Lithium iron phosphate battery electrolyte and preparation method thereof

Abstract

The lithium iron phosphate battery electrolyte is prepared from the following raw materials: lithium salt, a solvent, an additive and a flame retardant, the invention also discloses a preparation method of the lithium iron phosphate battery electrolyte. And under the protection of nitrogen, mixing the solvent, the additive and the lithium salt to obtain a mixed solution, and uniformly stirring the mixed solution and the flame retardant to obtain the lithium iron phosphate battery electrolyte. The prepared flame-retardant electrolyte for the lithium iron phosphate battery is good in flame retardant property, the flame retardant contains a nitrogen-containing, sulfur-containing and phosphorus-containing combination, and the flame retardant property of the flame retardant is greatly improved through the synergistic effect. When the composite material is applied to a battery electrolyte, film formation on the surface of a negative electrode is facilitated, the composite material plays a crucial role in high-temperature stability of a positive electrode material, and the safety of a lithium iron phosphate battery is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and relates to a lithium iron phosphate battery electrolyte and a preparation method thereof. BACKGROUND

[0002] New energy power generation technology is changing rapidly, and the construction of electrochemical energy storage power stations has also increased rapidly. The lithium iron phosphate battery system is widely used in the energy storage field due to its low cost, long service life, environmental protection and relatively high safety. However, it still has safety risks under extreme conditions such as internal and external short circuits, and thermal runaway may cause fires or even explosions. The energy storage power station has large capacity, high battery density and large total power, and once a fire breaks out, the fire will be difficult to control, which will seriously threaten personal and property safety. Therefore, it is crucial to develop intrinsically safe lithium iron phosphate batteries.

[0003] Currently, the lithium iron phosphate battery mainly uses a carbonate system electrolyte, which is easy to decompose and produce gas under extreme conditions, and is flammable and explosive. In order to improve the safety of the battery and prevent fire accidents in the energy storage power station, electrolyte regulation is imperative. In recent years, researchers have developed a variety of phosphoric acid ester or phosphazene flame retardants, hoping to achieve intrinsic safety of the electrolyte. However, a single phosphorus-containing flame retardant has poor flame retardant efficiency, and a large amount of addition is required to be effective, and high content of flame retardant will cause the electrochemical performance such as battery capacity and cycle life to decline. Therefore, the development of an electrolyte with flame retardant performance and without affecting the electrochemical performance of the lithium iron phosphate battery has become a key problem to be solved. SUMMARY

[0004] The purpose of the present application is to provide a lithium iron phosphate battery electrolyte and a preparation method thereof, which solves the problem of poor flame retardant effect of the lithium battery electrolyte mentioned in the background.

[0005] The purpose of the present application can be achieved by the following technical solutions:

[0006] A lithium iron phosphate battery electrolyte comprises the following raw materials: lithium salt, solvent, additive and flame retardant.

[0007] The content mass ratio of the flame retardant, additive and solvent is 1:1.2-21.6:12-100.

[0008] The concentration of the lithium salt is 0.52-1.7 mol / L.

[0009] The lithium iron phosphate battery electrolyte is prepared by the following steps:

[0010] Step A1, under nitrogen protection, 16-33℃, the solvent, additive and lithium salt are mixed at a speed of 150-900rpm for 7-35 minutes to obtain a mixed solution.

[0011] Step A2: The mixed solution and the flame retardant are stirred at 16-35 °C at a speed of 400-1000 rpm for 6-20 min to obtain the lithium phosphate iron battery electrolyte.

[0012] Further, the additive is one or more of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, and vinylene sulfate mixed in any ratio.

[0013] Further, the lithium salt is one of lithium hexafluorophosphate, lithium triflate, lithium bisfluorosulfonylimide, and lithium bis-trifluoromethanesulfonimide.

[0014] Further, the solvent is at least two of vinyl carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate mixed in any ratio.

