Use of unsaturated phosphate-based lithium fluoroborate in electrolyte and preparation method therefor

Abstract

A use of an unsaturated phosphate-based lithium fluoroborate in an electrolyte and a preparation method therefor. The unsaturated phosphate-based lithium fluoroborate is added into the electrolyte at an additive amount of 0.01-15 wt% of the total mass of the electrolyte. The preparation method for the unsaturated phosphate-based lithium fluoroborate comprises the following steps: reacting an unsaturated phosphonic acid with inorganic lithium salts to obtain an unsaturated lithium phosphonate reaction solution; and introducing a boron trifluoride gas into the unsaturated lithium phosphonate reaction solution or adding a boron trifluoride complex for a reaction to obtain the unsaturated phosphate-based lithium fluoroborate. Applying the unsaturated phosphate-based lithium fluoroborate to the electrolyte has the advantages of reducing the initial impedance of batteries, decreasing gas generation during storage, improving the cycle performance and the like.

Description

Application of unsaturated phosphate-based lithium fluoroborate in electrolyte and preparation method thereof Technical Field

[0001] The present invention relates to the field of electrolytes, and in particular to a preparation method of unsaturated phosphate-based lithium fluoroborate and application thereof in lithium ion battery electrolytes. Background Art

[0002] As an indispensable component of lithium-ion batteries, electrolyte additives are mainly responsible for constructing a stable electrode / electrolyte interface film to achieve electronic insulation and facilitate lithium ion transmission. Under the influence of different functional groups in the additives, the composition and structure of the battery interface film change, ultimately affecting the battery's cycle life, high-temperature storage, and low-temperature discharge performance.

[0003] Compounds with different functional groups or heteroatoms often exhibit different effects. For example, boron-containing additives can dissolve LiF inorganic lithium salts on the surface of the interfacial film due to the electron-deficient effect of boron atoms, thereby reducing the battery's internal resistance and improving low-temperature performance. The "P=O" group in phosphorus-containing additives strongly interacts with transition metal elements on the positive electrode surface, forming a stable positive electrode interfacial film on the surface, improving the battery's storage and cycling performance. Unsaturated functional groups such as vinyl and acetylene groups can undergo reduction polymerization at the negative electrode to form a stable interfacial film, reducing the reduction reaction between the solvent and the negative electrode surface and reducing battery gassing. The F atoms in the additives can form LiH at the positive and negative electrode interfaces, increasing the stability and mechanical strength of the interfacial film and improving the battery's cycle life. However, compounds containing multiple advantageous functional groups or heteroatoms often have complex structures and are difficult to prepare and obtain.

[0004] Chinese patent CN112751080A discloses an unsaturated phosphate compound and its synthesis method, wherein unsaturated vinyl phosphate is used as an electrolyte additive to reduce battery impedance and improve rate and cycle performance. This patent utilizes excess lithium hydride (LiH) to react with an unsaturated phosphonic acid to produce the unsaturated phosphate. However, due to the high reactivity of LiH and the resulting high exothermic reaction, the unsaturated bonds are susceptible to thermal polymerization at high temperatures, generating insoluble impurities. Furthermore, due to the low solubility of both LiH and lithium vinyl phosphate in carbonates, effective separation of the reaction product and the raw materials is difficult, resulting in a low reaction yield.

[0005] Therefore, developing compounds with multiple advantageous functional groups or heteroatoms, simplifying the synthesis method, and improving the reaction yield and product purity are one of the difficulties in the current development and design of new additives. Summary of the Invention

[0006] In order to solve the problems existing in the background technology, the present invention provides a new type of unsaturated phosphate-based lithium fluoroborate for use in the electrolyte, which contains multiple advantageous functional groups such as F, P, B and unsaturated bonds, and can form stable SEI film and CEI film at the positive and negative electrode interfaces, which can effectively reduce the initial impedance of the battery, reduce storage gas production, and further improve the cycle performance of the battery.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] An application of unsaturated phosphate-based lithium fluoroborate in an electrolyte, specifically, adding the unsaturated phosphate-based lithium fluoroborate represented by the following structure (I) to the electrolyte in an amount of 0.01 to 15 wt% of the total mass of the electrolyte:

