Lithium ion battery fire-retardant electrolyte, preparation method and application thereof

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a lithium ion battery flame-retardant electrolyte and a preparation method and application thereof. The lithium ion battery flame-retardant electrolyte comprises a non-water solvent, a lithium salt, a flame retardant and a bistrifluoromethanesulfonimide salt, the lithium salt is selected from lithium hexafluorophosphate or lithium bisfluorosulfonimide, the flame retardant is fluorinated phosphazene organic matter, the flame retardant accounts for 0.5wt%-20wt% of the total mass of the electrolyte, the lithium ion battery flame-retardant electrolyte provided by the application introduces the bistrifluoromethanesulfonimide salt, the bistrifluoromethanesulfonimide anion has a larger anion radius compared with a bisfluorosulfonimide group, the migration number of lithium ions in the electrolyte is effectively improved, and the rate performance of the battery is improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a flame-retardant electrolyte for lithium-ion batteries, its preparation method, and its application. Background Technology

[0002] Lithium-ion battery electrolyte is a key material in lithium-ion batteries, essentially their "blood," determining their critical performance. Improving the flame-retardant properties of lithium-ion battery electrolytes enhances the safety of end products such as electric vehicles, digital 3C products, and energy storage power stations, preventing property damage and personal injury caused by fires resulting from battery rupture or accidental short circuits.

[0003] Currently, lithium-ion battery electrolytes are usually organic systems. Commonly used solvents for electrolytes are alkyl carbonates. Chain carbonates typically use ethylene carbonate (EC) and propylene carbonate (PC), while linear carbonates typically use dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). These solvents have very low flash points, especially chain carbonate solvents, making lithium-ion battery electrolytes extremely flammable.

[0004] To improve the safety of lithium-ion batteries, common practices in electrolyte safety include using high-boiling-point solvents such as γ-butyrolactone, certain sulfone solvents, and even large amounts of propylene carbonate. However, these high-boiling-point solvents only have non-flammable properties and cannot provide flame retardant performance. Furthermore, their use results in higher viscosity. For example, γ-butyrolactone has a viscosity of 1.5 mPa·s, while hexafluorocyclotriphosphazene, which has a higher viscosity, has a viscosity of only 1.2 mPa·s at the same temperature. Moreover, the amount of high-boiling-point solvents used is much higher than the amount of additives used. Therefore, the use of the above-mentioned high-boiling-point solvents will result in higher viscosity, which will seriously affect the rate performance of the battery and increase the internal resistance of the battery.

[0005] Some flame-retardant additives have also emerged, especially phosphate esters. Currently commonly used flame-retardant additives include trimethyl phosphate (TMP), triethyl phosphate (TEP), dimethyl methyl phosphate (DMMP), and triphenyl phosphate (TPP). Appropriate addition can shorten the self-extinguishing time of the electrolyte. However, they are not compatible with graphite anodes and cannot form a stable SEI film. Therefore, electrolytes containing the above additives cannot guarantee the cycle stability of the battery.

[0006] In recent years, fluorinated phosphazene flame retardants have been used in electrolytes. Because they contain abundant phosphorus, nitrogen and fluorine elements, the synergistic effect of the three greatly improves the flame retardant performance of the electrolyte. At the same time, fluorinated phosphazene flame retardants can generate a LiF-rich SEI film at the negative electrode and generate alkyl-substituted phosphazenes that migrate to the positive electrode to participate in the film formation at the positive electrode.

[0007] To ensure stable safety during cycling, fluorinated phosphazenes are typically added at concentrations between 5% and 15% in carbonate electrolytes. While this ensures the flame-retardant properties of the electrolyte, the high viscosity of these additives (e.g., hexafluorocyclotriphosphazene has a viscosity of 1.2 mPa·s at room temperature) leads to a decrease in battery rate performance, hindering the widespread adoption of this commercial solution. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a flame-retardant electrolyte for lithium-ion batteries, its preparation method, and its application. By adding bis(trifluoromethanesulfonyl)imide salt additives, the flame-retardant performance of the electrolyte is ensured while effectively increasing the transference number of lithium ions and improving the rate performance of the battery. At the same time, it combines with the anions in the lithium salt to improve the cycle stability of the battery.

[0009] To achieve the above-mentioned technical effects, the present invention adopts the following technical solution:

[0010] A flame-retardant electrolyte for lithium-ion batteries comprises a non-aqueous solvent, a lithium salt, a flame retardant, and a bis(trifluoromethanesulfonyl)imide salt. The lithium salt is selected from lithium hexafluorophosphate (LiPF6) or lithium bis(trifluoromethanesulfonyl)imide (LiFSI). The flame retardant is a fluorinated phosphazene organic compound. The bis(trifluoromethanesulfonyl)imide salt is selected from any of the compounds shown in the following formula.

