A local high-concentration lithium metal battery electrolyte, a preparation method and application thereof

Abstract

The application discloses a local high-concentration lithium metal battery electrolyte and a preparation method and application thereof. The electrolyte comprises a lithium salt, an organic solvent, an additive and a diluent; the organic solvent comprises at least one of methyl formate, methyl acetate, methyl propionate, methyl butyrate, ethyl propionate, ethyl acetate, ethyl butyrate, methyl trifluoroacetate and ethyl trifluoroacetate; the additive comprises at least one of fluoroethylene carbonate, lithium difluorobisoxalate phosphate and lithium difluorophosphate; and the diluent is a fluorine-containing ether compound. The local high-concentration electrolyte has the advantages of high-voltage resistance of high-concentration electrolyte and inhibition of corrosion of sulfonimide-based electrolyte on aluminum foil, meanwhile, the problems of high viscosity, low conductivity and poor impregnation with a separator of high-concentration electrolyte are overcome, the cycle performance and low-temperature performance of the lithium metal battery are significantly improved, and the local high-concentration electrolyte is particularly suitable for application in a low-temperature high-voltage lithium battery. The preparation method is simple, the cost is low, and the method is suitable for industrial production.

Description

TECHNICAL FIELD

[0001] The application relates to a metal battery electrolyte, in particular to a local high-concentration lithium metal battery electrolyte, a preparation method and application thereof, and belongs to the technical field of lithium metal batteries. BACKGROUND

[0002] Lithium ion batteries have the advantages of high energy density, long cycle life, no memory effect, etc., and are widely used in small electronic devices such as mobile phones and notebook computers. In the field of large-scale energy storage such as smart grids and electric vehicles, it has also been popularized. Lithium metal batteries are considered to be the next generation of high-energy density storage devices due to their extremely high theoretical specific capacity (3860 mAh / g) and the lowest electrochemical potential (-3.040 V compared with the standard hydrogen electrode). High-voltage lithium metal batteries assembled by high-voltage cathodes and lithium metal have received more and more attention.

[0003] However, at lower temperatures, especially below 0 DEG C, the discharge capacity and discharge voltage of lithium batteries decrease sharply, which cannot meet the growing demand of portable electronic products and electric vehicles under low temperature conditions. At the same time, during the charging and discharging process of lithium metal batteries, the uncontrollable growth of lithium dendrites and the low coulombic efficiency (CE) limit the application of high-voltage LMBs. In addition, with the continuous development of high-voltage cathode materials, such as high-nickel LiNi x Co y Mn z O2(x+y+z=1). As a link between the cathode and the anode, the electrolyte also plays a very important role in the battery system. However, the traditional commercial carbonate electrolyte has limited oxidation stability (about 4.3 V), which limits its application in high-voltage battery fields. In addition, under low temperature conditions, the lower limit of the temperature operating window of the traditional commercial electrolyte is also low (≥-20 DEG C), the lithium ion migration rate is reduced, the impedance is high, and the capacity provided is also small. Therefore, it is particularly important to develop an electrolyte that can be compatible with high-voltage materials under low temperature conditions.

[0004] With the increase of lithium salt concentration, the interaction between the cations and anions of lithium salt and the solvent is enhanced, the content of free solvent molecules is greatly reduced or even disappears, a special salt-solvent coordination structure is formed, a new type of electrolyte with special structure is obtained, and the electrochemical window is widened, the lithium ion migration rate is reduced, and the reaction kinetics rate is improved, thereby improving the high-voltage and low-temperature performance of the lithium ion battery. However, high-concentration electrolyte still has some disadvantages, such as high viscosity, low ionic conductivity, high cost of lithium salt, etc., which also limits its further application. Local high-concentration electrolyte is an improvement based on high-concentration electrolyte, that is, Li +The "diluent" is used to reduce the apparent concentration. This not only ensures that the microstructure of the high-concentration electrolyte is not destroyed, but also reduces the viscosity and economic cost of the electrolyte, improves the ionic conductivity, and increases the possibility of its application in the field of low-temperature high-voltage. SUMMARY

[0005] In view of the high viscosity, poor wettability, low ionic conductivity, poor low-temperature adaptability and other deficiencies of the existing lithium metal battery electrolyte, the first object of the present application is to provide a local high-concentration lithium metal battery electrolyte. The electrolyte has a high electrochemical window, good conductivity, good low-temperature and high-pressure adaptability, and can be widely used in low-temperature and high-pressure lithium metal batteries.