[0015] Further, the flame retardant is prepared by the following steps:

[0016] Step S1, add p-hydroxybenzaldehyde and THF to a three-necked flask, mix well, then add triethylamine dropwise, add diphenyl chlorophosphate dropwise at -10 °C, react for 5-7 hours, remove THF under reduced pressure, wash with saturated brine, 5% sodium hydroxide solution, and distilled water in turn, and dry to obtain intermediate 1;

[0017] The reaction process is as follows:

[0018]

[0019] Step S2, add 4-methyl-1, 2-phenylenediamine and DMF to a three-necked flask, mix well, then add Na2S2O5 and MgSO4, stir well, and slowly add intermediate 1 dropwise, react for 7-8 hours to obtain intermediate 2;

[0020] The reaction process is as follows:

[0021]

[0022] Step S3, dissolve intermediate 2 in tetrahydrofuran, and react with chlorine under light for 1-2 h to obtain intermediate 3;

[0023] The reaction process is as follows:

[0024]

[0025] Step S4, 4, 4'-diamino diphenyl sulfone is added to a three-necked flask, nitrogen is introduced, acetonitrile, triethylamine is added, the temperature is raised to 40-45 DEG C, after 4, 4'-diamino diphenyl sulfone is dissolved, benzene phosphorus dichloride is added dropwise, the temperature is lowered to 22-25 DEG C, stirring for 3 hours, after uniform stirring, the temperature is raised to 76-80 DEG C, reaction for 15-20 hours, after reaction, the temperature is lowered to room temperature, methyl tert-butyl ether is used as eluent, hot acetonitrile is used to wash the unreacted monomer, after filtration and drying, intermediate 4 is obtained;

[0026] The reaction process is as follows:

[0027]

[0028] Step S5, intermediate 4, N, N-diisopropyl ethylamine, tetrahydrofuran are added to a reaction kettle, nitrogen is introduced for protection, under the conditions of 200-300 r / min rotation speed and 0-5 DEG C temperature, stirring for 10-20 min, then intermediate 3 is added, reaction for 3-5 h, to obtain a flame retardant.

[0029] The reaction process is as follows:

[0030]

[0031] Further, the amount ratio of p-hydroxybenzaldehyde, THF, triethylamine and diphenyl chlorophosphate in step S1 is 23.4-28.3 g: 160-180 mL: 13.2-14.8 g: 35.2-37.1 g.

[0032] Further, the amount ratio of 4-methyl-1, 2-phenylenediamine, DMF, Na2S2O5, MgSO4 and intermediate 1 in step S2 is 14.1-15.8 g: 200-220 mL: 25.6-27.7 g: 15.7-17.3 g: 45.8-48.4 g.

[0033] Further, the molar ratio of intermediate 2 and chlorine gas in step S3 is 2:1.

[0034] Further, the amount ratio of 4, 4'-diamino diphenyl sulfone, acetonitrile, triethylamine and benzene phosphorus dichloride in step S4 is 67.3-73.2 g: 510-550 mL: 77.3-78.5 g: 27.4-29.2 g.

[0035] Further, the amount ratio of intermediate 4, N, N-diisopropyl ethylamine, tetrahydrofuran and intermediate 3 in step S5 is 23.5-26.2 g: 13.6-15.2 g: 80-120 mL: 27.5-28.6 g.

[0036] A preparation method of a lithium iron phosphate battery electrolyte comprises the following steps:

[0037] Step A1, under nitrogen protection, 16-33℃, solvent, additive and lithium salt are mixed at 150-900rpm for 7-35min to obtain a mixed solution;

[0038] Step A2: the mixed solution and flame retardant are stirred at 400-1000rpm for 6-20min at 16-35℃ to obtain a lithium iron phosphate battery electrolyte.