[0011] Equation (I),

[0012] Formula (II)

[0013] Formula (III) or

[0014] Equation (Ⅳ),

[0015] Among them, R1-R 11 All are alkyl groups with 1-4 carbon atoms, and the flame retardant and bis(trifluoromethanesulfonyl)imide salt together account for 3wt%-20wt% of the total mass of the electrolyte.

[0016] Fluorinated phosphazene additives are commonly used flame retardant additives; however, excessive addition can lead to decreased electrolyte rate performance due to high viscosity. Adding bis(trifluoromethanesulfonyl)imide salt to the electrolyte can effectively reduce the amount of phosphazene used, thereby controlling the electrolyte viscosity and increasing ion transport number to improve rate performance. Simultaneously, the cationic portion of bis(trifluoromethanesulfonyl)imide salt also provides dehydration and deacidification effects, further improving battery cycle stability. Other additives in the electrolyte are commonly used film-forming additives, primarily ensuring battery cycle stability.

[0017] The lithium-ion battery flame-retardant electrolyte provided by this invention, by introducing bis(trifluoromethanesulfonyl)imide salt, wherein the bis(trifluoromethanesulfonyl)imide anion, compared to The bis(fluorosulfonyl)imide group has a larger anionic radius, effectively increasing the transport number of lithium ions in the electrolyte, thereby improving the battery rate performance. Furthermore, when lithium hexafluorophosphate is selected as the lithium salt, the cation in the bis(trifluoromethanesulfonyl)imide salt is a Lewis acid, which can react with... Combined, to prevent the former from decomposing into HF and This effectively improves the cycle stability of the battery. Meanwhile, the bis(trifluoromethanesulfonyl)imide salt is also rich in N, P, and F elements, which can further enhance the flame retardant properties of the electrolyte.

[0018] Preferably, lithium hexafluorophosphate is selected as the lithium salt.

[0019] Preferably, the non-aqueous solvent is selected from at least two of carbonates such as ethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate, and carboxylic acid esters such as ethyl acetate, propyl acetate, propyl propionate, and methyl propionate; more preferably, the non-aqueous solvent accounts for 60wt%-90wt% of the total mass of the electrolyte.

[0020] Preferably, the lithium salt accounts for 8.0 wt%-18.0 wt% of the total mass of the electrolyte.

[0021] Preferably, the flame-retardant electrolyte for lithium-ion batteries further includes additives, which are selected from at least one of vinylene carbonate, fluoroethylene carbonate, 1,3-propenesulfonate lactone, lithium difluorophosphate, vinyl sulfate, and methyl disulfonate; more preferably, the additives account for 0.5wt%-10wt% of the total mass of the electrolyte.

[0022] Preferably, the flame-retardant electrolyte for lithium-ion batteries has a viscosity of ≤4.5 s·g. -1 Self-extinguishing time.

[0023] This invention also provides a method for preparing the above-mentioned flame-retardant electrolyte for lithium-ion batteries. In an argon-filled glove box, a non-aqueous solvent is mixed thoroughly. A flame retardant, bis(trifluoromethanesulfonyl)imide salt, and additives are then added to the non-aqueous solvent and mixed thoroughly. Lithium salt is then slowly added to the mixed solution, and the mixture is stirred until homogeneous to obtain the flame-retardant electrolyte for lithium-ion batteries. Preferably, the glove box components contain less than 1 ppm of water and less than 1 ppm of oxygen.

[0024] The present invention also provides the application of the above-mentioned flame-retardant electrolyte for lithium-ion batteries or the flame-retardant electrolyte for lithium-ion batteries prepared by the above-mentioned preparation method as an electrolyte in the preparation of lithium-ion batteries.

[0025] Preferably, the lithium-ion battery made from the flame-retardant electrolyte is placed in a constant temperature chamber at 25°C and charged to 4.5V with a constant current and constant voltage of 1C, with a cutoff current of 0.05C, and discharged to 3.0V with a constant current of 1C. The capacity retention rate after 500 cycles is 80.5%-83.5%.

