Application of lithium bis(fluoroborate) difluorophosphate as a main lithium salt and its electrolyte

Abstract

The present invention discloses an application of lithium difluorophosphate fluoroborate as a main lithium salt and an electrolyte thereof. The application includes using lithium difluorophosphate fluoroborate represented by the following formula (I) as a main lithium salt in a lithium-ion battery electrolyte in an addition amount within the range of (10 wt%, 30 wt%]: In the formula, x + y = 4, x ≥ 0 and y ≥ 1, and x and y are positive integers. When lithium difluorophosphate fluoroborate of the present invention is used as a main lithium salt, it has good thermal stability, solubility and conductivity, and can increase the battery interface stability and inhibit gas generation during high-temperature storage of the battery.

Description

Technical Field

[0001] The present invention relates to the field of battery electrolytes, especially lithium-ion battery electrolytes, and particularly relates to the application of lithium difluorophosphate fluoroborate as a main lithium salt, and a lithium-ion battery electrolyte containing the lithium difluorophosphate fluoroborate. Background Art

[0002] The electrolyte system is an essential part of lithium-ion batteries, and its properties determine the performance of lithium-ion batteries. For example, the decomposition voltage of the electrolyte limits the maximum voltage of the battery, and its conductivity limits the magnitude of the discharge current of the battery. Currently, most electrolyte systems are composed of dissolving electrolyte salts in organic aprotic solvents, and the typical electrolyte salt used commercially is lithium hexafluorophosphate (LiPF6). Although LiPF6-based electrolytes have advantages such as good conductivity and can passivate aluminum current collectors, LiPF6 has poor stability and is prone to hydrolysis to produce HF, PF5, etc., which accelerate the occurrence of side reactions of the electrolyte, accelerate the discoloration and decomposition of the electrolyte, etc., thus leading to a decline in battery performance.

[0003] In addition to lithium hexafluorophosphate, the prior art also mentions that lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium difluoro bis(oxalato)phosphate (LiDFOP), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), etc. can be used as main lithium salts. However, due to factors such as conductivity, solubility, compatibility with aluminum current collectors, or safety, these substances cannot replace lithium hexafluorophosphate as the main lithium salt in industrial production, and can only be used as additives, or need to be used in combination with lithium hexafluorophosphate as the main lithium salt.

[0004] The stability of lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide as main lithium salts is better than that of LiPF6, and it is not easy to produce HF. However, when the potential exceeds 3.6V vs. Li+ / Li, it will form [(CF3SO2)2N]3Al with the Al current collector, and this substance is stable and very soluble, which will continuously corrode the aluminum current collector, causing an increase in internal short circuit of the battery and attenuation of battery performance.

[0005] Lithium tetrafluoroborate has good stability, is not prone to hydrolysis to produce HF, and there is no phenomenon of corroding the Al current collector. However, the conductivity of LiBF4 in the electrolyte is low, and its cycle performance and rate performance are poor. Generally, it is only used as an additive, and using it as the main lithium salt cannot meet the battery cycle performance.

[0006] Lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluoro bis(oxalato)phosphate, etc. have good film-forming effects at the electrode / electrolyte interface and can improve the electrochemical performance of batteries. However, the solubility of these salts is poor, and the oxalate group in the anion is prone to decomposition upon heating to generate gases such as CO2, which exacerbates battery swelling. Therefore, they can only be used as electrolyte additives and cannot be added in large amounts as the main lithium salt.

[0007] Lithium perchlorate is explosive and has poor safety, and lithium hexafluoroarsenate (LiAsF6) is highly toxic. These defects limit their use as the main lithium salt in electrolytes.

[0008] Therefore, it is very necessary to find a main lithium salt with good stability, solubility, high conductivity, and no deterioration of battery performance, and which can completely replace lithium hexafluorophosphate in industrial production. Summary of the Invention

[0009] To solve the above technical problems, the present invention provides an application of lithium difluorophosphate fluoroborate as a main lithium salt and a lithium-ion battery electrolyte containing the lithium difluorophosphate fluoroborate.

[0010] The object of the present invention is achieved by the following technical solutions:

[0011] The application of lithium difluorophosphate fluoroborate as a main lithium salt includes: using the lithium difluorophosphate fluoroborate shown in the following formula (I) as the main lithium salt in a lithium-ion battery electrolyte at an addition amount of (10 wt%, 30 wt%]:

[0012]

[0013] In the formula, x + y = 4, x ≥ 0 and y ≥ 1, and x and y are positive integers.

