Use of unsaturated phosphato lithium fluoroborate in electrolyte and preparation method thereof

Abstract

The application discloses application of unsaturated phosphonic group lithium fluoroborate in electrolyte and a preparation method thereof. Specifically, unsaturated phosphonic group lithium fluoroborate is added into electrolyte in an adding amount of 0.01-15wt% of the total mass of the electrolyte; the preparation method of the unsaturated phosphonic group lithium fluoroborate comprises the following steps: unsaturated phosphonic acid is reacted with inorganic lithium salt to obtain an unsaturated lithium phosphonate reaction solution, and then boron trifluoride gas is introduced into the unsaturated lithium phosphonate reaction solution or a boron trifluoride complex is added to react to obtain the unsaturated phosphonic group lithium fluoroborate. The unsaturated phosphonic group lithium fluoroborate has the advantages of reducing initial impedance of a battery, reducing storage gas production, improving cycle performance and the like when being applied to the electrolyte.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electrolyte, in particular to a preparation method of unsaturated phosphonic acid group lithium fluoroborate and its application in lithium ion battery electrolyte. BACKGROUND

[0002] As an indispensable part of lithium ion battery, electrolyte additives are mainly responsible for building a stable electrode / electrolyte interface film to achieve electron 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 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 the LiF inorganic lithium salt on the surface of the interface film due to the electron-deficient effect of boron atoms, thereby reducing the internal resistance of the battery and improving the low-temperature performance of the battery. The "P=O" of phosphorus-containing additives has a strong interaction with transition metal elements on the positive electrode surface, thereby forming a stable positive electrode interface film on the surface, improving the storage performance and cycle performance of the battery. Unsaturated functional groups, such as vinyl and acetylene groups, can be reduced and polymerized on the negative electrode to form a stable interface film, reduce the reduction reaction of solvents with the negative electrode surface, and reduce the gas production of the battery. The F atoms in the additive can form LiH at the positive and negative electrode interface, increase the stability and mechanical strength of the interface film, and improve the cycle life of the battery. However, compounds containing multiple advantageous functional groups or heteroatoms often have complex structures and are difficult to prepare.

[0004] Chinese patent CN112751080A discloses an unsaturated phosphate compound and a synthesis method thereof. The unsaturated vinyl lithium phosphate as an electrolyte additive can reduce the battery impedance and improve the rate and cycle performance. The patent uses excess LiH to react with unsaturated phosphonic acid to obtain unsaturated phosphate. On the one hand, due to the high reactivity of LiH, the reaction releases a large amount of heat, and the unsaturated bond is prone to thermal polymerization at high temperatures to generate insoluble impurities. On the other hand, both LiH and unsaturated vinyl lithium phosphate have low solubility in carbonates, making it difficult to effectively separate the reaction products and raw materials, resulting in 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

[0006] In order to solve the problems in the background art, the application provides a new application of unsaturated phosphonic fluoroboric acid lithium in electrolyte, which contains F, P, B and unsaturated bond multiple advantage functional groups, can form stable SEI film and CEI film at the interface of positive and negative electrodes, can effectively reduce the initial impedance of the battery, reduce the storage gas production, and further improve the cycle performance of the battery.

[0007] The purpose of the application is achieved by the following technical solutions:

[0008] The application relates to an application of unsaturated phosphonic fluoroboric acid lithium in electrolyte, in particular, unsaturated phosphonic fluoroboric acid lithium shown in the following formula (I) is added into electrolyte in an adding amount of 0.01-15wt% of the total mass of electrolyte:

[0009]

[0010] 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 phosphonic fluoroboric acid lithium is selected from at least one of the following structures:

[0011]

[0012] Preferably, the unsaturated phosphonic fluoroboric acid lithium is added into electrolyte in an adding amount of 0.1-10wt% of the total mass of electrolyte; more preferably, the unsaturated phosphonic fluoroboric acid lithium is added into electrolyte in an adding amount of 0.5-5.0wt% of the total mass of electrolyte.

