A carboxylate-based electrolyte for low-temperature lithium batteries

Abstract

The present invention discloses a carboxylate-based electrolyte for low-temperature lithium batteries, and belongs to the technical field of lithium battery electrolytes. The present invention comprises lithium salts, carboxylate solvents, fluorinated compound diluents and additives. The use of fluorinated compounds as diluents improves the physical properties, electrochemical stability and safety of carboxylate-based electrolytes, adjusts the solvation structure and interface reaction, and improves the compatibility of electrolytes with electrode materials. It is suitable for a variety of commercial lithium-ion batteries and new lithium battery systems, and can achieve excellent low-temperature electrochemical performance. The present invention provides a reference for the design and optimization of low-temperature electrolytes and has a wide range of application value.

Description

Technical Field

[0001] The present invention relates to the technical field of low-temperature electrolytes for lithium batteries, and in particular to a carboxylate-based electrolyte for low-temperature lithium batteries. Background Art

[0002] Lithium-ion batteries (LIBs) are widely used in portable electronics, electric and hybrid vehicles, and renewable energy storage devices due to their advantages such as high energy density, low self-discharge, and stable cycle performance. However, the performance of commercial lithium-ion batteries degrades in low-temperature environments, mainly including reduced energy and power density, difficulty in charging, shortened cycle life, and safety issues. As an important component of lithium-ion batteries, the performance degradation of electrolytes under low temperature conditions is particularly significant. In lithium-ion electrolytes, cyclic carbonates, especially ethylene carbonate (EC), are often used as polar solvents because EC has a strong ability to dissolve lithium salts and can form a stable solid electrolyte interface (SEI) to reduce side reactions, thereby improving compatibility with most electrode materials. However, carbonate electrolytes based on EC usually have a high freezing point and high lithium ion solvation energy, resulting in slow physical properties and desolvation kinetics in low-temperature applications.

[0003] In response to the above problems, short-chain carboxylates such as ethyl acetate (EA) and methyl propionate (MP) are used instead of carbonates as polar solvents for electrolytes, which can effectively improve the physical properties of electrolytes such as viscosity and conductivity at low temperatures and reduce polarization. Carboxylates with weak binding energy with lithium ions can also reduce the energy barrier of the desolvation process of lithium ions at low temperatures, effectively improving the kinetics of electrochemical reactions. However, short-chain carboxylates have strong reactivity and are difficult to form a stable protective SEI layer at the electrode interface. They have serious side reactions with lithium metal negative electrodes and low coulombic efficiency. In graphite negative electrodes, solvent co-intercalation will occur, destroying the structure of the electrode material. Short-chain carboxylates have a low flash point and there is a risk of fire or even explosion. Due to defects in stability, safety and compatibility with electrode materials, short-chain carboxylates are often used only as co-solvents in carbonate-based electrolyte systems. However, this type of electrolyte still uses carbonate as the main solvent to combine with lithium ions, making it difficult to give full play to the advantages of carboxylate desolvation kinetics at low temperatures. Since it is difficult to stably adapt carboxylate-based electrolytes to commercial electrode materials such as graphite and new electrode materials such as lithium metal and organic positive electrodes, there are few lithium battery electrolytes that directly use carboxylate as the main solvent. Summary of the invention

[0004] In order to overcome the limitations of current carboxylate-based electrolytes, the purpose of the present invention is to provide a carboxylate-based electrolyte for a low-temperature lithium battery. By introducing a fluorinated compound diluent and a small amount of additives into a highly concentrated carboxylate-based electrolyte system, a unique low-temperature electrolyte system is formed, the lithium ion solvation structure in the electrolyte is changed, the desolvation energy of lithium ions is reduced, and the battery reaction kinetics are improved. Fluorinated compounds have good physical properties and electrochemical stability, and can inhibit the side reactions of carboxylates and electrode materials. The addition of fluorinated compounds can also promote the participation of lithium salt anions in the solvation structure, thereby forming a stable, dense and low-impedance anion-derived inorganic SEI layer, which is not only suitable for lithium-ion batteries, but also for new lithium battery systems such as lithium negative electrodes and sulfurized polyacrylonitrile positive electrodes, improving the room temperature and low temperature performance of the battery.

