Lithium metal battery low-temperature electrolyte and preparation method and application thereof

Abstract

The invention discloses a lithium metal battery low-temperature electrolyte and a preparation method and application thereof. According to the invention, highly dissociated lithium bis (trifluoromethanesulfonimide) is used as a lithium salt, a methyl acetate solvent with low melting point and low viscosity is added as an anti-freezing agent, and a dimethyl sulfite solvent and a fluoroethylene carbonate solvent which are easy to coordinate with lithium ions are added as a negative electrode film-forming agent; the low-temperature electrolyte with low melting point, low viscosity and high ionic conductivity is obtained, and the battery capacity and the cycle life of the lithium metal battery at low temperature can be effectively improved and prolonged. The lithium iron phosphate half battery is subjected to reversible charge-discharge cycles for 50 circles at-40 DEG C, and the specific discharge capacity can be stabilized at 85.27 mAh g <-1 >. The preparation method is simple, low in cost, low in equipment investment and suitable for batch production.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and in particular relates to a low-temperature electrolyte for lithium metal batteries and a preparation method and use thereof. Background Art

[0002] With the rapid development of electric vehicles, portable wearable devices, aerospace and other fields, higher requirements are placed on the energy density, cycle life and low temperature performance of lithium-ion batteries (LIBs). However, when commercial lithium-ion batteries work at low temperatures (i.e., below -20°C), they suffer from serious capacity loss, poor cycle life, difficulty in charging and even failure. Among them, the slow diffusion of lithium ions in the graphite anode is considered to be one of the main reasons for the poor low temperature performance of lithium-ion batteries. Lithium metal anodes with electroplating / stripping processes have advantages over graphite anodes with lithium ion insertion / deinsertion mechanisms in low temperature applications. Compared with lithium-ion batteries, lithium metal batteries (LMBs) have a low energy density of 0.53 g cm -3 mass density and an anodic potential of -3.04 V and up to 3860 mAh g -1 The theoretical energy density is obviously much higher than the graphite anode (372 mAh g -1 ), so lithium metal batteries are called the "holy grail electrode" and are one of the most promising candidate materials for high-performance electric vehicles and high-power portable electronic devices.

[0003] However, lithium metal batteries still face a series of problems when operating at low temperatures. This is mainly related to the electrolyte and the electrolyte / electrode interface derived from the electrolyte. Traditional carbonate-based electrolytes often have defects such as high viscosity, decreased ionic conductivity, low lithium ion diffusion rate, and large electrolyte / interface resistance under low temperature conditions. This seriously limits the application of lithium metal batteries in cold environments, greatly reducing battery capacity and cycle life, and may even cause lithium precipitation, posing a safety hazard. Although the actual operating temperature of the battery can be increased by external heating, this will consume additional energy.

[0004] Therefore, the design of a rational low-temperature electrolyte is currently the most cost-effective solution to improve the low-temperature performance of LMBs. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a low-temperature electrolyte for lithium metal batteries and a preparation method and use thereof in view of the existing technology. The technology is used to improve the battery performance of lithium metal batteries in low-temperature environments.

[0006] The technical solution adopted by the present invention to solve the above technical problems is: a low-temperature electrolyte for lithium metal batteries, the electrolyte comprising a lithium salt and a solvent, wherein the solvent comprises methyl acetate, dimethyl sulfite and fluoroethylene carbonate;

[0007] It is further defined that the lithium salt is lithium bis(trifluoromethanesulfonyl imide);

[0008] It is further defined that the molecular weight of lithium bis(trifluoromethanesulfonyl imide) is 287.08;

[0009] It is further defined that the molecular weight of fluoroethylene carbonate is 106.05;

[0010] It is further defined that the molecular weight of dimethyl sulfite is 110.13;

[0011] It is further defined that methyl acetate has a molecular weight of 74.08, a viscosity of 0.381 mPa·s, and a melting point of -98.1°C;

[0012] It is further defined that the concentration of lithium salt in the low temperature electrolyte is 0.1 mol L -1 ~1.0mol L -1 ;

[0013] It is further defined that the volume percentage of fluoroethylene carbonate in the solvent is 10% to 40%, the volume percentage of dimethyl sulfite is 20% to 60%, and the volume percentage of methyl acetate is 30% to 90%.

[0014] The present invention also provides a method for preparing the lithium metal battery low-temperature electrolyte, which specifically comprises the following steps:

[0015] 1) In a glove box under an argon atmosphere, methyl acetate, dimethyl sulfite and fluoroethylene carbonate are mixed according to a ratio to obtain a mixed solution;

[0016] 2) Weigh a certain amount of lithium bis(trifluoromethanesulfonyl)imide as a lithium salt, dissolve it in the mixed solution of methyl acetate, dimethyl sulfite and fluoroethylene carbonate, and stir it at room temperature to form a uniform solution, which is the low-temperature electrolyte for lithium metal batteries.

