Electrolyte, lithium metal battery and electronic device

Abstract

The invention discloses an electrolyte, a lithium metal battery and an electronic device, and belongs to the technical field of battery materials. The electrolyte comprises lithium salts and an organic solvent, the lithium salts comprise a first lithium salt and a second lithium salt, and the organic solvent comprises fluorocarboxylic ester and fluoroamide; wherein the first lithium salt is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluoro (oxalato) phosphate, lithium tetrafluoro (oxalato) phosphate and lithium tetrafluoroborate, and the second lithium salt comprises lithium bis (trifluoromethylsulfonyl) imide and lithium difluoro (oxalato) borate in a mass ratio of 1: (1-10). According to the electrolyte, the lithium metal battery can be charged / discharged at-40 DEG C and 0.1 C, the constant-current charging ratio exceeds 80%, and the application field and range of the lithium metal battery are widened.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to an electrolyte, a lithium metal battery and an electronic device. Background Art

[0002] The deployment of rechargeable lithium-ion batteries is essential for the operation of modern portable electronics in extreme environments. The reduction in battery energy density at ultra-low temperatures (-30°C and below) has limited the electric field, submarine and military installations, and space exploration. In principle, the former problem can be solved by increasing the baseline energy density of the battery or by improving the charge transfer kinetics to mitigate the energy loss at low temperatures. -1 ) instead of conventional graphite anode (specific capacity 372 mAh g -1 ) is considered to push the battery energy density to 300Wh·kg -1 The above are effective steps. However, the cycle stability of lithium metal anode is poor. Irreversible lithium will be formed during the cycle, which reduces the cycle coulomb efficiency of lithium metal and limits the cycle stability of actual lithium metal batteries. On the other hand, like lithium-ion batteries, the operating voltage of lithium metal batteries will drop severely in an environment below -30°C. This can be attributed to the large ion transfer impedance in the electrolyte and the migration impedance of the solid electrolyte interface at the electrode interface. Most importantly, Li + The desolvation energy is considered to be the main contributor to the impedance and is related to the interface treatment Li + / solvent binding energy. In general, significant lithium reversibility at temperatures as low as -15 °C can be achieved by using low melting point and / or low polarity solvents, novel salt additives, and surface functionalization. Although great progress has been made, methods to directly improve desolvation kinetics are still largely unknown. Therefore, reducing Li + The desolvation energy is crucial to the reversibility and deposition behavior of lithium metal at low temperatures.

[0003] The related prior art discloses a lithium-ion secondary electrolyte, including electrolyte solvent, lithium bis(fluorosulfonyl)imide, lithium difluorooxalatoborate, fluoroethylene carbonate, lithium difluorophosphate and vinyl sulfate, but the lithium-ion battery can only maintain 72% of the initial capacity retention rate at minus 40 degrees. In addition, the related prior art obtains a solvent system with a higher dielectric constant and low viscosity by combining linear carboxylic acid esters and vinyl carbonate, so that the capacity retention rate at minus 20 degrees is 40%, which obviously cannot meet the use requirements under low temperature conditions. Summary of the invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention provides an electrolyte, which can significantly improve the charge and discharge retention rate of lithium metal batteries at low temperatures, and the constant current charging ratio exceeds 80%, thereby improving the rechargeability of lithium metal batteries at low temperatures and broadening the application of lithium metal batteries at low temperatures.

[0005] The invention also provides a method for preparing the electrolyte.

[0006] The present invention also provides a lithium metal battery comprising the electrolyte.

[0007] The present invention also provides an electronic device comprising the lithium metal battery.

[0008] According to an embodiment of the first aspect of the present invention, an electrolyte is provided, the electrolyte comprising a lithium salt and an organic solvent, the lithium salt comprising a first lithium salt and a second lithium salt, the organic solvent comprising a fluorocarboxylic acid ester and a fluoroamide, the first lithium salt being selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorooxalate phosphate, lithium tetrafluorooxalate phosphate and lithium tetrafluoroborate, and the second lithium salt comprising lithium bis(trifluoromethylsulfonyl)imide and lithium difluorooxalate borate in a mass ratio of 1:1 to 10.