[0014] Specifically, the lithium difluorophosphate fluoroborate is selected from at least one of the following structural formulas:

[0015]

[0016] Preferably, the lithium difluorophosphate fluoroborate is selected from the structural formulas shown in the following formula (I) and / or (II):

[0017]

[0018] The above addition amount of (10 wt%, 30 wt%] in the present invention is a semi-open and semi-closed interval, indicating that the addition amount of lithium difluorophosphate fluoroborate is between 10 wt% and 30 wt%, but does not include 10 wt% and includes 30 wt%.

[0019] Preferably, lithium bis(fluoroborate) difluorophosphate is used as the main lithium salt in the lithium-ion battery electrolyte at an addition amount of [12 wt%, 20 wt%], that is, the addition amount of lithium bis(fluoroborate) difluorophosphate is 12 wt% to 20 wt%.

[0020] More preferably, lithium bis(fluoroborate) difluorophosphate is used as the main lithium salt in the lithium-ion battery electrolyte at an addition amount of [12 wt%, 16 wt%], that is, the addition amount of lithium bis(fluoroborate) difluorophosphate is 12 wt% to 16 wt%.

[0021] The lithium bis(fluoroborate) difluorophosphate described in the present invention can replace lithium hexafluorophosphate in the lithium-ion battery electrolyte, so that lithium hexafluorophosphate does not need to be added to the electrolyte, improving the stability of the electrolyte and not easily generating substances such as HF and PF5 to accelerate side reactions during the use of the battery.

[0022] The present invention also provides a lithium-ion battery electrolyte, which includes a main lithium salt, a non-aqueous solvent, and the lithium bis(fluoroborate) difluorophosphate described in any one of the above, and the addition amount of the lithium bis(fluoroborate) difluorophosphate in the electrolyte is (10 wt%, 30 wt%], preferably the addition amount is [12 wt%, 20 wt%], and more preferably the addition amount is [12 wt%, 16 wt%].

[0023] In order to improve the comprehensive performance of the battery, the electrolyte further includes a basic additive, and the basic additive is selected from at least one of sulfonate compounds, sulfate compounds, fluorinated carbonate compounds, unsaturated carbonate compounds or fluorinated lithium salt compounds, and the dosage accounts for 0.1 wt% to 5.0 wt% of the total amount of the electrolyte. The selection of the basic additive and its addition amount can be selected according to the performance requirements of different electrolyte formulations, and no specific limitation is made here.

[0024] The sulfonate compounds are selected from at least one of 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone or methylene methanedisulfonate;

[0025] The sulfate compounds are selected from at least one of ethylene sulfate, 4-methyl ethylene sulfate, 4-fluoroethylene sulfate or 4,4'-bis(ethylene sulfate);

[0026] The fluorinated carbonate compounds are selected from at least one of ethyl fluorocarbonate, difluoroethylene carbonate or propylene trifluoromethyl carbonate;

[0027] The unsaturated carbonate compounds are selected from vinylene carbonate and / or vinyl ethylene carbonate;

[0028] The fluorine-containing lithium salt compound is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium tetrafluorooxalate phosphate, lithium difluorobis(oxalate)phosphate, lithium difluorooxalate borate, and lithium trioxalate phosphate.

[0029] The non-aqueous solvent used in the present invention can be a commonly used solvent in electrolytes. Preferably, the non-aqueous solvent is selected from at least one of C3-C6 carbonate compounds, C3-C8 carboxylate compounds, sulfone compounds, or ether compounds.

[0030] Among them: the C3-C6 carbonate compounds are selected from at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, fluorinated ethylene carbonate, or difluorinated ethylene carbonate;

[0031] The C3-C8 carboxylate compounds are selected from at least one of γ-butyrolactone, methyl acetate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl acetate, propyl propionate, or fluorinated ethyl acetate;

[0032] The sulfone compounds are selected from at least one of sulfolane, dimethyl sulfoxide, dimethyl sulfone, or diethyl sulfone;

[0033] The ether compounds are selected from at least one of triglyme, tetraglyme, or 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0034] The present invention also provides a lithium-ion battery, including a positive electrode, a negative electrode, a separator, and the lithium-ion battery electrolyte described in any one of the above.