[0013] The electrolyte also comprises:

[0014] A main lithium salt, the main lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bisfluorosulfonylimide and lithium bis(trifluoromethylsulfonyl)imide, and the molar concentration in the electrolyte is 0.4-1.6mol / L; preferably, the main lithium salt is lithium hexafluorophosphate or lithium bisfluorosulfonylimide, and the molar concentration in the electrolyte is 0.6-1.2mol / L;

[0015] A non-aqueous solvent, the non-aqueous organic solvent is a combination of a cyclic solvent and a linear solvent, the cyclic solvent is selected from at least one of ethylene carbonate, propylene carbonate, fluorinated ethylene carbonate, gamma-butyrolactone or delta-valerolactone; and the linear solvent is selected from at least one of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, ethyl propionate, n-propyl propionate and methyl propyl carbonate;

[0016] The base additive is at least one selected from fluoroethylene carbonate, ethylene carbonate, tris(trimethylsilyl)phosphate, 1,3-propane sultone, lithium difluorophosphate, lithium bisfluorosulfonylimide, lithium difluorophosphate bisoxalate or lithium difluorophosphate boric oxalate, and the amount of any base additive is 0.1-5.0 wt% of the total electrolyte.

[0017] 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 phosphonic fluoroboric acid lithium of structural formula I-1, which can further reduce the storage gas production of the battery and improve the high-temperature cycle performance compared with the use of unsaturated phosphonic fluoroboric acid lithium alone.

[0018] 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 bisfluorosulfonylimide and 1.0-2.0 wt% unsaturated phosphonic fluoroboric acid lithium of structural formula I-1, which can not only reduce the initial impedance of the battery, improve the normal temperature and high-temperature cycle performance, but also further inhibit the high-temperature storage gas production and improve the comprehensive performance of the battery compared with the use of unsaturated phosphonic fluoroboric acid lithium alone.

[0019] The application also provides a lithium ion battery comprising a positive electrode, a negative electrode, a separator and any of the above-mentioned electrolytes.

[0020] The positive 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; and the negative active material is one of a graphite negative electrode, a silicon-carbon negative electrode and a lithium metal negative electrode.

[0021] The application also provides a preparation method of the above-mentioned unsaturated phosphonic fluoroboric acid lithium, which comprises the following steps:

[0022] (1) reacting an unsaturated phosphonic acid with an inorganic lithium salt in a first solvent to obtain an unsaturated phosphonic acid lithium reaction solution, the inorganic lithium salt is at least one selected from lithium carbonate, lithium bicarbonate, lithium oxide or lithium hydroxide; preferably lithium bicarbonate, lithium oxide or lithium hydroxide, and more preferably lithium carbonate;

[0023] For example, lithium carbonate, the reaction is as follows:

[0024]

[0025] wherein R is as defined above;

[0026] (2) the unsaturated phosphonic acid lithium reaction solution is reacted with boron trifluoride gas or boron trifluoride complex, and an unsaturated phosphonic fluoroboric acid lithium reaction solution is obtained, and the reaction is as follows:

[0027]

[0028] wherein R is as defined above.

[0029] In step (1), inorganic lithium salt and unsaturated phosphonic acid are used as raw materials, the reaction heat is reduced by controlling the reaction temperature and the feeding ratio, and the thermal polymerization of the unsaturated bond in the raw materials or reaction products to form dimers or even polymers is avoided, so as to improve the reaction yield and product purity.

[0030] 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 the molar ratio is (1.8-2.1):1, and more preferably the molar ratio is about 2:1.

[0031] Meanwhile, the reaction temperature in step (1) is controlled to be -10-50℃, and the reaction time is 0.5-24h; preferably the reaction temperature is 0-10℃, and the reaction time is 10-12h.

[0032] In step (1), the first solvent is selected from at least one of water, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, methyl propionate, γ-butyrolactone, diethyl ether, ethylene glycol dimethyl ether, acetonitrile, phenylacetonitrile or propionitrile, and is preferably at least one of dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate.