[0039] The present application provides a lithium iron phosphate battery electrolyte and a preparation method thereof. The newly prepared flame retardant is used in combination with lithium salt, solvent and specific additives to prepare a lithium iron phosphate battery electrolyte with good flame retardant performance. The nitrogen-containing, sulfur-containing and phosphorus-containing combination in the flame retardant has a synergistic effect to greatly improve the flame retardant performance of the flame retardant. The excellent gas phase flame retardant effect and condensed phase flame retardant effect are derived therefrom. In the combustion process, the flame retardant releases non-combustible nitrogen-containing and sulfur-containing gas, which can dilute the concentration of degradable combustible gas and oxygen, reduce the heat release in the combustion process, and block the combustion reaction by capturing H· and HO· free radicals to terminate the free radical chain reaction. The phosphorus-containing component decomposes to generate acid sources such as phosphoric acid and metaphosphoric acid with strong dehydrating property, which catalyzes the dehydration and carbonization of the substrate and forms a cross-linked and dense carbon layer on the surface of the substrate to act as a physical barrier to block the transmission of heat, oxygen and combustible gas between the two phases, preventing further combustion and diffusion. Therefore, the application of the flame retardant to the battery electrolyte is beneficial to the formation of a film on the negative electrode surface and plays a crucial role in the high-temperature stability of the positive electrode material, thereby improving the safety of the lithium iron phosphate battery. DETAILED DESCRIPTION

[0040] The technical solutions in the embodiments of the present application will be clearly and completely described below in combination with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0041] Embodiment 1

[0042] The flame retardant is prepared by the following steps:

[0043] Step S1, 23.4g of p-hydroxybenzaldehyde, 160mL of THF are added to a three-necked flask, mixed uniformly, then 13.2g of triethylamine is added dropwise, 35.2g of diphenyl chlorophosphate is added dropwise at-10℃ for 5 hours, THF is removed under reduced pressure, and then washed with saturated brine, 5% sodium hydroxide solution and distilled water in sequence, and dried to obtain intermediate 1;

[0044] Step S2, 14.1g 4-methyl-1,2-phenylenediamine, 200mL DMF, mixed uniformly, then 25.6g Na2S2O5 and 15.7g MgSO4 were added, stirred uniformly, 45.8g intermediate 1 was slowly added dropwise, reacted for 7 hours to obtain intermediate 2;

[0045] Step S3, intermediate 2 was dissolved in tetrahydrofuran, reacted with chlorine under light conditions for 1h to obtain intermediate 3, wherein the molar ratio of intermediate 2 to chlorine was 2:1;

[0046] Step S4, 67.3g 4,4'-diaminodiphenyl sulfone was added to a three-necked flask, nitrogen was introduced, 510mL acetonitrile and 77.3g triethylamine were added, and the temperature was raised to 40℃. After the 4,4'-diaminodiphenyl sulfone was dissolved, 27.4g phenylphosphoryl dichloride was added dropwise, the temperature was lowered to 22℃, and stirred for 3 hours. After being stirred uniformly, the temperature was raised to 76℃ and reacted for 15 hours. After the reaction was completed, the temperature was lowered to room temperature, the eluent was washed with methyl tert-butyl ether, and the unreacted monomer was washed with hot acetonitrile. After filtration and drying, intermediate 4 was obtained.

[0047] Step S5, 23.5g intermediate 4, 13.6g N,N-diisopropylethylamine, and 80mL tetrahydrofuran were added to a reaction kettle, nitrogen was introduced for protection, and stirred at a rotation speed of 200r / min and a temperature of 0℃ for 10min. Then 27.5g intermediate 3 was added and reacted for 3h to obtain a flame retardant.

[0048] Example 2

[0049] The flame retardant was prepared by the following steps:

[0050] Step S1, 25.6g p-hydroxybenzaldehyde, 170mL THF were added to a three-necked flask, mixed uniformly, then 13.9g triethylamine was added dropwise, 36.1g diphenyl chlorophosphate was added dropwise at-10℃, and reacted for 6 hours. THF was removed under reduced pressure, and then saturated brine, 5% sodium hydroxide solution, and distilled water were used for washing in sequence, and dried to obtain intermediate 1;

[0051] Step S2, 14.9g 4-methyl-1,2-phenylenediamine, 210mL DMF were added to a three-necked flask, mixed uniformly, then 26.5g Na2S2O5 and 16.4g MgSO4 were added, stirred uniformly, 46.7g intermediate 1 was slowly added dropwise, and reacted for 7 hours to obtain intermediate 2;