[0026] The beneficial effects of this invention are:

[0027] 1. The lithium-ion battery flame-retardant electrolyte provided by this invention, by introducing bis(trifluoromethanesulfonyl)imide salt, wherein the bis(trifluoromethanesulfonyl)imide anion, compared to The bis(trifluoromethanesulfonyl)imide group has a larger anionic radius, effectively increasing the transport number of lithium ions in the electrolyte, thereby improving the battery rate performance. When lithium hexafluorophosphate is selected as the lithium salt, the cation in the bis(trifluoromethanesulfonyl)imide salt is a Lewis acid, which can react with the Lewis base... Combined, to prevent the former from decomposing into HF and This effectively improves the cycle stability of the battery. Meanwhile, the bis(trifluoromethanesulfonyl)imide salt is also rich in N, P, and F elements, which can further enhance the flame retardant properties of the electrolyte.

[0028] 2. The lithium-ion battery flame-retardant electrolyte provided by this invention has a maximum flame retardant content of 4.5 s·g. -1 The self-extinguishing time range; after the battery is made, it is placed in a constant temperature chamber at 25℃ and charged to 4.5V with a constant current and constant voltage of 1C. The cutoff current is 0.05C, and it is discharged to 3.0V with a constant current of 1C. The capacity retention rate range after 500 cycles is 80.5%-83.5%. Detailed Implementation

[0029] The present invention will be further described below with reference to embodiments and comparative examples.

[0030] The reagents and experimental equipment used in the examples and comparative examples are all commercially available and commonly used reagents and experimental equipment in the art, and their specific sources will not be repeated here.

[0031] This invention provides a flame-retardant electrolyte for lithium-ion batteries, comprising a non-aqueous solvent, a lithium salt, a flame retardant, and a bis(trifluoromethanesulfonyl)imide salt. The lithium salt is selected from lithium hexafluorophosphate (LiPF6) or lithium bis(trifluoromethanesulfonyl)imide (LiFSI). The flame retardant is a fluorinated phosphazene organic compound. The bis(trifluoromethanesulfonyl)imide salt is selected from any of the compounds shown in the following formula.

[0032] Equation (I),

[0033] Formula (II)

[0034] Formula (III) or

[0035] Equation (Ⅳ),

[0036] Among them, R1-R 11 All are alkyl groups with 1-4 carbon atoms, and the flame retardant and bis(trifluoromethanesulfonyl)imide salt together account for 3wt%-20wt% of the total mass of the electrolyte.

[0037] Preferably, lithium hexafluorophosphate is selected as the lithium salt.

[0038] Preferably, the non-aqueous solvent is selected from at least two of carbonates such as ethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate, and carboxylic acid esters such as ethyl acetate, propyl acetate, propyl propionate, and methyl propionate; more preferably, the non-aqueous solvent accounts for 60wt%-90wt% of the total mass of the electrolyte.

[0039] Preferably, the lithium salt accounts for 8.0wt%-18.0wt% of the total mass of the electrolyte, and the molar concentration of the lithium salt is 1.0M-1.3M.

[0040] Preferably, the flame-retardant electrolyte for lithium-ion batteries further includes additives, which are selected from at least one of vinylene carbonate, fluoroethylene carbonate, 1,3-propenesulfonate lactone, lithium difluorophosphate, vinyl sulfate, and methyl disulfonate; more preferably, the additives account for 0.5wt%-10wt% of the total mass of the electrolyte.

[0041] The flame-retardant electrolytes for lithium-ion batteries provided in each embodiment are prepared by the following method:

[0042] In a glove box filled with argon (moisture < 1 ppm, oxygen < 1 ppm), the non-aqueous solvent is mixed. Flame retardant, bis(trifluoromethanesulfonyl)imide salt and additives are added to the non-aqueous solvent and mixed. Lithium salt is then slowly added to the mixed solution and stirred until homogeneous to obtain the flame-retardant electrolyte for lithium-ion batteries.

[0043] The preparation methods of the lithium-ion battery flame-retardant electrolytes provided in each comparative example differ from those in the examples in that bis(trifluoromethanesulfonyl)imide salt is not added.

[0044] The component content of the lithium-ion battery flame-retardant electrolytes provided in each embodiment and comparative example is shown in the table below:

[0045] Table 1 Component Content

[0046]

[0047] Experimental Example 1

[0048] Self-extinguishing time (SET) test

[0049] Absorb 0.05-0.10 g of electrolyte using a spherical core with a diameter of 0.3-0.5 cm, and then normalize the quench time according to the sample weight. Results are divided into three categories: if SET < 6 s·g -1 The electrolyte is considered "non-flammable"; such as 6 s·g -1 <SET<20s·g -1 It is considered "flame retardant"; if SET > 20 s·g -1 The electrolytes prepared in the comparative and example cases were considered "flammable". The self-extinguishing time data were obtained by testing the electrolytes prepared in the above manner, and the test results are shown in Table 2.