[0006] The second object of the present application is to provide a preparation method of a local high-concentration lithium metal battery electrolyte. The method is simple, easy to operate, low in cost and suitable for industrial production.

[0007] The third object of the present application is to provide an application of a local high-concentration lithium metal battery electrolyte. The electrolyte can be used as a lithium metal battery electrolyte, which can make the lithium metal battery have high discharge capacity, stable long cycle performance, high coulomb efficiency and good safety performance under extremely low temperature conditions.

[0008] In order to achieve the above technical purposes, the present application provides a local high-concentration lithium metal battery electrolyte, which comprises a lithium salt, an organic solvent, an additive and a diluent; the organic solvent comprises at least one of methyl formate, methyl acetate, methyl propionate, methyl butyrate, ethyl propionate, ethyl acetate, ethyl butyrate, methyl trifluoroacetate and ethyl trifluoroacetate; the additive comprises at least one of fluoroethylene carbonate, lithium difluorobisoxalate phosphate and lithium difluorophosphate; and the diluent is a fluorine-containing ether compound.

[0009] The present application adopts a reagent with high dielectric constant, low melting point and small viscosity as a solvent of the electrolyte. On the one hand, the local high-concentration electrolyte can dissolve more lithium salt and increase the lithium salt concentration. On the other hand, the local high-concentration electrolyte can still be in a liquid state at an extremely low temperature (-70℃), and the electrolyte does not freeze, has high conductivity and low viscosity. At the same time, by introducing an additive containing special components as a film-forming agent and a fluorinated ether compound as a diluent, the film-forming additive can form a stable interfacial film on the electrode surface, effectively stabilize the electrode / electrolyte interface, and improve the cycle stability of the lithium metal battery. The diluent solves the problems of high viscosity, low conductivity and poor impregnation with the separator of the high-concentration electrolyte, improves the cycle performance and low-temperature performance of the lithium metal battery, and the degradation of the fluorinated ether compound can form a high-fluorine-content passivation solid electrolyte film on the electrode surface, which can effectively improve the cycle stability and coulombic efficiency of the battery. Through the synergistic effect of the special organic solvent, additive and diluent of the present application, the local high-concentration lithium metal battery electrolyte has better electrochemical window and conductivity, and can adapt to low temperature and high pressure conditions.

[0010] As a preferred scheme, the lithium salt includes lithium bis(trifluoromethanesulfonyl)imide and / or lithium bis(fluorosulfonyl)imide.

[0011] As a preferred scheme, the organic solvent includes methyl propionate. The solvent has high dielectric constant and low melting point, which ensures that it does not freeze at low temperature and dissolves as many lithium salts as possible.

[0012] As a preferred scheme, the additive includes fluorinated ethylene carbonate to protect and improve the intercalation kinetics of Li+, change the chemical composition and properties of the positive and negative electrode surface interfacial film, and facilitate the operation of the battery under low temperature and high voltage conditions.

[0013] As a preferred scheme, the diluent includes 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether.

[0014] As a preferred scheme, the local concentration of the lithium salt in the electrolyte is 2-7 mol / L, preferably 3-5 mol / L, further preferably 3-4 mol / L, and more preferably 3 mol / L.

[0015] As a preferred scheme, the additive accounts for 1%-20% of the volume percentage of the electrolyte, preferably 10%.

[0016] As a preferred scheme, the diluent accounts for 30%-70% of the volume percentage of the electrolyte, preferably 45%-55%.

[0017] The application further provides a preparation method of the local high-concentration lithium metal battery electrolyte.

[0018] The local high-concentration lithium metal battery electrolyte developed and prepared in the application has an electrical conductivity of 0.655-0.22 ms / cm at-40 DEG C.

[0019] In the preparation process, the lithium salt is coordinated with the organic solvent to form a stable lithium salt solution, and the additive and the diluent can be mutually soluble with the organic solvent, so as to reduce the viscosity of the electrolyte, improve the electrical conductivity and the diaphragm infiltration of the electrolyte, and improve the cycle performance and low-temperature performance of the lithium metal battery.

[0020] As a preferred scheme, the concentration of the lithium salt solution is 3-5 mol / L.