[0009] The lithium ion solvation system constructed by the present invention using fluorinated solvents will reduce the amount of solvent in the solvation sheath due to the electron-withdrawing ability of F atoms, and will be beneficial to the introduction of anions, so that the electrolyte system has a reduced lithium ion desolvation energy, which is beneficial to the desolvation / insertion and stripping / deposition of lithium ions under low temperature conditions. Specifically, the electrolyte for lithium metal batteries of the present invention uses a low melting point and low viscosity fluorinated carboxylic acid ester solvent as the main component, which is beneficial to improve the low-temperature conductivity of the electrolyte system. Fluoroamide has a high dielectric constant, which ensures sufficient ionic conductivity in the electrolyte, and it is Li + There is almost no solvation between them, and no desolvation is required during lithium ion transfer, which can significantly improve the kinetics of lithium ion transfer and thus improve the rate performance of the battery system. In addition, the N element in fluoroamide is beneficial to enhance the stability of the interface film and improve the low-temperature performance of the battery.

[0010] The present invention adopts a composite of lithium bis(trifluoromethylsulfonyl imide) and lithium difluorooxalate borate as the second lithium salt. The lithium bis(trifluoromethylsulfonyl imide) can well adapt to the surface and volume changes of the metal lithium negative electrode during the deintercalation / embedding and stripping / deposition of lithium ions under a low temperature environment. The introduction of lithium difluorooxalate borate induces the lithium bis(trifluoromethylsulfonyl imide to enter the inner layer of the lithium ion solvation structure, and then is preferentially reduced to derive a SEI film, providing a F source for the SEI film, which can effectively reduce the decomposition of the fluorinated solvent, effectively reduce the interface impedance, and improve the low-temperature performance of the electrolyte.

[0011] The present invention further controls the mass ratio of lithium bis(trifluoromethylsulfonyl)imide and lithium difluorooxalatoborate to 1:1-10, which is helpful to derive an inorganic-rich SEI layer. When the mass ratio is too high, an organic-rich SEI layer is easily derived. On the contrary, when the mass ratio is too low, the viscosity of the electrolyte is too large, resulting in unstable contact between the electrolyte and the electrode interface, thereby increasing the interface impedance or forming irreversible lithium metal deposition.

[0012] According to some embodiments of the present invention, the electrolyte further includes an additive, and the mass of the additive is 0.5% to 8% of the mass of the organic solvent, for example, 1% to 8%, 1% to 6%, 2% to 6%, 1% to 5%, 2% to 5%, 1% to 4%, 1% to 3%.

[0013] According to some embodiments of the present invention, the additive is selected from at least one of magnesium salt and perfluorobutane sulfonate.

[0014] According to some embodiments of the present invention, the additive includes magnesium salt and perfluorobutane sulfonate in a mass ratio of 1:(0.5-2) (e.g., 1:0.5-1.5, 1:0.5-1, 1:0.5-0.8, 1:0.6-1, 1:0.6-0.8).

[0015] Magnesium salts are conducive to the formation of an interface layer where inorganic lithium salts and magnesium metal coexist, effectively reducing surface side reactions, improving the interface stability and Coulomb efficiency of the metal lithium negative electrode, reducing interphase resistance, promoting Li ion diffusion, and obtaining uniform lithium deposition. In addition, magnesium metal can provide uniform lithiophilic nucleation sites, and small and stable potential polarization can be obtained during the cycle process, thereby inhibiting low-temperature lithium precipitation. Perfluorobutyl sulfonate also has a stabilizing effect on the metal lithium negative electrode interface, and can further cooperate with magnesium salts to improve the low-temperature performance of lithium metal batteries.

[0016] According to some embodiments of the present invention, the magnesium salt is selected from at least one of magnesium nitrate and magnesium chloride.

[0017] According to some preferred embodiments of the present invention, the magnesium salt is magnesium nitrate, which may be due to: NO3 - It can promote the decomposition of bis(trifluoromethylsulfonyl)imide and difluorooxalate borate, increase the content of LiF in the SEI layer, and alleviate the growth of lithium dendrites under low temperature conditions. In addition, the solubility of the generated lithium nitrate in the organic solvent of the present invention is limited, which avoids the Mg 2+ The further spontaneous reduction of , the coordination between the magnesium salt and solvent molecules and the Li + and NO3 - The strong interaction between Li + The number of surrounding solvent molecules is reduced by optimizing Li + Desolvation kinetics enable excellent low-temperature performance of lithium metal batteries.