[0009] In the formula, R is selected from C2-C4 alkenyl, C2-C4 alkynyl, C2-C4 haloalkenyl or C2-C4 haloalkynyl; preferably, R is selected from C2-C4 alkenyl or C2-C4 alkynyl; more preferably, the unsaturated phosphate lithium fluoroborate is selected from at least one of the following structures:

[0010] Preferably, the unsaturated lithium phosphate fluoroborate of the present invention is added to the electrolyte in an amount of 0.1 to 10 wt% of the total mass of the electrolyte; more preferably, the unsaturated lithium phosphate fluoroborate is added to the electrolyte in an amount of 0.5 to 5.0 wt% of the total mass of the electrolyte.

[0011] The electrolyte of the present invention further comprises:

[0012] A main lithium salt, wherein the main lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide, and the molar concentration in the electrolyte is 0.4 to 1.6 mol / L; preferably, the main lithium salt is lithium hexafluorophosphate or lithium bis(fluorosulfonyl)imide, and the molar concentration in the electrolyte is 0.6 to 1.2 mol / L;

[0013] A non-aqueous solvent, wherein the non-aqueous organic solvent is a combination of a cyclic solvent and a linear solvent, wherein the cyclic solvent is selected from at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, γ-butyrolactone, or δ-valerolactone; and the linear solvent is selected from at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, ethyl propionate, n-propyl propionate, and methylpropyl carbonate;

[0014] A basic additive, wherein the basic additive is selected from at least one of fluoroethylene carbonate, ethylene carbonate, tris(trimethylsilyl) phosphate, 1,3-propane sultone, lithium difluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(oxalatodifluorophosphate) or lithium difluorooxalatoborate, and the amount of any one basic additive used accounts for 0.1 to 5.0 wt% of the total amount of the electrolyte.

[0015] In a preferred embodiment, the electrolyte comprises: 1.0-1.2 mol / L lithium hexafluorophosphate, 0.2-1.0 wt% 1,3-propane sultone and 1.0-2.0 wt% unsaturated lithium phosphate fluoroborate represented by structural formula I-1. Compared with adding unsaturated lithium phosphate fluoroborate alone, it can not only further reduce the storage gas production of the battery, but also improve the high temperature cycle performance.

[0016] In another preferred embodiment, the electrolyte comprises: 1.0-1.2 mol / L lithium hexafluorophosphate, 0.5-5.0 wt% fluoroethylene carbonate, 0.5-2.0 wt% lithium bis(fluorosulfonyl)imide, and 1.0-2.0 wt% unsaturated lithium phosphate fluoroborate represented by structural formula I-1. Compared with the use of unsaturated lithium phosphate fluoroborate alone, it can not only reduce the initial impedance of the battery and improve the room temperature and high temperature cycle performance, but also further inhibit high temperature storage gas production and improve the overall performance of the battery.

[0017] The present invention also provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and any one of the above-mentioned electrolytes.

[0018] The positive electrode active material is a nickel-cobalt-manganese ternary material, a nickel-cobalt-aluminum ternary material, a lithium cobaltate material or a lithium iron phosphate material. The nickel-cobalt-manganese ternary material is Li(Ni x Co y Mn z )O2, x>0, y>0, z>0, x+y+z=1, the nickel-cobalt-aluminum ternary material is Li(Ni x Co y Al z )O2, x>0, y>0, z>0, x+y+z=1; the negative electrode active material is one of a graphite negative electrode, a silicon-carbon negative electrode, and a lithium metal negative electrode.

