High-voltage electrolyte additive, electrolyte, lithium ion battery and preparation method

Abstract

The invention provides a high-voltage electrolyte additive, an electrolyte, a lithium ion battery and a preparation method, and belongs to the technical field of lithium ion batteries. The high-voltage electrolyte additive comprises methyl pentafluorobenzoate and bis (2, 2, 2-trifluoroethoxyl) methane, under the synergistic effect of fluoro aromatic ester of MFPB and a perfluoroether chain of BTFEM, a planar rigid structure of MFPB covers a high-activity crystal face of a positive electrode, a flexible fluoroether chain of BTFEM fills interface gaps, and the two cooperate to form a compact composite protective layer. Besides, the dynamic ester exchange balance of the MFPB repairs interfacial film microcracks, the BTFEM redistributes charges through the modes of MFPB electron absorption, BTFEM Li < + > enrichment and the like, the redistributed charges can inhibit oxygenolysis of the electrolyte and lithium dendrite growth, the purpose of stabilizing the electrolyte-electrode interface is achieved, the electrochemical window of the electrolyte is widened to be larger than or equal to 4.5 V, and the electrochemical performance of the electrolyte is improved. And long-acting oxidation resistance and interface stabilization protection under a high-voltage working condition are realized. In addition, the MFPB can enhance the solubility of the electrolyte, the BTFEM can optimize the transmission path of Li < + >, after the MFPB and the BTFEM are compounded, the capacity fading under the high-voltage working condition can be obviously slowed down, efficient reaction kinetics can be maintained, and the comprehensive performance is far better than that of a traditional system.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries and relates to a high-voltage electrolyte additive, an electrolyte, a lithium ion battery and a preparation method. Background Art

[0002] Lithium-ion batteries are secondary batteries that rely on the movement of lithium ions between the positive and negative electrodes to work. Lithium-ion batteries have the advantages of high energy density, high average output voltage, long cycle life, and good safety performance. With the widespread application of lithium-ion batteries in electric vehicles, energy storage systems and other fields, higher requirements are placed on the energy density and safety of batteries.

[0003] High-voltage lithium-ion batteries have attracted much attention because they can provide higher energy density. However, the stability of the electrolyte under high voltage has become one of the main bottlenecks restricting its development. At present, the electrolyte commonly used in high-voltage lithium-ion batteries is a carbonate electrolyte, which is prone to oxidative decomposition and generates an unstable interface film under high voltage (≥4.3V), resulting in an increase in the interface impedance between the electrode and the electrolyte, a decrease in the stability of the electrolyte, and then causing battery capacity decay and shortened cycle life, and even causing safety issues. In addition, the solubility and conductivity of the electrolyte under high voltage are poor, resulting in a decrease in the electrochemical reaction rate of the battery under high voltage, further affecting the charge and discharge performance of the battery. In addition, the decomposition products of the electrolyte under high voltage may cause irreversible damage to the electrode material, resulting in a significant decrease in the capacity retention rate of the battery.

[0004] In order to improve the performance of lithium-ion batteries at high voltage, the research of electrolyte additives has become the key. For example, Lei (Journal of Electroanalytical Chemistry, 2019, 846,113141) et al. added 2 wt% lithium difluorophosphate (LiPO2F2) to carbonate electrolyte and found that the high voltage cycle life of NCM523 / graphite battery system was significantly enhanced regardless of low or high temperature environment. Liu (ACS Appl Mater Interfaces, 2019, 11(19): 17435-17443) et al. studied the application of boron-containing electrolyte additive trimethyl borate (TMB) in LiCoO2 batteries. After adding 2 wt% TMP to the basic carbonate electrolyte, when the charging voltage increased to 4.5V, TMB can effectively improve the high-voltage electrochemical performance of lithium-ion batteries, where TMB can be preferentially oxidized and then form a strong passivation layer on the cathode surface.

[0005] However, most of the existing high-voltage electrolyte additives improve the stability of the electrolyte through a single mechanism, such as improving the electrolyte's antioxidant properties or enhancing the stability of the electrode interface, and often cannot take into account the electrolyte's solubility, conductivity, and electrode interface stability at the same time. Therefore, it is of great significance to develop a composite additive that can simultaneously improve the electrolyte's solubility, conductivity, and high-voltage stability. Summary of the invention

[0006] The purpose of the present invention is to provide a high voltage electrolyte additive, an electrolyte, a lithium ion battery and a preparation method to solve the problem that the existing lithium ion battery has poor performance under high voltage.