[0033] In step (2) of the present application, the unsaturated phosphonic acid lithium is reacted with boron trifluoride gas or boron trifluoride complex to obtain an unsaturated phosphonic fluoroboric acid lithium reaction solution, and the reaction solution is concentrated by reduced pressure distillation to obtain unsaturated phosphonic fluoroboric acid lithium, and the reduced pressure distillation temperature is 0-60℃, and preferably the reduced pressure distillation temperature is 20-30℃. Due to the solvation of unsaturated phosphonic fluoroboric acid lithium, the unsaturated phosphonic fluoroboric acid lithium obtained after concentration by reduced pressure distillation is a concentrated solution.

[0034] Specifically, in step (2), the boron trifluoride complex is selected from at least one of boron trifluoride diethyl ether complex, boron trifluoride ethylene glycol dimethyl ether complex, boron trifluoride dimethyl carbonate complex, boron trifluoride pyridine complex, boron trifluoride ethylamine complex, boron trifluoride butyl ether complex, boron trifluoride methyl ether complex, boron trifluoride acetonitrile complex, boron trifluoride piperidine complex, boron trifluoride phenol complex, boron trifluoride tetrahydrofuran complex, boron trifluoride dimethyl sulfide complex or boron trifluoride morpholine complex.

[0035] 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), more preferably the molar ratio is 1:(2.0-2.2).

[0036] In step (2), the reaction temperature is 0-50℃, and the reaction time is 0.5-24h; preferably, the reaction temperature is 10-30℃, and the reaction time is 2-4h.

[0037] Compared with the prior art, the present application has the following technical effects:

[0038] 1. The present application provides a preparation method of unsaturated phosphonic group lithium fluoroborate, which has simple process and high reaction yield, and further improves the reaction yield and product purity by optimizing the reaction conditions to reduce the generation of polymers.

[0039] 2. The present application provides the application of unsaturated phosphonic group lithium fluoroborate in lithium ion battery electrolyte, which can form film on the positive and negative electrodes of the battery, reduce the initial impedance of the battery, reduce the gas production amount during high-temperature storage, and improve the room temperature and high-temperature cycle performance of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 NMR nuclear magnetic hydrogen spectrum of lithium salt 1# prepared in Example 1 of the present application;

[0041] Figure 2 NMR nuclear magnetic fluorine spectrum of lithium salt 1# prepared in Example 1 of the present application;

[0042] Figure 3 NMR nuclear magnetic phosphorus spectrum of lithium salt 1# prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0043] The present application will be further described in conjunction with specific examples, but the present application is not limited to these specific embodiments. Those skilled in the art should realize that the present application encompasses all alternatives, improvements and equivalents within the scope of the claims.

[0044] Example 1

[0045] The present embodiment provides a preparation method of ethylene phosphonic acid lithium trifluoroborate (I-1), which specifically comprises the following steps:

[0046] (1) In a dry room with a dew point of -40℃, 0.1 mol (7.40 g) of lithium carbonate and 100 g of DMC solvent were added to a reaction bottle, and then stirred in a 0℃ low-temperature cold bath pot for 30 min to cool the reaction liquid to 0℃; 0.1 mol (10.80 g) of vinyl phosphonic acid was dissolved in 10 g of DMC, and slowly added to the reaction liquid through a constant pressure dropping funnel, and reacted at 0℃ for 10 h to obtain a solution containing lithium ethylene phosphonate;

[0047] (2) 0.21 mol (33.18 g) of boron trifluoride dimethyl carbonate complex was added to the above solution containing lithium ethylene phosphonate, and stirred at room temperature for 4 h, then filtered to remove the insoluble solid, and then removed the excess DMC solvent and BF3 by reduced pressure distillation, and then removed water for 30 min by using dry molecular sieve, and finally obtained a DMC concentrated solution containing compound (I-1), which was recorded as lithium salt 1#.

[0048] Figure 1 、 Figure 2 and Figure 3 The NMR spectrum of lithium salt 1# is shown in the following figure, wherein Figure 1 is the H spectrum, wherein δ = 5.7-6.2 ppm (m, 3H) corresponds to the H on the vinyl group, and δ = 3.66 ppm (s) corresponds to the methyl hydrogen on the DMC; Figure 2 is the F spectrum, and a peak is observed at δ = -148.05 ppm; Figure 3 is the P spectrum, and a peak is observed at δ = 11.79 ppm. The structure of the compound was optimized by using DFT / B3LYP function and 6-31** basis set, and the nuclear magnetic peak position of the above compound was calculated by using DFT / B3LYP function and 6-311** basis set. Through the synergistic analysis of the calculation results and the test results, it is matched with the above nuclear magnetic spectrum results, and has high reliability. Therefore, it can be confirmed that the compound in lithium salt 1# is compound (I-1).