[0035] The active material of the positive electrode is selected from nickel cobalt manganese ternary materials, nickel cobalt aluminum ternary materials, lithium cobaltate materials, or lithium iron phosphate materials; among them, the nickel cobalt manganese ternary material is Li(Ni x Co y Mn z )O2, x≥0.5, y>0, z>0, x + y + z = 1; the nickel cobalt aluminum ternary material is Li(NixCo y Al z) O2, x≥0.8, y>0, z>0, x + y + z = 1.

[0036] The active material of the negative electrode is graphite, silicon-carbon, silicon monoxide, silicon, tin, metallic lithium, or a composite material thereof.

[0037] Compared with the prior art, the beneficial effects of the present invention include:

[0038] The present invention uses lithium bis(fluoroborate) difluorophosphate as the main lithium salt in the electrolyte of lithium-ion batteries. It not only has good stability, solubility, and conductivity, and there are no phenomena such as corrosion of aluminum current collectors and deterioration of battery performance. Moreover, when it completely replaces lithium hexafluorophosphate as the main lithium salt, it can form a good SEI / CEI film at the positive and negative electrode interfaces of the battery, protect the positive and negative electrode interfaces of the battery, increase the stability of the battery interface, significantly inhibit gas generation during high-temperature storage of the battery, and has a cycle performance comparable to that of lithium hexafluorophosphate. Detailed implementation manners

[0039] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited to these specific implementation manners. Those skilled in the art should recognize that the present invention covers all alternative solutions, improvement solutions, and equivalent solutions that may be included within the scope of the claims.

[0040] I. Preparation of electrolyte

[0041] Example 1

[0042] In a glove box filled with argon (moisture < 5 ppm, oxygen content < 10 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were uniformly mixed according to a mass ratio of EC:EMC:DEC = 3:5:2. Lithium bis(fluoroborate) difluorophosphate (Compound 1) was slowly added to the mixed solution to a mass percentage of 10.5%, and then vinylene carbonate (VC) accounting for 1.0% of the mass of the electrolyte was added to obtain the electrolyte of this example.

[0043] Example 2

[0044] The operation of this example is the same as that of Example 1, except that: the mass content of lithium bis(fluoroborate) difluorophosphate (Compound 1) is 12.5%, and the mass content of vinylene carbonate (VC) is 1.0% to obtain the electrolyte of this example.

[0045] Example 3

[0046] The operation of this example is the same as that of Example 1, except that: the mass content of lithium bis(fluoroborate) difluorophosphate (Compound 1) is 14.5%, and the mass content of vinylene carbonate (VC) is 1.0% to obtain the electrolyte of this example.

[0047] Example 4

[0048] The operation of this example is the same as that of Example 1, except that: the mass content of lithium bis(fluoroborate) difluorophosphate (Compound 1) is 16.0%, and the mass content of vinylene carbonate (VC) is 1.0% to obtain the electrolyte of this example.

[0049] Example 5

[0050] The operation of this example is the same as that of Example 1, with the only difference being that the mass content of lithium bis(fluoroborate) monofluorophosphate (Compound 1) is 20.0%, and the mass content of vinylene carbonate (VC) is 1.0%, obtaining the electrolyte of this example.

[0051] Example 6

[0052] The operation of this example is the same as that of Example 3, with the only difference being that vinylene sulfate (DTD) is used instead of vinylene carbonate (VC), and the mass content of vinylene sulfate (DTD) is 1.0%, obtaining the electrolyte of this example.

[0053] Example 7

[0054] The operation of this example is the same as that of Example 3, with the only difference being that fluoroethylene carbonate (FEC) is used instead of vinylene carbonate (VC), and the mass content of fluoroethylene carbonate (FEC) is 1.0%, obtaining the electrolyte of this example.

[0055] Example 8

[0056] The operation of this example is the same as that of Example 3, with the only difference being that lithium bis(fluoroborate) difluorophosphate (Compound 2) is used instead of lithium bis(fluoroborate) monofluorophosphate, and the mass content of lithium bis(fluoroborate) difluorophosphate is 14.5%, and the mass content of vinylene carbonate (VC) is 1.0%, obtaining the electrolyte of this example.

[0057] Example 9

[0058] The operation of this example is the same as that of Example 3, with the only difference being that lithium bis(fluoroborate) trifluorophosphate (Compound 3) is used instead of lithium bis(fluoroborate) monofluorophosphate, and the mass content of lithium bis(fluoroborate) trifluorophosphate is 14.5%, and the mass content of vinylene carbonate (VC) is 1.0%, obtaining the electrolyte of this example.