[0007] To achieve the above object, the present invention adopts the following technical solutions: In a first aspect, the present application provides a high voltage electrolyte additive, which includes methyl pentafluorobenzoate (English name: methyl pentafluorobenzoate; abbreviated as: MFPB) and bis(2,2,2-trifluoroethoxy)methane (English name: 1,4-bis(2,2,2-trifluoroethoxy)benzene; abbreviated as: BTFEM).

[0008] MFPB has a fluorinated aromatic ester structure C6F5COOCH3, and its planar rigid structure can be preferentially adsorbed on the positive electrode surface and oxidized to form a dense CF bond interface film. The strong electron-withdrawing effect of the CF bond interface film can significantly inhibit the catalytic oxidation reaction of transition metal ions under high voltage, thereby inhibiting the oxidative decomposition of the electrolyte; at the same time, the CF bond interface film removes acidic byproducts of the electrolyte, such as HF, through dynamic ester exchange equilibrium, thereby delaying the degradation of the CEI membrane (English name: Chemical-Electrochemical Interface; Chinese name: Chemical-Electrochemical Interface Membrane) and continuously repairing CEI membrane defects.

[0009] BTFEM has a perfluorinated ether chain CF3OCH2-, which promotes Li + Desolvation and adsorption of PF6 - Free anions such as fluorinated ether can reduce the interface polarization; at the same time, the interface gaps are filled by flexible molecular chains, thereby forming a composite protective layer with MFPB. In addition, the perfluoroether chain is >5.0Vvs.Li / Li + The inherent oxidation potential and the weak coordination of ether oxygen synergistically broaden the electrochemical window of the electrolyte to 4.5V, so that the discharge specific capacity of the lithium-ion battery remains at 139 mAh / g after 150 cycles at a high cut-off voltage of 4.5V, and the capacity retention rate reaches 97%.

[0010] Under the synergistic effect of the fluorinated aromatic ester of MFPB and the perfluoroether chain of BTFEM, the planar rigid structure of MFPB covers the highly active crystal surface of the positive electrode, and the flexible fluorinated ether chain of BTFEM fills the interfacial gaps, and the two cooperate to form a dense composite protective layer. In addition, the dynamic ester exchange equilibrium of MFPB repairs the microcracks of the interfacial film, and BTFEM absorbs electrons through MFPB and enriches Li + The redistributed charge can inhibit the oxidative decomposition of the electrolyte and the growth of lithium dendrites, thereby stabilizing the electrolyte-electrode interface, widening the electrochemical window of the electrolyte to ≥4.5 V, and achieving long-term antioxidant and interface stabilization protection under high voltage conditions. In addition, MFPB can also enhance the solubility of the electrolyte, and BTFEM can optimize Li + The combination of the two can significantly slow down the capacity decay under high voltage conditions, maintain efficient reaction kinetics, and the overall performance far exceeds that of traditional systems.

[0011] In the present application, the mass ratio of MFPB and BTFEM is (1-3):(1-3).

[0012] In a second aspect, the present application provides an electrolyte, comprising the high-voltage electrolyte additive in the first aspect, wherein the added amount of MFPB is 1-3% of the total mass of the electrolyte.

[0013] In addition, the electrolyte also includes a basic electrolyte composed of ethylene carbonate (English name: Ethylene carbonate; abbreviated: EC), dimethyl carbonate (English name: Dimethyl carbonate; abbreviated: DMC), diethyl carbonate (English name: Diethylcarbonate; abbreviated: DEC) and lithium salt, which is an existing carbonate electrolyte. In the basic electrolyte, the volume ratio of EC, DMC and DEC is 1:1:1, the concentration of lithium salt in the basic electrolyte is 1M, and the lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium bis(oxalatoborate).

[0014] In a third aspect, the present application provides a method for preparing an electrolyte, comprising: S01: Ethylene carbonate, dimethyl carbonate and diethyl carbonate are mixed after removing water under an argon atmosphere, lithium salt is added, and stirred until completely dissolved to form a basic electrolyte.