[0021] The application further provides an application of the local high-concentration lithium metal battery electrolyte, which is used as a lithium metal battery electrolyte.

[0022] The electrolyte developed in the application is used to construct an NCM811||Li battery, and the discharge specific capacity of the battery is 155-143 mAh / g at-40 DEG C and a 0.2C discharge rate.

[0023] The electrolyte developed in the application is used to construct an NCM811||Li battery, and when the volume ratio of the solvent, the additive and the diluent is 4:1:5, and the concentration of the lithium salt in the organic solvent is 3 mol / L, the discharge specific capacity of the product obtained at-50 DEG C is 115 mAh / g.

[0024] The electrolyte developed in the application is used to construct an NCM811||Li battery, and when the volume ratio of the solvent, the additive and the diluent is 4:1:5, and the concentration of the lithium salt in the organic solvent is 3 mol / L, the product obtained can still be stably and rapidly charged and discharged at-40 DEG C and a voltage range of 2.8-4.6 V, the specific capacity reaches 152 mAh / g, can be long-cycled for 50 cycles, and the coulombic efficiency is above 99.9%. This effect is far superior to the prior art and other schemes in the development process of the application.

[0025] The lithium metal battery provided by the application is based on ternary materials as positive electrode materials and metal lithium sheets as negative electrodes, and can perform stable, fast and reversible long-acting charge and discharge cycles in the case of a voltage range of 2.8-4.6V at-40℃.

[0026] Compared with the prior art, the application has the following beneficial effects:

[0027] (1) The local high-concentration electrolyte has the advantages of high-voltage resistance of high-concentration electrolyte and inhibition of corrosion of sulfonimide-based electrolyte on aluminum foil, while overcoming the problems of high viscosity, low conductivity and poor impregnation with the separator of high-concentration electrolyte, greatly improving the cycle performance and low-temperature performance of the lithium metal battery, effectively improving the defects of low-temperature energy loss, small voltage range and serious lithium dendrite of the lithium metal battery, and being particularly suitable for application in low-temperature high-voltage lithium batteries;

[0028] (2) The reagent with high dielectric constant, low melting point and small viscosity is selected as the solvent of the lithium salt, more lithium salt can be dissolved, the local lithium salt concentration is increased, the electrolyte is still in a liquid state at extremely low temperature (-70℃), the electrolyte does not freeze, the conductivity is high, and the viscosity is low;

[0029] (3) The film-forming additive and the fluorinated ether compound diluent are introduced into the electrolyte, wherein the film-forming additive can form a stable interface film on the electrode surface, which can effectively stabilize the electrode / electrolyte interface and improve the cycle stability of the lithium metal battery; the degradation reduction of the fluorinated ether compound can form a high-fluorine-content passivation solid electrolyte film on the electrode surface, which can effectively improve the cycle stability and coulombic efficiency of the battery;

[0030] (4) The preparation process is simple, the cost is low, and it is suitable for industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The electrochemical window diagram of the electrolyte prepared for Examples 1-3 and Comparative Example 4.

[0032] Figure 2 The conductivity test diagram of the electrolyte prepared for Examples 1-3 at different temperatures.

[0033] Figure 3 The discharge performance diagram of the NCM811||Li battery using the electrolyte prepared for Examples 1-3 and Comparative Examples 1 and 4 at-40℃ and 0.2C discharge rate.

[0034] Figure 4 The comparison diagram of the 0.2C rate discharge test curves of the NCM811||Li battery using the electrolyte prepared for Example 1 at different temperatures.

[0035] Figure 5 Cycle performance and coulombic efficiency plots for NCM811||Li cells using the electrolyte prepared in Example 1 at -40°C, 0.2C, 2.8-4.6V voltage range.

[0036] Figure 6 Consists of (a), (b), (c) three figures, wherein (a) is the top view scanning electron microscope image after lithium plating on copper using the electrolyte prepared in Example 1; (b) is the top view scanning electron microscope image after lithium plating on copper using the electrolyte prepared in Example 2; (c) is the top view scanning electron microscope image after lithium plating on copper using the electrolyte prepared in Comparative Example 3.

[0037] Figure 7 Discharge performance plots for NCM811||Li cells using the electrolyte prepared in Example 1 and Comparative Examples 5, 6 at -40°C, 0.2C discharge rate.