[0005] The technical solution of the present invention is:

[0006] A low-temperature lithium battery carboxylate-based electrolyte is prepared by mixing a lithium salt, a carboxylate organic solvent, a fluorinated compound diluent, and an additive in a molar ratio of 1:(0.1-10):(1-20):(0.01-1).

[0007] Preferably, the electrolyte is prepared by mixing lithium salt, carboxylic acid ester organic solvent, fluorinated compound diluent and additive in a molar ratio of 1:(0.8-5.0):(2-15):(0.05-0.4).

[0008] The preparation process of the carboxylate-based electrolyte is as follows: first, dissolving lithium salt in a carboxylate-based organic solvent, stirring for 2 to 24 hours at 25 to 60° C., and then adding a fluorinated compound diluent and an additive to obtain the carboxylate-based electrolyte.

[0009] In the carboxylate-based electrolyte, the lithium salt is one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiDFOB), lithium bis(oxalatoborate) (LiBOB), lithium trifluorooxalatoborate (LiTFOB), lithium trifluoromethanesulfonate (LiTf), lithium bis(trifluorosulfonylimide) (LiFSI), lithium bis(trifluorosulfonylimide) (LiTFSI), and lithium trifluoroacetate (LiTFA).

[0010] In the carboxylate-based electrolyte, the carboxylate organic solvent is one or more of the molecular formula R1COOR2, wherein R1 and R2 are alkyl or cycloalkyl groups with a carbon number of less than 6 (such as methyl propionate (MP), ethyl acetate (EA), ethyl butyrate (EB), etc.).

[0011] In the carboxylate-based electrolyte, the fluorinated compound diluent is one or more of fluorinated ethers, fluorinated carboxylates, fluorinated alkanes, and fluorinated aromatic hydrocarbons.

[0012] In the carboxylate-based electrolyte, the additive is one or more of fluoroethylene carbonate (FEC), lithium nitrate (LiNO3), propylene sulfite (PS), difluoroethylene carbonate (DFEC), methyl trifluoroethyl carbonate (FEMC), vinyl sulfate (DTD), dimethyl succinate (DMS), ethylene carbonate (VC), and lithium difluorophosphate (LiPO2F2).

[0013] The operating temperature of the low-temperature lithium battery carboxylate-based electrolyte is -80 to 60°C.

[0014] The advantages and beneficial effects of the present invention are:

[0015] 1. The present invention provides a carboxylate-based electrolyte for use in low-temperature lithium batteries.

[0016] 2. The present invention uses a fluorinated compound diluent to improve the physical properties, electrochemical stability, electrode material compatibility and safety of the electrolyte.

[0017] 3. The carboxylate-based electrolyte proposed in the present invention significantly improves the room temperature cycle stability and high rate capacity retention of lithium iron phosphate and graphite.

[0018] 4. The carboxylate-based electrolyte proposed in the present invention significantly improves the stability and coulombic efficiency of the lithium metal negative electrode.

[0019] 5. The carboxylate-based electrolyte proposed in the present invention significantly improves the low-temperature charge and discharge capacity, cycle stability and high-rate performance of lithium-ion batteries.

[0020] 6. The carboxylate-based electrolyte proposed in the present invention realizes the extremely low temperature discharge of lithium batteries using sulfide polyacrylonitrile material as the positive electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The present invention is a flow chart for preparing the carboxylate-based electrolyte.

[0022] Figure 2 The room temperature cycle performance of a lithium iron phosphate half-cell using the carboxylate-based electrolyte of the present invention at a 1C charge and discharge rate.

[0023] Figure 3 The room temperature rate performance of a lithium iron phosphate half-cell using the carboxylate-based electrolyte of the present invention at charge and discharge rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C, and 20C.

[0024] Figure 4 The room temperature cycle performance of a graphite half-cell using the carboxylate-based electrolyte of the present invention at a 1C charge and discharge rate.

[0025] Figure 5 In order to use the carboxylate-based electrolyte of the present invention at 1 mA cm -2 Current density and 1mAh cm -2 Coulombic efficiency in capacity-cycled lithium-copper half-cells.

