Preparation method of lithium metal battery electrolyte based on fluorine-containing phosphate additive

Abstract

The invention discloses a preparation method of a lithium metal battery electrolyte based on a fluorine-containing phosphate additive, and relates to a preparation method of a lithium metal battery electrolyte. The technical problems that an existing lithium metal battery using commercial electrolyte is poor in stability and short in service life are solved. The method comprises the following steps: 1, drying a lithium salt and a fluorine-containing phosphate additive, wherein the fluorine-containing phosphate additive is one or more of lithium monofluorophosphate, lithium difluorophosphate, stannous fluorophosphate and lithium difluoro bis (oxalate) phosphate; 2, adding an ester additive into the mixed ester solvent to obtain an ester-based mixed solvent; and 3, adding a lithium salt and the fluorine-containing phosphate additive into the ester-based mixed solvent, and stirring to obtain the lithium metal battery electrolyte based on the fluorine-containing phosphate additive. By adopting the electrolyte, a battery is assembled by taking nickel cobalt lithium manganate NCM811 as a positive electrode and taking lithium as a negative electrode, and the capacity retention ratio is 75%-80% after 500 cycles under the current density of 0.5 C. The invention can be used in the lithium battery field.

Description

Technical Field

[0001] The invention relates to a lithium metal battery electrolyte and a preparation method thereof. Background Art

[0002] In order to alleviate the pollution caused by traditional fossil energy, the development of new energy technologies is becoming more urgent. Lithium-ion batteries are widely used in portable electronic devices, electric vehicles, and energy storage devices. Existing lithium-ion batteries generally use graphite as the negative electrode, and the theoretical capacity is only 372mAh g -1 , which cannot meet the growing market demand. Lithium metal batteries using lithium metal as negative electrodes have a capacity of 3860mAh g -1 The high theoretical capacity of Li[Ni] is considered to be a very promising candidate for the next generation of negative electrodes. Among the various positive electrode materials suitable for lithium metal batteries, Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 (NCM811) exhibits the most balanced performance, with high energy density and cycle stability, and is an ideal material for high-capacity positive electrodes. However, existing lithium metal batteries use commercial electrolytes, and lithium metal batteries have problems such as easy interface instability, severe cracking of the SEI film, short cycle life, and low safety factor, which leads to deterioration of lithium metal battery performance and short life. Summary of the invention

[0003] The present invention aims to solve the technical problems of poor stability and short life of existing lithium metal batteries using commercial electrolytes, and provides a method for preparing a lithium metal battery electrolyte based on a fluorine-containing phosphate additive.

[0004] The method for preparing a lithium metal battery electrolyte based on a fluorine-containing phosphate additive of the present invention is carried out according to the following steps:

[0005] 1. placing a lithium salt and a fluorine-containing phosphate additive in a vacuum drying oven at a temperature of 40 to 60° C. and vacuum drying for 0.5 to 5 days to obtain a dry lithium salt and a fluorine-containing phosphate additive, and storing them in an environment where the water oxygen value is less than 0.01 ppm; wherein the lithium salt is one or more of lithium hexafluorophosphate, potassium hexafluorophosphate, lithium bistrifluoromethanesulfonyl imide and lithium perchlorate; and the fluorine-containing phosphate additive is one or more of lithium monofluorophosphate, lithium difluorophosphate, stannous fluorophosphate and lithium difluorobisoxalate phosphate;

[0006] 2. In an environment where the water oxygen value is less than 0.01 ppm, firstly weigh solvent A, solvent B and solvent C according to the volume ratio of solvent A, solvent B and solvent C of 1:(1.8-2.2):(1.8-2.2), mix and stir for 20-30 minutes to obtain a mixed solution; then add an ester additive to the mixed solution, stir for 2-4 hours at room temperature to obtain an ester-based mixed solvent; wherein the solvent A is ethylene carbonate, propylene carbonate or butylene carbonate; the solvent B is dimethyl carbonate, diethyl carbonate, methyl propionate or methyl acetate; the solvent C is ethyl methyl carbonate or ethyl acetate; the ester additive is one or a combination of vinylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, methyl (trimethylsilyl) carbonate and vinyl sulfate;

[0007] 3. In an environment where the water oxygen value is less than 0.01 ppm, add the lithium salt and the fluorine-containing phosphate additive to the ester-based mixed solvent obtained in step 2, and stir at room temperature for 12 to 36 hours to obtain a lithium metal battery electrolyte based on the fluorine-containing phosphate additive.

