Flame-retardant low-temperature electrolyte for lithium metal battery with thin lithium negative electrode as well as preparation method and application of flame-retardant low-temperature electrolyte

Abstract

The invention relates to the technical field of flame-retardant low-temperature electrolyte, in particular to flame-retardant low-temperature electrolyte for a lithium metal battery with a thin lithium negative electrode, a preparation method and application. Comprising soluble lithium salt and an organic solvent, the organic solvent comprises the following components in parts by mass: 10-40 parts of ethylene carbonate; 10 to 40 parts of methyl ethyl carbonate; 0 to 30 parts of ethyl propionate; 10 to 40 parts of diethyl carbonate; 0 to 30 parts of ethyl 2-fluoropropionate; 0 to 30 parts of ethyl 2, 2-difluoropropionate; 10 to 50 parts of ethyl 3, 3, 3-trifluoropropionate; and 0-30 parts of one or a combination of more of the components. The fluorine-containing solvent containing the fluorine-containing group and the multifunctional lithium salt are used, the cycle performance of the LiLi symmetric battery can be remarkably improved, the coulombic efficiency of the LiCu half-battery can be remarkably improved, the coulombic efficiency of the LiCu half-battery can be remarkably improved by using the fluorine-containing solvent containing the fluorine-containing group and the multifunctional lithium salt, and the coulombic efficiency of the LiCu half-battery can be remarkably improved by using the fluorine-containing solvent containing the fluorine-containing group and the multifunctional lithium salt. The capacity retention ratio of the thin lithium negative electrode NCM811 total battery can be obviously improved, and meanwhile, the cycle life is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of flame retardant low-temperature electrolytes, and in particular to a flame retardant low-temperature electrolyte for a thin lithium negative electrode lithium metal battery, a preparation method and an application thereof. Background Art

[0002] Lithium metal has a high theoretical specific capacity (3860 mAh g -1 ) and low redox potential (-3.04V vs standard hydrogen electrode), is the key material for realizing high energy density battery. However, the lithium metal negative electrode often has uneven deposition during the deposition / stripping process, which leads to the growth of lithium dendrites and the generation of dead lithium, resulting in excessive active lithium consumption and shortening the battery life. To supplement the active lithium, people usually use thick lithium metal negative electrode, but this will reduce the energy density of lithium metal battery, and the excess lithium will cause more safety problems. The use of limited thickness lithium negative electrode (<50μm) can achieve higher energy density and safety. The use of bare copper negative electrode and thin lithium negative electrode to replace the traditional lithium excess negative electrode, almost all the active lithium in the battery comes from the positive electrode, and there is basically no excess lithium, so the volume can be minimized and the energy density can be maximized. However, since there is no active lithium to supplement during the cycle, the low reversibility of lithium deintercalation / deposition leads to active lithium loss and rapid battery capacity decay.

[0003] In order to realize practical lithium metal batteries under the condition of limiting the excess of lithium negative electrode, people have adopted methods such as current collector modification, construction of artificial interface layer, electrolyte design, etc., and have achieved certain results. Among them, reasonable electrolyte design has been proved to be one of the simplest, most promising and most economical methods to extend the life of lithium metal negative electrode. Fluorinated solvents generally have lower HOMO energy level and LUMO energy level due to the high electronegativity, high ion potential and low polarizability of F atoms. They will preferentially decompose on the electrode surface to form LiF-rich solid electrolyte interface film (SEI) and cathode-electrolyte interface layer (CEI), which can effectively improve the interfacial chemical properties of lithium metal negative electrode.

[0004] For lithium metal anodes, the LiF-rich SEI film with high Young’s modulus can effectively regulate the Li + The uniform deposition of lithium metal can inhibit the growth of dendrites and greatly improve the reversibility of lithium metal deintercalation / deposition. At the same time, the weak solvation tendency of fluorinated solvents can reduce the proportion of solvent molecules in the lithium ion solvation sheath, promote the participation of anions in the interfacial reaction, and improve the interfacial performance of lithium metal. However, the single fluorinated solvent system still faces challenges such as uncontrollable SEI dynamic evolution and insufficient compatibility with high-voltage cathodes, and it is urgent to further optimize the interfacial function through electrolyte engineering.