[0015] DMC and DEC were placed in sealed containers containing 3Å or 4Å molecular sieves, respectively, and then placed in a vacuum glove box to avoid lithium salt hydrolysis and solvent oxidation side reactions. The water content of DMC and DEC was removed by standing under argon atmosphere and room temperature with a water and oxygen content of ≤0.1ppm, so that the moisture content of DMC and DEC was ≤20ppm. Since EC has a high melting point and is solid at room temperature, it needs to be removed by water at 70°C. Specifically, EC was placed in a sealed container containing 3Å or 4Å molecular sieves, and then placed in a vacuum glove box; the water content of EC was removed by standing under argon atmosphere, a water and oxygen content of ≤0.1ppm, and 70°C, so that the moisture content of EC was ≤20ppm.

[0016] The dehydrated EC, DMC and DEC were mixed evenly in a volume ratio of 1:1:1, lithium salt was added, and the mixture was shaken and mixed until completely dissolved to form a basic electrolyte with a lithium salt concentration of 1 M.

[0017] S02: Methyl pentafluorobenzoate and bis(2,2,2-trifluoroethoxy)methane are sequentially added to the basic electrolyte, and after sealing, the mixture is oscillated at a rotation speed of ≥200 rpm until a uniform and transparent electrolyte is formed.

[0018] In a vacuum glove box, MFPB and BTFEM were added to the basic electrolyte in sequence, and then sealed and shaken at a speed of ≥200rpm for 2h until a uniform and transparent electrolyte was formed. The addition amount of MFPB and BTFEM was 1-3% of the total mass of the electrolyte, respectively.

[0019] In a fourth aspect, the present application provides a lithium-ion battery, comprising the high-voltage electrolyte additive of the first aspect, or comprising the electrolyte of the second aspect.

[0020] The present invention has the following beneficial effects: (1) The high-voltage electrolyte additive formed by the composite of MFPB and BTFEM breaks through the oxidation stability limit of lithium-ion batteries with voltages of 4.5 V and above through the synergistic effect of fluorinated molecules.

[0021] (2) MFPB has a fluorinated aromatic ester structure C6F5COOCH3, and its planar rigid structure can be preferentially adsorbed on the cathode surface and oxidized to form a dense CF bond interface film. The strong electron-withdrawing effect of the CF bond interface film can significantly inhibit the catalytic oxidation reaction of transition metal ions under high voltage, thereby inhibiting the oxidative decomposition of the electrolyte; at the same time, the CF bond interface film removes acidic byproducts of the electrolyte, such as HF, through dynamic ester exchange equilibrium, thereby delaying the degradation of the CEI membrane and continuously repairing the defects of the CEI membrane.

[0022] (3) BTFEM has a perfluorinated ether chain CF3OCH2-, which promotes Li + Desolvation and adsorption of PF6- Free anions such as fluorinated ether can reduce the interface polarization; at the same time, the interface gaps are filled by flexible molecular chains, thereby forming a composite protective layer with MFPB. In addition, the perfluoroether chain is >5.0Vvs.Li / Li + The inherent oxidation potential and the weak coordination of ether oxygen synergistically broaden the electrochemical window of the electrolyte to 4.5V, so that the discharge specific capacity of the lithium-ion battery remains at 139 mAh / g after 150 cycles at a high cut-off voltage of 4.5V, and the capacity retention rate reaches 97%.

[0023] (4) Under the synergistic effect of the fluorinated aromatic ester of MFPB and the perfluoroether chain of BTFEM, the planar rigid structure of MFPB covers the highly active crystal surface of the positive electrode, and the flexible fluorinated ether chain of BTFEM fills the interfacial gaps, and the two synergistically form a dense composite protective layer. In addition, the dynamic ester exchange equilibrium of MFPB repairs the microcracks of the interfacial film, and BTFEM absorbs electrons through MFPB and enriches Li + The redistributed charge can inhibit the oxidative decomposition of the electrolyte and the growth of lithium dendrites, thereby stabilizing the electrolyte-electrode interface, widening the electrochemical window of the electrolyte to ≥4.5V, and achieving long-term antioxidant and interface stabilization protection under high voltage conditions. In addition, MFPB can also enhance the solubility of the electrolyte, and BTFEM can optimize Li + The combination of the two can significantly slow down the capacity decay under high voltage conditions, maintain efficient reaction kinetics, and the overall performance far exceeds that of traditional systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a test diagram of the 2C rate current cycling performance of the lithium ion battery prepared in Comparative Example 1 of the present application at a voltage of 3-4.5V; Figure 2 This is a test diagram of the 2C rate current cycling performance of the lithium-ion battery prepared in Example 9 of the present application at a voltage of 3-4.5V; Figure 3 The discharge curves of the lithium-ion battery prepared in Comparative Example 1 of the present application at the 3rd, 50th, 100th and 150th cycles; Figure 4 The discharge curves of the lithium-ion battery prepared in Example 9 of the present application at the 3rd, 50th, 100th and 150th cycles; Figure 5 CV curve diagram of the lithium-ion battery prepared in Comparative Example 1 of the present application; Figure 6 This is a CV curve diagram of the lithium-ion battery prepared in Example 9 of the present application; Figure 7 The battery impedance diagram of the lithium-ion battery prepared in Example 9 and Comparative Example 1 of the present application after 3 cycles; Figure 8 The battery impedance diagram of the lithium-ion battery prepared in Example 9 and Comparative Example 1 of the present application after 50 cycles; Fig. 9 The battery impedance diagram of the lithium-ion battery prepared in Example 9 and Comparative Example 1 of the present application after 150 cycles. DETAILED DESCRIPTION

