Phosphorus, nitrogen and fluorine-containing flame-retardant electrolyte additive, preparation method, phosphorus, nitrogen and fluorine-containing flame-retardant electrolyte and application thereof in lithium batteries

Abstract

The present invention provides a phosphorus, nitrogen and fluorine-containing flame-retardant electrolyte additive, a preparation method, a phosphorus, nitrogen and fluorine-containing flame-retardant electrolyte and its application in lithium batteries. The preparation method of the additive comprises: subjecting phosphorus trichloride and trifluoroalcohol to a dehydrochlorination reaction in an ice-water bath to obtain an intermediate product; then subjecting the intermediate product to a Mannich reaction with formaldehyde and a diamine, and purifying the intermediate product to obtain a phosphorus, nitrogen and fluorine-containing flame-retardant electrolyte additive. The flame-retardant electrolyte additive has good affinity with a mixture of lithium salts and organic solvents, and at the same time, phosphorus, nitrogen and fluorine have a synergistic flame-retardant effect, so that it can greatly improve the safety performance of the electrolyte without affecting the conductivity and viscosity of the electrolyte. In addition, the phosphorus, nitrogen and fluorine-containing flame-retardant electrolyte additive of the present invention can achieve a self-extinguishing or even non-flammable effect when added in a small amount, and has broad application prospects in the fields of energy storage and electric vehicles.

Description

Technical Field

[0001] The present invention relates to the field of energy and new material technology, and in particular to a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive, a preparation method, a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte and applications thereof in lithium batteries. Background Art

[0002] Lithium batteries have become a crucial energy storage device. They are widely used in power storage, mobile communications, military equipment, aerospace, and other fields, and have promising applications in energy storage and electric vehicles. However, while their high operating voltage and high energy density give them excellent electrochemical performance, they also pose serious safety risks.

[0003] At present, the electrolyte of commercial lithium-ion batteries is mainly composed of flammable liquid organic substances such as ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate. Their flash point and boiling point are both low, which may cause battery bulging and even serious safety accidents. Since a large amount of flammable materials (such as liquid ester electrolytes) are used in lithium batteries, once the battery undergoes thermal runaway and the internal pressure exceeds the tolerable range, extremely high temperature and flammable electrolyte vapor will be ejected, causing rapid combustion or even explosion. In recent years, combustion and explosion accidents of lithium batteries have greatly threatened the safety of people and property. Therefore, introducing flame-retardant electrolyte additives with high flash points and boiling points into the electrolyte is of great significance for effectively improving the safety performance of the electrolyte. Summary of the Invention

[0004] The object of the present invention is to provide a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive, which has a high flash point and boiling point and has good compatibility with a mixture of lithium salt and organic solvent.

[0005] Another object of the present invention is to provide a method for preparing a phosphorus, nitrogen and fluorine-containing flame-retardant electrolyte additive, wherein phosphorus trichloride and trifluoroalcohol undergo a dehydrochlorination reaction, followed by a Mannich reaction with formaldehyde and diamine to ultimately obtain the phosphorus, nitrogen and fluorine-containing flame-retardant electrolyte additive. The method has controllable parameters and is suitable for industrial large-scale production.

[0006] The third object of the present invention is to provide a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte having good safety performance.

[0007] The fourth object of the present invention is to provide a method for preparing a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte, which is simple to operate and has controllable parameters and is suitable for industrial large-scale production.

[0008] The fifth object of the present invention is to provide the use of the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte in the preparation of lithium-ion batteries.

[0009] The present invention solves the technical problem by adopting the following technical solutions.

[0010] The present invention proposes a flame retardant electrolyte additive containing phosphorus, nitrogen and fluorine, the structural formula of the flame retardant electrolyte additive is:

[0011]

[0012]

[0013] Wherein, R1 is any one of the following groups: -CH2-, -CH2-CH2-, -CH2-CH2-CH2-; R2 is any one of the following groups: -CH2-CH2-, -CH(CH3)-CH2-, -CH2-CH2-CH2-CH2-, -CH2-CH2-CH2-CH2-CH2-CH2-; R3 is any one of the following groups: -CH2-, -CH2-CH2-, -CH2-CH2-CH2-CH2-.