[0050] Table 2 Self-extinguishing time

[0051]

[0052] Non-flammable: This is mainly due to excessive addition of flame retardant in the electrolyte, resulting in non-flammability. Typically, a flame retardant dosage >10wt% will cause this phenomenon. The self-extinguishing time is also related to the amount added and the solvent. The higher the amount of flame retardant additive, the shorter the self-extinguishing time. At the same time, the addition of the aforementioned bis(trifluoromethanesulfonyl)imide salt will also reduce the self-extinguishing time; the higher the amount added, the shorter the self-extinguishing time.

[0053] Experimental Example 2

[0054] Battery performance test

[0055] In an argon-filled glove box (moisture < 1 ppm, oxygen < 1 ppm), the lithium battery electrolytes prepared in Comparative Examples 1-6 and the lithium-ion battery electrolytes prepared in Examples 1-10 were respectively injected into lithium cobalt oxide as the positive electrode and artificial graphite (90%) and SiO2 as the negative electrodes. X Of the 10% of soft-pack batteries with a rated capacity of 1mAh, the battery underwent a room temperature cycle performance test, the specific test method of which is as follows:

[0056] The battery was placed in a constant temperature chamber at 25°C and charged to 4.5V with a constant current and constant voltage of 1C, with a cutoff current of 0.05C. Then it was discharged to 3.0V with a constant current of 1C. This cycle was repeated for 500 cycles. The discharge capacity of the first cycle and the discharge capacity of the 500th cycle were recorded. The capacity retention rate was calculated using the following formula: 1C = 1mA.

[0057] The test results are shown in Table 3.

[0058] Table 3 Capacity retention rate

[0059]

[0060] Table 3 shows the calculation method for capacity retention after 500 laps:

[0061] 500-cycle capacity retention rate (%) = (500th cycle discharge capacity / 1st cycle discharge capacity) × 100%.

[0062] Compared with the comparative example, the main reason for the improved cycle stability of the battery is that the dehydration and deacidification functions of the bis(trifluoromethanesulfonate)imine salt are significantly enhanced.

[0063] Similarly, because the amount of film-forming additives added is different from that of the above-mentioned additives, it also has a certain impact on the cycle stability of the battery. Generally speaking, within a reasonable range, the higher the amount added, the better the cycle stability.

[0064] Experimental Example 3

[0065] Ratio Performance Test

[0066] Using the battery prepared in Experiment Example 2, it was charged to 4.5V with currents of 0.5C / 1.0C / 3C / 5C, charged at constant voltage to a current of 0.05C, and discharged at 0.5C / 1.0C / 3C / 5C. Each current density was cycled 10 times, and the discharge capacity of the fifth cycle at each current density was recorded.

[0067] The test results are shown in Table 4.

[0068] Table 4 Discharge Capacity Data

[0069]

[0070] The reason why Comparative Example 4 has the best performance is that compared with Comparative Example 1, the lithium salt concentration is lower and the ionic conductivity is lower, so the rate performance is worse. Compared with Comparative Example 4, the main reason for the poor rate performance of Comparative Example 3 is that the amount of flame retardant additive added in Comparative Example 3 is higher and the electrolyte viscosity is higher. Compared with Comparative Example 2, the main reason for the superior rate performance of Comparative Example 4 is that the above-mentioned carboxylic acid ester additive has better electrolyte conductivity and viscosity than Comparative Example 2.

[0071] Compared with Comparative Example 1, Examples 1 and 2 are mainly improved by adding the above-mentioned bis(trifluoromethanesulfonyl)imide salt additive, which increases the lithium-ion transference number and thus improves the rate performance of the battery.

[0072] Compared with Comparative Example 2, Examples 3 and 4 also improved battery rate performance because the addition of the above-mentioned bis(trifluoromethanesulfonyl)imide salt additive increased the lithium-ion transference number. The main difference between Examples 3 and 4 is that the dosage of bis(trifluoromethanesulfonyl)imide lithium salt was higher, so Example 4 had better performance.

[0073] Compared with Comparative Example 3, Examples 5 and 6 also improved their rate performance due to the introduction of lithium bis(trifluoromethanesulfonyl)imide. The poor performance of the other examples was due to the high viscosity of the electrolyte caused by the excessive amount of flame retardant additive. The superior performance of Example 6 was mainly due to the higher dosage of lithium bis(trifluoromethanesulfonyl)imide added.