[0018] According to some embodiments of the present invention, the perfluorobutanesulfonate is selected from at least one of lithium perfluorobutanesulfonate or potassium perfluorobutanesulfonate.

[0019] According to some embodiments of the present invention, the organic solvent further comprises a fluorocarbonate.

[0020] Fluorinated carbonate solvents participate in the lithium ion solvation structure. The electron-absorbing fluorinated groups lower the LUMO energy level of the molecule, which then undergoes reduction at a low potential and participates in the derivation of the SEI film to obtain a solid electrolyte layer rich in LiF, thereby increasing the transmission efficiency of lithium ions at the interface.

[0021] According to some embodiments of the present invention, the mass ratio of the fluoroamide to the fluorocarboxylic acid ester is 1:3-40, for example, 1:3-35, 1:3-30, 1:3-25, 1:3-20, 1:5-30, 1:5-25, 1:5-20, 1:10-30, 1:10-25, 1:10-20, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:18, 1:20, 1:23, 1:25, 1:27, 1:30, 1:35, 1:40.

[0022] According to some embodiments of the present invention, the mass ratio of the fluorocarbonate to the fluorocarboxylate is 1:2-80, for example, 1:2-75, 1:2-70, 1:2-60, 1:2-50, 1:2-40, 1:2-30, 1:2-25, 1:2-20, 1:2-10, 1:3-60, 1:3-40, 1:3-20, 1:3-10, 1:4-40, 1:4-20, 1:4-10, 1:4-6, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:12, 1:15, 1:18, 1:20, 1:25.

[0023] Controlling the mass ratio between solvents within the above range helps to obtain a lower lithium ion desolvation energy, and lithium ions are more easily separated from the solvation structure, thereby improving the charge / discharge performance of the battery.

[0024] According to some embodiments of the present invention, the fluorocarboxylic acid ester is selected from at least one of ethyl 4,4,4-trifluorobutyrate (ETFB), methyl difluoroacetate (MDFA), ethyl difluoroacetate, methyl difluoropropionate, propyl trifluoroacetate (PFTA), 2-trifluoromethylethyl acetate, and ethyl heptafluorobutyrate.

[0025] According to some embodiments of the present invention, the fluoroamide is selected from at least one of N,N-dimethyltrifluoroacetamide (FDMA), N-methyltrifluoroacetamide, N-methyltrifluoroformamide and N,N-diethyl-2,2,2-trifluoroacetamide.

[0026] Fluoroamides, especially FDMA, have relatively lower binding energy with lithium ions compared to carbonates / carboxylates and their fluorinated derivatives, and can thus coordinate the lithium ion solvation structure, reduce the desolvation energy of lithium ions, and improve the low-temperature performance of the electrolyte.

[0027] According to some embodiments of the present invention, the fluorinated carbonate is at least one selected from fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, methyl trifluoroethyl carbonate, and bistrifluoroethyl carbonate.

[0028] According to some embodiments of the present invention, in the second lithium salt, the mass ratio of lithium bis(trifluoromethylsulfonyl)imide to lithium difluorooxalatoborate is 1:1-10, for example, 1:1-8, 1:2-8, 1:4-8, 1:4-6, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10.

[0029] According to some embodiments of the invention, the mass of the second lithium salt is 0.5% to 12% of the mass of the organic solvent, for example, 0.5% to 10%, 0.5% to 8%, 0.5% to 6%, 0.5% to 4%, 1% to 10%, 1% to 8%, 1% to 4%, 2% to 4%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%.

[0030] An appropriate content of the second lithium salt is helpful to derive an inorganic-rich SEI layer or CEI layer. Specifically, when the content is too high, it causes an increase in economic cost; conversely, when the content is too low, the effect of deriving an inorganic SEI or CEI layer is not ideal.

[0031] According to some embodiments of the present invention, the concentration of the first lithium salt in the electrolyte is 0.5 to 2 mol / L.