[0059] Example 10

[0060] The operation of this comparative example is the same as that of Example 3, with the only difference being that lithium bis(fluoroborate) tetrafluorophosphate (Compound 4) is used instead of lithium bis(fluoroborate) monofluorophosphate, and the mass content of lithium bis(fluoroborate) tetrafluorophosphate is 14.5%, and the mass content of vinylene carbonate (VC) is 1.0%, obtaining the electrolyte of this example.

[0061] Comparative Example 1

[0062] The operation of this comparative example is the same as that of Example 1, except that: lithium hexafluorophosphate (LiPF6) is used instead of mono-substituted lithium fluoroborate difluorophosphate, and the mass content of lithium hexafluorophosphate (LiPF6) is 12.5%, and the mass content of vinylene carbonate (VC) is 1.0% to obtain the electrolyte of this comparative example.

[0063] Comparative Example 2

[0064] The operation of this comparative example is the same as that of Example 1, except that: lithium hexafluorophosphate (LiPF6) is used instead of mono-substituted lithium fluoroborate difluorophosphate, and the mass content of lithium hexafluorophosphate (LiPF6) is 14.5%, and the mass content of vinylene carbonate (VC) is 1.0% to obtain the electrolyte of this comparative example.

[0065] Comparative Example 3

[0066] The operation of this comparative example is the same as that of Example 2, except that: on the basis of Example 2, lithium hexafluorophosphate (LiPF6) with a mass content of 12.5% is further added.

[0067] Comparative Example 4

[0068] The operation of this comparative example is the same as that of Comparative Example 1, except that: on the basis of Comparative Example 1, mono-substituted lithium fluoroborate difluorophosphate with a mass content of 1.0% is further added.

[0069] II. Performance Test

[0070] Four kinds of lithium fluoroborate difluorophosphates are dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (the mass ratio of EC:DMC is 50:50), and the conductivity test is carried out at 25°C. The test results are shown in Table 1 below.

[0071] Table 1 Conductivity Test Results Table of Lithium Fluoroborate Difluorophosphate

[0072]

[0073] It can be seen from Table 1 above that the electrolyte with lithium fluoroborate difluorophosphate as the main salt has relatively good conductivity, and with the increase of the addition amount, the conductivity of the electrolyte increases, and when the addition amount is 16.0%, the conductivity is basically equivalent to that of LiPF6.

[0074] The lithium-ion battery electrolytes of the above examples and comparative examples are respectively made into soft-pack lithium-ion power batteries with a capacity of 1500 mAh. The lithium-ion power batteries include a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte, and battery accessories. The positive electrode active material is a nickel-cobalt-manganese ternary material or a nickel-cobalt-aluminum ternary material or a lithium cobaltate material or a lithium iron phosphate material; among them, the positive electrode active material is a high-nickel ternary positive electrode LiNi 0.83Co 0.07 Mn 0.2 O₂, and the negative electrode active material is high-capacity graphite. The preparation process is as follows: The positive electrode sheet, separator, and negative electrode sheet are wound together into a core, sealed with an aluminum-plastic film, and then baked to make the electrode moisture meet the requirements. After baking, the electrolyte is injected into the battery cell, and the finished soft-pack battery cell is obtained through the processes of standing, formation, grading, and aging.

[0075] Performance tests were carried out on the prepared lithium-ion power battery (soft-pack battery cell). The specific test items and methods are as follows:

[0076] (1) 60°C high-temperature storage test: Charge the battery to 100% SOC, store it in an oven at 60 ± 2°C for 28 days, measure the volume before and after storage, and obtain the volume expansion rate of the single battery before and after storage at 60°C; measure the DCR value after storage at room temperature, and calculate the percentage value compared with the initial DCR, which is recorded as the discharge DCR change rate;

[0077] (2) 45°C high-temperature cycle test: The battery is cycled in an oven at 45 ± 1°C with a charge-discharge current of 1C / 1C, calculate the discharge capacity per week, cycle to 500 weeks, stop cycling, and calculate the capacity retention rate after cycling.

[0078] The test results are shown in Table 2 below.