[0025] The technical solution of the present invention is further explained and illustrated by means of specific embodiments below.

[0026] Example 1 An embodiment of the present application provides a high voltage electrolyte additive, which includes MFPB and BTFEM in a mass ratio of 3:3.

[0027] Example 2 An embodiment of the present application provides a high voltage electrolyte additive, which includes MFPB and BTFEM in a mass ratio of 1:3.

[0028] Example 3 An embodiment of the present application provides a high voltage electrolyte additive, which includes MFPB and BTFEM in a mass ratio of 3:1.

[0029] Example 4 An embodiment of the present application provides a high voltage electrolyte additive, which includes MFPB and BTFEM in a mass ratio of 2:1.

[0030] Example 5 An embodiment of the present application provides a high-voltage electrolyte additive, which includes MFPB and BTFEM in a mass ratio of 1:2.

[0031] Example 6 An embodiment of the present application provides an electrolyte, which includes a basic electrolyte and the high-voltage electrolyte additive in Example 1, wherein the basic electrolyte includes EC, DMC, DEC in a volume ratio of 1:1:1 and lithium hexafluorophosphate with a concentration of 1M in the basic electrolyte, and the addition amount of MFPB in the high-voltage electrolyte additive is 3% of the total mass of the electrolyte.

[0032] The preparation method of the electrolyte comprises: S601: DMC and DEC are placed in sealed containers containing 3Å molecular sieves, respectively, and then placed in a vacuum glove box. Stand and remove water in an argon atmosphere at room temperature with a water and oxygen content of 0.1ppm, so that the moisture content of DMC and DEC is ≤20ppm. EC is placed in a sealed container containing 3Å molecular sieves, and then placed in a vacuum glove box; stand and remove water in an argon atmosphere at 70°C with a water and oxygen content of 0.1ppm, so that the moisture content of EC is ≤20ppm. After dehydration, EC, DMC and DEC are mixed evenly in a volume ratio of 1:1:1, lithium hexafluorophosphate is added, and the mixture is shaken and mixed until completely dissolved to form a basic electrolyte with a lithium hexafluorophosphate concentration of 1M.

[0033] S602: In a vacuum glove box, add 3% and 3% by weight of MFPB and BTFEM respectively to the basic electrolyte, seal it, and oscillate it at 200 rpm for 2 h until a uniform and transparent electrolyte is formed.

[0034] Example 7 An embodiment of the present application provides an electrolyte, which includes a basic electrolyte and the high-voltage electrolyte additive in Example 2, wherein the basic electrolyte includes EC, DMC, DEC in a volume ratio of 1:1:1 and lithium tetrafluoroborate with a concentration of 1M in the basic electrolyte, and the addition amount of MFPB in the high-voltage electrolyte additive is 1% of the total mass of the electrolyte.

[0035] The preparation method of the electrolyte comprises: S701: DMC and DEC were placed in sealed containers containing 4Å molecular sieves, respectively, and then placed in a vacuum glove box. In an argon atmosphere, the water and oxygen content was 0.06ppm, and the water content of DMC and DEC was allowed to stand and be removed at room temperature, so that the water content of DMC and DEC was ≤20ppm. EC was placed in a sealed container containing 3Å molecular sieves, and then placed in a vacuum glove box; in an argon atmosphere, the water and oxygen content was 0.06ppm, and the temperature was 70°C, the water content of EC was ≤20ppm. After dehydration, EC, DMC and DEC were mixed evenly in a volume ratio of 1:1:1, lithium tetrafluoroborate was added, and the mixture was shaken and mixed until completely dissolved to form a basic electrolyte with a lithium tetrafluoroborate concentration of 1M.

