Electrolyte, lithium metal battery and electronic device

Abstract

This invention discloses an electrolyte, a lithium metal battery, and an electronic device, belonging to the field of battery material technology. The electrolyte comprises a lithium salt and an organic solvent. The lithium salt includes a first lithium salt and a second lithium salt, and the organic solvent includes fluorocarboxylic acid esters and fluoroamides. The first lithium salt is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorooxalate phosphate, lithium tetrafluorooxalate phosphate, and lithium tetrafluoroborate. The second lithium salt comprises lithium bis(trifluoromethanesulfonyl)imide and lithium difluorooxalate borate in a mass ratio of 1:1 to 10. The electrolyte of this invention enables lithium metal batteries to achieve charge / discharge at -40°C and 0.1C, with a constant current charge ratio exceeding 80%, thus broadening the application fields and scope of lithium metal batteries.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and in particular to an electrolyte, a lithium metal battery, and an electronic device. Background Technology

[0002] The deployment of rechargeable lithium-ion batteries is crucial for the operation of modern portable electronics in extreme environments. The reduction in battery energy density at ultra-low temperatures (-30°C and below) limits applications in electric vehicles, underwater and military devices, and space exploration. In principle, the problem can be addressed by increasing the battery's baseline energy density or by mitigating energy loss at low temperatures through improved charge transfer kinetics. Using lithium metal (specific capacity 3860 mAh·g)... -1 ) to replace conventional graphite anodes (specific capacity 372 mAh·g) -1 It is considered to have pushed battery energy density to 300 Wh·kg -1 The above steps are effective. However, lithium metal anodes have poor cycle stability, and irreversible lithium forms during cycling, reducing the coulombic efficiency of lithium metal and thus limiting the cycle stability of practical lithium metal batteries. On the other hand, like lithium-ion batteries, lithium metal batteries experience a significant voltage drop below -30°C. This can be attributed to the high ion transport impedance in the electrolyte and the migration impedance at the solid-electrolyte interface of the electrode. Most importantly, Li... + The desolvation energy is considered to be the main contributor to impedance, and is related to the Li at the interface. + The solvent binding energy is relevant. Generally, 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. Despite considerable progress, methods to directly improve desolvation kinetics remain largely unknown. Therefore, reducing Li... + Desolvation energy is crucial for the reversibility and deposition behavior of lithium metal at low temperatures.

[0003] Existing technologies disclose a lithium-ion secondary electrolyte comprising an electrolyte solvent, lithium bis(fluorosulfonyl)imide, lithium difluorooxalate borate, fluoroethylene carbonate, lithium difluorophosphate, and ethylene sulfate. However, lithium-ion batteries in this prior art can only retain 72% of their initial capacity at -40°C. Furthermore, existing technologies utilize a combination of linear carboxylic acid esters and ethylene carbonate to obtain a solvent system with a high dielectric constant and low viscosity, resulting in a capacity retention of only 40% at -20°C, which clearly fails to meet the requirements for use under low-temperature conditions. Summary of the Invention

[0004] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention provides an electrolyte that can significantly improve the charge-discharge retention rate of lithium metal batteries at low temperatures, with a constant current charge ratio exceeding 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 present 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 including the lithium metal battery.

[0008] According to an embodiment of a 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(trifluoromethanesulfonylimide) and lithium difluorooxalate borate in a mass ratio of 1:1 to 10.

[0009] This invention employs a lithium-ion solvation system constructed with a fluorinated solvent. Due to the electron-withdrawing ability of the fluorine atom, the amount of solvent in the solvation sheath is reduced, and the introduction of anions is facilitated. This results in a lower lithium-ion desolvation energy in the electrolyte system, which is beneficial for lithium-ion deintercalation / extraction and stripping / deposition at low temperatures. Specifically, the lithium metal battery electrolyte of this invention uses a low-melting-point, low-viscosity fluorinated carboxylic acid ester solvent as the main component, which is beneficial for improving the low-temperature conductivity of the electrolyte system. Fluorinated amides have a high dielectric constant, ensuring sufficient ionic conductivity in the electrolyte, and their interaction with Li... + There is almost no solvation between them, so desolvation is not required during lithium-ion transport, which can significantly improve the kinetic performance of lithium-ion transport and thus improve the rate performance of the battery system. In addition, the nitrogen element in fluoroamide is beneficial to enhance the stability of the interfacial film and improve the low-temperature performance of the battery.