[0005] Based on this, the present invention provides a flame-retardant low-temperature electrolyte for a thin lithium negative electrode lithium metal battery, a preparation method and an application to solve the technical problems raised above. Summary of the invention

[0006] The purpose of the present invention is to provide a flame retardant low-temperature electrolyte for a thin lithium negative electrode lithium metal battery, a preparation method and an application thereof, so as to solve the problems raised by the above-mentioned background technology.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention provides a flame-retardant low-temperature electrolyte for a thin lithium negative electrode lithium metal battery, comprising a soluble lithium salt and an organic solvent;

[0009] The organic solvent comprises, by mass, one or more of the following: ethylene carbonate: 10-40 parts; ethyl methyl carbonate: 10-40 parts; ethyl propionate: 0-30 parts; diethyl carbonate: 10-40 parts; ethyl 2-fluoropropionate: 0-30 parts; ethyl 2,2-difluoropropionate: 0-30 parts; ethyl 3,3,3-trifluoropropionate: 10-50 parts; fluoroethylene carbonate: 0-30 parts;

[0010] The soluble lithium salt comprises, by mass, one or more of the following: lithium hexafluorophosphate: 20-60 parts; lithium difluorooxalatoborate: 0-40 parts; lithium bisoxalatoborate: 0-40 parts; lithium tetrafluoroborate: 20-60 parts; lithium bistrifluoromethylsulfonyl imide: 0-40 parts.

[0011] Preferably, the molar concentration of the soluble lithium salt in the electrolyte is 1 mol / L.

[0012] Based on the above-mentioned flame-retardant low-temperature electrolyte for thin lithium negative electrode lithium metal battery, the present invention also proposes a method for preparing the flame-retardant low-temperature electrolyte for thin lithium negative electrode lithium metal battery, according to any of claims 1-2, the flame-retardant low-temperature electrolyte for thin lithium negative electrode lithium metal battery, comprising the following steps:

[0013] S1. Prepare materials and equipment:

[0014] Select and prepare at least one of ethylene carbonate, ethyl methyl carbonate, ethyl propionate, diethyl carbonate, ethyl 2-fluoropropionate, ethyl 2,2-difluoropropionate, ethyl 3,3,3-trifluoropropionate, and fluoroethylene carbonate;

[0015] Select and prepare at least one of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium tetrafluoroborate, and lithium bistrifluoromethylsulfonyl imide. The purity of the lithium salt must also meet high standards. Prepare glove boxes, molecular sieves, pipettes, magnetic stirrers, weighing bottles, beakers, volumetric flasks, and experimental equipment and containers. Ensure that all equipment has been cleaned and dried before use.

[0016] S2. In a glove box filled with high-purity argon, purify and dehydrate the selected organic solvent using 4A molecular sieves to remove water from the solvent to avoid adverse effects during the preparation of the electrolyte;

[0017] S3. According to the required electrolyte formula, use precise weighing tools to weigh out the required amount of various organic solvents. In the glove box, use a pipette or a magnetic stirrer to mix the weighed organic solvents evenly. During the mixing process, be careful to avoid introducing air and moisture. Slowly add the required amount of soluble lithium salt to the evenly mixed solvent. Stir continuously during the addition process to ensure that the lithium salt can be evenly dissolved in the solvent. Continue stirring until the lithium salt is completely dissolved in the solvent to form a uniform electrolyte. During this process, pay attention to controlling the temperature and stirring speed to avoid precipitation or crystallization. Use a volumetric flask to make the electrolyte volume constant to ensure that the molar concentration of the soluble lithium salt in the electrolyte reaches 1 mol / L.

[0018] S4. Observe the appearance of the electrolyte to ensure that it is transparent, uniform, and free of precipitation. Perform necessary performance tests on the electrolyte, including conductivity, viscosity, and flash point, to ensure that it meets the use requirements;

[0019] S5. Store the prepared electrolyte in a dry, cool, dark place to prevent it from deteriorating or degrading;

[0020] S6. Match the prepared electrolyte with the positive electrode, negative electrode, and separator, and assemble them according to the standard battery assembly process to obtain a lithium metal battery. During the assembly process, pay attention to controlling the operating conditions and ambient humidity to ensure the performance and quality of the battery.