[0049] Further, the concentration of compound (I-1) in the DMC solution is calculated by nuclear magnetic H spectrum to be 37.6 wt%, and a total of 66.73 g of reduced pressure distillation concentrated solution is obtained, and the actual mass of compound (I-1) is 25.09 g (0.098 mol), and the reaction yield is 98%.

[0050] Example 2

[0051] The operation of this example is the same as that of Example 1, except that in step (1), the amount of the raw material lithium carbonate is adjusted to 0.08 mol (5.92 g), and a DMC concentrate containing compound (I-1) is finally obtained, which is recorded as Lithium Salt 1# in the same manner as in Example 1.

[0052] The actual mass of compound (I-1) obtained is 19.46 g (0.076 mol), and the reaction yield is 95%.

[0053] Example 3

[0054] 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, and the amount used is 0.1 mol (3.0 g), and a DMC concentrate containing compound (I-1) is finally obtained, which is recorded as Lithium Salt 1# in the same manner as in Example 1.

[0055] The actual mass of compound (I-1) obtained is 24.05 g (0.093 mol), and the reaction yield is 93%.

[0056] Example 4

[0057] The operation of this example is the same as that of Example 1, except that in step (2), there is no operation of removing excess DMC and BF3 by vacuum distillation, and a DMC mixture containing compound (I-1) and BF3 is obtained, which is recorded as Lithium Salt 2#.

[0058] The concentration of compound (I-1) in the DMC solution is calculated to be 21.02% by H-NMR spectrum, and after removing the insoluble matter by filtration, 117.76 g of reaction liquid is obtained, and the actual mass of compound (I-1) is 25.42 g (0.098 mol), and the reaction yield is 98%.

[0059] Example 5

[0060] 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 by DEC, and a DEC concentrate containing compound (I-1) is obtained, which is recorded as Lithium Salt 3#

[0061] The concentration of compound (I-1) in the DEC solution is calculated to be 39.4 wt% by H-NMR spectrum, and a total of 63.03 g of concentrated liquid is obtained by vacuum distillation, and the actual mass of compound (I-1) is 24.83 g (0.097 mol), and the reaction yield is 97%.

[0062] Example 6

[0063] 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, and the amount used is 0.2 mol (4.8 g), to obtain a DMC concentrate containing compound (I-1), which is referred to as lithium salt 4#.

[0064] The actual mass of compound (I-1) obtained is 24.05 g (0.093 mol), and the reaction yield is 93%.

[0065] Example 7

[0066] The operation of this example is the same as that of Example 1, except that in step (1), the raw material vinyl phosphoric acid is replaced by propenyl phosphoric acid, and the amount used is 0.1 mol (12.20 g), to obtain a DMC concentrate containing compound (I-2), which is referred to as lithium salt 5#.

[0067] The actual mass of compound (I-2) obtained is 25.79 g (0.097 mol), and the reaction yield is 97%.

[0068] Example 8

[0069] The operation of this example is the same as that of Example 1, except that in step (1), the raw material vinyl phosphoric acid is replaced by butynyl phosphoric acid, and the amount used is 0.1 mol (13.40 g), to obtain a DMC concentrate containing compound (I-6), which is referred to as lithium salt 6#.

[0070] The actual mass of compound (I-6) obtained is 26.16 g (0.094 mol), and the reaction yield is 94%.

[0071] Comparative Example 1

[0072] The operation of this example is the same as that of Example 1, except that in step (1), the reaction 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.

[0073] The actual mass of compound (I-1) obtained is 22.91 g (0.097 mol), and the reaction yield is 89.5%.