[0010] This invention employs a combination of lithium bis(trifluoromethanesulfonyl)imide and lithium difluorooxalateborate as the second lithium salt. Lithium bis(trifluoromethanesulfonyl)imide can well adapt to the changes in the surface and volume of the lithium metal anode during lithium ion deintercalation / intercalation and stripping / deposition under low-temperature conditions. The introduction of lithium difluorooxalateborate induces lithium bis(trifluoromethanesulfonyl)imide to enter the inner layer of the lithium ion solvation structure, and is then preferentially reduced to derive the SEI film, providing an F source for the SEI film. This can effectively reduce the decomposition of the fluorinated solvent, effectively reduce the interfacial impedance, and improve the low-temperature performance of the electrolyte.

[0011] The present invention further controls the mass ratio of lithium bis(trifluoromethanesulfonyl)imide to lithium difluorooxalateborate at 1:1 to 10, which helps to generate an SEI layer rich in inorganic matter. When the mass ratio is too high, it is easy to generate an SEI layer rich in organic matter; conversely, when the mass ratio is too low, the electrolyte viscosity is too high, which causes unstable contact between the electrolyte and the electrode interface, thereby increasing the interfacial impedance or forming irreversible lithium metal deposition.

[0012] According to some embodiments of the present invention, the electrolyte further includes additives, the mass of which 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%, or 1% to 3%.

[0013] According to some embodiments of the present invention, the additive is selected from at least one of magnesium salts and perfluorobutyl sulfonates.

[0014] According to some embodiments of the present invention, the additive comprises magnesium salt and perfluorobutyl 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 facilitate the formation of an interfacial layer consisting of inorganic lithium salts and metallic magnesium, effectively reducing surface side reactions, improving the interfacial stability and coulombic efficiency of the lithium metal anode, lowering interphase resistance, promoting Li ion diffusion, and achieving uniform lithium deposition. Furthermore, magnesium metal provides uniform lithiophilic nucleation sites, resulting in small and stable potential polarization during cycling, thereby suppressing low-temperature lithium deposition. Perfluorobutyl sulfonate also has a stabilizing effect on the lithium metal anode interface and can further synergistically improve the low-temperature performance of lithium metal batteries in conjunction with magnesium salts.

[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, possibly because: NO3 - It can promote the decomposition of bis(trifluoromethanesulfonyl)imide and difluorooxalateborate, increase the LiF content in the SEI layer, and alleviate the growth of lithium dendrites under low-temperature conditions. Furthermore, the generated lithium nitrate has limited solubility in the organic solvent of this invention, avoiding the formation of Mg... 2+ Further spontaneous reduction, coordination between magnesium salt and solvent molecules, and Li + and NO3 - The strong interaction between them makes 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 perfluorobutyl sulfonate is selected from at least one of lithium perfluorobutyl sulfonate or potassium perfluorobutyl sulfonate.