[0032] According to an embodiment of the second aspect of the present invention, there is provided a method for preparing the above-mentioned electrolyte, the preparation method comprising: mixing a lithium salt and an organic solvent to obtain the electrolyte.

[0033] According to some embodiments of the invention, the mixing is performed in a glove box.

[0034] According to some embodiments of the present invention, the glove box is filled with an inert gas, such as argon, nitrogen, or helium.

[0035] According to some embodiments of the present invention, the water oxygen value of the glove box is less than 0.1 ppm.

[0036] According to an embodiment of the third aspect of the present invention, there is provided a lithium metal battery, wherein the lithium metal battery comprises the above-mentioned electrolyte.

[0037] The lithium metal battery of the present invention further comprises a positive electrode sheet, a negative electrode sheet and a separator arranged between the positive electrode sheet and the negative electrode sheet.

[0038] The present invention does not impose any special restrictions on the positive electrode plate, as long as the purpose of the present invention can be achieved. For example, the positive electrode plate includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The present invention does not impose any special restrictions on the positive current collector. For example, the positive current collector can be made of metal foil or porous metal plate, such as foil or porous plate of metals such as aluminum, copper, nickel, titanium or silver or their alloys, such as aluminum foil. The positive active material layer contains positive active material. The present invention does not impose any special restrictions on the type of positive active material. For example, in some embodiments, the positive active material of the lithium metal battery can include lithium nickel cobalt manganese oxide (NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium iron manganese phosphate or lithium titanate. At least one of the above. The present invention does not impose any special restrictions on the thickness of the positive electrode current collector and the positive electrode active material layer. For example, the thickness of the positive electrode current collector is 5μm to 20μm, preferably 6μm to 18μm. The thickness of the single-sided positive electrode active material layer is 30μm to 120μm. The positive electrode active material layer can be arranged on one surface in the thickness direction of the positive electrode current collector, or on two surfaces in the thickness direction of the positive electrode current collector. The positive electrode active material layer of the present invention may also contain a conductive agent and a binder. In some embodiments of the present invention, the mass ratio of the positive electrode material to the conductive agent and the binder is 80-90:5-10:5-10. In some embodiments of the present invention, the conductive agent is selected from at least one of carbon black conductive agent SP, carbon nanotubes CNT, carbon nanofibers, flake graphite, carbon dots, and graphene. In some embodiments of the present invention, the binder is selected from at least one of styrene-butadiene rubber SBR, polypropylene alcohol, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate PAALi, polyimide, polyamide-imide, polyvinylidene fluoride PVDF, polyvinyl alcohol PVA, polytetrafluoroethylene PTFE, polyvinyl butyral PVB, water-based acrylic resin, carboxymethyl cellulose CMC, and sodium carboxymethyl cellulose CMC-Na. The positive electrode can be prepared by coating. For example, the positive electrode active material, binder, conductive agent and organic solvent are first mixed in a predetermined proportion, and the organic solvent can be N-methylpyrrolidone (NMP). The mixed material is stirred to a uniform system to obtain a positive electrode slurry; then the positive electrode slurry is coated on the positive electrode current collector, and after drying, rolling and other processes, the positive electrode is obtained. The single-sided surface density of the positive electrode material slurry coated with aluminum foil can be 1.35mAh cm -2 ~2.5mAh cm -2 .

[0039] The negative electrode of the lithium metal battery of the present invention adopts a lithium-containing metal sheet, such as a lithium alloy sheet or a lithium metal sheet, and the lithium alloy is, for example, one or more of Li-Sn alloy, Li-Sn-O alloy, Li-Mg alloy, Li-B alloy and Li-Al alloy. The negative electrode can also include a negative electrode current collector and a lithium-containing metal sheet disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector includes two opposite surfaces in its own thickness direction, and the lithium-containing metal sheet is stacked on any one or both of the two surfaces of the negative electrode current collector. The negative electrode current collector can use materials such as metal foil or porous metal plate, for example, foil or porous plate of metals such as copper, nickel, titanium or iron or their alloys, such as copper foil. The negative electrode plate is a metal lithium foil with a thickness of 200 to 500 μm and a diameter of 16 mm.