[0017] The present invention also provides the use of the lithium metal battery low temperature electrolyte. The lithium metal battery low temperature electrolyte is used as a lithium metal battery low temperature electrolyte with lithium iron phosphate as the positive electrode to improve the battery capacity and cycle life of the lithium metal battery under low temperature conditions below 0°C. Under low temperature conditions of -40°C, the battery capacity is improved by 0.2C charge and discharge rate (1.0C = 170mA g -1 ) for charge and discharge test, the lithium metal battery has 85.27mAh g -1The initial discharge capacity is 82.8% of the initial capacity after 50 stable cycles. At room temperature, the discharge capacity reaches 151.91 mAh g -1 ; At 0℃, the discharge capacity reaches 136.3mAh g -1 ; At 10°C, the discharge capacity reaches 142.84 mAh g -1 .

[0018] Compared with the prior art, the present invention has the following characteristics:

[0019] In the present invention, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is used as a lithium salt, and the -CF3SO2 group in its structure with strong electron absorption is used to promote the negative charge to leave the structural domain, reduce the ion association pairs, and make the lithium salt highly dissociated, thereby improving the ion conductivity at low temperatures. Methyl acetate (MA) in the solvent has a low melting point and low viscosity, and as an antifreeze, it effectively improves the migration rate of lithium ions at low temperatures. Dimethyl sulfite (DMS) is easy to coordinate with lithium ions, which can promote the interaction between anions and solvent molecules, and improve Li + Fluoroethylene carbonate (FEC) is used as a negative electrode film-forming agent to promote the formation of SEI interface, and a low-temperature electrolyte with low melting point, low viscosity and high ionic conductivity is obtained. The low-temperature electrolyte provided by the present invention is based on lithium iron phosphate as the positive electrode material and lithium sheet as the positive electrode. It can be reversibly charged and discharged at -40°C, and the discharge specific capacity can reach 85.27mAh g -1 , can stably cycle 50 times; at room temperature, the discharge capacity reaches 151.91mAh g -1 ; At 0℃, the discharge capacity reaches 136.3mAh g -1 ; At 10°C, the discharge capacity reaches 142.84 mAh g -1 . BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The charge and discharge cycle diagram of the lithium metal battery low-temperature electrolyte prepared in Example 1 of the present invention applied to a lithium iron phosphate half-cell at -40°C;

[0021] Figure 2 The charge and discharge cycle diagram of the lithium metal battery low-temperature electrolyte prepared in Example 1 of the present invention applied to a lithium iron phosphate half-cell at 0°C;

[0022] Figure 3 This is a charge and discharge cycle diagram of the lithium metal battery low-temperature electrolyte prepared in Example 2 of the present invention applied to a lithium iron phosphate half-cell at room temperature. DETAILED DESCRIPTION

[0023] The present invention is further described in detail below with reference to the embodiments.

[0024] In order for current lithium metal batteries to work stably in low temperature environments, the electrolyte needs to have the following characteristics: (1) low freezing point to maintain liquid state; (2) high ionic conductivity at low temperatures to ensure sufficient Li+ transfer; (3) ability to build a stable low-resistance interface at the electrolyte / electrode interface; (4) maintain low solubility energy of lithium ions to prevent rapid capacity decay. Currently, the low-temperature performance of lithium metal batteries is mainly improved by introducing low melting point and low viscosity solvents, using highly dissociated lithium salts and adding low-temperature electrolyte additives, or using high-concentration electrolytes, local high-concentration electrolytes, and perfluorinated electrolytes.

[0025] Example 1

[0026] In an argon-filled glove box, prepare a 10 mL glass bottle, weigh 1.722 g of lithium bis(trifluoromethanesulfonyl)imide and add it to the bottle, then add 3 mL of methyl acetate, 2.4 mL of dimethyl sulfite and 0.6 mL of fluoroethylene carbonate (solvent volume ratio of 5:4:1) in sequence, stir evenly at room temperature to obtain a lithium salt concentration of 1.0 mol L -1 Secondly, in a glove box filled with argon, the positive electrode material, polyolefin (PP) diaphragm, negative electrode material, positive electrode shell, positive electrode material, low temperature electrolyte, PP diaphragm, negative electrode lithium sheet, gasket, spring sheet, and negative electrode shell are assembled in order to obtain a button battery, wherein the positive electrode material is lithium iron phosphate and the negative electrode material is a lithium sheet;

[0027] The obtained button batteries were placed in ultra-low temperature refrigerators at -40°C and 0°C for 8 hours, respectively. After standing, the button batteries were installed in the ultra-low temperature refrigerators at -40°C and 0°C in the Blue Electric high-precision battery test system and charged and discharged at a rate of 0.2C (1.0C = 170mA g -1 ) to conduct charge and discharge cycle test, the test results are as follows Figure 1 As shown in Figure 2, the discharge capacity can reach 85.27 mAh g after 50 cycles of stable charge and discharge at -40 °C. -1 , the capacity retention rate is as high as 82.8%; the discharge capacity reaches 136.3mAh g after 200 stable charge and discharge cycles at 0℃ -1 , the capacity retention rate is as high as 94.8%.