[0008] Furthermore, the concentration of the ester additive in the ester-based mixed solvent in step 2 is 1.5% to 3%.

[0009] Furthermore, the normal temperature in steps 2 and 3 is 25-35°C.

[0010] Furthermore, the lithium ion concentration in the lithium metal battery electrolyte based on the fluorine-containing phosphate additive described in step three is 1 to 1.5 mol / L.

[0011] Furthermore, the molar ratio of the lithium salt to the fluorine-containing phosphate additive in step three is 1:(0.05-0.2).

[0012] The present invention introduces ester additives and fluorine-containing phosphate additives into an ester-based electrolyte solvent to form an electrolyte, which is used in a lithium metal battery with nickel cobalt manganese oxide as a positive electrode and lithium as a negative electrode. The fluorine-containing phosphate additive effectively promotes the generation of protective LiF, thereby inhibiting the parasitic reaction of lithium metal with the ester-based solvent and the dissolved transition metal, strengthening SEI, and protecting the positive electrode ternary material; effectively reducing lithium dendrite growth, changing the lithium ion solvation structure, inhibiting irreversible parasitic reactions between lithium metal and the electrolyte, improving the lithium deposition morphology, strengthening CEI, and improving the stability of the lithium negative electrode, so that the lithium metal battery system can perform long-term charge and discharge cycles with high capacity, and significantly improving the cycle life of the lithium battery. The present invention is used in the field of lithium batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1This is the positive electrode XRD spectrum of a battery assembled with a commercial electrolyte and a lithium metal battery electrolyte based on a fluorine-containing phosphate additive prepared in Example 1 after 200 cycles of charge and discharge;

[0014] Figure 2 This is a scanning electron microscope image of the positive electrode of a commercial electrolyte battery;

[0015] Figure 3 is a scanning electron microscope image of the positive electrode of the lithium metal battery electrolyte battery based on the fluorine-containing phosphate additive prepared in Example 1;

[0016] Figure 4 This is a scanning electron microscope image of the negative electrode of a commercial electrolyte battery;

[0017] Figure 5 is a scanning electron microscope image of the negative electrode of the lithium metal battery electrolyte battery based on the fluorine-containing phosphate additive prepared in Example 1;

[0018] Figure 6 It is a cycle performance curve diagram of a commercial electrolyte battery and a lithium metal battery electrolyte battery based on a fluorine-containing phosphate additive prepared in Example 1. DETAILED DESCRIPTION

[0019] The beneficial effects of the present invention are verified by the following examples.

[0020] Embodiment 1: The preparation method of the lithium metal battery electrolyte based on the fluorine-containing phosphate additive of this embodiment comprises the following steps:

[0021] 1. Place lithium hexafluorophosphate and lithium difluorophosphate in a vacuum drying oven at 50°C for 1 day to obtain dry lithium hexafluorophosphate and lithium difluorophosphate, and store them in an environment where the water oxygen value is less than 0.01 ppm;

[0022] 2. In an environment where the water oxygen value is less than 0.01 ppm, first weigh 1.96 mL of ethylene carbonate, 3.92 mL of diethyl carbonate and 3.92 mL of ethyl methyl carbonate and mix and stir for 20 minutes to obtain a mixed solution; then add 0.2 mL of fluoroethylene carbonate to the mixed solution, stir at room temperature for 3 hours, and obtain an ester-based mixed solvent;

[0023] 3. In an environment where the water oxygen value is less than 0.01 ppm, 1.52 g of lithium hexafluorophosphate and 0.108 g of lithium difluorophosphate are added to the ester-based mixed solvent obtained in step 2, and stirred at room temperature for 24 hours to obtain a lithium metal battery electrolyte based on a fluorine-containing phosphate additive.