[0040] The lithium metal battery of the present invention does not impose any special restrictions on the diaphragm. For example, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer may be a non-woven fabric, a film or a composite film having a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate or polyimide. Optionally, polypropylene porous membranes, polyethylene porous membranes, polypropylene non-woven fabrics, polyethylene non-woven fabrics, polypropylene-polyethylene-polypropylene porous composite membranes, single-sided ceramic-coated polyethylene-based films, polypropylene-based films and polyimide films may be used. The present invention does not impose any special restrictions on the thickness of the diaphragm. For example, the thickness of the diaphragm may be 5 μm to 500 μm.

[0041] According to an embodiment of a fourth aspect of the present invention, there is provided a method for preparing a lithium metal battery, comprising the following steps:

[0042] The positive electrode sheets and the negative electrode sheets are alternately stacked, and an isolation film is provided between the positive electrode sheets and the negative electrode sheets to isolate the positive electrode from the negative electrode to obtain a battery cell, or the battery cell can be obtained by winding; the battery cell is placed in a shell, an electrolyte is injected, and it is sealed to obtain a lithium metal battery.

[0043] According to an embodiment of a fifth aspect of the present invention, there is provided an electronic device, the electronic device comprising the above-mentioned lithium metal battery.

[0044] According to some embodiments of the present invention, there are at least the following beneficial effects:

[0045] The electrolyte of the present invention can enable the lithium metal battery to be charged / discharged at -40°C and 0.1C, with a constant current charging ratio exceeding 80%, thereby broadening the application field and scope of the lithium metal battery.

[0046] Other features and advantages of the present invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. DETAILED DESCRIPTION

[0047] The following will be combined with the embodiments to clearly and completely describe the concept of the present invention and the technical effects produced, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0048] In the description of the present invention, unless otherwise clearly defined, terms such as setting, installing, connecting, etc. should be understood in a broad sense, and technicians in the relevant technical field can reasonably determine the specific meanings of the above terms in the present invention based on the specific content of the technical solution.

[0049] In the description of the present invention, if there is a description of first, second, etc., it is only for the purpose of distinguishing technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features or implicitly indicating the order of the indicated technical features.

[0050] The words "preferably", "more preferably", etc. in the present invention refer to embodiments of the present invention that may provide certain beneficial effects in certain circumstances. However, other embodiments may also be preferred under the same circumstances or other circumstances. In addition, the description of one or more preferred embodiments does not imply that other embodiments are not applicable, nor is it intended to exclude other embodiments from the scope of the present invention.

[0051] "And / or" is used to indicate that one or both of the stated situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0052] When a numerical range is disclosed herein, the above range is considered to be continuous and includes the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0053] If there is no special explanation, the actual meaning of “about” in the present invention is that the error is allowed to be within the range of ±10%, for example, about 100 is actually 100±10%×100.

[0054] In the description of the present invention, the description with reference to the terms "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0055] In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following embodiments, unless otherwise specified, are conventional methods in the art.

[0056] The full names of the abbreviations used in this article are as follows:

[0057] ETFB refers to ethyl 4,4,4-trifluorobutyrate.

[0058] FEC refers to fluoroethylene carbonate.

[0059] LiTFSI refers to lithium bis(trifluoromethanesulfonyl)imide.

[0060] LiDOFB refers to lithium difluorooxalate borate.

[0061] FDMA refers to N,N-dimethyltrifluoroacetamide,

[0062] MDFA refers to methyl difluoroacetate.

[0063] EC refers to ethylene carbonate.

[0064] EMC refers to ethyl methyl carbonate.

[0065] VC refers to vinyl carbonate.

[0066] TFEB refers to 2,2,2-trifluoroethyl butyrate;

[0067] KPBS refers to potassium perfluorobutanesulfonate, CAS number is 29420-49-3;

[0068] LiNFSA refers to lithium perfluorobutanesulfonate, CAS number 131651-65-5.

[0069] Examples 1-12

[0070] Examples 1-12 respectively provide an electrolyte, and the composition of the electrolyte is shown in Table 1 below.