[0021] The flame-retardant low-temperature electrolyte for thin lithium negative electrode lithium metal battery is applied to the lithium metal battery: the electrolyte is matched with the positive electrode, the negative electrode and the separator, and the lithium metal battery is assembled, and the obtained lithium metal battery has excellent cycle life and battery capacity;

[0022] The positive electrode material is LiNi 0.8 Co 0.1 Mn 0.1 O 2 、LiMn 2 O 4 、LiFePO 4 or LiCoO 2 , preferably LiNi 0.8 Co 0.1 Mn 0.1 O 2 ;

[0023] The negative electrode is copper foil, 6.6 μm lithium foil or 450 μm lithium foil;

[0024] The membrane material is glass fiber, polyethylene or polypropylene.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The present invention uses a fluorinated solvent and a multifunctional lithium salt containing a fluorinated group. The fluorinated solvent and the multifunctional lithium salt can be controllably decomposed preferentially on the surface of lithium metal to induce the formation of an inorganic SEI layer rich in LiF. The high interface energy and excellent ionic conductivity of LiF can inhibit dendrite growth. The present invention uses a fluorinated solvent and a multifunctional lithium salt containing a fluorinated group. The fluorinated solvent and the multifunctional lithium salt can change the solvation structure of the electrolyte to form anion-rich coordination, so that more anions enter the solvation sheath to form a denser and more stable interface layer. The present invention uses a fluorinated solvent and a multifunctional lithium salt containing a fluorinated group, which have excellent low-temperature discharge performance and flame retardant effect. The present invention uses a fluorinated solvent and a multifunctional lithium salt containing a fluorinated group, which can significantly improve the cycle performance of a Li||Li symmetric battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Graph showing the cycle performance of Li||Li symmetric batteries using Example 1, Example 2 and the comparative example;

[0028] Figure 2 Graph showing the coulombic efficiency of Li||Cu half-cells using Example 1, Example 2 and the comparative example;

[0029] Figure 3 Illustrations for flame retardant self-extinguishing experiments using electrolytes of Example 1, Example 2 and the comparative example;

[0030] Figure 4 The cycle performance diagram of the thin lithium||NCM811 full battery using Example 1, Example 2 and the comparative example at 0.2C charging and 0.5C discharge rate;

[0031] Figure 5 This is the discharge performance diagram of the thin lithium||NCM811 full battery at low temperature using the control example;

[0032] Figure 6 This is a discharge performance diagram of a thin lithium||NCM811 full battery using Example 1 at low temperature;

[0033] Figure 7 This is a discharge performance diagram of the thin lithium||NCM811 full battery using Example 2 at low temperature. DETAILED DESCRIPTION

[0034] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] See also Figures 1 to 7 , the present invention provides a flame retardant low temperature electrolyte for a thin lithium negative electrode lithium metal battery, comprising a soluble lithium salt and an organic solvent;

[0036] The organic solvent includes, by mass, one or more of the following: ethylene carbonate: 10-40 parts; ethyl methyl carbonate: 10-40 parts; ethyl propionate: 0-30 parts; diethyl carbonate: 10-40 parts; ethyl 2-fluoropropionate: 0-30 parts; ethyl 2,2-difluoropropionate: 0-30 parts; ethyl 3,3,3-trifluoropropionate: 10-50 parts; fluoroethylene carbonate: 0-30 parts;

[0037] The soluble lithium salt includes, by mass, one or more of the following: lithium hexafluorophosphate: 20-60 parts; lithium difluorooxalatoborate: 0-40 parts; lithium bisoxalatoborate: 0-40 parts; lithium tetrafluoroborate: 20-60 parts; lithium bistrifluoromethylsulfonyl imide: 0-40 parts;

[0038] Preferably, the molar concentration of the soluble lithium salt in the electrolyte is 1 mol / L;

[0039] Based on the above-mentioned flame-retardant low-temperature electrolyte for thin lithium negative electrode lithium metal battery, the present invention also proposes a method for preparing a flame-retardant low-temperature electrolyte for thin lithium negative electrode lithium metal battery, according to any one of claims 1-2, comprising the following steps:

[0040] S1. Prepare materials and equipment:

[0041] Select and prepare at least one of ethylene carbonate, ethyl methyl carbonate, ethyl propionate, diethyl carbonate, ethyl 2-fluoropropionate, ethyl 2,2-difluoropropionate, ethyl 3,3,3-trifluoropropionate, and fluoroethylene carbonate;

[0042] Select and prepare at least one of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium tetrafluoroborate, and lithium bistrifluoromethylsulfonyl imide. The purity of the lithium salt must also meet high standards. Prepare glove boxes, molecular sieves, pipettes, magnetic stirrers, weighing bottles, beakers, volumetric flasks, and experimental equipment and containers. Ensure that all equipment has been cleaned and dried before use.

[0043] S2. In a glove box filled with high-purity argon, purify and dehydrate the selected organic solvent using 4A molecular sieves to remove water from the solvent to avoid adverse effects during the preparation of the electrolyte;

[0044] S3. According to the required electrolyte formula, use precise weighing tools to weigh out the required amount of various organic solvents. In the glove box, use a pipette or a magnetic stirrer to mix the weighed organic solvents evenly. During the mixing process, be careful to avoid introducing air and moisture. Slowly add the required amount of soluble lithium salt to the evenly mixed solvent. Stir continuously during the addition process to ensure that the lithium salt can be evenly dissolved in the solvent. Continue stirring until the lithium salt is completely dissolved in the solvent to form a uniform electrolyte. During this process, pay attention to controlling the temperature and stirring speed to avoid precipitation or crystallization. Use a volumetric flask to make the electrolyte volume constant to ensure that the molar concentration of the soluble lithium salt in the electrolyte reaches 1 mol / L.

[0045] S4. Observe the appearance of the electrolyte to ensure that it is transparent, uniform, and free of precipitation. Perform necessary performance tests on the electrolyte, including conductivity, viscosity, and flash point, to ensure that it meets the use requirements;

[0046] S5. Store the prepared electrolyte in a dry, cool, dark place to prevent it from deteriorating or degrading;

[0047] S6. Match the prepared electrolyte with the positive electrode, the negative electrode, and the separator, and assemble them according to the standard battery assembly process to obtain a lithium metal battery. During the assembly process, pay attention to controlling the operating conditions and ambient humidity to ensure the performance and quality of the battery;

[0048] The flame-retardant low-temperature electrolyte for thin lithium negative electrode lithium metal battery is applied to lithium metal battery: the electrolyte is matched with the positive electrode, the negative electrode and the separator, and the lithium metal battery is assembled. The obtained lithium metal battery has excellent cycle life and battery capacity;

[0049] The positive electrode material is LiNi 0.8 Co 0.1 Mn 0.1 O 2 、LiMn 2 O 4 、LiFePO 4 or LiCoO 2 , preferably LiNi 0.8 Co 0.1 Mn 0.1 O 2 ;

[0050] The negative electrode is copper foil, 6.6μm lithium foil or 450μm lithium foil;

[0051] The diaphragm material is glass fiber, polyethylene or polypropylene;

[0052] Example 1

[0053] In a glove box filled with argon, 70% by volume of 333 ethyl trifluoropropionate (ETFP) and 30% by volume of diethyl carbonate (DEC) were mixed evenly using a pipette, and then lithium hexafluorophosphate (LiPF 6 ), and after it is completely dissolved, a 1 mol / L lithium salt electrolyte is prepared;

[0054] Example 2

[0055] In a glove box filled with argon, 70% by volume of 333 ethyl trifluoropropionate (ETFP) and 30% by volume of diethyl carbonate (DEC) were mixed evenly using a pipette, and then lithium tetrafluoroborate (LiBF 4 ), and after it is completely dissolved, a 1 mol / L lithium salt electrolyte is prepared;

[0056] It should also be noted that the preparation of the negative electrode includes the following steps:

[0057] A thin layer of lithium was electrochemically deposited on the surface of Cu foil using a Li||Cu battery. The diameter of the copper foil was 15 mm and the diameter of the lithium foil was 14 mm. The electrolytes of the control example, Example 1, and Example 2 were used to discharge at a constant current of 0.1 mA for 24 h. After the Cu foil was disassembled from the battery, it was washed in a dimethyl carbonate (DMC) solution and evaporated to dryness for subsequent battery assembly. The above process was carried out in a glove box filled with argon (H 2 O≤0.1ppm, O 2 ≤0.1ppm). The theoretical thin lithium negative electrode thickness is about 6.6μm. It should also be noted that the preparation of the electrolyte includes the following steps:

[0058] S1. Prepare materials and equipment:

[0059] Select and prepare at least one of ethylene carbonate, ethyl methyl carbonate, ethyl propionate, diethyl carbonate, ethyl 2-fluoropropionate, ethyl 2,2-difluoropropionate, ethyl 3,3,3-trifluoropropionate, and fluoroethylene carbonate;

[0060] Select and prepare at least one of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium tetrafluoroborate, and lithium bistrifluoromethylsulfonyl imide. The purity of the lithium salt must also meet high standards. Prepare glove boxes, molecular sieves, pipettes, magnetic stirrers, weighing bottles, beakers, volumetric flasks, and experimental equipment and containers. Ensure that all equipment has been cleaned and dried before use.

[0061] S2. In a glove box filled with high-purity argon, purify and dehydrate the selected organic solvent using 4A molecular sieves to remove water from the solvent to avoid adverse effects during the preparation of the electrolyte;

[0062] S3. According to the required electrolyte formula, use precise weighing tools to weigh out the required amount of various organic solvents. In the glove box, use a pipette or a magnetic stirrer to mix the weighed organic solvents evenly. During the mixing process, be careful to avoid introducing air and moisture. Slowly add the required amount of soluble lithium salt to the evenly mixed solvent. Stir continuously during the addition process to ensure that the lithium salt can be evenly dissolved in the solvent. Continue stirring until the lithium salt is completely dissolved in the solvent to form a uniform electrolyte. During this process, pay attention to controlling the temperature and stirring speed to avoid precipitation or crystallization. Use a volumetric flask to make the electrolyte volume constant to ensure that the molar concentration of the soluble lithium salt in the electrolyte reaches 1 mol / L.

[0063] S4. Observe the appearance of the electrolyte to ensure that it is transparent, uniform, and free of precipitation. Perform necessary performance tests on the electrolyte, including conductivity, viscosity, and flash point, to ensure that it meets the use requirements;

[0064] S5. Store the prepared electrolyte in a dry, cool, dark place to prevent it from deteriorating or degrading;

[0065] S6. Match the prepared electrolyte with the positive electrode, negative electrode, and separator, and assemble them according to the standard battery assembly process to obtain a lithium metal battery. During the assembly process, pay attention to controlling the operating conditions and ambient humidity to ensure the performance and quality of the battery.

[0066] It should also be noted that the assembly and testing of the battery includes the following steps:

[0067] A CR2025 button battery case was selected, and the battery assembly was carried out in a glove box filled with argon. 50 μl of electrolyte was added, and polypropylene was used as a separator.

[0068] Li||Li symmetric battery, two 14mm diameter lithium foils assembled together at 1mA·cm -2 Current density and 1mAh·cm -2 Test under surface capacity;

[0069] Thin Li||NCM811 battery, using the thin lithium negative electrode prepared previously, with a test voltage range of 3-4.3V, first charged and discharged 3 times at 0.1C current, then charged at 0.2C current, and discharged at 0.5C current for cycle testing;

[0070] Li||Cu half-cell, copper foil diameter is 15mm, lithium foil diameter is 14mm, first at 0.5mA·cm -2Discharge for 1h and then at 1mA·cm -2 Charge to 1V.

[0071] All batteries were tested on a NEWARE battery testing system.

[0072] Comparison Example

[0073] In a glove box filled with argon, 70% by volume of ethyl methyl carbonate (EMC) and 30% by volume of ethylene carbonate (EC) were mixed with a pipette, and then lithium hexafluorophosphate (LiPF6) was added to the mixed solvent. 6 ), and after it is completely dissolved, a 1 mol / L lithium salt electrolyte is obtained.

[0074] See also Figure 1 , Cyclic performance diagram of Li||Li symmetric battery using Example 1, Example 2 and Control Example;

[0075] The battery based on the electrolyte of Example 2 showed a stable cycle performance of up to 1200 h, and still maintained a low overpotential of about 40 mV at about 1100 h.