[0074] Comparative Example 2

[0075] This comparative example is used to obtain lithium ethylene phosphate. The specific operation is as follows: after the reaction in step (1) of Example 1 is completed, the insoluble material generated in the reaction is filtered and washed with DMC, and after drying, lithium ethylene phosphate with the following structure (II) is obtained:

[0076]

[0077] II. Electrolyte

[0078] Preparation of base electrolyte 1: In an argon-filled glove box (moisture <5 ppm, oxygen <10 ppm), ethylene carbonate (EC), methyl ethyl carbonate (EMC), 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 the base electrolyte.

[0079] Application Example 1: In the base electrolyte, 0.5wt% of compound (I-1) was added using lithium salt 1# to obtain the electrolyte.

[0080] Application Example 2: In the base electrolyte, 1.0wt% of compound (I-1) was added using lithium salt 1# to obtain the electrolyte.

[0081] Application Example 3: In the base electrolyte, 2.0wt% of compound (I-1) was added using lithium salt 1# to obtain the electrolyte.

[0082] Application Example 4: In the base electrolyte, 5.0wt% of compound (I-1) was added using lithium salt 1# to obtain the electrolyte.

[0083] Application Example 5: In the base electrolyte, 1.0wt% of compound (I-1) and 1.0wt% of vinylene carbonate (VC) were added using lithium salt 1# to obtain the electrolyte.

[0084] Application Example 6: In the base electrolyte, 1.0wt% of compound (I-1) and 1.0wt% of 1,3-propanesultone (PS) were added using lithium salt 1# to obtain the electrolyte.

[0085] Application Example 7: In the base electrolyte, 1.0wt% of compound (I-1) and 2.0wt% of fluoroethylene carbonate (FEC) were added using lithium salt 1# to obtain the electrolyte.

[0086] Application Example 8: In the base electrolyte, 1.0wt% of compound (I-1) was added using lithium salt 2# to obtain the electrolyte.

[0087] Application Example 9: In the base electrolyte, 1.0wt% of compound (I-1) was added using lithium salt 3# to obtain the electrolyte.

[0088] Application Example 10: In the base electrolyte, 1.0wt% of compound (I-1) was added using lithium salt 4# to obtain the electrolyte.

[0089] Application Example 11: In the base electrolyte, 1.0wt% of compound (I-2) was added using lithium salt 5# to obtain the electrolyte.

[0090] Application Example 12: In the base electrolyte, 1.0 wt% of compound (I-6) is added using lithium salt 6#, to obtain an electrolyte.

[0091] Application Example 13: In the base electrolyte, 1.0 wt% of compound (I-1), 1.0 wt% of fluoroethylene carbonate (FEC) and 1.0 wt% of lithium bisfluorosulfonylimide (LiFSI) are added using lithium salt 1#, to obtain an electrolyte.

[0092] Application Example 14: In the base electrolyte, 2.0 wt% of compound (I-1) is added using lithium salt 4#, to obtain an electrolyte.

[0093] Application Example 15: In the base electrolyte, 5.0 wt% of compound (I-1) is added using lithium salt 4#, to obtain an electrolyte.

[0094] Application Comparative Example 1: The base electrolyte is not treated, to obtain an electrolyte.

[0095] Application Comparative Example 2: In the base electrolyte, 1.0 wt% of VC is added, to obtain an electrolyte.

[0096] Application Comparative Example 3: In the base electrolyte, 1.0 wt% of PS is added, to obtain an electrolyte.

[0097] Application Comparative Example 4: In the base electrolyte, 2.0 wt% of FEC is added, to obtain an electrolyte.

[0098] Application Comparative Example 4: In the base electrolyte, 0.2 wt% of lithium ethylene phosphate is added, to obtain an electrolyte.

[0099] II. Electrochemical performance test

[0100] The electrolytes of the above application examples and application comparative examples are respectively made into soft package capacity 1500 mAh lithium ion batteries, which include positive electrode sheets, negative electrode sheets, separators, electrolytes and battery accessories, the positive electrode active material is nickel-cobalt-manganese ternary material Li(Ni 0.8 Co 0.1 Mn 0.1 )O2, and the negative electrode active material is artificial graphite.