[0079] Table 2 Battery Performance Test Results

[0080]

[0081] As can be seen from Table 2 above, using lithium difluorophosphate fluoroborate as the main salt of the electrolyte can ensure the normal operation of the battery. By comparing Examples 2, 3 and Comparative Examples 1, 2, it can be found that when lithium difluorophosphate fluoroborate is used to replace LiPF₆, the lithium battery can significantly inhibit gas generation during high-temperature storage while ensuring comparable cycle performance. By comparing Examples 1-5, it can be found that as the content of lithium difluorophosphate fluoroborate increases, the effect of inhibiting gas generation during high-temperature storage of the battery is enhanced. In Comparative Examples 1 and 2, due to the poor thermal stability of LiPF₆, as the content increases, the gas generation of the battery intensifies. In addition, by comparing Examples 3, 8-10, it can be found that as the degree of substitution in lithium difluorophosphate fluoroborate increases, the stability of the possible compound decreases, so the effect of inhibiting gas generation during high-temperature storage of the battery gradually decreases, indicating that the performance of monosubstituted lithium difluorophosphate fluoroborate is relatively optimal.

Claims

1. Application of lithium difluorophosphate fluoroborate as a main lithium salt, characterized in that: Lithium difluorophosphate fluoroborate represented by the following formula (I) is used as the main lithium salt in a lithium-ion battery electrolyte at an addition amount within the range of (10 wt%, 30 wt%]: In the formula, x + y = 4, x ≥ 0 and y ≥ 1, and x and y are positive integers.

2. The application of lithium difluorophosphate fluoroborate as the main lithium salt according to claim 1, wherein: The lithium difluorophosphate fluoroborate is selected from at least one of the following structural formulas:

3. Use of lithium difluorophosphate fluoroborate as the main lithium salt according to claim 1 or 2, characterized in that: Lithium difluorophosphate fluoroborate is used as the main lithium salt in a lithium-ion battery electrolyte at an addition amount of 12.0 wt% to 20 wt%.

4. Use of lithium difluorophosphate fluoroborate as a main lithium salt according to claim 1, characterized in that: The lithium difluorophosphate fluoroborate replaces lithium hexafluorophosphate and is used in a lithium-ion battery electrolyte.

5. A lithium-ion battery electrolyte, comprising a main lithium salt and a non-aqueous solvent, characterized in that: The main lithium salt is the lithium difluorophosphate fluoroborate according to any one of claims 1-4, and the addition amount in the electrolyte is (10 wt%, 30 wt%].

6. The lithium ion battery electrolyte according to claim 5, characterized in that: The addition amount of the lithium difluorophosphate fluoroborate in the electrolyte is 12.0 wt% to 20 wt%.

7. The lithium-ion battery electrolyte according to claim 5, wherein: The electrolyte further includes a basic additive, and the basic additive is selected from at least one of sulfonate compounds, sulfate compounds, fluorinated carbonate compounds, unsaturated carbonate compounds, or fluorinated lithium salt compounds, and the dosage accounts for 0.1 wt% to 5.0 wt% of the total amount of the electrolyte; The sulfonate compounds are selected from at least one of 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, or methylene methanedisulfonate; The sulfate compounds are selected from at least one of vinylene sulfate, 4-methyl vinylene sulfate, 4-fluoro vinylene sulfate, or 4,4'-bis(vinylene sulfate); The fluorinated carbonate compounds are selected from at least one of ethyl fluorocarbonate, difluoro vinylene carbonate, or propylene trifluoromethyl carbonate; The unsaturated carbonate compounds are selected from vinylene carbonate and / or vinyl vinylene carbonate; The fluorinated lithium salt compounds are selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium tetrafluorooxalate phosphate, lithium difluorodioxalate phosphate, lithium difluorooxalate borate, or lithium trioxalate phosphate; 8. The electrolyte for a lithium-ion battery according to claim 5, wherein: The non-aqueous solvent is selected from at least one of C3-C6 carbonate compounds, C3-C8 carboxylate compounds, sulfone compounds, or ether compounds; 9. The lithium-ion battery electrolyte according to claim 8, wherein: The C3-C6 carbonate compounds are selected from at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, fluoroethylene carbonate, or difluoroethylene carbonate; The C3-C8 carboxylate compounds are selected from at least one of γ-butyrolactone, methyl acetate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl acetate, propyl propionate, or fluoroethyl acetate; The sulfone compounds are selected from at least one of sulfolane, dimethyl sulfoxide, dimethyl sulfone, or diethyl sulfone; The ether compounds are selected from at least one of triglyme, tetraglyme, or 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

10. A lithium-ion battery, comprising a positive electrode, a negative electrode, and a separator, characterized in that: The lithium-ion battery further includes the lithium-ion battery electrolyte according to any one of claims 5-9.

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