Then slowly add 1.871 g of lithium bis(fluorosulfonyl)imide (LiFSI). After the solution is fully dissolved, add 0.936 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), that is, mix them in a molar ratio of 3.6:7.05:2:1, and stir until it is completely dissolved to obtain an electrolyte for a lithium metal battery, denoted as Example 14.

[0066] Example 15 This example provides a method for preparing an electrolyte for a lithium metal battery. The difference from Example 1 lies in the addition amount of the lithium salt. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), take 1.864 g of 1,2 - dimethoxypropane (DMP) and 8.168 g of 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether (TTE) and mix them. Then slowly add 2.413 g of lithium bis(fluorosulfonyl)imide (LiFSI). After the solution is fully dissolved, mix them in a molar ratio of 1.39:2.73:1, and stir until it is completely dissolved to obtain an electrolyte for a lithium metal battery, denoted as Example 15.

[0067] Example 16 This example provides a method for preparing an electrolyte for a lithium metal battery. The difference from Example 1 lies in the addition amount of the lithium salt. The specific steps are as follows: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), 1.864 g of 1,2-dimethoxypropane (DMP) and 8.168 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) were taken and mixed, and then 3.227 g of lithium bis(fluorosulfonyl)imide (LiFSI) was slowly added. After the solution was fully dissolved, they were mixed in a molar ratio of 1.4:2.04:1 and stirred until completely dissolved to obtain a lithium metal battery electrolyte, denoted as Example 16.

[0068] Comparative Example 1 This comparative example provides a method for preparing a lithium metal battery electrolyte, which is different from Example 1 in the type of lithium salt, and specifically includes the following steps: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), 1.864 g of 1,2-dimethoxypropane (DMP) and 8.168 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) were taken and mixed, and then 4.306 g of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was slowly added. After the solution was fully dissolved, they were mixed in a molar ratio of 1.2:2.35:1 and stirred until completely dissolved to obtain a lithium metal battery electrolyte, denoted as Comparative Example 1.

[0069] Comparative Example 2 This comparative example provides a method for preparing a lithium metal battery electrolyte, which is different from Example 1 in the type of solvent, and includes the following steps: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), 1.615 g of dimethyl carbonate (DMC) and 8.168 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) were taken and mixed, and then 2.806 g of lithium bis(fluorosulfonyl)imide (LiFSI) was slowly added. After the solution was fully dissolved, they were mixed in a molar ratio of 1.2:2.35:1 and stirred until completely dissolved to obtain a lithium metal battery electrolyte, denoted as Comparative Example 2.

[0070] Comparative Example 3 This comparative example provides a method for preparing a lithium metal battery electrolyte, including the following steps: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), 1.864 g of 1,2-dimethoxypropane (DMP) and 3.382 g of fluorobenzene (FB) were taken and mixed, and then 2.806 g of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was slowly added. After the solution was fully dissolved, they were mixed in a molar ratio of 1.2:2.35:1 and stirred until completely dissolved to obtain a lithium metal battery electrolyte, denoted as Comparative Example 3.

[0071] Test Section Li-Cu Half-cell Assembly: Use a circular lithium battery with a diameter of 15.8 mm as the symmetric electrode, a ceramic-coated double-sided film as the separator, and the lithium metal battery electrolytes prepared in all examples and comparative examples as the electrolyte. Assemble CR2032 lithium-lithium button batteries respectively in an argon glove box. Conduct constant current charge-discharge tests on the button symmetric batteries, and continuously cycle 100 weeks at a current density of 0.98 mA. Use a circular lithium sheet with a diameter of 12 mm as the negative electrode, a circular copper sheet with a diameter of 15.8 mm as the positive electrode, a ceramic-coated double-sided film as the separator, and the lithium metal battery electrolytes prepared in all examples and comparative examples as the electrolyte. Assemble CR2032 button lithium-copper button batteries in an argon glove box, and then conduct constant capacity charge-discharge tests by continuously cycling 100 weeks at a capacity density of 3 mAh. The test results are shown in Table 1.

[0072] Full-cell Assembly: To explore its application in actual scenarios, full-cell assembly is carried out. The positive electrode uses one of the common materials on the market, such as lithium iron phosphate (LFP), lithium nickel cobalt manganese oxide (NMC), lithium cobalt oxide (LCO), etc.; the negative electrode uses metallic lithium. Conduct the assembly of an Ah-level soft-pack battery, and conduct electrochemical performance tests under the conditions of a charging rate of 0.2C and a discharging rate of 1C. The attached drawings are the test diagrams of the 2.3 Ah soft-pack batteries corresponding to Examples 1 and 2.