[0101] The preparation process is as follows: the positive electrode sheet, the separator and the negative electrode sheet are wound together into a roll core, and the roll core is sealed with an aluminum plastic film and then baked to make the electrode moisture meet the requirements. After baking, the battery core is subjected to electrolyte injection, standing, formation, capacity distribution and aging processes to obtain a finished soft package battery core.

[0102] The lithium ion power battery (soft package cell) prepared was subjected to performance test, and the specific test items and methods were as follows:

[0103] (1) 60°C high temperature storage test: before storage, 1 cycle was carried out at 0.5C / 0.5C room temperature, the 1st cycle discharge capacity, internal resistance and volume were recorded, then the battery was charged to 4.20V at 0.5C current and charged to constant voltage until the current decreased to 0.05C, and then the battery was placed in a 60°C constant temperature oven for 60 days, and then 2 cycles were carried out at 0.5C / 0.5C room temperature, the 1st cycle discharge capacity after high temperature storage, the 2nd cycle discharge capacity, the internal resistance after storage and the volume were recorded. The capacity retention rate, the capacity recovery rate, the internal resistance increase rate and the volume expansion rate of the battery after storage were calculated according to the following formula:

[0104] Capacity retention rate = 1st cycle discharge capacity after high temperature storage (after storage) / 1st cycle discharge capacity (before storage) * 100%.

[0105] Capacity recovery rate = 2nd cycle discharge capacity after high temperature storage (after storage) / 1st cycle discharge capacity (before storage) * 100%.

[0106] Volume expansion rate = (volume after storage - volume before storage) / volume before storage * 100%.

[0107] (2) 25°C high temperature cycle test: the battery was cycled at 1C / 1C charge-discharge current in a 25±1°C room temperature environment, the discharge capacity of each week was calculated, and the cycle was stopped when the capacity retention rate was lower than 80%.

[0108] (3) 45°C high temperature cycle test: the battery was cycled at 1C / 1C charge-discharge current in a 45±1°C oven, the discharge capacity of each week was calculated, and the cycle was stopped when the capacity retention rate was lower than 80%.

[0109] The test results are shown in Table 1 as follows:

[0110] Table 1 Battery performance test results

[0111]

[0112]

[0113] According to the data in Table 1, by comparing application examples 1-4 and the application of comparative example 1, it can be seen that compound (I-1) used as an electrolyte additive can reduce the initial impedance of the battery, improve the normal temperature and high temperature cycle performance, and also has excellent storage gas inhibition effect. With the gradual increase of the amount of lithium salt additive, within a certain range, the comprehensive performance of the battery gradually improves, but as the content further increases, part of the performance begins to decline, and the best use amount is about 1.0-2.0wt%. In the application of comparative example, the solubility of compound (II) in the electrolyte is only 0.2wt%, and it cannot reach the addition amount of 1.0wt% otherwise it will precipitate solid, and its improvement effect on the battery is weak, while compound (I-1) has good solubility in the solvents used in the electrolyte, and the comprehensive performance of the battery is improved more obviously, and has a wider application range and practical value.

[0114] By comparing application examples 2, 5-7, 13 and application comparative example 1, it can be seen that compound (I-1) and VC, PS, FEC and LiFSI and other basic additives can further improve the comprehensive performance of the battery.

[0115] By comparing application examples 2, 8 and 10, application example 8 does not use reduced pressure distillation for lithium salt 2#, and there is a small amount of BF3 in the reaction solution, and in application example 10, the amount of H2O generated by the reaction of lithium hydroxide with acid is large, which is difficult to remove with molecular sieves, and the existence of BF3 and H2O can deteriorate the storage gas inhibition effect and the cycle performance. By further comparing application example 10, application example 14 and application example 15, it can be seen that with the increase of the amount of lithium salt 2#, the water content in the electrolyte is higher, and the battery performance is also more deteriorated. Therefore, during preparation, by selecting appropriate raw materials and reasonable post-processing schemes, the existence of impurities should be minimized to improve the reaction efficiency and maximize the effect of the material in the application process.