[0071] The electrolyte solution was prepared in an argon-protected glove box (H2O≤0.1ppm, O2≤0.1ppm). The components were mixed according to the proportions in Table 1 and stirred evenly to form a mixed solution, thereby obtaining a low-temperature electrolyte for lithium metal batteries.

[0072] Comparative Examples 1-8

[0073] Comparative Examples 1-8 respectively provide an electrolyte, and the composition of the electrolyte is shown in Table 1 below.

[0074] The preparation method of the electrolyte of the comparative example is carried out with reference to that of Examples 1-12.

[0075] Table 1 Composition of the electrolyte of the embodiment and the comparative example

[0076]

[0077]

[0078]

[0079] The % in the table refers to the mass percentage of each component based on the solvent+second lithium salt+additive.

[0080] Application Examples

[0081] This application example also provides a lithium-ion battery, including an electrolyte, a positive electrode plate, a negative electrode plate, and a separator between the positive electrode plate and the negative electrode plate, and the electrolyte is the above-mentioned electrolyte.

[0082] The preparation steps of the lithium-ion battery include:

[0083] Preparation of positive electrode sheet: NCM811 positive electrode (8.3 mg cm -2 , about 1.47 mAh cm -2 ) is prepared by mixing 80 weight percent NCM811 powder, 10 weight percent carbon black and 10 weight percent polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidone (NMP), then coating the mixture on aluminum foil, drying (120°C for 12 hours) and rolling (compacted density: 3.2-2.5), and then cutting into the required size. The coated surface density is 8.3±0.5 mg·cm -2 .

[0084] Preparation of negative electrode sheet: Use lithium metal as negative electrode, thickness specification: 20μm, cut according to the size of positive electrode sheet.

[0085] Isolation membrane: Polypropylene (PP) microporous membrane is selected.

[0086] Preparation of lithium metal batteries: Battery assembly: stack the positive electrode, negative electrode and separator in a certain order to form a battery cell. Put the battery cell into a plastic shell. Use welding or bonding technology to connect the pole ear to the battery shell. Inject electrolyte and then seal the battery. Battery activation: Perform a 0.1C charge and discharge cycle on the battery at room temperature for one week to activate the battery and improve its performance. Then leave it at 45°C for 24 hours, then charge it to 4.3V at 0.1C constant current and constant voltage for battery testing. Among them, 50μL of electrolyte is injected into each button battery, and 3.0g Ah is injected into each lithium metal soft pack battery. -1 .

[0087] Test Case

[0088] The electrolytes prepared in all the embodiments and comparative examples were assembled into lithium metal batteries and tested according to the following test scheme:

[0089] 1) Stand at 25℃ for 24h;

[0090] 2) 0.01C constant current discharge for 4h;

[0091] 3) Let stand for 0.5h;

[0092] 4) 0.1C constant current charging to 4.3V, 4.3V constant voltage charging to a cut-off current of 0.01C;

[0093] 5) Let stand for 10 minutes;

[0094] 6) Stand at -40℃ for 8h;

[0095] 7) Discharge at a constant current to 2.5 V and obtain the -40°C discharge capacity retention rate;

[0096] 8) After discharging to 2.5V at -40℃, let it stand for 2min, and then charge to 4.3V at 0.1C constant current.

[0097] The test results are shown in Table 2.

[0098] Table 2 Performance test results of batteries of embodiments and comparative examples

[0099] <![CDATA[Normal temperature capacity (mAh·g -1 )]]> <![CDATA[Capacity at -40 °C (mAh·g -1 )]]> Capacity retention rate (%) Example 1 201 161 80.1 Example 2 201 162 80.6 Example 3 202 165 81.7 Example 4 199 172 86.4 Example 5 203 194 95.6 Example 6 203 189 93.1 Example 7 202 182 90.1 Example 8 202 180 89.1 Example 9 201 178 88.6 Example 10 202 183 90.6 Embodiment 11 203 169 83.3 Example 12 200 191 95.5 Comparative Example 1 200 32 16.0 Comparative Example 2 201 103 51.2 Comparative Example 3 197 109 55.3 Comparative Example 4 199 86 43.2 Comparative Example 5 198 70 35.4 Comparative Example 6 200 101 50.5 Comparative Example 7 201 97 48.3 Comparative Example 8 196 114 58.2