[0019] The present invention also provides a method for preparing any of the above-mentioned unsaturated phosphate lithium fluoroborate, the preparation method comprising the following steps:

[0020] (1) in a first solvent, reacting an unsaturated phosphonic acid with an inorganic lithium salt to obtain an unsaturated lithium phosphonate reaction solution, wherein the inorganic lithium salt is selected from at least one of lithium carbonate, lithium bicarbonate, lithium oxide, or lithium hydroxide; preferably lithium bicarbonate, lithium oxide, or lithium hydride, more preferably lithium carbonate;

[0021] Taking lithium carbonate as an example, the reaction formula is as follows:

[0022] Wherein, R is defined as above;

[0023] (2) Boron trifluoride gas or a boron trifluoride complex is introduced into the unsaturated lithium phosphonate reaction solution to obtain an unsaturated lithium phosphate fluoroborate reaction solution. The reaction formula is as follows:

[0024] The definition of R in the formula is the same as described above.

[0025] In step (1), inorganic lithium salt and unsaturated phosphonic acid are used as raw materials. By controlling the reaction temperature and feed ratio, the reaction heat is reduced to avoid thermal polymerization of unsaturated bonds in the raw materials or reaction products to form dimers or even polymers, thereby improving the reaction yield and product purity.

[0026] Therefore, in step (1), the molar ratio of lithium in the inorganic lithium salt to the unsaturated phosphonic acid is (1.6-2.2):1, preferably (1.8-2.1):1, and more preferably about 2:1.

[0027] Meanwhile, the reaction temperature in step (1) is controlled to be -10 to 50° C., and the reaction time is 0.5 to 24 h; preferably, the reaction temperature is 0 to 10° C., and the reaction time is 10 to 12 h.

[0028] In step (1), the first solvent is selected from at least one of water, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, methyl propionate, γ-butyrolactone, ether, ethylene glycol dimethyl ether, acetonitrile, benzyl cyanide or propionitrile, preferably at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate.

[0029] In step (2) of the present invention, unsaturated lithium phosphonate reacts with boron trifluoride gas or a boron trifluoride complex to obtain an unsaturated lithium phosphate fluoroborate reaction solution. The reaction solution is concentrated by vacuum distillation to obtain unsaturated lithium phosphate fluoroborate. The vacuum distillation temperature is 0 to 60° C., preferably 20 to 30° C. Due to the solvation of the unsaturated lithium phosphate fluoroborate, the unsaturated lithium phosphate fluoroborate obtained after vacuum distillation is a concentrated solution.

[0030] Specifically, in step (2), the boron trifluoride complex is selected from at least one of a boron trifluoride ethyl ether complex, a boron trifluoride glycol dimethyl ether complex, a boron trifluoride dimethyl carbonate complex, a boron trifluoride pyridine complex, a boron trifluoride ethylamine complex, a boron trifluoride butyl ether complex, a boron trifluoride methyl ether complex, a boron trifluoride acetonitrile complex, a boron trifluoride piperidine complex, a boron trifluoride phenol complex, a boron trifluoride tetrahydrofuran complex, a boron trifluoride dimethyl sulfide complex or a boron trifluoride morpholine complex.

[0031] In step (2), the molar ratio of unsaturated lithium phosphate to boron trifluoride or boron trifluoride complex is 1:(1.8-3), preferably the molar ratio is 1:(1.9-2.5), and more preferably the molar ratio is 1:(2.0-2.2).

[0032] In step (2), the reaction temperature is 0-50° C., and the reaction time is 0.5-24 h; preferably, the reaction temperature is 10-30° C., and the reaction time is 2-4 h.

[0033] Compared with the prior art, the present invention has the following technical effects:

[0034] 1. The present invention proposes a method for preparing unsaturated lithium phosphate fluoroborate, which has a simple process and high reaction yield. By optimizing the reaction conditions, the formation of polymers is reduced, and the reaction yield and product purity are further improved.