[0036] S702: In a vacuum glove box, add 1% and 3% by weight of MFPB and BTFEM respectively to the basic electrolyte, seal it, and oscillate it at 300 rpm for 2 h until a uniform and transparent electrolyte is formed.

[0037] Example 8 An embodiment of the present application provides an electrolyte, which includes a basic electrolyte and the high-voltage electrolyte additive in Example 3, wherein the basic electrolyte includes EC, DMC, DEC in a volume ratio of 1:1:1 and lithium bis(oxalatoborate) with a concentration of 1M in the basic electrolyte, and the amount of MFPB added in the high-voltage electrolyte additive is 3% of the total mass of the electrolyte.

[0038] The preparation method of the electrolyte comprises: S801: DMC and DEC were placed in sealed containers containing 4Å molecular sieves, respectively, and then placed in a vacuum glove box. Under argon atmosphere, water and oxygen content of 0.05ppm, the solution was allowed to stand and dehydrate at room temperature, so that the moisture content of DMC and DEC was ≤20ppm. EC was placed in a sealed container containing 4Å molecular sieves, and then placed in a vacuum glove box; under argon atmosphere, water and oxygen content of 0.05ppm, and 70°C, the solution was allowed to stand and dehydrate, so that the moisture content of EC was ≤20ppm. After dehydration, EC, DMC and DEC were mixed evenly in a volume ratio of 1:1:1, lithium bis(oxalatoborate) was added, and the mixture was shaken and mixed until completely dissolved to form a basic electrolyte with a lithium bis(oxalatoborate) concentration of 1M.

[0039] S802: In a vacuum glove box, add 3% and 1% by weight of MFPB and BTFEM respectively to the basic electrolyte, seal it, and oscillate it at 250 rpm for 2 h until a uniform and transparent electrolyte is formed.

[0040] Example 9 The present invention provides a lithium-ion battery, and the preparation method of the lithium-ion battery includes: S901: After mixing the positive electrode active material lithium iron phosphate (English name: ferrous lithium phosphate; abbreviated as: LFP), the conductive agent carbon black Super-P and the binder polyvinylidene fluoride (English name: polyvinylidenedifluoride; abbreviated as: PVDF) in a mass ratio of 8:1:1, add N-methylpyrrolidone (English name: N-Methylpyrrolidone; abbreviated as: NMP) and stir to form a uniform and viscous slurry.

[0041] S902: The viscous slurry is evenly coated on the aluminum foil current collector, placed in a vacuum drying oven, and dried at 80°C for 12 hours. After drying, the aluminum foil current collector is cut into a circular positive electrode sheet with a diameter of 14 mm, and the positive electrode sheet is weighed so that the active material loading of each positive electrode sheet is about 3.6 mg / cm 2 , put it in the glove box for later use.

[0042] S903: Graphite, carbon black and PVDF were mixed evenly in a mass ratio of 8:1:1, and evenly coated on a copper sheet using a 100μm coater, and placed in a vacuum drying oven and dried at 110°C for 24 hours. After drying, the copper sheet was sliced ​​into a circular negative electrode sheet with a diameter of 14mm, and the negative electrode sheet was weighed so that the density of each negative electrode sheet was 3.5mg / cm 2 , put it in the glove box for later use.

[0043] S904: In a glove box, place the side of the positive electrode sheet coated with the viscous slurry on the negative electrode shell, place a polypropylene separator, drop 100 μL of the electrolyte prepared in Example 6, then place the negative electrode sheet, gasket and spring in sequence, cover the positive electrode shell to form a lithium-ion battery. Use a button battery sealing machine to seal and package the lithium-ion battery.

[0044] Example 10 The present invention provides a lithium-ion battery, and the preparation method of the lithium-ion battery includes: S1001: After mixing the positive electrode active material LFP, the conductive agent Super-P and the binder PVDF in a mass ratio of 8:1:1, NMP was added and stirred to form a uniform and viscous slurry.