[0076] Batteries based on other electrolytes failed one by one within 800 h due to short circuits caused by lithium dendrite growth (Example 1 > Control Example), indicating that fluorinated solvents and multifunctional lithium salts greatly improved the long-cycle stability of lithium metal negative electrodes;

[0077] See also Figure 2 , Coulombic efficiency diagram of Li||Cu half-cell using Example 1, Example 2 and Control Example;

[0078] The first coulombic efficiency of Example 1 and Example 2 is much higher than that of EC / EMC. In the subsequent cycles, the coulombic efficiency of Example 2 slowly increases and stabilizes at around 98.5%. After 500 cycles, the coulombic efficiency is still 98.56%, indicating that the fluorinated solvent and the multifunctional lithium salt enhance the reversibility of lithium deposition / stripping.

[0079] See also Figure 3 , using the electrolyte flame retardant self-extinguishing test diagram of Example 1, Example 2 and the control example;

[0080] In the self-extinguishing experiment, 0.2 ml of Example 1 and Example 2 still did not ignite after multiple contacts with the muzzle flame of the butane spray gun, while the control electrolyte had a combustion phenomenon, indicating that the addition of the fluorinated solvent enhanced the flame retardant property of the electrolyte;

[0081] See also Figure 4 , Cyclic performance diagram of thin lithium||NCM811 full battery using Example 1, Example 2 and Control Example at 0.2C charge and 0.5C discharge rate;

[0082] Under the thin lithium condition, the battery capacity of the control example dropped sharply after 10 cycles, from 205.7 mAh g -1 Down to 7.31 mAh g -1 ;

[0083] The battery of Example 1 steadily decreased in capacity within 100 cycles, from 198.79 mAh·g in the first cycle to -1 Down to 59.66mAh g -1 and;

[0084] The battery of Example 2 still has a capacity of 129.18 mAh·g after 100 cycles. -1 , the capacity retention rate is 67.35%, and the average CE is 99.80%, indicating that the fluorinated solvent and the multifunctional lithium salt improve the electrochemical performance of lithium metal batteries under thin lithium conditions, showing higher specific capacity and long cycle life;

[0085] See also Figures 5 to 7 , the discharge performance diagram of the thin lithium||NCM811 full battery at low temperature using the control example, embodiment 1 and embodiment 2 respectively;

[0086] The battery of Example 2 under thin lithium conditions exhibits a higher room temperature discharge capacity retention rate at low temperatures (the initial discharge voltage is 4.3 V and the cut-off voltage is 3 V), which is 191.78 mAh·g at 25°C, 0°C, -20°C, and -40°C, respectively. -1 (100%), 190.5mAh·g -1 (99.33%), 170mAh·g -1 (88.64%), 145.6mAh·g -1 (75.92%), Example 1 can release 143.8 mAh·g at -40°C -1 The capacity retention rate is 67.85%;

[0087] The discharge capacity of the control sample at -40℃ is only 91.7 mAh·g -1 , the retention rate is 44.57%, low temperature discharge capacity Example 2> Example 1> Control Example;

[0088] The above electrochemical tests show that fluorinated solvents and multifunctional lithium salts can improve the cycle stability of NCM811 lithium metal batteries under thin lithium conditions, extend their life, and broaden their low-temperature electrochemical performance;

[0089] It can be seen from the above embodiments that the present invention, starting from the perspective of electrolyte, introduces fluorinated solvents and multifunctional lithium salts into the lithium metal battery electrolyte to prepare the electrolyte, which can improve the lithium metal interface performance and deposition uniformity while stably cycling under thin lithium negative electrode conditions, thereby achieving high energy density of the battery, and has good practical value and universality.

[0090] In the description of this specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. 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 can be combined in any one or more embodiments or examples in a suitable manner.