[0024] The lithium ion concentration in the lithium metal battery electrolyte based on fluorine-containing phosphate additives prepared in this embodiment is 1.1 mol / L. The lithium metal battery electrolyte based on fluorine-containing phosphate additives prepared in Example 1 and the commercial electrolyte were respectively used to assemble the battery with nickel cobalt manganese oxide NCM811 as the positive electrode and lithium as the negative electrode. The commercial electrolyte is a customized electrolyte purchased from KLD New Energy Technology Co., Ltd., model KLD-LP361, and the formula of the commercial electrolyte is: 1M LiPF6 in EC:DEC:EMC=1:2:2Vol%;

[0025] The assembled battery was charged and discharged for 200 times. The positive electrode after 200 times of charge and discharge was subjected to X-ray diffraction test, and the obtained XRD spectrum was as follows: Figure 1 As shown, from Figure 1 It can be seen that the positive electrode-electrolyte interface generated by the lithium metal battery electrolyte based on fluorine-containing phosphate additives prepared in the embodiment can effectively inhibit the decomposition and deactivation of the high-nickel positive electrode material NCM811 during the cycle process, reduce lithium-nickel mixing, and thus maintain the structural stability of the positive electrode material.

[0026] The positive electrode was tested by scanning electron microscope after 200 charge and discharge cycles. Figure 2 This is a scanning electron microscope image of the positive electrode of a commercial electrolyte battery; Figure 3 is a scanning electron microscope image of the positive electrode of the lithium metal battery electrolyte battery based on the fluorine-containing phosphate additive prepared in Example 1; Figure 2 It can be seen that after 200 cycles of the commercial electrolyte battery, slight cracks exist on the surface of NCM811 particles. Figure 3 It can be seen that the NCM811 particles on the surface of the positive electrode of the lithium metal battery electrolyte battery based on the fluorine-containing phosphate additive prepared in Example 1 have no cracks.

[0027] The negative electrode after 200 charge and discharge cycles was tested by scanning electron microscope. Figure 4 This is a scanning electron microscope image of the negative electrode of a commercial electrolyte battery; Figure 5 is a scanning electron microscope image of the negative electrode of the lithium metal battery electrolyte battery based on the fluorine-containing phosphate additive prepared in Example 1; Figure 4 It can be seen that after the commercial electrolyte is circulated, the lithium dendrites grow seriously and have a large specific surface area, which accelerates the consumption of the electrolyte and causes side reactions. Figure 5 It can be seen that the lithium metal negative electrode using the lithium metal battery electrolyte based on the fluorine-containing phosphate additive prepared in Example 1 is flat and smooth, and the lithium deposition is uniform.

[0028] The commercial electrolyte battery and the lithium metal battery electrolyte battery based on fluorine-containing phosphate additive prepared in Example 1 were subjected to cycle capacity test at a current density of 0.5C, and the cycle performance test results were as follows: Figure 6As shown, from Figure 6 It can be seen that at a current density of 0.5C, the battery using the electrolyte of Example 1 has a capacity retention rate of 80% after 500 cycles, while the battery using the commercial electrolyte has a capacity retention rate of 51% after 500 cycles. This indicates that the lithium metal battery electrolyte based on the fluorine-containing phosphate additive prepared in Example 1 can perform long-term charge and discharge cycles with high capacity, significantly improving the cycle life of the lithium battery.

[0029] Embodiment 2: The preparation method of the lithium metal battery electrolyte based on the fluorine-containing phosphate additive of this embodiment comprises the following steps:

[0030] 1. Place lithium hexafluorophosphate and lithium difluorobisoxalate phosphate in a vacuum drying oven at a temperature of 50° C. for 1 day to obtain dry lithium hexafluorophosphate and lithium difluorobisoxalate phosphate, and store them in an environment where the water oxygen value is less than 0.01 ppm;

[0031] 2. In an environment where the water oxygen value is less than 0.01 ppm, first weigh 1.96 mL of propylene carbonate, 3.92 mL of dimethyl carbonate and 3.92 mL of ethyl acetate and mix and stir for 20 minutes to obtain a mixed solution; then add 0.2 mL of vinyl ethylene carbonate to the mixed solution, stir at room temperature for 3 hours, and obtain an ester-based mixed solvent;

[0032] 4. In an environment where the water oxygen value is less than 0.01 ppm, add 1.52 g of lithium hexafluorophosphate and 1.72 g of lithium difluorobisoxalate phosphate to the ester-based mixed solvent obtained in step 2, and stir at room temperature for 24 hours to obtain a lithium metal battery electrolyte based on a fluorine-containing phosphate additive.