[0052] Step S3, intermediate 2 was dissolved in tetrahydrofuran, reacted with chlorine under light conditions for 2h to obtain intermediate 3, wherein the molar ratio of intermediate 2 to chlorine was 2:1;

[0053] Step S4, 71.7g of 4,4'-diaminodiphenyl sulfone was added to a three-necked flask, nitrogen was introduced, 530mL of acetonitrile and 77.8g of triethylamine were added, and the temperature was raised to 43°C. After 4,4'-diaminodiphenyl sulfone was dissolved, 28.3g of phenylphosphonic dichloride was added dropwise. The temperature was lowered to 24°C and stirred for 3 hours. After being stirred uniformly, the temperature was raised to 78°C and reacted for 18 hours. After the reaction was completed, the temperature was lowered to room temperature. The solvent was eluted with methyl tert-butyl ether. The unreacted monomer was washed away with hot acetonitrile. After filtration and drying, intermediate 4 was obtained.

[0054] Step S5, 24.6g of intermediate 4, 14.5g of N,N-diisopropylethylamine, and 100mL of tetrahydrofuran were added to a reaction kettle. Nitrogen was introduced for protection. The stirring speed was 250r / min, and the temperature was 3°C. After stirring for 15min, 27.9g of intermediate 3 was added, and the reaction was carried out for 4h to prepare the flame retardant.

[0055] Example 3

[0056] The flame retardant was prepared by the following steps:

[0057] Step S1, 28.3g of p-hydroxybenzaldehyde and 180mL of THF were added to a three-necked flask. After being mixed uniformly, 14.8g of triethylamine was added dropwise. 37.1g of diphenyl chlorophosphate was added dropwise at -10°C and reacted for 7 hours. THF was removed under reduced pressure. It was washed with saturated brine, 5% sodium hydroxide solution, and distilled water in sequence, and dried to obtain intermediate 1.

[0058] Step S2, 15.8g of 4-methyl-1,2-phenylenediamine and 220mL of DMF were added to a three-necked flask. After being mixed uniformly, 27.7g of Na2S2O5 and 17.3g of MgSO4 were added. After being stirred uniformly, 48.4g of intermediate 1 was added dropwise and reacted for 8 hours to obtain intermediate 2.

[0059] Step S3, intermediate 2 was dissolved in tetrahydrofuran, and chlorine was reacted under light for 2h to prepare intermediate 3, wherein the molar ratio of intermediate 2 to chlorine was 2:1.

[0060] Step S4, 73.2g of 4,4'-diaminodiphenyl sulfone was added to a three-necked flask, nitrogen was introduced, 550mL of acetonitrile and 78.5g of triethylamine were added, and the temperature was raised to 45°C. After 4,4'-diaminodiphenyl sulfone was dissolved, 29.2g of phenylphosphonic dichloride was added dropwise. The temperature was lowered to 25°C and stirred for 3 hours. After being stirred uniformly, the temperature was raised to 80°C and reacted for 20 hours. After the reaction was completed, the temperature was lowered to room temperature. The solvent was eluted with methyl tert-butyl ether. The unreacted monomer was washed away with hot acetonitrile. After filtration and drying, intermediate 4 was obtained.

[0061] Step S5, 26.2g of intermediate 4, 15.2g of N,N-diisopropylethylamine, 120mL of tetrahydrofuran were added into a reaction kettle, and nitrogen was introduced for protection, and stirring was carried out at a rotation speed of 300r / min and a temperature of 5℃ for 20min, then 28.6g of intermediate 3 was added, and reaction was carried out for 5h to prepare the flame retardant.

[0062] Example 4

[0063] A lithium iron phosphate battery electrolyte comprises the following raw materials: lithium hexafluorophosphate, a solvent, vinylene carbonate and a flame retardant.

[0064] The content mass ratio of the flame retardant, the vinylene carbonate and the solvent is 1:1.2:12.

[0065] The concentration of the lithium hexafluorophosphate is 0.52mol / L.