[0045] S1002: The viscous slurry is evenly coated on the aluminum foil current collector, placed in a vacuum drying oven, and dried at 80°C for 12 hours. After drying, the aluminum foil current collector is cut into circular positive electrode sheets with a diameter of 14 mm, and the positive electrode sheets are weighed so that the active material loading of each positive electrode sheet is about 4 mg / cm 2 , put it in the glove box for later use.

[0046] S1003: Graphite, carbon black and PVDF were mixed evenly in a mass ratio of 8:1:1, and evenly coated on a copper sheet using a 100μm coater, and placed in a vacuum drying oven and dried at 110°C for 24h. After drying, the copper sheet was sliced ​​into a circular negative electrode sheet with a diameter of 14mm, and the negative electrode sheet was weighed so that the density of each negative electrode sheet was 4mg / cm 2 , put it in the glove box for later use.

[0047] S1004: In a glove box, place the side of the positive electrode sheet coated with the viscous slurry on the negative electrode shell, place a polypropylene separator, drop 100 μL of the high-voltage electrolyte prepared in Example 7, and then place the negative electrode sheet, gasket and spring in sequence, cover the positive electrode shell to form a lithium-ion battery. Use a button battery sealing machine to seal and package the lithium-ion battery.

[0048] Embodiment 11 The present invention provides a lithium-ion battery, and the preparation method of the lithium-ion battery includes: S1101: After mixing the positive electrode active material LFP, the conductive agent Super-P and the binder PVDF in a mass ratio of 8:1:1, NMP is added and stirred to form a uniform and viscous slurry.

[0049] S1102: The viscous slurry is evenly coated on the aluminum foil current collector, placed in a vacuum drying oven, and dried at 80°C for 12 hours. After drying, the aluminum foil current collector is cut into circular positive electrode sheets with a diameter of 14 mm, and the positive electrode sheets are weighed so that the active material loading of each positive electrode sheet is about 3 mg / cm 2 , put it in the glove box for later use.

[0050] S1103: Graphite, carbon black and PVDF were mixed evenly in a mass ratio of 8:1:1, and evenly coated on a copper sheet using a 100μm coater, and placed in a vacuum drying oven and dried at 110°C for 24h. After drying, the copper sheet was sliced ​​into a circular negative electrode sheet with a diameter of 14mm, and the negative electrode sheet was weighed so that the density of each negative electrode sheet was 3mg / cm 2 , put it in the glove box for later use.

[0051] S1104: In a glove box, place the side of the positive electrode sheet coated with the viscous slurry on the negative electrode shell, place a polypropylene separator, drop 100 μL of the high-voltage electrolyte prepared in Example 8, and then place the negative electrode sheet, gasket and spring in sequence, cover the positive electrode shell to form a lithium-ion battery. Use a button battery sealing machine to seal and package the lithium-ion battery.

[0052] Comparative Example 1 The comparative example of the present application provides a lithium ion battery, and the preparation method of the lithium ion battery comprises: D101: After mixing the positive electrode active material LFP, the conductive agent Super-P and the binder PVDF in a mass ratio of 8:1:1, NMP was added and stirred to form a uniform and viscous slurry.

[0053] D102: The viscous slurry was evenly coated on the aluminum foil current collector, placed in a vacuum drying oven, and dried at 80°C for 12 hours. After drying, the aluminum foil current collector was cut into circular positive electrode sheets with a diameter of 14 mm, and the positive electrode sheets were weighed so that the active material loading of each positive electrode sheet was about 3.6 mg / cm 2 , put it in the glove box for later use.

[0054] D103: Graphite, carbon black and PVDF were mixed evenly in a mass ratio of 8:1:1, and evenly coated on a copper sheet using a 100μm coater, and placed in a vacuum drying oven and dried at 110°C for 24h. After drying, the copper sheet was sliced ​​into a circular negative electrode sheet with a diameter of 14mm, and the negative electrode sheet was weighed so that the density of each negative electrode sheet was 3.5mg / cm2 , put it in the glove box for later use.

[0055] D104: In a glove box, place the side of the positive electrode sheet coated with the viscous slurry on the negative electrode shell, place a polypropylene separator, add 100μL of basic electrolyte, and then place the negative electrode sheet, gasket and shrapnel in sequence, and cover the positive electrode shell to form a lithium-ion battery. Use a button battery sealing machine to seal and package the lithium-ion battery.