[0033] A battery was assembled using the lithium metal battery electrolyte based on fluorine-containing phosphate additives prepared in Example 2, with lithium nickel cobalt manganese oxide NCM811 as the positive electrode and lithium as the negative electrode. The cycle capacity test was conducted at a current density of 0.5C, and the capacity retention rate was 75% after 500 cycles.

[0034] The electrolyte added with fluorine-containing phosphate and ester additives in the present invention can effectively adjust lithium deposition in the lithium metal system, can effectively maintain the stability of CEI / SEI, and achieve a higher capacity retention rate of the lithium metal system, and the preparation process is simple and easy to implement.

Claims

1. A method for preparing a lithium metal battery electrolyte based on a fluorine-containing phosphate additive, characterized in that: The method proceeds as follows:

1. placing a lithium salt and a fluorine-containing phosphate additive in a vacuum drying oven at a temperature of 40 to 60° C. and vacuum drying for 0.5 to 5 days to obtain a dry lithium salt and a fluorine-containing phosphate additive, and storing them in an environment where the water oxygen value is less than 0.01 ppm; wherein the lithium salt is one or more of lithium hexafluorophosphate, potassium hexafluorophosphate, lithium bistrifluoromethanesulfonyl imide and lithium perchlorate; and the fluorine-containing phosphate additive is one or more of lithium monofluorophosphate, lithium difluorophosphate, stannous fluorophosphate and lithium difluorobisoxalate phosphate; 2. In an environment where the water oxygen value is less than 0.01 ppm, firstly weigh solvent A, solvent B and solvent C according to the volume ratio of solvent A, solvent B and solvent C of 1:(1.8-2.2):(1.8-2.2), mix and stir for 20-30 minutes to obtain a mixed solution; then add an ester additive to the mixed solution, stir for 2-4 hours at room temperature to obtain an ester-based mixed solvent; wherein the solvent A is ethylene carbonate, propylene carbonate or butylene carbonate; the solvent B is dimethyl carbonate, diethyl carbonate, methyl propionate or methyl acetate; the solvent C is ethyl methyl carbonate or ethyl acetate; the ester additive is one or a combination of vinylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, methyl (trimethylsilyl) carbonate and vinyl sulfate; 3. In an environment where the water oxygen value is less than 0.01 ppm, add the lithium salt and the fluorine-containing phosphate additive to the ester-based mixed solvent obtained in step 2, and stir at room temperature for 12 to 36 hours to obtain a lithium metal battery electrolyte based on the fluorine-containing phosphate additive.

2. The method for preparing a lithium metal battery electrolyte based on a fluorine-containing phosphate additive according to claim 1, characterized in that: The concentration of the ester additive in the ester-based mixed solvent described in step 2 is 1.5% to 3%.

3. The method for preparing a lithium metal battery electrolyte based on a fluorine-containing phosphate additive according to claim 1 or 2, characterized in that: The normal temperature in steps 2 and 3 is 25-35°C.

4. The method for preparing a lithium metal battery electrolyte based on a fluorine-containing phosphate additive according to claim 1 or 2, characterized in that: The lithium ion concentration in the lithium metal battery electrolyte based on the fluorine-containing phosphate additive described in step three is 1 to 1.5 mol / L.

5. The method for preparing a lithium metal battery electrolyte based on a fluorine-containing phosphate additive according to claim 1, characterized in that: The molar ratio of the lithium salt to the fluorine-containing phosphate additive in step three is 1:(0.05-0.2).

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