[0026] Figure 6 The -30°C cycle performance of a lithium iron phosphate graphite full battery using the carboxylate-based electrolyte of the present invention at a charge and discharge rate of 0.2C.

[0027] Figure 7 The -60°C discharge curve of a polyacrylonitrile half-cell using the carboxylate-based electrolyte of the present invention at a rate of 0.05C.

[0028] Figure 8 The room temperature performance of a lithium cobalt oxide half-cell using the carboxylate-based electrolyte of the present invention at a charge and discharge rate of 0.1C.

[0029] Fig. 9 The -40°C charge and discharge performance of a lithium iron phosphate half-cell using the carboxylate-based electrolyte of the present invention at a charge and discharge rate of 0.05C.

[0030] Fig.10 The -40°C charge and discharge performance of a lithium iron phosphate half-cell using the carboxylate-based electrolyte of the present invention at a charge and discharge rate of 0.1C.

[0031] Fig.11 The carboxylate-based electrolyte of the present invention is used to generate 1 mA cm at a low temperature of -20 °C. -2 Current density and 1mAhcm -2 Coulombic efficiency in capacity-cycled lithium-copper half-cells. DETAILED DESCRIPTION

[0032] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with specific embodiments.

[0033] Example 1

[0034] Figure 1The flow chart for the preparation of the carboxylate-based electrolyte for the low-temperature lithium battery. First, lithium bis(fluorosulfonyl)imide was dissolved in the polar carboxylate solvent methyl propionate at a molar ratio of 1:1.8, and magnetically stirred at 25°C for 2 hours. The fluorinated compound diluent methyl pentafluoropropionate was added to the solution, and the molar ratio of the lithium salt to the fluorinated compound diluent was 1:5. Then the fluoroethylene carbonate additive was added, and the molar ratio of the lithium salt to the additive was 1:0.58 to obtain a carboxylate-based low-temperature electrolyte. The lithium battery using the carboxylate-based electrolyte exhibits excellent performance at both room temperature and low temperature. Specific room temperature performance: At a charge and discharge rate of 1C, the capacity retention rate of the lithium iron phosphate half-cell using the carboxylate-based electrolyte reached 98.01% after 300 cycles, and had a coulombic efficiency of 99.75% ( Figure 2 ). At 20C high rate charge and discharge, the average capacity reaches 117mAh / g ( Figure 3 After 200 cycles at 1C charge and discharge rate, the graphite half-cell has a capacity retention rate of 97.09% and a coulombic efficiency of 99.94% ( Figure 4 ). The lithium copper half-cell is 1 mA cm -2 Current density and 1mAh cm -2 After 100 cycles of capacity cycling, the average Coulomb efficiency is 96.71% ( Figure 5 ). Under low temperature conditions: After 130 cycles of low-temperature charge and discharge at -30°C at a rate of 0.2C, the capacity retention rate of the lithium iron phosphate graphite full battery is 84.34%, and the coulombic efficiency is 99.73% ( Figure 6 ). Under -60℃ discharge conditions, the discharge capacity of the polyacrylonitrile half-cell reaches 434mAh / g ( Figure 7 ).

[0035] Example 2

[0036] The difference between Example 2 and Example 1 is that the types of lithium salts are different. First, lithium hexafluorophosphate is dissolved in polar carboxylate solvent methyl propionate at a molar ratio of 1:1.8, and magnetic stirring is carried out at 25°C for 2 hours. The fluorochemical diluent methyl pentafluoropropionate is added to the solution, and the molar ratio of lithium salt to fluorochemical diluent is 1:5. Then, fluoroethylene carbonate additive is added, and the molar ratio of lithium salt to additive is 1:0.58 to obtain a carboxylate-based low-temperature electrolyte. The lithium cobalt oxide half-cell using the carboxylate-based electrolyte has a discharge specific capacity of 188.9 mAh / g ( Figure 8 ).