[0014] The present invention provides a method for preparing a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive, comprising the following steps:

[0015] S1. Dissolve phosphorus trichloride in a first solvent and cool to 0-5°C, add tert-butanol dropwise, and stir for 50-70 minutes. Then add trifluoromethanol dropwise, react for 12-16 hours, heat under reflux, and distill under reduced pressure to obtain an intermediate product.

[0016] S2. After the intermediate product, formaldehyde and diamine react in a second solvent, purification is carried out to obtain the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive.

[0017] The invention provides a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte, comprising a lithium salt, an organic solvent mixture and the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive.

[0018] The present invention provides a method for preparing a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte, comprising the following steps:

[0019] The lithium salt, the organic solvent mixture and the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive are mixed and stirred for 1 to 10 hours to obtain the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte.

[0020] The present invention provides application of the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte in the preparation of lithium ion batteries.

[0021] The beneficial effects of the phosphorus, nitrogen and fluorine-containing flame-retardant electrolyte additive, preparation method, phosphorus, nitrogen and fluorine-containing flame-retardant electrolyte and application thereof in lithium batteries according to the embodiments of the present invention are:

[0022] The phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte additive of the present invention contains a large amount of flame-retardant elements such as phosphorus, nitrogen, and fluorine, which can provide a good synergistic flame retardant effect. This additive can significantly improve the safety performance of lithium battery electrolytes without affecting the electrolyte conductivity and viscosity, and thus has broad application prospects in fields such as energy storage and electric vehicles.

[0023] 2. The phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte additive of the present invention has high flash and boiling points and good compatibility with the lithium salt and organic solvent mixture in the electrolyte. Mixing it with the lithium salt and organic solvent mixture can produce an electrolyte with excellent safety performance. Using this electrolyte in lithium batteries can reduce the risk of lithium battery explosion. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0025] Figure 1 This is a hydrogen nuclear magnetic resonance spectrum of the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive of Example 1 of the present invention;

[0026] Figure 2 This is the nuclear magnetic resonance fluorine spectrum of the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive of Example 1 of the present invention;

[0027] Figure 3 The figure is a flow chart for preparing the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive of the present invention. DETAILED DESCRIPTION

[0028] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0029] The phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive, preparation method, phosphorus, nitrogen and fluorine-containing flame retardant electrolyte and application thereof in lithium batteries according to embodiments of the present invention are described in detail below.

[0030] An embodiment of the present invention provides a phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte additive, the structural formula of which is:

[0031]

[0032] Wherein, R1 is any one of the following groups: -CH2-, -CH2-CH2-, -CH2-CH2-CH2-; R2 is any one of the following groups: -CH2-CH2-, -CH(CH3)-CH2-, -CH2-CH2-CH2-CH2-, -CH2-CH2-CH2-CH2-CH2-CH2-; R3 is any one of the following groups: -CH2-, -CH2-CH2-, -CH2-CH2-CH2-CH2-.

[0033] The structure of the phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte additive of the present invention contains a large amount of flame-retardant elements such as phosphorus, nitrogen, and fluorine. Compounds containing flame-retardant elements such as phosphorus, nitrogen, and fluorine have significant flame-retardant effects in the condensed phase and gas phase, decomposing into free radicals at high temperatures or catalyzing the cracking of combustible materials into carbon, thereby blocking combustion. By introducing fluorine into the structure, a better synergistic flame-retardant effect can be achieved, further improving the flame-retardant efficiency of the resulting electrolyte. In addition, when the amount of flame-retardant additive is relatively small, the electrolyte can achieve a self-extinguishing or even non-flammable effect.

[0034] Reference Figure 3 As shown, the present invention provides a method for preparing a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive, comprising the following steps:

[0035] S1, phosphorus trichloride is dissolved in the first solvent and cooled to 0~5 DEG C, stirred 50~70min after dripping tert-butyl alcohol, then dripping trifluoro alcohol and reacting 12~16h, heating to reflux, distilling under reduced pressure, obtain intermediate product. Preferably, the present invention adopts constant pressure dropping funnel to carry out the dripping of tert-butyl alcohol, and stirs in an ice bath after dripping, then drips trifluoro alcohol using constant pressure dropping funnel and reacts in an ice-water bath. After the reaction terminates, heat reflux 3 hours to remove excess hydrogen chloride gas in the solution, and removes residual solvent by the method for distilling under reduced pressure to obtain intermediate product.