[0056] After the lithium ion batteries prepared in Example 9 and Comparative Example 1 were left to stand for 12 hours, cycle performance test, constant current charge and discharge performance test, cyclic voltammetry curve test and electrochemical impedance test were performed respectively, wherein the lithium ion battery prepared in Example 9 was marked as LEDD+MB, and the lithium ion battery prepared in Comparative Example 1 was marked as LEDD. The specific test contents are as follows: 1. Cycle performance test The lithium ion batteries prepared in Example 9 and Comparative Example 1 were charged and discharged for 150 cycles at 25°C at a rate of 2C in the range of 3-4.5V to obtain the attached Figure 1 , 2 .

[0057] By the attached Figure 1 , 2 It can be seen that the first-cycle discharge specific capacities of the lithium-ion battery LEDD prepared in Comparative Example 1 and the lithium-ion battery LEDD+MB prepared in Example 9 are 124 mAh / g and 137 mAh / g, respectively, which indicates that the use of MFPB and BTFEM composite additives has limited improvement in the first-cycle discharge. During the cycle of 50 to 150 cycles, the difference in discharge capacity between the lithium-ion battery LEDD+MB and the lithium-ion battery LEDD significantly expanded. After 150 cycles, the capacity retention rates of the lithium-ion battery LEDD and the lithium-ion battery LEDD+MB were 80% and 97%, respectively, which indicates that the electrolyte is bound to have capacity decay in high-voltage long cycles, and the use of MFPB and BTFEM composite additives can significantly inhibit capacity loss. In addition, the lithium-ion battery LEDD+MB has a more stable coulombic efficiency than the lithium-ion battery LEDD, which ultimately verifies that the MFPB and BTFEM composite additives have a decisive optimization effect on the high-voltage performance of lithium salt / graphite batteries.

[0058] 2. Constant current charge and discharge performance test The lithium ion batteries prepared in Example 9 and Comparative Example 1 were subjected to cyclic charge and discharge at room temperature and a 2C rate with a current range of 3-4.5V, and their discharge curves were measured at the 3rd, 50th, 100th and 150th cycles to obtain the attached Figure 3 , 4 .

[0059] By the attached Figure 3 , 4 It can be seen that the discharge capacity and voltage platform of the lithium-ion battery LEDD prepared in Comparative Example 1 significantly decrease with the increase in the number of cycles, which may be caused by the increase in the internal resistance of the battery due to battery polarization. The lithium-ion battery LEDD+MB prepared in Example 9 exhibits a more stable discharge specific capacity and voltage platform at a high rate, which indicates that the addition of the MFPB and BTFEM composite additives improves the stability of the electrode / electrolyte interface and alleviates the polarization of the electrode.

[0060] 3. Cyclic voltammetry curve test In order to understand the electrochemical redox behavior of the battery during the charge and discharge process, the lithium ion batteries prepared in Example 9 and Comparative Example 1 were subjected to CV tests in the voltage range of 3-4.5 V on a CHI660E electrochemical workstation. Figure 5 , 6 The CV curve shown in the figure, where the scanning rate is 1mV / s.

[0061] By the attached Figure 5 , 6 It can be seen that the lithium ion batteries prepared in Example 9 and Comparative Example 1 showed similar redox peaks, wherein the reduction peak was located at about 3.1 V, indicating that lithium ions were embedded in the LiFePO4 positive electrode material; the oxidation peak was located at about 3.8 V, indicating that lithium ions were released from the LiFePO4 positive electrode material.

[0062] In addition, the lithium-ion batteries prepared in Example 9 and Comparative Example 1 both showed obvious polarization reactions in the first cycle. Compared with the oxidation peak in the second charging process, the lithium-ion battery LEDD+MB prepared in Example 9 had a smaller potential difference, i.e., 0.103 vs. 0.152 V. These differences are due to the fact that the electrolyte in the lithium-ion battery LEDD+MB has fewer interfacial side reactions. Comparing the second and third CV curves, the lithium-ion battery LEDD+MB shows better reversibility, which indicates that no more electrolyte decomposition occurs after the interfacial film is formed in the first cycle, which is consistent with the improved cycle stability and efficiency performance mentioned above.

[0063] 4. Electrochemical impedance test The alternating current impedance spectrum (EIS) of the battery can reflect the internal impedance of the battery and the obstacles to the deintercalation and extraction of lithium ions at the interface. In order to understand the effects of LEDD and MB composite additives on the positive electrode, it is necessary to test the changes in electrochemical impedance of different electrolytes in the LFP / graphite system. Specifically, on the CHI660E electrochemical workstation, the5 The EIS test was performed on the lithium ion batteries prepared in Example 9 and Comparative Example 1 after 3, 50 and 100 cycles within a frequency range of Hz, and the attached Figure 7-9 In the EIS graph, the semicircle in the high-frequency region corresponds to the surface film resistance ( R f ) and the charge transfer resistance ( R ct ), while the low-frequency oblique line is similar to Li + is related to the diffusion process.