[0066] The lithium iron phosphate battery electrolyte is prepared by the following steps:

[0067] Step A1, under nitrogen protection, the solvent, the vinylene carbonate and the lithium hexafluorophosphate were mixed at a rotation speed of 150rpm for 7min at 16℃ to obtain a mixed solution.

[0068] Step A2: the mixed solution and the flame retardant were stirred at a rotation speed of 400rpm for 6min at 16℃ to obtain the lithium iron phosphate battery electrolyte.

[0069] The solvent is a mixture of vinyl carbonate and propylene carbonate in any ratio.

[0070] Example 5

[0071] A lithium iron phosphate battery electrolyte comprises the following raw materials: lithium triflate, a solvent, fluoroethylene carbonate and a flame retardant.

[0072] The content mass ratio of the flame retardant, the fluoroethylene carbonate and the solvent is 1:16.8:25.

[0073] The concentration of the lithium triflate is 0.8mol / L.

[0074] The lithium iron phosphate battery electrolyte is prepared by the following steps:

[0075] Step A1, under nitrogen protection, the solvent, the fluoroethylene carbonate and the lithium triflate were mixed at a rotation speed of 450rpm for 21min at 22℃ to obtain a mixed solution.

[0076] Step A2: the mixed solution and the flame retardant were stirred at a rotation speed of 600rpm for 12min at 25℃ to obtain the lithium iron phosphate battery electrolyte.

[0077] The solvent is vinylene carbonate, methyl ethyl carbonate and diethyl carbonate mixed in any ratio.

[0078] Example 6

[0079] A lithium iron phosphate battery electrolyte, comprising the following raw materials: lithium bisfluorosulfonylimide, a solvent, vinyl ethylene carbonate and a flame retardant;

[0080] The content mass ratio of the flame retardant, vinyl ethylene carbonate and solvent is 1:21.6:100.

[0081] The concentration of the lithium bisfluorosulfonylimide is 1.7 mol / L.

[0082] The lithium iron phosphate battery electrolyte is prepared by the following steps:

[0083] Step A1, under nitrogen protection, 33℃, the solvent, vinyl ethylene carbonate and lithium bisfluorosulfonylimide are mixed at a speed of 900 rpm for 35 minutes to obtain a mixed solution;

[0084] Step A2: the mixed solution and the flame retardant are stirred at 35℃ at a speed of 1000 rpm for 20 min to obtain a lithium iron phosphate battery electrolyte.

[0085] The solvent is dimethyl carbonate, methyl ethyl carbonate mixed in any ratio.

[0086] Comparative Example 1

[0087] The lithium iron phosphate battery electrolyte produced by Jinan Qiangxin Chemical Co., Ltd.

[0088] Comparative Example 2

[0089] The preparation method of the lithium iron phosphate battery electrolyte of Comparative Example 2 refers to Example 4, except that no flame retardant is added.

[0090] The lithium iron phosphate battery electrolytes obtained in Examples 4-6 and Comparative Examples 1-2 are tested for the following properties: 18g lithium iron phosphate, conductive carbon black and polyvinylidene fluoride are added to 20mL of NMP in a mass ratio of 7:1:1, stirred (speed 500rpm) for 9h, to prepare a positive electrode slurry, the positive electrode slurry is coated on an aluminum foil, vacuum baked at 110℃ for 6h to obtain a positive electrode sheet I, the positive electrode sheet I is cut into a circular sheet with a diameter of 10mm to obtain the positive electrode sheet; 18g of graphite, conductive carbon black, sodium carboxymethyl cellulose and styrene-butadiene copolymer are added to 30mL of ultrapure water in a mass ratio of 7:0.5:0.5:1, stirred (speed 500rpm) for 7h, to prepare a negative electrode slurry, the negative electrode slurry is coated on a copper foil, vacuum baked at 105℃ for 7h to obtain a negative electrode sheet I, the negative electrode sheet I is cut into a circular sheet with a diameter of 12mm to obtain the negative electrode sheet;

[0091] The capacity ratio of the positive electrode sheet and the negative electrode sheet is 1:1.1; a CR2032 type button cell is assembled in the order of negative electrode shell-negative electrode sheet-separator-positive electrode sheet-gasket-elastic sheet-positive electrode shell, and an electrolyte is added between the negative electrode sheet and the positive electrode sheet, and the electrolyte is the electrolyte prepared in the above examples and comparative examples.