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

[0020] Fluorinated carbonate solvents participate in the solvation structure of lithium ions. The electron-withdrawing fluorinated groups lower the LUMO energy level of the molecule, thereby causing reduction at low potentials and participating in the derivation of the SEI film to obtain a LiF-rich solid electrolyte layer, which increases the transport 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 to 40, for example, 1:3 to 35, 1:3 to 30, 1:3 to 25, 1:3 to 20, 1:5 to 30, 1:5 to 25, 1:5 to 20, 1:10 to 30, 1:10 to 25, 1:10 to 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 fluorocarboxylic acid ester is 1:2 to 80, for example 1:2 to 75, 1:2 to 70, 1:2 to 60, 1:2 to 50, 1:2 to 40, 1:2 to 30, 1:2 to 25, 1:2 to 20, 1:2 to 10, 1:3 to 60, 1:3 to 40, 1:3 to 20, 1:3 to 10, 1:4 to 40, 1:4 to 20, 1:4 to 10, 1:4 to 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, making it easier for lithium ions to detach from the solvation structure, thereby improving the battery's charge / discharge performance.

[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-trifluoromethyl ethyl 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 a relatively low binding energy with lithium ions compared to carbonates / carboxylic esters and their fluorinated derivatives. This allows them to coordinate the lithium ion solvation structure, reduce the lithium ion desolvation energy, and improve the low-temperature performance of the electrolyte.

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

[0028] According to some embodiments of the present invention, in the second lithium salt, the mass ratio of lithium bis(trifluoromethanesulfonyl)imide to lithium difluorooxalateborate is 1:1 to 10, for example 1:1 to 8, 1:2 to 8, 1:4 to 8, 1:4 to 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 present 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 amount of second lithium salt helps to derive an inorganic-rich SEI or CEI layer. Specifically, if the content is too high, it will increase the economic cost; conversely, if the content is too low, the effect of deriving an inorganic-rich SEI or CEI layer will be unsatisfactory.

[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 a second aspect of the present invention, a method for preparing the above-described electrolyte is provided, the 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 carried out 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 oxygen level in the glove box is <0.1 ppm.

[0036] According to a third aspect of the present invention, a lithium metal battery is provided, the lithium metal battery comprising the electrolyte described above.

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

[0038] This invention does not impose any special limitations on the positive electrode sheet, as long as it achieves the purpose of this invention. For example, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. This invention does not impose any special limitations on the positive current collector; for example, the positive current collector can be a metal foil or a porous metal plate, such as a foil or porous plate made of metals or alloys thereof, such as aluminum foil, copper, nickel, titanium, or silver. The positive active material layer includes a positive active material. This invention does not impose any special limitations on the type of positive active material; for example, in some embodiments, the positive active material of a lithium metal battery can include at least one of 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 manganese iron phosphate, or lithium titanate. This invention does not impose special limitations 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 5 μm to 20 μm, preferably 6 μm to 18 μm. The thickness of the single-sided positive electrode active material layer is 30 μm to 120 μm. The positive electrode active material layer can be disposed on one surface or on two surfaces in the thickness direction of the positive electrode current collector. The positive electrode active material layer of this invention may also contain a conductive agent and a binder. In some embodiments of this invention, the mass ratio of the positive electrode material to the conductive agent and binder is 80-90:5-10:5-10. In some embodiments of this invention, the conductive agent is selected from at least one of carbon black conductive agent SP, carbon nanotubes (CNTs), 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), polyacrylamide, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate (PAALi), polyimide, polyamide-imide, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTEE), polyvinyl butyral (PVB), waterborne 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 mixed in a predetermined ratio. The organic solvent can be N-methylpyrrolidone (NMP). The mixture is stirred until a homogeneous system is obtained to obtain a positive electrode slurry. Then, the positive electrode slurry is coated onto a positive electrode current collector, and after drying, rolling, and other processes, the positive electrode is obtained. The surface density of the positive electrode slurry coated on aluminum foil can be 1.35 mAh cm⁻¹. -2 ~2.5mAh cm -2 .

[0039] The negative electrode of the lithium metal battery of the present invention employs a lithium-containing metal sheet, such as a lithium alloy sheet or a lithium metal sheet, wherein 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 may 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 opposing surfaces in its thickness direction, and the lithium-containing metal sheet is stacked on either or both of the two surfaces of the negative electrode current collector. The negative electrode current collector can be made of materials such as metal foil or porous metal plates, for example, foils or porous plates of metals or alloys thereof such as copper, nickel, titanium, or iron, such as copper foil. The negative electrode sheet is a lithium metal foil with a thickness of 200–500 μm and a diameter of 16 mm.