[0028] Example 2

[0029] In an argon-filled glove box, prepare a 10 mL glass bottle, weigh 1.722 g of lithium bis(trifluoromethanesulfonyl)imide and add it to the bottle, then add 3 mL of methyl acetate, 2.4 mL of dimethyl sulfite and 0.6 mL of fluoroethylene carbonate (solvent volume ratio of 5:4:1) in sequence, stir evenly at room temperature to obtain a lithium salt concentration of 1.0 mol L -1Secondly, in a glove box filled with argon, the positive electrode material, polyolefin (PP) diaphragm, negative electrode material, positive electrode shell, positive electrode material, low temperature electrolyte, PP diaphragm, negative electrode lithium sheet, gasket, spring sheet, and negative electrode shell are assembled in order to obtain a button battery, wherein the positive electrode material is lithium iron phosphate and the negative electrode material is a lithium sheet;

[0030] The obtained button battery was installed in the Blue Electric high-precision battery test system at room temperature and charged and discharged at a 1C rate (1.0C = 170mA g -1 ) to conduct charge and discharge cycle test, the test results are as follows Figure 3 As shown in the figure, the battery can be stably charged and discharged for 300 times at room temperature, and the discharge capacity can reach 151.91 mAh g -1 , the capacity retention rate is as high as 98%.

[0031] Example 3

[0032] In an argon-filled glove box, prepare a 10 mL glass bottle, weigh 1.722 g of lithium bis(trifluoromethanesulfonyl)imide and add it to the bottle, then add 4.2 mL of methyl acetate, 1.2 mL of dimethyl sulfite and 0.6 mL of fluoroethylene carbonate (solvent volume ratio of 7:2:1) in sequence, stir evenly at room temperature to obtain a lithium salt concentration of 1 mol L -1 Secondly, in a glove box filled with argon, the positive electrode material, polyolefin (PP) diaphragm, negative electrode material, positive electrode shell, positive electrode material, low temperature electrolyte, PP diaphragm, negative electrode lithium sheet, gasket, spring sheet, and negative electrode shell are assembled in order to obtain a button battery, wherein the positive electrode material is lithium iron phosphate and the negative electrode material is a lithium sheet;

[0033] The obtained button battery was installed in the Blue Electric high-precision battery testing system at room temperature and the charge and discharge cycle test was carried out at a 1C charge and discharge rate.

[0034] Example 4

[0035] In an argon-filled glove box, prepare a 10 mL glass bottle, weigh 1.722 g of lithium bis(trifluoromethanesulfonyl)imide and add it to the bottle, then add 1.8 mL of methyl acetate, 3.6 mL of dimethyl sulfite and 0.6 mL of fluoroethylene carbonate (solvent volume ratio is 3:6:1) in sequence, stir evenly at room temperature to obtain a lithium salt concentration of 1 mol L -1 Secondly, in a glove box filled with argon, the positive electrode material, polyolefin (PP) diaphragm, negative electrode material, positive electrode shell, positive electrode material, low temperature electrolyte, PP diaphragm, negative electrode lithium sheet, gasket, spring sheet, and negative electrode shell are assembled in order to obtain a button battery, wherein the positive electrode material is lithium iron phosphate and the negative electrode material is a lithium sheet;

[0036] The obtained button battery was installed in the Blue Electric high-precision battery testing system at room temperature and the charge and discharge cycle test was carried out at a 1C charge and discharge rate.

Claims

1. A low-temperature electrolyte for lithium metal batteries, characterized in that: The electrolyte comprises a lithium salt and a solvent, wherein the solvent comprises methyl acetate, dimethyl sulfite and fluoroethylene carbonate; and the lithium salt is lithium bis(trifluoromethanesulfonyl)imide.

2. A method for preparing a low-temperature electrolyte for a lithium metal battery as claimed in claim 1, characterized in that: The preparation method comprises the following steps: In an argon atmosphere in a glove box, a certain amount of lithium bis(trifluoromethanesulfonyl)imide is weighed as a lithium salt, and solvents methyl acetate, dimethyl sulfite and fluoroethylene carbonate are added according to a ratio, and mixed and stirred at room temperature to obtain a mixed solution; that is, the lithium metal battery low-temperature electrolyte; The concentration of the lithium salt in the low-temperature electrolyte of the lithium metal battery is 0.1 mol L -1 ~1.0mol L -1 ; The volume percentage of fluoroethylene carbonate in the solvent is 10% to 40%, the volume percentage of dimethyl sulfite is 20% to 60%, and the volume percentage of methyl acetate is 30% to 90%.

3. The use of a low-temperature electrolyte for lithium metal batteries according to claim 1, characterized in that: The electrolyte is a lithium metal low-temperature electrolyte with lithium iron phosphate as the positive electrode. At a low temperature of -40°C, the charge and discharge rate is 0.2C, 1.0C = 170mAg -1 , the charge and discharge test showed that the lithium metal battery had 85.27mAh g -1 The initial discharge capacity is 82.8% of the initial capacity after 50 stable cycles. At room temperature, the discharge capacity reaches 151.91 mAh g -1 ; At 0℃, the discharge capacity reaches 136.3mAh g -1 ; At 10°C, the discharge capacity reaches 142.84 mAh g -1 ; The electrolyte has applications in the field of low-temperature batteries.

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