[0036] Furthermore, in a preferred embodiment of the present invention, the first solvent is selected from one of acetone, butanone, dichloromethane, chloroform, dioxane, and tetrahydrofuran.

[0037] Furthermore, in a preferred embodiment of the present invention, the molar ratio of the phosphorus trichloride, the tert-butanol and the trifluoroalcohol is 1:0.8~1.2:1.8~2.2, the trifluoroalcohol is selected from one of trifluoroethanol, 3,3,3-trifluoropropanol and 4,4,4-trifluorobutanol, and the dripping time of the trifluoroalcohol is 0.5~1h.

[0038] S2. After the intermediate product, formaldehyde and diamine react in a second solvent, purification is carried out to obtain the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive.

[0039] Furthermore, in a preferred embodiment of the present invention, the second solvent is selected from a solution of hydrochloric acid in tetrahydrofuran, a solution of hydrochloric acid in dichloromethane, a solution of triethylamine in tetrahydrofuran, or a solution of triethylamine in dichloromethane; the formaldehyde is selected from a solution of triformaldehyde, paraformaldehyde, and aqueous formaldehyde; and the diamine is selected from a solution of piperazine, ethylenediamine, 1,2-propylenediamine, butanediamine, and hexamethylenediamine. Preferably, when the second solvent is an acidic solution, its pH is approximately 5; when the second solvent is an alkaline solution, its pH is approximately 9.

[0040] Furthermore, in a preferred embodiment of the present invention, the reaction temperature is 30-60° C., and the reaction time is 4-12 hours.

[0041] The synthetic route of the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive (Formula I) of the present invention is as follows:

[0042]

[0043] Wherein, R1 is any one of the following groups: -CH2-, -CH2-CH2-, -CH2-CH2-CH2-; R2 is any one of the following groups: -CH2-CH2-, -CH(CH3)-CH2-, -CH2-CH2-CH2-CH2-, -CH2-CH2-CH2-CH2-CH2-CH2-.

[0044] The present invention first uses phosphorus trichloride and trifluoroalcohol to undergo a dehydrochlorination reaction, followed by a Mannich reaction with formaldehyde and diamine to ultimately produce a phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte additive. The phosphorus, nitrogen, and fluorine in the additive structure exhibit a synergistic effect and exhibit excellent compatibility with lithium salt and organic solvent mixtures. This overcomes the shortcomings of electrolyte additives in electrolyte systems, such as poor compatibility and low flame retardancy, and provides technical support for improving the safety performance of lithium battery electrolytes.

[0045] The invention provides a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte, comprising a lithium salt, an organic solvent mixture and the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive.

[0046] The present invention provides a method for preparing a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte, comprising the following steps:

[0047] The lithium salt, the organic solvent mixture and the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive are mixed and stirred for 1 to 10 hours to obtain the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte.

[0048] In a preferred embodiment, in the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte, the mass percentage of the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive is 5 to 30 wt %; and the concentration of lithium hexafluorophosphate is 0.5 to 1.0 mol / L.

[0049] Furthermore, in a preferred embodiment of the present invention, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium difluorobis(oxalatophosphate), and lithium bis(trifluoromethylsulfonyl)imide, and the organic solvent mixture is a carbonate organic solvent mixture. Preferably, the organic solvent mixture is an ethylene carbonate (EC) / dimethyl carbonate (DMC) mixed solution, and the volume ratio of EC to DMC in the EC / DMC mixed solution is 1:1.

[0050] The present invention also provides the use of the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte in the preparation of lithium ion batteries.

[0051] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0052] Example 1

[0053] This embodiment provides a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive, which is prepared according to the following method:

[0054] 1 mol of phosphorus trichloride and 200 mL of dichloromethane were added to a 500 mL flask and cooled to 0°C in an ice bath. Then, 100 mL of a 0.01 mol / mL solution of tert-butyl alcohol in dichloromethane was slowly added dropwise using a constant pressure dropping funnel over approximately 45 minutes. The mixture was stirred in an ice bath for one hour. Then, 100 mL of a 0.02 mol / mL solution of trifluoroethanol in dichloromethane was slowly added dropwise using a constant pressure dropping funnel over approximately 30 minutes. After completion of the addition, the mixture was allowed to react for 12 hours with stirring. The solution was then heated to 47°C and refluxed for three hours to remove hydrogen chloride. The dichloromethane was then removed by rotary evaporation to obtain an intermediate product. Subsequently, 1 mol of paraformaldehyde, 0.5 mol of piperazine, and a predetermined amount of triethylamine in dichloromethane were added to adjust the solution's pH to 8.5-9. The mixture was allowed to react at 70°C for 4 hours. After purification, a phosphorus-nitrogen-fluorine flame-retardant electrolyte additive (TFPFP) was obtained.