[0073] Table 1. Performance of 2.3 Ah Soft-pack Batteries Prepared from Lithium Metal Battery Electrolytes in Each Example and Comparative Example Example Electrolyte composition Molar ratio Number of cycles Capacity retention rate Example 1 DMP:TTE:LiFSI 1.2：2.35：1 200 98% Example 2 DMP:LiFSI 1.2：1 140 88% Example 3 DMB:TTE:LiFSI 1.2：2.35：1 150 89% Example 4 DMB:LiFSI 1.2：1 120 85% Example 5 TGME:TTE:LiFSI 1.2：2.35：1 130 89% Example 6 TGME:LiFSI 1.2：1 89 84% Example 7 THF:TTE:LiFSI 1.2：2.35：1 120 87% Example 8 THF:LiFSI 1.2:1 80 85% Example 9 2-ME-THF:TTE:LiFSI 1.2：2.35：1 130 88% Example 10 THP:TTE:LiFSI 1.2：2.35：1 150 88% Example 11 DMP:PFPN:LiFSI 1.2：2.35：1 180 86% Example 12 DMP:TFEO:LiFSI 1.2：2.35：1 166 86% Example 13 DMP:TFTFE:LiFSI 1.2：2.35：1 155 88% Example 14 DMP:TTE:LiFSI:LiTFSI 3.6：7.05：2：1 150 89% Example 15 DMP:TTE:LiFSI 1.2：2.35：0.8 164 88% Example 16 DMP:TTE:LiFSI 1.2：2.35：1.2 158 86% Comparative example 1 DMP:TTE:LiTFSI 1.2：2.35：1 70 72% Comparative example 2 DMC:TTE:LiFSI 1.2：2.35：1 100 69% Comparative example 3 DMP:FB:LiFSI 1.2：2.35：1 90 75% 。

[0074] To solve the technical problems of low capacity retention rate and few cycle numbers of existing lithium batteries, the present invention provides an electrolyte. This electrolyte introduces a diluent with weak solvation ability. The diluted electrolyte has lower viscosity, better wettability, and excellent ionic conductivity. These diluents have little influence on the solvation structure of the original electrolyte solvent because they cannot dissolve lithium salts, but have great miscibility with the binary or multicomponent system composed of salt / solvent. Its essence lies in selecting inert solvents and diluents with non-polar structures and low donor abilities, usually hydrohaloethers with high oxidation stability.

[0075] It can be seen from Examples 1-8 that after adding a diluent, the wetting ability of the electrolyte can be improved, the viscosity can be reduced, and thus the ionic conductivity of the electrolyte can be increased. While improving the cycle life of the full cell, the capacity retention rate of the battery is also improved. The electrolyte of Example 1 was selected to assemble the battery. During the assembly of the button cell, a 15.6 mm lithium metal sheet and an aluminized stainless steel battery case were used. The copper foil, separator and lithium metal were assembled into lithium-copper and lithium-lithium half cells in a glove box filled with argon protection, and the above electrolyte was dropped on both sides of the separator. The assembled button cell was tested for Coulombic efficiency, as Figure 1 The deposition capacity was 3.39 mAh / cm2 and the current density was 6.78 mA / cm 2 Under the condition of a large current, it can also stably cycle 178 times, and the Coulombic efficiency remains 99.6%; in contrast, as Figure 2 shown in Example 2, the electrolyte without diluent failed after 88 cycles under the conditions of this deposition capacity and current density. As Figure 3 The electrolyte in Example 1 was applied to a 2.3 Ah soft-pack battery, which stably cycled 200 times at 25 °C room temperature, with a capacity retention rate of 98% and an average Coulombic efficiency of 99.6%; as Figure 4 The electrolyte in Example 2 was applied to a 2.3 Ah soft-pack battery, which stably cycled 140 times at 25 °C room temperature. Although the capacity retention rate was above 88% and the average Coulombic efficiency was 99%, the specific capacity was low. Compared with Example 1, the battery capacity was less than 0.5 Ah at a 1C discharge rate. The above tests show that the addition of a diluent can make lithium ions deposit evenly on the surface of the copper foil, delay the large-area uneven growth of lithium dendrites, and then pierce the separator to cause battery short circuit, realizing the stable deposition and stripping of metallic lithium.