[0037] Example 3

[0038] The difference between Example 3 and Example 1 is that the carboxylate polar solvents are different. First, lithium bis(fluorosulfonyl)imide is dissolved in the polar carboxylate solvent ethyl acetate at a molar ratio of 1:1.8, and magnetically stirred at 25°C for 2 hours. The fluorochemical diluent methyl pentafluoropropionate is added to the solution, and the molar ratio of lithium salt to fluorochemical diluent is 1:5. Then, a fluoroethylene carbonate additive is added, and the molar ratio of lithium salt to additive is 1:0.58 to obtain a carboxylate-based low-temperature electrolyte. The lithium iron phosphate half-cell using the carboxylate-based electrolyte has a charge and discharge specific capacity of 92mAh / g (cycled at a rate of 0.05C at -40°C). Fig. 9 ).

[0039] Example 4

[0040] The difference between Example 4 and Example 1 is that the fluorinated compound diluent is different. First, lithium bis(fluorosulfonyl)imide is dissolved in the polar carboxylate solvent methyl propionate at a molar ratio of 1:1.8, and magnetically stirred at 25°C for 2 hours. The fluorinated compound diluent 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether is added to the solution, and the molar ratio of lithium salt to fluorinated compound diluent is 1:5. Then, fluoroethylene carbonate additive is added, and the molar ratio of lithium salt to additive is 1:0.58 to obtain a carboxylate-based low-temperature electrolyte. The lithium iron phosphate half-cell using the carboxylate-based electrolyte has a charge and discharge specific capacity of 96mAh / g (cycled at -40°C at a rate of 0.1C). Fig.10 ).

[0041] Example 5

[0042] The difference between Example 5 and Example 1 lies in the ratio of polar carboxylate solvent and the type and ratio of additives. First, lithium bis(fluorosulfonyl)imide was dissolved in polar carboxylate solvent methyl propionate at a molar ratio of 1:1.2, and magnetically stirred at 25°C for 2 hours. Methyl pentafluoropropionate, a fluorinated compound diluent, was added to the solution, and the molar ratio of lithium salt to fluorinated compound diluent was 1:5. Then lithium nitrate additive was added, and the molar ratio of lithium salt to additive was 1:0.2 to obtain a carboxylate-based low-temperature electrolyte. The lithium-copper half-cell using the carboxylate-based electrolyte operated at -20°C at 1 mA cm -2 Current density and 1mAhcm -2 Capacity cycle 20 times, with 83% Coulombic efficiency ( Fig.11 ).

[0043] The results of the embodiments show that the present invention improves the electrode compatibility of the carboxylate-based electrolyte, optimizes the physical and chemical properties of the electrode-electrolyte interface, and further promotes the low-temperature electrochemical reaction kinetics of the carboxylate-based electrolyte, thereby improving the cycle stability and rate performance of the battery, and significantly improving the low-temperature charge and discharge capacity of the battery.

Claims

1. A low-temperature lithium battery carboxylate-based electrolyte, characterized in that: The electrolyte is prepared by mixing lithium salt, carboxylic acid ester organic solvent, methyl pentafluoropropionate and additives in a molar ratio of 1:(0.8-5.0):(2-15):(0.05-0.4).

2. The low-temperature lithium battery carboxylate-based electrolyte according to claim 1, characterized in that: The lithium salt is one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiDFOB), lithium bis(oxalatoborate) (LiBOB), lithium trifluorooxalatoborate (LiTFOB), lithium trifluoromethanesulfonate (LiTf), lithium bis(trifluorosulfonylimide) (LiFSI), lithium bis(trifluorosulfonylimide) (LiTFSI) and lithium trifluoroacetate (LiTFA).

3. The low-temperature lithium battery carboxylate-based electrolyte according to claim 1, characterized in that: The carboxylic acid ester organic solvent is one or more of the molecular formula R1COOR2, wherein R1 and R2 are alkyl or cycloalkyl groups with less than 6 carbon atoms.

4. The low-temperature lithium battery carboxylate-based electrolyte according to claim 1, characterized in that: The additive is one or more of fluoroethylene carbonate (FEC), lithium nitrate (LiNO3), propylene sulfite (PS), difluoroethylene carbonate (DFEC), methyl trifluoroethyl carbonate (FEMC), vinyl sulfate (DTD), dimethyl succinate (DMS), ethylene carbonate (VC) and lithium difluorophosphate (LiPO2F2).

5. The low-temperature lithium battery carboxylate-based electrolyte according to claim 1, characterized in that: The operating temperature of the lithium battery electrolyte is -80 to 60°C.

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