[0055] The synthetic route and molecular structure of the target product TFPFP are as follows:

[0056]

[0057] Example 2

[0058] This embodiment provides a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte, which is prepared according to the following method:

[0059] Ethylene carbonate and dimethyl carbonate were mixed in a volume ratio of 1:1 to obtain an organic solvent mixture. Then, lithium hexafluorophosphate (1M) and the phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte additive of Example 1 were added. After thorough stirring for 1 hour, a phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte (LiPF6 / EC / DMC / TFPFP) was obtained. The mass percentage of TFPFP in the phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte was 5 wt%.

[0060] Example 3

[0061] This embodiment provides a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte, which is prepared according to the following method:

[0062] Ethylene carbonate and dimethyl carbonate were mixed in a volume ratio of 1:1 to obtain an organic solvent mixture. Then, lithium hexafluorophosphate (1M) and the phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte additive of Example 1 were added. After thorough stirring for 1 hour, a phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte (LiPF6 / EC / DMC / TFPFP) was obtained. The mass percentage of TFPFP in the phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte was 10 wt%.

[0063] Example 4

[0064] This embodiment provides a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte, which is prepared according to the following method:

[0065] Ethylene carbonate and dimethyl carbonate were mixed in a volume ratio of 1:1 to obtain an organic solvent mixture. Then, lithium hexafluorophosphate (1M) and the phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte additive of Example 1 were added. After thorough stirring for 1 hour, a phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte (LiPF6 / EC / DMC / TFPFP) was obtained. The mass percentage of TFPFP in the phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte was 20 wt%.

[0066] Example 5

[0067] This embodiment provides a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte, which is prepared according to the following method:

[0068] Ethylene carbonate and dimethyl carbonate were mixed in a volume ratio of 1:1 to obtain an organic solvent mixture. 1M lithium hexafluorophosphate and the phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte additive of Example 1 were then added. After thorough stirring for 1 hour, a phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte (LiPF6 / EC / DMC / TFPFP) was obtained. The mass percentage of TFPFP in the phosphorus, nitrogen, and fluorine-containing flame-retardant electrolyte was 30 wt%.

[0069] Comparative Example 1

[0070] This comparative example provides an electrolyte, which is prepared according to the following method:

[0071] Ethylene carbonate and dimethyl carbonate were mixed uniformly in a volume ratio of 1:1 to obtain an organic solvent mixture, and then lithium hexafluorophosphate (1M) was added. After sufficient stirring for 1 hour, an electrolyte solution (LiPF6 / EC / DMC) was obtained.

[0072] Comparative Example 2

[0073] This comparative example provides an electrolyte, which is prepared according to the following method:

[0074] Ethylene carbonate and dimethyl carbonate were mixed in a volume ratio of 1:1 to obtain an organic solvent mixture. Lithium hexafluorophosphate (1M) and the conventional additive hexamethylphosphoric triamide (HMPA) were then added. After thorough stirring for 1 hour, an electrolyte solution (LiPF6 / EC / DMC / HMPA) was obtained. The mass percentage of HMPA in the electrolyte solution was 5 wt%.

[0075] Test Example 1

[0076] This test case uses 1 H NMR and 19 The structure of the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive of Example 1 was characterized by F NMR.

[0077] like Figure 1 Shown is the nuclear magnetic resonance hydrogen spectrum of the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive solution of Example 1. Figure 2 The figure shows the nuclear magnetic resonance fluorine spectrum of the phosphorus, nitrogen and fluorine flame retardant electrolyte additive solution of Example 1. Figure 1 It can be seen that the peak around 4.55ppm comes from the hydrogen atoms in CF3CH2-O, the peak around 3ppm is attributed to the hydrogen atoms in O=P-CH2-N, and the characteristic peak at 2.65ppm comes from piperazine. The ratio of the integral area of ​​the graph is 2:1:2, which is consistent with the ratio of the number of hydrogen atoms in the structural formula of 8:4:8.