[0064] By the attached Figure 7-9 It can be seen that after 3 cycles, the impedance of the lithium ion battery LEDD prepared in Comparative Example 1 is about 50 Ω, and the battery impedance of the lithium ion battery LEDD+MB prepared in Example 9 is about 90 Ω, which may be related to the formation of a dense protective film on the surface of the positive electrode. After 50 cycles, the battery impedance of the lithium ion battery LEDD prepared in Comparative Example 1 rises sharply, while the impedance increase of the lithium ion battery LEDD+MB prepared in Example 9 is significantly reduced. After 150 cycles, the impedance of the lithium ion battery LEDD+MB prepared in Example 9 is still lower than that of the lithium ion battery LEDD, which verifies that the MFPB / BTFEM composite additive can effectively inhibit the accumulation of electrode impedance and help long-cycle stability.

[0065] The above test results show that MFPB dynamically repairs the microcracks of CEI film through fluorinated aromatic esters to maintain the smooth flow of charge transfer paths; while the perfluoroether chain of BTFEM blocks the deposition of byproducts such as LiF aggregates by virtue of the molecular sieving effect, and its flexible configuration adapts to the change of interface volume. The synergistic effect of MFPB and BTFEM can form a low-impedance interface film during the cycle and reduce the Li + The diffusion energy barrier is improved, the stability of the electrode / electrolyte interface is enhanced, and ultimately the simultaneous optimization of the cycle performance and rate performance of LiFePO4 / graphite batteries under high voltage conditions is achieved.

[0066] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A high voltage electrolyte additive, characterized in that: These include methyl pentafluorobenzoate and bis(2,2,2-trifluoroethoxy)methane.

2. The high voltage electrolyte additive according to claim 1, characterized in that: The mass ratio of the methyl pentafluorobenzoate to the bis(2,2,2-trifluoroethoxy)methane is (1-3):(1-3).

3. An electrolyte, characterized in that: The invention comprises the high voltage electrolyte additive as claimed in claim 1 or 2.

4. The electrolyte according to claim 3, characterized in that The added amount of the methyl pentafluorobenzoate is 1-3% of the total mass of the electrolyte.

5. The electrolyte according to claim 3, characterized in that The electrolyte also includes a basic electrolyte consisting of ethylene carbonate, dimethyl carbonate, diethyl carbonate and lithium salt.

6. The electrolyte according to claim 5, characterized in that The concentration of the lithium salt in the basic electrolyte is 1 M, and the lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium bis(oxalatoborate).

7. The electrolyte according to claim 5, characterized in that The volume ratio of the ethylene carbonate, dimethyl carbonate and diethyl carbonate is 1:1:

1.

8. A method for preparing the electrolyte according to any one of claims 5 to 7, characterized in that: include: Ethylene carbonate, dimethyl carbonate and diethyl carbonate are mixed after removing water under an argon atmosphere, lithium salt is added, and stirred until completely dissolved to form a basic electrolyte; Methyl pentafluorobenzoate and bis(2,2,2-trifluoroethoxy)methane are sequentially added to the basic electrolyte, and after sealing, the mixture is shaken at a rotation speed of ≥200 rpm until a uniform and transparent electrolyte is formed.

9. The method for preparing an electrolyte according to claim 8, characterized in that: The preparation of the basic electrolyte comprises: Ethylene carbonate, dimethyl carbonate and diethyl carbonate are respectively placed in a sealed container containing molecular sieves, and allowed to stand to remove water under an argon atmosphere and a water and oxygen content of ≤0.1ppm, so that the water content of ethylene carbonate, dimethyl carbonate and diethyl carbonate is ≤20ppm; The ethylene carbonate, dimethyl carbonate and diethyl carbonate after dehydration are mixed, lithium salt is added, and stirred until completely dissolved to form a basic electrolyte.

10. A lithium ion battery, characterized in that: The invention comprises the high voltage electrolyte additive according to claim 1 or 2, or the electrolyte according to any one of claims 3 to 7.

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