[0040] The lithium metal battery of the present invention does not impose special limitations on the separator. For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, membrane, or composite membrane with 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 nonwoven fabrics, polyethylene nonwoven fabrics, polypropylene-polyethylene-polypropylene porous composite membranes, single-sided ceramic-coated polyethylene-based membranes, polypropylene-based membranes, and polyimide membranes may be used. The present invention does not impose special limitations on the thickness of the separator; for example, the thickness of the separator may be from 5 μm to 500 μm.

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

[0042] A battery cell is obtained by alternately stacking positive and negative electrode sheets and placing a separator between the positive and negative electrode sheets to isolate the positive and negative electrodes. Alternatively, the battery cell can be obtained by winding the electrodes. The battery cell is then placed in a casing, injected with electrolyte, and sealed to obtain a lithium metal battery.

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

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

[0045] The electrolyte of this invention enables lithium metal batteries to be charged / discharged at -40°C and 0.1C with a constant current charging ratio exceeding 80%, thus broadening the application fields and scope of lithium metal batteries.

[0046] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. Detailed Implementation

[0047] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0048] In the description of this invention, unless otherwise explicitly defined, terms such as "set up," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0049] In the description of this invention, the use of terms such as "first," "second," etc., is for the purpose of distinguishing technical features only and should not be construed as indicating or implying relative importance, or implicitly indicating the number of technical features indicated, or implicitly indicating the order of the technical features indicated.

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

[0051] "And / or" is used to indicate that one or both of the described 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 range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0053] Unless otherwise specified, the term "about" in this invention actually means that the error is allowed to be within ±10%, for example, about 100 is actually 100 ± 10% × 100.

[0054] In the description of this invention, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0055] In this invention, unless otherwise specified, all equipment and raw materials are available from the market or commonly used in the industry. Unless otherwise specified, the methods in the following embodiments 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 vinylene carbonate.

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

[0067] KPBS refers to potassium perfluorobutyl sulfonate, CAS number 29420-49-3;

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

[0069] Examples 1-12

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

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

[0072] Comparative Examples 1-8

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

[0074] The preparation method of the comparative electrolyte is as described in Examples 1-12.

[0075] Table 1. Composition of the electrolytes in the examples and comparative examples.

[0076]

[0077]

[0078]

[0079] The percentages in the table refer to the mass percentage of each component relative to the solvent + second lithium salt + additives.

[0080] Application examples

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

[0082] The manufacturing steps of this lithium-ion battery include:

[0083] Preparation of the positive electrode: NCM811 positive electrode (8.3 mg·cm⁻¹) -2 Approximately 1.47 mAh·cm -2 The process involves mixing 80% by weight of NCM811 powder, 10% by weight of carbon black, and 10% by weight of polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidone (NMP), coating the mixture onto aluminum foil, drying the coated electrode (120°C for 12 hours), rolling it (compacted density: 3.2-2.5), and then cutting it to the desired size. The areal density of the coating is 8.3 ± 0.5 mg·cm³. -2 .

[0084] Preparation of negative electrode sheet: Lithium metal is used as the negative electrode, with a thickness specification of 20μm, and it is cut according to the size of the positive electrode sheet.

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

[0086] Lithium metal battery fabrication: Battery assembly: The positive electrode, negative electrode, and separator are stacked together in a specific order to form a battery cell. The battery cell is then encased in a plastic casing. The tabs are connected to the battery casing using welding or bonding techniques. Electrolyte is injected, and then the battery is sealed. Battery activation: The battery is cycled at 0.1C for one cycle at room temperature to activate it and improve its performance. It is then placed at 45°C for 24 hours, followed by constant current and constant voltage charging at 0.1C to 4.3V for battery testing. Specifically, 50 μL of electrolyte is injected into each coin cell, and 3.0 g Ah is injected into each lithium metal pouch cell. -1 .