[0038] Figure 8 Discharge performance plots for NCM811||Li cells using the electrolyte prepared in Example 1 and Comparative Example 7 at -40°C, 0.2C discharge rate. DETAILED DESCRIPTION

[0039] The application is further described in conjunction with the specific embodiments, but the application is not limited to these specific embodiments. Those skilled in the art should recognize that the application encompasses all alternatives, modifications and equivalents possible within the scope of the claims.

[0040] Example 1

[0041] A low-temperature high-voltage local high-concentration electrolyte, taking bis-trifluoromethanesulfonylimide lithium as a lithium salt, methyl propionate as a solvent, fluoroethylene carbonate as an additive, and 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether as a diluent. The preparation method is as follows: dissolving bis-trifluoromethylsulfonylimide lithium in dimethyl carbonate to make the concentration reach 3 mol / L, stirring the lithium salt uniformly and dissolving to obtain a high-concentration electrolyte. Then, adding fluoroethylene carbonate to the solution, stirring uniformly until dissolving. Finally, adding 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether to the solution, stirring uniformly to obtain a local high-concentration electrolyte, which is referred to as 3MMFH electrolyte. The volume ratio of the solvent, the additive and the diluent is 4:1:5.

[0042] Example 2

[0043] A high-voltage low-temperature local high-concentration electrolyte, lithium bis-trifluoromethanesulfonimide as lithium salt, methyl propionate as solvent, fluoroethylene carbonate as additive, 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether as diluent. Its preparation method is: dissolving lithium bis-trifluoromethyl sulfonimide in dimethyl carbonate to make the concentration reach 4mol / L, stirring the lithium salt uniformly and dissolving to obtain a high-concentration electrolyte. Then, add fluoroethylene carbonate to the solution, stir uniformly until dissolved. Finally, add 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether to the solution, stir uniformly to obtain a local high-concentration electrolyte, the obtained local high-concentration electrolyte is simply referred to as 4MMFH electrolyte. Among them, the volume ratio of the solvent, additive and diluent is 4:1:5.

[0044] Example 3

[0045] A high-voltage low-temperature local high-concentration electrolyte, lithium bis-trifluoromethanesulfonimide as lithium salt, methyl propionate as solvent, fluoroethylene carbonate as additive, 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether as diluent. Its preparation method is: dissolving lithium bis-trifluoromethyl sulfonimide in dimethyl carbonate to make the concentration reach 5mol / L, stirring the lithium salt uniformly and dissolving to obtain a high-concentration electrolyte. Then, add fluoroethylene carbonate to the solution, stir uniformly until dissolved. Finally, add 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether to the solution, stir uniformly to obtain a local high-concentration electrolyte, the obtained local high-concentration electrolyte is simply referred to as 5MMFH electrolyte. Among them, the volume ratio of the solvent, additive and diluent is 4:1:5.

[0046] Comparative Example 1

[0047] A commonly used low-temperature electrolyte, lithium salt is lithium hexafluorophosphate, solvent is ethylene carbonate, dimethyl carbonate, methyl ethyl carbonate. Its preparation method is: preparing a solution by mixing ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate in a volume ratio of 1:1:1, then dissolving lithium bis-trifluoromethyl sulfonimide in the above solution to make the concentration reach 1mol / L, the obtained electrolyte is simply referred to as EDD111.

[0048] Comparative Example 2

[0049] A low-concentration electrolyte, lithium bis-trifluoromethanesulfonimide as lithium salt, methyl propionate as solvent, fluoroethylene carbonate as additive. Its preparation method is: dissolving lithium bis-trifluoromethyl sulfonimide in dimethyl carbonate to make the concentration 1mol / L. Then, add fluoroethylene carbonate to the solution, stir uniformly to obtain a low-concentration electrolyte. The obtained low-concentration electrolyte is simply referred to as 1MMF electrolyte. Among them, the volume ratio of the solvent and additive is 4:1.