[0076] Comparing the electrochemical data of Comparative Examples 1, 11, 12, 13 and Comparative Example 3, without changing the ratio of lithium salt, solvent, and diluent, and replacing the type of diluent, chain-like hydrofluoroethers (TTE, PFPN, TFEO, TFTFE) and cyclic (FB) are selected, and TTE exhibits excellent electrochemical performance. In the electrolyte system, solvent molecules (ethers, esters, sulfolane, acetonitrile, etc.) can act as electron donors to coordinate with Li+, and the negative charge is mainly concentrated on oxygen and nitrogen atoms. The donor unit has a strong negative surface electrostatic potential, generating a stronger negative potential than the diluent. Due to the presence of fluorine atoms in hydrofluoroethers, the negative charge on the oxygen atom surface is dispersed and is almost evenly distributed on oxygen and fluorine atoms. This means that the diluent is not a satisfactory electron donor, and it also shows that the diluent does not deeply participate in the solvation shell, but it does not mean that it does not participate in the solvation structure at all. To a certain extent, it also affects the overall solvation structure of the electrolyte. Based on Example 1 of the present invention, 5 kinds of hydrofluoroethers are selected as diluents, and long-term cycling tests are carried out on the assembled 2.3 Ah soft-pack batteries respectively. Among them, TTE not only has a certain cost advantage, but also has a significant improvement in electrochemical performance.

[0077] Comparing Examples 1, 15, and 16, it can be found that under the same conditions of solvent and diluent, adjusting the concentration of lithium salt has an impact on the battery cycle life. Generally speaking, the effect of lithium salt: solvent: diluent = (1~2.5): (1.6~3): (0.7~2) is better. As Figure 3 In Example 1, the electrolyte was applied to a 2.3 Ah soft-pack battery, and it stably cycled 200 times at 25 °C room temperature, with a capacity retention rate of 98% and an average Coulombic efficiency of 99.6%; in Example 15 electrolyte at room temperature, the 2.3 Ah soft-pack battery stably cycled 164 times, with a capacity retention rate of 88% and an average Coulombic efficiency of 99.5%; in Example 16 electrolyte at room temperature, the 2.3 Ah soft-pack battery stably cycled 158 times, with a capacity retention rate of 86% and an average Coulombic efficiency of 99.4%; an appropriate lithium salt concentration has a certain impact on the cycle stability of the battery. The above examples can directly prove that the electrolyte constructed in this application has the potential for practical application.

[0078] By comparing Comparative Example 1, 14 and Comparative Example 1, it can be found that by changing the type of lithium salt without changing the concentration, LiFSI is more compatible with ether-based electrolytes. The electrolyte in Example 1 was applied to a 2.3 Ah soft-pack battery and stably cycled 200 times at 25 °C room temperature, with a capacity retention rate of 98% and a Coulombic efficiency of 99.6%. In contrast, Example 14 adopted a dual-salt system and stably cycled 150 times at 25 °C room temperature, with a capacity retention rate of 89% and a Coulombic efficiency of 99.1%. The cycle stability was significantly reduced, and the Coulombic efficiency decreased by 0.5%. Comparative Example 1 used a single-salt system of LiTFSI and stably cycled 70 times at 25 °C room temperature, with a capacity retention rate of 72% and an average Coulombic efficiency of 98.9%. Compared with Example 1, the cycle life was greatly shortened, and the Coulombic efficiency decreased by 0.7%. From the above experimental data, it can be seen that under the same concentration conditions, as the proportion of LiFSI increases, the long-cycle performance and stable Coulombic efficiency of the battery are greatly improved.

[0079] By comparing Example 1, 3, 5, 7, 9, 10 and Comparative Example 2, without changing the dilution factor of the lithium salt concentration, the compatibility of 7 solvents, namely DMP, DMB, G3, THF, THP, 2-Me-THF, and DMC, as electrolytes for the lithium metal system was compared. Among them, there are chain ethers (DMP, DMB, G3), cyclic ethers (THF, THP, 2-Me-THF), and esters (DMC). It can be found from the comparison of the cycle performance of 2.3 Ah soft-pack battery cells in Table 1 that the application of chain ethers in the lithium metal system has significantly better electrochemical performance than cyclic ethers and esters. Since the chain of chain ether DMP is shorter than that of DMB and G3, its solvation ability is relatively weak, which also increases the interaction between anions and lithium ions, promotes the formation of anion-derived SEI, and can produce a relatively large proportion of aggregates (AGGs), thereby increasing the uniform stability of SEI and making lithium deposition more uniform, further increasing the cycle life of the metal battery. Therefore, the selection of DMP as the solvent results in the best electrochemical performance of the electrolyte.