[0035] 2. The present invention proposes the use of unsaturated lithium phosphate fluoroborate in lithium-ion battery electrolyte, which can form a film on the positive and negative electrodes of the battery, reduce the initial impedance of the battery, reduce the gas production during high-temperature storage, and improve the battery's room-temperature and high-temperature cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG1 is the NMR nuclear magnetic hydrogen spectrum of lithium salt 1# prepared in Example 1 of the present invention;

[0037] FIG2 is the NMR spectrum of lithium salt 1# prepared in Example 1 of the present invention;

[0038] FIG3 is the NMR phosphorus spectrum of lithium salt 1# prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0039] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all possible alternatives, improvements and equivalents within the scope of the claims.

[0040] Example 1

[0041] This embodiment provides a method for preparing lithium vinylphosphotrifluoroborate (I-1), which specifically comprises the following steps:

[0042] (1) In a dry room with a dew point of -40°C, 0.1 mol (7.40 g) of lithium carbonate and 100 g of DMC solvent were added to a reaction flask, and then stirred in a 0°C low-temperature cold bath for 30 min to cool the reaction solution to 0°C; 0.1 mol (10.80 g) of vinylphosphonic acid was dissolved in 10 g of DMC and slowly added to the reaction solution through a constant pressure dropping funnel, and the reaction was carried out at 0°C for 10 h to obtain a solution containing lithium vinylphosphonate;

[0043] (2) 0.21 mol (33.18 g) of dimethyl boron trifluoride carbonate complex was added to the above solution containing lithium vinyl phosphate. After stirring at room temperature for 4 h, the insoluble solid was removed by filtration. The excess DMC solvent and BF3 were removed by vacuum distillation. The water was then removed using a dry molecular sieve for 30 min to finally obtain a DMC concentrate containing compound (I-1), which was recorded as lithium salt 1#.

[0044] Figures 1, 2, and 3 show the NMR spectra of lithium salt 1#. Figure 1 shows the H spectrum, with δ = 5.7-6.2 ppm (m, 3H), corresponding to the H on the vinyl group, and δ = 3.66 ppm (s), corresponding to the methyl hydrogen on the DMC group. Figure 2 shows the F spectrum, with a peak observed at δ = -148.05 ppm. Figure 3 shows the P spectrum, with a peak observed at δ = 11.79 ppm. The structure of the compound was optimized using the DFT / B3LYP function and the 6-31** basis set, and the NMR peak positions of the above compound were calculated using the DFT / B3LYP function and the 6-311** basis set. The calculated results were analyzed in conjunction with the test results and matched the above NMR spectrum results with high confidence. Therefore, it can be confirmed that the compound in lithium salt 1# is compound (I-1).

[0045] Further calculation by H-NMR spectrum showed that the concentration of compound (I-1) in DMC solution was 37.6 wt %. A total of 66.73 g of vacuum distillation concentrate was obtained. The actual mass of compound (I-1) was 25.09 g (0.098 mol), and the reaction yield was 98%.

[0046] Example 2

[0047] The operation of this example is the same as that of Example 1, except that in step (1), the amount of raw material lithium carbonate is adjusted to 0.08 mol (5.92 g), and finally a DMC concentrate containing compound (I-1) is obtained, which is the same as that of Example 1 and is recorded as lithium salt 1#.

[0048] The actual mass of the obtained compound (I-1) was 19.46 g (0.076 mol), and the reaction yield was 95%.

[0049] Example 3

[0050] The operation of this example is the same as that of Example 1, except that in step (1), the raw material lithium carbonate is replaced by lithium oxide in an amount of 0.1 mol (3.0 g). Finally, a DMC concentrate containing compound (I-1) is obtained, which is the same as that of Example 1 and is recorded as lithium salt 1#.

[0051] The actual mass of the obtained compound (I-1) was 24.05 g (0.093 mol), and the reaction yield was 93%.

[0052] Example 4

[0053] The operation of this embodiment is the same as that of Example 1, except that in step (2), no vacuum distillation is performed to remove excess DMC and BF3, thereby obtaining a DMC mixed solution containing compound (I-1) and BF3, which is recorded as lithium salt 2#.