[0074] Compared with Comparative Example 4, Examples 7 and 8 also showed improved rate performance due to the introduction of lithium bis(trifluoromethanesulfonylimide) salt. Their better performance compared to other examples was due to the introduction of carboxylic acid ester solvents, which have lower viscosity than carbonates, thereby improving the overall rate performance.

[0075] Compared with Comparative Example 2, the decrease in cycle stability of Comparative Example 5 is mainly due to the corrosion of the current collector by lithium bis(fluorosulfonyl)imide, which is a lithium salt, leading to a certain decrease in cycle stability. In Comparative Example 6, the presence of lithium hexafluorophosphate can passivate the current collector, so the cycle stability is not affected. Similarly, because the anionic radius of lithium bis(fluorosulfonyl)imide is larger than that of lithium hexafluorophosphate, the ion transference number is improved to a certain extent, thus improving the rate performance. The reason why the rate performance of Examples 9 and 10 is better than that of Comparative Example 4 is the same as the above reasons.

Claims

1. A flame-retardant electrolyte for lithium-ion batteries, characterized in that, It includes non-aqueous solvents, lithium salts, flame retardants, and bis(trifluoromethanesulfonyl)imide salts. The lithium salts are selected from lithium hexafluorophosphate or lithium(trifluorosulfonyl)imide, and the flame retardants are fluorinated phosphazene organic compounds. The bis(trifluoromethanesulfonyl)imide salt is selected from any of the compounds shown in the following formula. Equation (I), Formula (II) Formula (III) or Equation (Ⅳ), Among them, R1-R 11 All are alkyl groups with 1-4 carbon atoms. The flame retardant and bis(trifluoromethanesulfonyl)imide salt together account for 3wt%-20wt% of the total mass of the electrolyte.

2. The lithium-ion battery flame-retardant electrolyte as described in claim 1, characterized in that, The lithium salt used is lithium hexafluorophosphate.

3. The lithium-ion battery flame-retardant electrolyte as described in claim 1, characterized in that, The non-aqueous solvent is selected from at least two of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, ethyl acetate, propyl acetate, propyl propionate, or methyl propionate.

4. The lithium-ion battery flame-retardant electrolyte as described in claim 3, characterized in that, The non-aqueous solvent accounts for 60wt%-90wt% of the total mass of the electrolyte.

5. The lithium-ion battery flame-retardant electrolyte as described in claim 1, characterized in that, Lithium salts account for 8.0 wt% to 18.0 wt% of the total mass of the electrolyte.

6. The lithium-ion battery flame-retardant electrolyte as described in claim 1, characterized in that, The flame-retardant electrolyte for lithium-ion batteries also includes additives, which are selected from at least one of vinylene carbonate, fluoroethylene carbonate, 1,3-propenesulfonate lactone, lithium difluorophosphate, vinyl sulfate, or methyl disulfonate.

7. The lithium-ion battery flame-retardant electrolyte as described in claim 6, characterized in that, The additive accounts for 0.5wt%-10wt% of the total mass of the electrolyte.

8. The lithium-ion battery flame-retardant electrolyte as described in claim 1, characterized in that, The flame-retardant electrolyte for lithium-ion batteries has a strength of ≤4.5 s·g. -1 Self-extinguishing time.

9. The method for preparing the flame-retardant electrolyte for lithium-ion batteries according to any one of claims 1-8, characterized in that, In an argon-filled glove box, a non-aqueous solvent is mixed. Flame retardant, bis(trifluoromethanesulfonyl)imide salt, and additives are added to the non-aqueous solvent and mixed. Lithium salt is then slowly added to the mixed solution and stirred until homogeneous to obtain a flame-retardant electrolyte for lithium-ion batteries.

10. The preparation method according to claim 9, characterized in that, The glove box components contain less than 1 ppm of moisture and less than 1 ppm of oxygen.

11. The application of the lithium-ion battery flame-retardant electrolyte as described in any one of claims 1-8 or the lithium-ion battery flame-retardant electrolyte prepared by the preparation method as described in any one of claims 9-10 as an electrolyte in the preparation of lithium-ion batteries.

12. The application as described in claim 11, characterized in that, The lithium-ion battery made from the flame-retardant electrolyte was placed in a constant temperature chamber at 25°C and charged to 4.5V with a constant current and constant voltage of 1C. The cutoff current was 0.05C, and the battery was discharged to 3.0V with a constant current of 1C. The capacity retention rate after 500 cycles was 80.5%-83.5%.

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