[0092] The battery assembled above is subjected to charge-discharge test, and the charge-discharge test method is as follows: in the voltage range of 2.5-4.0V, first activate three times at 0.1C rate, then perform charge-discharge cycle 200 times at 0.5C rate, and calculate the capacity retention rate. The formula for calculating the capacity retention rate is: (discharge specific capacity after 200 times of 0.5C rate cycle / first discharge specific capacity) x 100%; the test standard of electrolyte self-extinguishing time is: glass wool is formed into a small ball, m grams of electrolyte is added to it, and a lighter is used to ignite the electrolyte. Record the total time t seconds from the start of the electrolyte combustion to the flame extinguishing, and the electrolyte self-extinguishing time η can be calculated according to the formula η = m / t. The test results are shown in Table 1; the test results are shown in Table 1:

[0093] Table 1

[0094]

[0095] As can be seen from Table 1, compared with Comparative Example 1-2, the lithium iron phosphate battery electrolyte prepared in Examples 4-6 has better flame retardant effect, and the battery capacity retention rate is better.

[0096] In the description of the specification, the description of the reference terms "one embodiment", "example", "specific example" and the like means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are contained in at least one embodiment or example of the present application. In the specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0097] The above is only an example and description of the present application, and those skilled in the art can make various modifications or supplements or use similar ways to replace the described specific embodiments, as long as they do not deviate from the invention or exceed the scope defined by the present claims, which shall belong to the protection scope of the present application.

Claims

1. A lithium iron phosphate battery electrolyte, characterized in that: Including the following raw materials: lithium salt, solvent, additives and flame retardant; The mass ratio of the flame retardant, additive and solvent is 1:1.2-21.6:12-100; The concentration of the lithium salt is 0.52-1.7 mol / L; The flame retardant is prepared by the following steps: Step S1, p-hydroxybenzaldehyde and THF are added to a three-necked flask, and after mixing evenly, triethylamine is added dropwise, and diphenyl chlorophosphate is added dropwise at -10°C to react to obtain intermediate 1; Step S2, add 4-methyl-1,2-phenylenediamine and DMF into a three-necked flask, mix well, then add Na2S2O5 and MgSO4, stir well, slowly add intermediate 1 dropwise and react to obtain intermediate 2; Step S3, dissolving intermediate 2 in tetrahydrofuran and reacting with chlorine gas under light conditions to obtain intermediate 3; Step S4, adding 4,4'-diaminodiphenyl sulfone to a three-necked flask, introducing nitrogen, adding acetonitrile and triethylamine, and adding phenylphosphoryl dichloride dropwise to react to obtain intermediate 4; Step S5: add intermediate 4, N,N-diisopropylethylamine, and tetrahydrofuran into a reaction kettle, introduce nitrogen for protection, add intermediate 3 for reaction, and prepare a flame retardant.

2. The lithium iron phosphate battery electrolyte according to claim 1, characterized in that: The additive is one or more of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate and vinylene sulfate mixed in any proportion.

3. The lithium iron phosphate battery electrolyte according to claim 1, characterized in that: The lithium salt is one of lithium hexafluorophosphate, lithium trifluoromethanesulfonate, lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide.

4. The lithium iron phosphate battery electrolyte according to claim 1, wherein: The solvent is a mixture of at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate in any proportion.

5. The method for preparing a lithium iron phosphate battery electrolyte according to claim 1, wherein: The method comprises the following preparation steps: Step A1, under nitrogen protection, at 16-33° C., mixing the solvent, additive, and lithium salt at 150-900 rpm for 7-35 minutes to obtain a mixed solution; Step A2: stirring the mixed solution and the flame retardant at 16-35° C. and 400-1000 rpm for 6-20 min to obtain a lithium iron phosphate battery electrolyte.

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