[0054] The concentration of compound (I-1) in the DMC solution was calculated to be 21.02% by H NMR spectroscopy. After filtering out the insoluble matter, 117.76 g of the reaction solution was obtained. The actual mass of compound (I-1) was 25.42 g (0.098 mol), and the reaction yield was 98%.

[0055] Example 5

[0056] The operation of this example is the same as that of Example 1, except that in step (1), the DMC used in the reaction is replaced with DEC to obtain a DEC concentrate containing compound (I-1), which is recorded as lithium salt 3#.

[0057] The concentration of compound (I-1) in the DEC solution was calculated by H NMR spectroscopy to be 39.4 wt%. A total of 63.03 g of vacuum distillation concentrate was obtained. The actual mass of compound (I-1) was 24.83 g (0.097 mol), and the reaction yield was 97%.

[0058] Example 6

[0059] The operation of this example is the same as that of Example 1, except that in step (1), the raw material lithium carbonate is replaced by lithium hydroxide in an amount of 0.2 mol (4.8 g), and a DMC concentrate containing compound (I-1) is obtained, which is recorded as lithium salt 4#.

[0060] The actual mass of the obtained compound (I-1) was 24.05 g (0.093 mol), and the reaction yield was 93%.

[0061] Example 7

[0062] The operation of this example is the same as that of Example 1, except that in step (1), the raw material vinyl phosphate is replaced with allyl phosphate in an amount of 0.1 mol (12.20 g), and a DMC concentrate containing compound (I-2) is obtained, which is recorded as lithium salt 5#.

[0063] The actual mass of the obtained compound (I-2) was calculated to be 25.79 g (0.097 mol), and the reaction yield was 97%.

[0064] Example 8

[0065] The operation of this example is the same as that of Example 1, except that in step (1), the raw material vinyl phosphate is replaced by butynyl phosphate in an amount of 0.1 mol (13.40 g), and a DMC concentrate containing compound (I-6) is obtained, which is recorded as lithium salt 6#.

[0066] The actual mass of the obtained compound (I-6) was calculated to be 26.16 g (0.094 mol), and the reaction yield was 94%.

[0067] Comparative Example 1

[0068] The operation of this comparative example is the same as that of Example 1, except that the reaction in step (1) is carried out without a cold bath and the reaction temperature is about 25°C. However, since the reaction is exothermic, the temperature rises sharply during the reaction.

[0069] The actual mass of the obtained compound (I-1) was 22.91 g (0.097 mol), and the reaction yield was 89.5%.

[0070] Comparative Example 2

[0071] This comparative example is used to obtain lithium ethylene phosphate. The specific operation is as follows: after the reaction of step (1) of Example 1 is completed, the insoluble matter generated by the reaction is filtered, washed with DMC, and dried to obtain lithium ethylene phosphate shown in the following structure (II):

[0072] 2. Electrolyte

[0073] Preparation of basic electrolyte 1: In an argon-filled glove box (moisture <5 ppm, oxygen <10 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were uniformly mixed in a mass ratio of EC:EMC:DEC = 3:5:2, and then lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to a molar concentration of 1.0 mol / L to obtain a basic electrolyte.

[0074] Application Example 1: In a basic electrolyte, lithium salt 1# was used to add 0.5 wt % of compound (I-1) to obtain an electrolyte.

[0075] Application Example 2: In a basic electrolyte, 1.0 wt% of compound (I-1) was added using lithium salt 1# to obtain an electrolyte.

[0076] Application Example 3: In a basic electrolyte, 2.0 wt% of compound (I-1) was added using lithium salt 1# to obtain an electrolyte.

[0077] Application Example 4: In a basic electrolyte, 5.0 wt% of compound (I-1) was added to lithium salt 1# to obtain an electrolyte.