[0087] Test case

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

[0089] 1) Let stand at 25℃ for 24 hours;

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

[0091] 3) Let stand for 0.5 hours;

[0092] 4) Charge at a constant current of 0.1C to 4.3V, then charge at a constant voltage of 4.3V until the cutoff current is 0.01C;

[0093] 5) Let stand for 10 minutes;

[0094] 6) Let stand at -40℃ for 8 hours;

[0095] 7) Constant current discharge to 2.5V to obtain the discharge capacity retention rate at -40℃;

[0096] 8) Discharge to 2.5V at -40℃, let stand for 2 minutes, and then charge to 4.3V at a constant current of 0.1C.

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

[0098] Table 2 shows the performance test results of the batteries in the examples and comparative examples.

[0099] <![CDATA[Room 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 Example 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 batteries assembled with the electrolytes prepared in Examples 1-12 exhibit excellent low-temperature performance, achieving charge / discharge capability at -40°C and 0.1C with a constant current charge ratio exceeding 80%. Furthermore, compared to Examples 1 and 3, even with increased LiTFSI dosage in the absence of lithium difluorooxalate borate (LiDOFB), the low-temperature performance significantly decreased. This can be attributed to the synergistic effect of the two additives, which leads to a greater amount of anions (DOFB). - TFSI- LiDFOB enters the inner layer of the solvation structure and participates in the derivation of the SEI film. Simultaneously, LiDFOB derivates an inorganic-rich CEI layer at the NCM811 interface, which can improve the electrode's oxidation resistance and reduce interfacial impedance. Comparative Examples 1 and 2 show that replacing FEC with FDMA improves the low-temperature discharge performance of lithium metal batteries. This is attributed to the lower binding energy of FDMA with lithium ions, which reduces the desolvation energy of lithium ions, thus further improving low-temperature performance. The degree of fluorination has a certain impact on low-temperature performance, and Comparative Example 2 demonstrates that the position of the fluorinated group in the molecular structure also affects low-temperature performance.

[0101] The foregoing description, in conjunction with specific embodiments, has provided a detailed account of the embodiments of the present invention. However, the present invention is not limited to the above embodiments, and various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention. Furthermore, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

Claims

1. An electrolyte, characterized in that, The mixture includes lithium salts and organic solvents. The lithium salts include a first lithium salt and a second lithium salt. The organic solvents include fluorocarboxylic acid esters and fluoroamides. The first lithium salt is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorooxalate phosphate, lithium tetrafluorooxalate phosphate, and lithium tetrafluoroborate. The second lithium salt includes lithium bis(trifluoromethanesulfonylimide) and lithium difluorooxalate borate in a mass ratio of 1:(1-10).

2. The electrolyte according to claim 1, characterized in that, The electrolyte also includes additives selected from at least one of magnesium salts and perfluorobutyl sulfonates.

3. The electrolyte according to claim 2, characterized in that, The additives include magnesium salts 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 selected from at least one of ethyl 4,4,4-trifluorobutyrate, methyl difluoroacetate, ethyl difluoroacetate, methyl difluoropropionate, propyl trifluoroacetate, 2-trifluoromethyl ethyl 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 fluorocarboxylic acid ester is 1:(2-80); And / or, the fluorocarbonate is selected from at least one of fluoroethylene carbonate, 1,2-difluoroethylene carbonate, methyltrifluoroethyl carbonate, and bis(trifluoroethyl) carbonate.

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

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

9. A lithium metal battery, characterized in that, The lithium metal battery includes the electrolyte as described in any one of claims 1-7 or the electrolyte prepared by the preparation method described in claim 8.

10. An electronic device, characterized in that, The electronic device includes the lithium metal battery as described in claim 9.

Images