[0091] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific implementation methods described. Obviously, many modifications and changes can be made according to the content of this specification. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can understand and use the present invention well. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A flame-retardant low-temperature electrolyte for a thin lithium negative electrode lithium metal battery, characterized in that: including a soluble lithium salt and an organic solvent; The organic solvent comprises, by mass, one or more of the following: ethylene carbonate: 10-40 parts; ethyl methyl carbonate: 10-40 parts; ethyl propionate: 0-30 parts; diethyl carbonate: 10-40 parts; ethyl 2-fluoropropionate: 0-30 parts; ethyl 2,2-difluoropropionate: 0-30 parts; ethyl 3,3,3-trifluoropropionate: 10-50 parts; fluoroethylene carbonate: 0-30 parts; The soluble lithium salt comprises, by mass, one or more of the following: lithium hexafluorophosphate: 20-60 parts; lithium difluorooxalatoborate: 0-40 parts; lithium bisoxalatoborate: 0-40 parts; lithium tetrafluoroborate: 20-60 parts; lithium bistrifluoromethylsulfonyl imide: 0-40 parts.

2. The flame-retardant low-temperature electrolyte for a thin lithium negative electrode lithium metal battery according to claim 1, characterized in that: The molar concentration of the soluble lithium salt in the electrolyte is 1 mol / L.

3. A method for preparing a flame-retardant low-temperature electrolyte for a thin lithium negative electrode lithium metal battery, according to any of claims 1-2, wherein the flame-retardant low-temperature electrolyte for a thin lithium negative electrode lithium metal battery is characterized in that: The following steps are involved: S1. Prepare materials and equipment: Select and prepare at least one of ethylene carbonate, ethyl methyl carbonate, ethyl propionate, diethyl carbonate, ethyl 2-fluoropropionate, ethyl 2,2-difluoropropionate, ethyl 3,3,3-trifluoropropionate, and fluoroethylene carbonate; Select and prepare at least one of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium tetrafluoroborate, and lithium bistrifluoromethylsulfonyl imide. The purity of the lithium salt must also meet high standards. Prepare glove boxes, molecular sieves, pipettes, magnetic stirrers, weighing bottles, beakers, volumetric flasks, and experimental equipment and containers. Ensure that all equipment has been cleaned and dried before use. S2. In a glove box filled with high-purity argon, purify and dehydrate the selected organic solvent using 4A molecular sieves to remove water from the solvent to avoid adverse effects during the preparation of the electrolyte; S3. According to the required electrolyte formula, use precise weighing tools to weigh out the required amount of various organic solvents. In the glove box, use a pipette or a magnetic stirrer to mix the weighed organic solvents evenly. During the mixing process, be careful to avoid introducing air and moisture. Slowly add the required amount of soluble lithium salt to the evenly mixed solvent. Stir continuously during the addition process to ensure that the lithium salt can be evenly dissolved in the solvent. Continue stirring until the lithium salt is completely dissolved in the solvent to form a uniform electrolyte. During this process, pay attention to controlling the temperature and stirring speed to avoid precipitation or crystallization. Use a volumetric flask to make the electrolyte volume constant to ensure that the molar concentration of the soluble lithium salt in the electrolyte reaches 1 mol / L. S4. Observe the appearance of the electrolyte to ensure that it is transparent, uniform, and free of precipitation. Perform necessary performance tests on the electrolyte, including conductivity, viscosity, and flash point, to ensure that it meets the use requirements; S5. Store the prepared electrolyte in a dry, cool, dark place to prevent it from deteriorating or degrading; S6. Match the prepared electrolyte with the positive electrode, negative electrode, and separator, and assemble them according to the standard battery assembly process to obtain a lithium metal battery. During the assembly process, pay attention to controlling the operating conditions and ambient humidity to ensure the performance and quality of the battery.

4. The use of the flame-retardant low-temperature electrolyte for a thin lithium negative electrode lithium metal battery according to any one of claims 1-2 in a lithium metal battery, characterized in that: The electrolyte is matched with a positive electrode, a negative electrode and a separator to assemble a lithium metal battery, wherein the obtained lithium metal battery has excellent cycle life and battery capacity; The positive electrode material is LiNi 0.8 Co 0.1 Mn 0.1 O2, LiMn2O4, LiFePO4 or LiCoO2, preferably LiNi 0.8 Co 0.1 Mn 0.1 O2; The negative electrode is copper foil, 6.6 μm lithium foil or 450 μm lithium foil; The membrane material is glass fiber, polyethylene or polypropylene.

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