[0050] Comparative Example 3

[0051] A low-concentration electrolyte, with lithium bis-trifluoromethanesulfonimide as the lithium salt, methyl propionate as the solvent, fluoroethylene carbonate as the additive, and 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether as the diluent. The preparation method is as follows: lithium bis-trifluoromethanesulfonimide is dissolved in dimethyl carbonate to a concentration of 1 mol / L, and the lithium salt is stirred uniformly and dissolved. Subsequently, fluoroethylene carbonate and 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether are added to the solution, and stirred uniformly to obtain a low-concentration electrolyte. The obtained low-concentration electrolyte is referred to as 1MMFH electrolyte. The volume ratio of the solvent, additive, and diluent is 4:1:5.

[0052] Comparative Example 4

[0053] A low-concentration electrolyte, with lithium bis-trifluoromethanesulfonimide as the lithium salt, methyl propionate as the solvent, fluoroethylene carbonate as the additive, and 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether as the diluent. The preparation method is as follows: lithium bis-trifluoromethanesulfonimide is dissolved in dimethyl carbonate to a concentration of 2 mol / L, and the lithium salt is stirred uniformly and dissolved. Subsequently, fluoroethylene carbonate and 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether are added to the solution, and stirred uniformly to obtain a low-concentration electrolyte. The obtained low-concentration electrolyte is referred to as 2MMFH electrolyte. The volume ratio of the solvent, additive, and diluent is 4:1:5.

[0054] Comparative Example 5

[0055] A local high-concentration electrolyte, with lithium bis-trifluoromethanesulfonimide as the lithium salt, methyl propionate as the solvent, fluoroethylene carbonate as the additive, and 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether as the diluent. The preparation method is as follows: lithium bis-trifluoromethanesulfonimide is dissolved in dimethyl carbonate to a concentration of 3 mol / L, and the lithium salt is stirred uniformly and dissolved to obtain a high-concentration electrolyte. Subsequently, fluoroethylene carbonate is added to the solution, and stirred uniformly to dissolve. Finally, 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether is added to the solution, and stirred uniformly to obtain a local high-concentration electrolyte. The obtained local high-concentration electrolyte is referred to as Comparative Example 5 electrolyte. The volume ratio of the solvent, additive, and diluent is 2:1:7.

[0056] Comparative Example 6

[0057] A locally high-concentration electrolyte is disclosed, using lithium bis(trifluoromethanesulfonyl)imide as the lithium salt, methyl propionate as the solvent, fluoroethylene carbonate as the additive, and 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether as the diluent. The preparation method is as follows: Lithium bis(trifluoromethanesulfonyl)imide is dissolved in dimethyl carbonate to a concentration of 3 mol / L. The lithium salt is stirred until homogeneous and dissolved, yielding a high-concentration electrolyte. Subsequently, fluoroethylene carbonate is added to the solution and stirred until dissolved. Finally, 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether is added to the solution and stirred until homogeneous, yielding a locally high-concentration electrolyte, referred to as Comparative Example 6 electrolyte. The volume ratio of the solvent, additive, and diluent is 6:1:3.

[0058] Comparative Example 7

[0059] A locally high-concentration electrolyte is disclosed, using lithium bis(trifluoromethanesulfonyl)imide as the lithium salt, methyl propionate as the solvent, and 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether as the diluent. The preparation method is as follows: lithium bis(trifluoromethanesulfonyl)imide is dissolved in dimethyl carbonate to a concentration of 3 mol / L. The lithium salt is stirred until homogeneous and dissolved, yielding a high-concentration electrolyte. Subsequently, 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether is added to the solution and stirred until homogeneous to obtain a locally high-concentration electrolyte. This locally high-concentration electrolyte is referred to as Comparative Example 7 electrolyte. The volume ratio of the solvent to the diluent is 4:5.

[0060] like Figure 1 The figure shows the electrochemical window obtained by linear voltammetry scanning tests using the locally high-concentration electrolytes of Examples 1, 2, and 3 of this invention and Comparative Example 4. As can be seen from the figure, the prepared locally high-concentration electrolytes are very stable below 5V and do not decompose. Compared with the previously reported electrochemical window of 4.3V for conventional commercial carbonates, this increases the upper limit of the electrochemical window and enhances the antioxidant properties of the electrolyte. Furthermore, with the increase of lithium salt concentration, the upper limit of the electrochemical window increases, the antioxidant properties of the electrolyte increase, and the passivation performance of the electrolyte on aluminum foil is also enhanced.