[0078] Application Example 5: In a basic electrolyte, 1.0 wt% of compound (I-1) and 1.0 wt% of vinylene carbonate (VC) were added using lithium salt 1# to obtain an electrolyte.

[0079] Application Example 6: In a basic electrolyte, 1.0 wt% of compound (I-1) and 1.0 wt% of 1,3-propane sultone (PS) were added using lithium salt 1# to obtain an electrolyte.

[0080] Application Example 7: In a basic electrolyte, 1.0 wt% of compound (I-1) and 2.0 wt% of fluoroethylene carbonate (FEC) were added using lithium salt 1# to obtain an electrolyte.

[0081] Application Example 8: In a basic electrolyte, 1.0 wt% of compound (I-1) was added using lithium salt 2# to obtain an electrolyte.

[0082] Application Example 9: In a basic electrolyte, 1.0 wt% of compound (I-1) was added using lithium salt 3# to obtain an electrolyte.

[0083] Application Example 10: In a basic electrolyte, 1.0 wt% of compound (I-1) was added using lithium salt 4# to obtain an electrolyte.

[0084] Application Example 11: In a basic electrolyte, 1.0 wt% of compound (I-2) was added using lithium salt 5# to obtain an electrolyte.

[0085] Application Example 12: In a basic electrolyte, 1.0 wt% of compound (I-6) was added using lithium salt 6# to obtain an electrolyte.

[0086] Application Example 13: In a basic electrolyte, 1.0 wt% of compound (I-1), 1.0 wt% of fluoroethylene carbonate (FEC) and 1.0 wt% of lithium bis(fluorosulfonyl)imide (LiFSI) were added using lithium salt 1# to obtain an electrolyte.

[0087] Application Example 14: In a basic electrolyte, 2.0 wt% of compound (I-1) was added using lithium salt 4# to obtain an electrolyte.

[0088] Application Example 15: In a basic electrolyte, 5.0 wt% of compound (I-1) is added using lithium salt 4# to obtain an electrolyte.

[0089] Comparative Application Example 1: The basic electrolyte was not subjected to any treatment to obtain an electrolyte.

[0090] Comparative Application Example 2: In the basic electrolyte, 1.0 wt% of VC was added to obtain an electrolyte.

[0091] Comparative Application Example 3: In the basic electrolyte, 1.0 wt% of PS was added to obtain an electrolyte.

[0092] Comparative Application Example 4: In the basic electrolyte, 2.0 wt% of FEC was added to obtain an electrolyte.

[0093] Comparative Application Example 4: In the basic electrolyte, 0.2 wt% of lithium ethylene phosphate was added to obtain an electrolyte.

[0094] 2. Electrochemical performance test

[0095] The electrolytes of the above application examples and comparative examples were respectively made into soft-pack lithium-ion batteries with a capacity of 1500 mAh. The lithium-ion batteries included a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte and battery auxiliary materials. The positive electrode active material was a nickel-cobalt-manganese ternary material Li (Ni 0.8 Co 0.1 Mn 0.1 )O2, the negative electrode active material is artificial graphite.

[0096] The preparation process is as follows: the positive electrode sheet, separator and negative electrode sheet are wound together into a core, sealed with aluminum-plastic film and then baked to ensure that the moisture content of the electrode meets the requirements. After baking, the battery cell is injected with electrolyte, and the finished soft-pack battery cell is obtained through the steps of standing, forming, capacity separation and aging.

[0097] The prepared lithium-ion power battery (soft-pack battery cell) was subjected to performance testing. The specific test items and methods are as follows:

[0098] (1) 60℃ high temperature storage test: Before storage, cycle the battery at 0.5C / 0.5C at room temperature for one cycle, record the discharge capacity, internal resistance and volume of the first cycle, then charge the battery at a constant current of 0.5C to 4.20V, and charge the battery at a constant voltage until the current drops to 0.05C. Place the battery in a 60℃ constant temperature oven for 60 days, then cycle the battery at 0.5C / 0.5C at room temperature for two cycles, record the discharge capacity of the first cycle after high temperature standing, the discharge capacity of the second cycle, the internal resistance and volume after storage. Calculate the capacity retention rate, capacity recovery rate, internal resistance growth rate and volume expansion rate of the battery after storage according to the following formula: Capacity retention rate = discharge capacity of the first cycle after high temperature standing (after storage) / discharge capacity of the first cycle (before storage) * 100%. Capacity recovery rate = discharge capacity of the second cycle after high temperature standing (after storage) / discharge capacity of the first week (before storage) * 100%. Volume expansion rate = (volume after storage - volume before storage) / volume before storage * 100%.

[0099] (2) 25℃ high temperature cycle test: The battery is cycled at a charge and discharge current of 1C / 1C in a room temperature environment of 25±1℃. The discharge capacity per week is calculated. The cycle is stopped when the capacity retention rate is less than 80%.

[0100] (3) 45°C high temperature cycle test: The battery is cycled in a 45±1°C oven at a charge and discharge current of 1C / 1C. The discharge capacity per week is calculated. The cycle is stopped when the capacity retention rate is less than 80%.

[0101] The test results are shown in Table 1 below:

[0102] Table 1 Battery performance test results

[0103] According to the data in Table 1 above, by comparing application examples 1 to 4 and comparative example 1, it can be seen that the use of compound (I-1) as an electrolyte additive can reduce the initial impedance of the battery, improve the room temperature and high temperature cycle performance, and also has an excellent storage gas production inhibition effect. As the amount of lithium salt added gradually increases, within a certain range, the overall performance of the battery gradually improves, but as the content further increases, some performance begins to decline, and the optimal usage amount is around 1.0 to 2.0 wt%. In the comparative example, the solubility of compound (II) in the electrolyte is only 0.2 wt%, and the addition amount cannot reach 1.0 wt%, otherwise solids will precipitate, and its effect on improving battery performance is weak. Compound (I-1) has good solubility in all solvents used in the electrolyte, and significantly improves the overall performance of the battery, and has a wider range of applications and practical value.

[0104] Comparison of Application Examples 2, 5 to 7, and 13 with Comparative Example 1 shows that the combined use of compound (I-1) and basic additives such as VC, PS, FEC, and LiFSI can further improve the overall performance of the battery.

[0105] Comparing Application Examples 2, 8, and 10, in Application Example 8, since lithium salt 2# has not been subjected to vacuum distillation, a small amount of BF3 is present in the reaction solution. In Application Example 10, the amount of H2O generated by the reaction of lithium hydroxide with acid is large and difficult to remove with molecular sieves. The presence of BF3 and H2O will deteriorate the storage gas production inhibition effect and cycle performance. Further comparison of Application Example 10, Application Example 14, and Application Example 15 shows that as the amount of lithium salt 2# used increases, the higher the water content in the electrolyte, the more obvious the deterioration of battery performance. Therefore, in the preparation process, it is necessary to minimize the presence of impurities by selecting suitable raw materials and reasonable post-processing solutions in order to improve the reaction efficiency and maximize the effect of the substances in the application process.

Claims

1. An application of unsaturated phosphate lithium fluoroborate in an electrolyte, characterized in that: Unsaturated lithium phosphate fluoroborate represented by the following structure (I) is added to the electrolyte in an amount of 0.01 to 15 wt % of the total mass of the electrolyte: In the formula, R is selected from C2-C4 alkenyl, C2-C4 alkynyl, C2-C4 haloalkenyl or C2-C4 haloalkynyl.

2. The use of the unsaturated phosphate-based lithium fluoroborate in an electrolyte according to claim 1, characterized in that: The unsaturated phosphate-based lithium fluoroborate is selected from at least one of the following structures:

3. The use of the unsaturated phosphate-based lithium fluoroborate in an electrolyte according to claim 1, characterized in that: The unsaturated phosphate-based lithium fluoroborate is added to the electrolyte in an amount of 0.5-5.0 wt % of the total mass of the electrolyte.