[0078] from Figure 2 It can be seen that only one characteristic peak is found in the figure. Figure 1 and Figure 2 The results all indicate the successful preparation of the final product TFPFP.

[0079] Test Example 2

[0080] In this test example, combustion tests were conducted on the phosphorus, nitrogen and fluorine-containing flame-retardant electrolytes of Examples 2 to 5 and the electrolytes of Comparative Examples 1 and 2. The combustion test results are shown in Table 1.

[0081] Table 1 Combustion test of phosphorus, nitrogen and fluorine-containing flame retardant electrolyte

[0082]

[0083]

[0084] As can be seen from Table 1, the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive prepared by the present invention has excellent self-extinguishing performance and safety performance compared with the traditional additive hexamethylphosphoric triamide.

[0085] The embodiments described above are some, but not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the invention as claimed, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are also within the scope of protection of the present invention.

Claims

1. A phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive, characterized in that: The structural formula of the flame retardant electrolyte additive is: Formula I or Formula II Wherein, R1 is any one of the following groups: -CH2-, -CH2-CH2-, -CH2-CH2-CH2-; R2 is any one of the following groups: -CH2-CH2-, -CH(CH3)-CH2-, -CH2-CH2-CH2-CH2-, -CH2-CH2-CH2-CH2-CH2-CH2-; R3 is any one of the following groups: -CH2-, -CH2-CH2-, -CH2-CH2-CH2-CH2-.

2. The method for preparing the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive according to claim 1, characterized in that: The following steps are involved: S1, dissolving phosphorus trichloride in a first solvent and cooling to 0-5°C, adding tert-butanol dropwise and stirring for 50-70 minutes, then adding trifluoroalcohol dropwise and reacting for 12-16 hours, heating to reflux, and distilling under reduced pressure to obtain an intermediate product; the trifluoroalcohol is selected from one of trifluoroethanol, 3,3,3-trifluoropropanol, and 4,4,4-trifluorobutanol; S2. After the intermediate product, formaldehyde and diamine react in a second solvent, purification is carried out to obtain the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive.

3. The preparation method according to claim 2, characterized in that In step S1, the first solvent is selected from one of acetone, butanone, dichloromethane, chloroform, dioxane, and tetrahydrofuran.

4. The preparation method according to claim 2, characterized in that In step S1, the molar ratio of the phosphorus trichloride, the tert-butyl alcohol and the trifluoroalcohol is 1:0.8-1.2:1.8-2.2, and the trifluoroalcohol is added dropwise for 0.5-1 h.

5. The preparation method according to claim 2, characterized in that In step S2, the second solvent is selected from one of a tetrahydrofuran solution of hydrochloric acid, a dichloromethane solution of hydrochloric acid, a tetrahydrofuran solution of triethylamine, and a dichloromethane solution of triethylamine; the formaldehyde is selected from one of triformaldehyde, paraformaldehyde, and a formaldehyde aqueous solution; and the diamine is selected from one of piperazine, ethylenediamine, 1,2-propylenediamine, butanediamine, and hexamethylenediamine.

6. The preparation method according to claim 2, characterized in that In step S2, the reaction temperature is 30-60° C., and the reaction time is 4-12 h.

7. A phosphorus, nitrogen and fluorine-containing flame retardant electrolyte, characterized in that: The invention comprises a lithium salt, an organic solvent mixture and the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive as claimed in claim 1.

8. The phosphorus, nitrogen and fluorine-containing flame retardant electrolyte according to claim 7, characterized in that: The lithium salt is selected from one or more of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium difluorodioxalatophosphate, and lithium bis(trifluoromethylsulfonyl)imide, and the organic solvent mixture is a carbonate organic solvent mixture.

9. A method for preparing a phosphorus, nitrogen and fluorine-containing flame retardant electrolyte according to any one of claims 7 or 8, characterized in that: The following steps are involved: The lithium salt, the organic solvent mixture and the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte additive are mixed and stirred for 1 to 10 hours to obtain the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte.

10. Use of the phosphorus, nitrogen and fluorine-containing flame retardant electrolyte according to any one of claims 7 or 8 in the preparation of lithium-ion batteries.

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