[0061] like Figure 2 The figure shows the conductivity of locally high-concentration electrolytes used in Examples 1, 2, and 3 of this invention. At -40°C, the conductivity of Examples 1, 2, and 3 are 0.652, 0.351, and 0.223 mS / cm, respectively, exhibiting very high ionic conductivity, ensuring good electrochemical performance of the batteries made using this electrolyte at extremely low temperatures. Furthermore, when the temperature drops to -70°C, the electrolyte remains non-liquid and does not solidify, guaranteeing smooth operation of the batteries at extremely low temperatures.

[0062] The electrolytes from Examples 1-3 and Comparative Examples 1 and 4 were used to construct NCM811||Li batteries, and the discharge performance of these batteries at -40°C and a discharge rate of 0.2C was tested. Figure 3 As shown in the figure, the use of locally high-concentration electrolyte significantly improves the low-temperature discharge performance of the battery. The discharge specific capacities of 3MMFH, 4MMFH, and 5MMFH are 155 mAh / g, 143 mAh / g, and 74 mAh / g, respectively. In contrast, the low-concentration electrolyte 2MMFH used in comparison only has 113 mAh / g, and the commonly used low-temperature electrolyte EDD111 in Comparative Example 1 only has 40 mAh / g. When the temperature drops to -50℃, the discharge specific capacity of the locally high-concentration electrolyte 3MMFH is still 115 mAh / g, while Comparative Example 1 cannot discharge normally at -50℃.

[0063] The electrolyte from Example 1 was used to construct an NCM811||Li battery, and the discharge performance of this battery at a current density of 0.2C at different temperatures was tested. Figure 4 As shown, the NCM811||Li lithium metal battery using this 3MMFH locally high-concentration electrolyte exhibits discharge specific capacities of 194, 174, and 155 mAh / g at -20℃, -30℃, and -40℃, respectively, representing 92%, 82%, and 73% of its room temperature capacity. Furthermore, its discharge specific capacity at -50℃ still reaches 115 mAh / g. Meanwhile, as... Figure 5 As shown, the NCM811||Li lithium metal battery fabricated using the locally high-concentration electrolyte of Example 1 exhibits stable and rapid charge-discharge performance at -40°C and within a voltage range of 2.8-4.6V, achieving a specific capacity of 152mAh / g and a long-term cycling capability of 50 cycles with a coulombic efficiency exceeding 99.9%. In contrast, under room temperature conditions, using a 0.5C current density and a voltage range of 2.8-4.6V for long-term cycling, Comparative Example 2 struggled to reach a charging voltage of 4.6V, and Comparative Example 3 showed a gradual decrease in coulombic efficiency during cycling, even exhibiting overcharging due to battery short circuits, ultimately failing to achieve effective long-term cycling. Examples 1 and 2, however, maintained stable coulombic efficiency at room temperature and within a voltage range of 2.8-4.6V, enabling stable long-term cycling.

[0064] The protective effect of Examples 1, 2, and Comparative Example 3 on the lithium metal anode was studied by fabricating Li||Cu batteries. A current density of 1 mA cm⁻¹ was used. -2 The capacity is 5mAh cm -2 The Li||Cu battery was disassembled and the Cu was analyzed using scanning electron microscopy (SEM) to determine the deposition morphology of Li on the Cu surface. Figure 6 As shown, Figure 6(c) In Comparative Example 3, a large number of needle-like dendrites and moss-like lithium crystals were present, forming a loose and thick deposition morphology. Lithium easily penetrated the separator, consuming the electrolyte and causing short circuits and significant polarization in the battery. In contrast, Figure 6 (a) Example 1 and Figure 6 (b) In Example 2, a flat and thin morphology was obtained, with a uniform and flat surface. The uniform and dense lithium deposition made it difficult for lithium to penetrate the separator, reducing the reaction area between lithium and the electrolyte. The electrolytes from Example 1 and Comparative Examples 5 and 6 were used to construct NCM811||Li batteries, and the discharge performance of these batteries at -40°C and a discharge rate of 0.2C was tested. Figure 7 As shown, the locally high-concentration electrolyte 3MMFH prepared using the preferred ratio can release a discharge capacity of 155 mAh / g at -40℃, while Comparative Example 5, with less solvent and more diluent, only releases a discharge capacity of 139 mAh / g at -40℃. When there is more solvent and less diluent, the electrolyte remains in a high-concentration state, with higher viscosity and lower conductivity, failing to fully form the special locally high-concentration electrolyte, thus reducing electrochemical performance. Comparative Example 6, with more solvent and less diluent, only releases a discharge capacity of 142 mAh / g at -40℃. Since the diluent cannot dissolve the lithium salt, it can affect the network structure of the solvent. When there is less solvent and more diluent, the excessive diluent disrupts the solvent structure, thus affecting the special coordination between the salt and the solvent, resulting in reduced electrochemical performance.