[0080] The technical features described above can be combined arbitrarily. Although not all possible combinations of these technical features are described, any combination of these technical features should be considered to be covered by this specification, as long as such a combination does not contain contradictions.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, and are not intended to limit them. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. An electrolyte for a lithium metal battery, characterized in that, It includes an organic solvent, a diluent and a lithium salt; the diluent is selected from at least one of halogenated aromatic hydrocarbons and R1-O-R2, wherein R1 and R2 each independently include at least one of the elements H, C, N, O, F, and P.

2. The electrolyte for a lithium metal battery according to claim 1, wherein In the components of the electrolyte, the molar ratio of the organic solvent: the diluent: the lithium salt is (1~2.5):(1.6~3):(0.7~2).

3. The electrolyte for a lithium metal battery according to claim 1, wherein R1 and R2 in the R1-O-R2 are each independently selected from one or more of phenyl, halogenated alkyl, and heterocyclic groups.

4. The electrolyte for a lithium metal battery according to claim 1, characterized in that, The organic solvent is at least one of R3-O-R4, wherein R3 and R4 each independently include at least one of the elements H, C, N, O, F, and P, and at least one branch is a methyl substituent.

5. The electrolyte of the lithium metal battery according to claim 1, characterized in that, The diluent molecular structural formula contains at least fluorine element; the number of main chain carbon atoms where the alkyl chain in the halogenated alkyl does not exceed 10; the halogenated aromatic hydrocarbon is selected from one or more of fluorobenzene, m-fluorotoluene, and p-fluorotoluene; the R1-O-R2 is selected from at least one of trifluoromethoxybenzene, 3-(trifluoromethoxy)fluorobenzene, 3-(trifluoromethoxy)phenol, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 3-(2,2,3,3-tetrafluoropropoxy)-1,2-epoxypropane, bis(2,2,2-trifluoroethyl) ether, 2,2,2-trifluoroethyl formate, tris(2,2,2-trifluoroethoxy)orthoformate, bis(2,2,2-trifluoroethoxy)methane, 1,1,2,2,-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1-ethoxy-1,1,2,2,2-pentafluoroethane, methoxy nonafluorobutane, and ethoxy nonafluorobutane.

6. The electrolyte of the lithium metal battery according to claim 1, wherein, The lithium salt includes at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium trifluoromethanesulfonate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium difluorophosphate, lithium difluorodioxalophosphate, or lithium tetrafluorodioxalophosphate; the organic solvent includes at least one of methyl n-butyl ether, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, ethylene glycol diethyl ether, propylene glycol dimethyl ether, ethylene glycol dibutyl ether, tetrahydrofuran, dimethyltetrahydrofuran, tetrahydropyran, 1,2-dimethoxypropane, 1,3-dimethoxypropane, 1,3-diethoxypropane, 1,4-dimethoxybutane, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, fluoroethylene carbonate, ethylene carbonate, vinylene carbonate, methyl butyrate, ethyl propionate, trimethyl phosphate, triethyl phosphate, and γ-butyrolactone.

7. In the components of the electrolyte, the molar ratio of the lithium salt: the organic solvent: the diluent is (1.1~1.5):(2.1~2.5):(0.8~1.8).

8. A method for preparing an electrolyte of a lithium metal battery, characterized in that, It includes the following steps: In a glove box filled with a protective gas (argon) (moisture < 10 ppm, oxygen content < 1 ppm), add the lithium salt to the organic solvent and stir until it is completely dissolved to obtain a lithium metal battery electrolyte.

9. The preparation method of an electrolyte for a lithium metal battery according to claim 8, wherein, It includes the following steps: In a glove box filled with a protective gas (argon) (moisture < 10 ppm, oxygen content < 1 ppm), a diluent is added to an organic solvent, and then a lithium salt is added, followed by stirring until it is completely dissolved to obtain an electrolyte for a lithium metal battery.

10. A lithium metal battery, characterized in that, Comprising the electrolyte for a lithium metal battery according to any one of claims 1-9.

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