4. The use of unsaturated lithium phosphate fluoroborate in an electrolyte according to any one of claims 1 to 3, characterized in that: The electrolyte also includes: A main lithium salt, wherein the main lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide; A non-aqueous solvent, wherein the non-aqueous organic solvent is a combination of a cyclic solvent and a linear solvent, wherein the cyclic solvent is selected from at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, γ-butyrolactone or δ-valerolactone; and the linear solvent is selected from at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, ethyl propionate, n-propyl propionate and methyl propyl carbonate; The basic additive is selected from at least one of fluoroethylene carbonate, ethylene carbonate, tris(trimethylsilyl) phosphate, 1,3-propylene sultone, lithium difluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)difluorophosphate or lithium difluorooxalatoborate, and the amount of any basic additive used accounts for 0.1 to 5.0% of the total amount of the electrolyte.

5. A lithium ion battery comprising a positive electrode, a negative electrode and a separator, characterized in that: The lithium-ion battery further comprises the electrolyte according to any one of claims 1-4.

6. The method for preparing the unsaturated lithium phosphate fluoroborate according to any one of claims 1 to 4, characterized in that: The preparation method comprises: (1) in a first solvent, an unsaturated phosphonic acid reacts with an inorganic lithium salt to obtain an unsaturated lithium phosphonate reaction solution, wherein the inorganic lithium salt is selected from at least one of lithium carbonate, lithium bicarbonate, lithium oxide or lithium hydroxide; (2) Boron trifluoride gas or a boron trifluoride complex is introduced into the unsaturated lithium phosphonate reaction solution to obtain an unsaturated lithium phosphate fluoroborate reaction solution.

7. The method for preparing unsaturated lithium phosphate fluoroborate according to claim 6, characterized in that: In step (1), the molar ratio of lithium in the inorganic lithium salt to the unsaturated phosphoric acid is (1.6-2.2):

1.

8. The method for preparing unsaturated lithium phosphate fluoroborate according to claim 6, characterized in that: In step (1), the reaction temperature is -10 to 50°C and the reaction time is 0.5 to 24 hours.

9. The method for preparing unsaturated lithium phosphate fluoroborate according to claim 6, characterized in that: The first solvent is selected from at least one of water, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, methyl propionate, γ-butyrolactone, ethyl ether, ethylene glycol dimethyl ether, acetonitrile, benzyl cyanide or propionitrile.

10. The method for preparing unsaturated lithium phosphate fluoroborate according to claim 6, characterized in that: In step (2), the boron trifluoride complex is selected from at least one of a boron trifluoride ethyl ether complex, a boron trifluoride glycol dimethyl ether complex, a boron trifluoride dimethyl carbonate complex, a boron trifluoride pyridine complex, a boron trifluoride ethylamine complex, a boron trifluoride butyl ether complex, a boron trifluoride methyl ether complex, a boron trifluoride acetonitrile complex, a boron trifluoride piperidine complex, a boron trifluoride phenol complex, a boron trifluoride tetrahydrofuran complex, a boron trifluoride dimethyl sulfide complex or a boron trifluoride morpholine complex.

11. The method for preparing unsaturated lithium phosphate fluoroborate according to claim 6, characterized in that: In step (2), the molar ratio of unsaturated lithium phosphonate to boron trifluoride gas or boron trifluoride complex is 1:(1.8-3).

12. The method for preparing unsaturated lithium phosphate fluoroborate according to claim 6, characterized in that: In step (2), the reaction temperature is 0 to 50° C. and the reaction time is 0.5 to 24 h.

13. The method for preparing unsaturated lithium phosphate fluoroborate according to claim 6, characterized in that: The unsaturated lithium phosphate fluoroborate reaction solution obtained in step (2) is subjected to reduced pressure distillation to obtain unsaturated lithium phosphate fluoroborate, and the reduced pressure distillation temperature is 0 to 60°C.

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