[0065] The electrolytes from Example 1 and Comparative Example 7 were used to construct NCM811||Li batteries, and the discharge performance of these batteries at -40°C and a discharge rate of 0.2C was tested. Figure 8 As shown, the locally high-concentration electrolyte 3MMFH prepared using the preferred ratio can release a discharge capacity of 155 mAh / g at -40℃, while the comparative example 7 without the additive fluoroethylene carbonate only releases a discharge capacity of 133 mAh / g at -40℃. The addition of the superior cathode film-forming additive fluoroethylene carbonate preferentially generates a passivation film that inhibits corrosion of the aluminum current collector and achieves better high-voltage stability to a certain extent. Furthermore, it can also alter the uniformity of the CEI film, thereby optimizing the electrochemical stability and low-temperature performance of the electrolyte.

[0066] As can be seen, the lithium metal battery provided by the present invention is based on ternary materials as positive electrode materials and lithium metal sheets as negative electrodes, and can perform stable, fast and reversible long-term charge-discharge cycles at -40℃ and a voltage range of 2.8-4.6V.

[0067] The above description is only a preferred embodiment of the present invention. Given that those skilled in the art can make appropriate changes and modifications to the above embodiments, the present invention is not limited to the specific embodiments described above, and some modifications and changes to the present invention should also fall within the protection scope of the claims of the present invention.

Claims

1. A locally high-concentration lithium metal battery electrolyte, characterized in that: The electrolyte comprises lithium salt, organic solvent, additive, and diluent; the organic solvent is methyl propionate; the additive is fluoroethylene carbonate; and the diluent is a fluorinated ether compound. The concentration of the lithium salt in the organic solvent is 3-4 mol / L; The additive accounts for 10% of the volume of the electrolyte; The diluent accounts for 45% to 55% of the volume of the electrolyte; The lithium salt includes at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, lithium difluorooxalate borate, and lithium dioxalate borate, wherein the lithium salt is insoluble in the diluent; The fluorinated ether compounds include at least one of 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3,-tetrafluoropropyl ether, 1,1,1,3,3,3-hexafluoroisopropylmethyl ether, 2,2,2-trifluoroethyl ether, and 1,1,2,2-tetrafluoroethyl methyl ether.

2. The method for preparing a locally high-concentration lithium metal battery electrolyte as described in claim 1, characterized in that: A lithium salt solution is formed by mixing lithium salt with an organic solvent, and then additives and diluents are added in sequence to obtain a locally high-concentration lithium metal battery electrolyte.

3. The application of the locally high-concentration lithium metal battery electrolyte as described in claim 1, characterized in that: As an electrolyte for lithium metal batteries.

4. The application of the locally high-concentration lithium metal battery electrolyte as described in claim 1, characterized in that: The developed electrolyte was used to construct an NCM811||Li battery, which exhibited a discharge specific capacity of 155~143 mAh / g at -40℃ and a discharge rate of 0.2C.

5. The application of a locally high-concentration lithium metal battery electrolyte according to claim 3, characterized in that: The developed electrolyte was used to construct NCM811||Li batteries. When the volume ratio of solvent, additive and diluent was 4:1:5 and the concentration of lithium salt in organic solvent was 3 mol / L, the resulting product had a discharge specific capacity of 115 mAh / g at -50℃.

6. The application of a locally high-concentration lithium metal battery electrolyte according to claim 3, characterized in that: The developed electrolyte was used to construct NCM811||Li batteries. When the volume ratio of solvent, additive and diluent was 4:1:5 and the concentration of lithium salt in organic solvent was 3 mol / L, the resulting product could still perform stable and rapid charge and discharge at -40℃ and voltage range of 2.8-4.6V, with a specific capacity of 152mAh / g and a long-term cycle life of 50 cycles, with a coulombic efficiency of over 99.9%.

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