[0100] The test results show that the lithium metal battery assembled with the electrolyte prepared in Examples 1-12 has excellent low temperature performance, and can be charged / discharged at -40°C and 0.1C, with a constant current charge ratio of more than 80%. In addition, in the absence of lithium difluorooxalatoborate (LiDOFB), even if the amount of LiTFSI is increased, the low temperature performance is significantly reduced, which can be attributed to the synergistic effect of the two additives, which makes more anions (DOFB - TFSI- ) enters the inner layer of the solvation structure and participates in the derivation of the SEI film. At the same time, LiDFOB will derive an inorganic-rich CEI layer at the NCM811 interface, which can improve the oxidation resistance of the electrode and reduce the interface impedance. Comparative Example 5 and Comparative Example 5, FEC is replaced with FDMA. The test results show that it is beneficial to improve the low-temperature discharge performance of lithium metal batteries. This is attributed to the lower binding energy between FDMA and lithium ions, which can reduce the desolvation energy of lithium ions, so the low-temperature performance is further improved. The degree of fluorination has a certain influence on the low-temperature performance, and Comparative Example 2 shows that the position of the fluorinated group in the molecular structure will also affect the low-temperature performance.

[0101] The above content describes the embodiments of the present invention in detail in conjunction with specific embodiments, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in the relevant technical field without departing from the purpose of the present invention. In addition, the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

Claims

1. An electrolyte, characterized in that: The invention comprises a lithium salt and an organic solvent, wherein the lithium salt comprises a first lithium salt and a second lithium salt, the organic solvent comprises a fluorocarboxylic acid ester and a fluoroamide, the first lithium salt is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorooxalate phosphate, lithium tetrafluorooxalate phosphate and lithium tetrafluoroborate, and the second lithium salt comprises lithium bistrifluoromethylsulfonyl imide and lithium difluorooxalate borate in a mass ratio of 1: (1 to 10).

2. The electrolyte according to claim 1, characterized in that The electrolyte further comprises an additive, wherein the additive is selected from at least one of a magnesium salt and a perfluorobutane sulfonate.

3. The electrolyte according to claim 2, characterized in that The additive comprises magnesium salt and perfluorobutyl sulfonate in a mass ratio of 1: (0.5-2).

4. The electrolyte according to claim 1, characterized in that The mass ratio of the fluoroamide to the fluorocarboxylic acid ester is 1:(3-40); And / or, the fluorocarboxylic acid ester is at least one selected from ethyl 4,4,4-trifluorobutyrate, methyl difluoroacetate, ethyl difluoroacetate, methyl difluoropropionate, propyl trifluoroacetate, 2-trifluoromethylethyl acetate, and ethyl heptafluorobutyrate; And / or, the fluoroamide is selected from at least one of N,N-dimethyltrifluoroacetamide, N-methyltrifluoroacetamide, N-methyltrifluoroformamide and N,N-diethyl-2,2,2-trifluoroacetamide.

5. The electrolyte according to claim 1, characterized in that The organic solvent also includes fluorocarbonate.

6. The electrolyte according to claim 5, characterized in that The mass ratio of the fluorocarbonate to the fluorocarboxylate is 1:(2-80); And / or, the fluorinated carbonate is at least one selected from fluorinated ethylene carbonate, 1,2-difluoroethylene carbonate, methyl trifluoroethyl carbonate, and bistrifluoroethyl carbonate.

7. The electrolyte according to claim 1, characterized in that The concentration of the first lithium salt in the electrolyte is 0.5 to 2 mol / L; and / or the mass of the second lithium salt is 0.5% to 12% of the mass of the organic solvent.

8. The method for preparing the electrolyte according to any one of claims 1 to 7, characterized in that: The preparation method comprises: mixing a lithium salt and an organic solvent to obtain the electrolyte.

9. A lithium metal battery, characterized in that: The lithium metal battery comprises the electrolyte according to any one of claims 1 to 7 or the electrolyte prepared by the preparation method according to claim 8.

10. An electronic device, characterized in that: The electronic device comprises the lithium metal battery as claimed in claim 9.

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