Claims

1. Use of unsaturated phosphonate-based lithium fluoroborate in an electrolyte, characterized in that: An unsaturated phosphonate-based lithium fluoroborate represented by the following structure (I) is added to an electrolyte in an amount of 0.01-15 wt% based on 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. Use of unsaturated phosphonate-based lithium fluoroborate in an electrolyte according to claim 1, characterized in that: The unsaturated phosphonate-based lithium fluoroborate is selected from at least one of the following structures:

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

4. Use of the unsaturated phosphonate-based lithium fluoroborate according to any one of claims 1 to 3 in an electrolyte, characterized in that: The electrolyte further comprises: a main lithium salt selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bisfluorosulfonimide, lithium bis-trifluoromethylsulfonimide; a non-aqueous organic solvent which is a combination of a cyclic solvent selected from at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, γ-butyrolactone or δ-valerolactone and a linear solvent selected from at least one of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, ethyl propionate, n-propyl propionate or methyl propyl carbonate; a base additive selected from at least one of fluoroethylene carbonate, ethylene carbonate, tris(trimethylsilyl)phosphate, 1,3-propene sultone, lithium difluorophosphate, lithium bisfluorosulfonimide, lithium bis-oxalato difluorophosphate or lithium difluoro oxalate borate, any base additive being in an amount of 0.1-5.0 wt% based on the total mass of the electrolyte.

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

6. Process for the preparation of unsaturated phosphonate-based lithium fluoroborate for use in electrolytes according to any one of claims 1-4, characterized in that: The preparation method comprises: (1) reacting an unsaturated phosphonic acid with at least one of lithium carbonate, lithium bicarbonate, lithium oxide or lithium hydroxide in a first solvent to obtain an unsaturated phosphonate lithium reaction solution, the reaction temperature being 0-10℃; (2) introducing boron trifluoride gas or adding a boron trifluoride complex into the unsaturated phosphonate lithium reaction solution to obtain an unsaturated phosphonate-based lithium fluoroborate reaction solution.

7. The process for the preparation of unsaturated phosphonite lithium fluoroborate according to claim 6, characterized by the fact that: In step (1), the molar ratio of lithium in lithium carbonate, lithium bicarbonate, lithium oxide or lithium hydroxide to the unsaturated phosphonic acid is (1.6-2.2):

1.

8. The process for the preparation of unsaturated phosphonate-based lithium fluoroborate according to claim 6, characterized by the fact that: In step (1), the reaction time is 0.5-24 h.

9. The process for the preparation of unsaturated phosphonite lithium fluoroborate according to claim 6, characterized by the fact that: The first solvent is selected from at least one of water, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, methyl propionate, γ-butyrolactone, diethyl ether, ethylene glycol dimethyl ether, acetonitrile, phenylacetonitrile or propionitrile.

10. The process for the preparation of unsaturated phosphonate fluoro-borate lithium salt according to claim 6, characterized by the fact that: In step (2), the boron trifluoride complex is selected from at least one of boron trifluoride diethyl ether complex, boron trifluoride ethylene glycol dimethyl ether complex, boron trifluoride dimethyl carbonate complex, boron trifluoride pyridine complex, boron trifluoride ethylamine complex, boron trifluoride butyl ether complex, boron trifluoride methyl ether complex, boron trifluoride acetonitrile complex, boron trifluoride piperidine complex, boron trifluoride phenol complex, boron trifluoride tetrahydrofuran complex, boron trifluoride dimethyl sulfide complex or boron trifluoride morpholine complex.

11. The process for the preparation of unsaturated phosphonite lithium fluoroborate according to claim 6, characterized by the fact that: In step (2), the molar ratio of the unsaturated phosphonate lithium to the boron trifluoride gas or the boron trifluoride complex is 1:(1.8-3).

12. The process for the preparation of unsaturated phosphonofluoroborate lithium salt according to claim 6, characterized by: In step (2), the reaction temperature is 0-50°C, and the reaction time is 0.5-24h.

13. The process for the preparation of unsaturated phosphonite lithium fluoroborate according to claim 6, characterized by the fact that: The unsaturated phosphonate-based lithium fluoroborate reaction solution obtained in step (2) is subjected to vacuum distillation to obtain unsaturated phosphonate-based lithium fluoroborate, and the vacuum distillation temperature is 0-60°C.

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