An electrolyte for lithium iron phosphate battery and its preparation method and application

Abstract

The present invention provides an electrolyte for a lithium iron phosphate battery, a preparation method and an application thereof. The electrolyte comprises, by mass percentage, 8-15% of a lithium salt, 81-90% of a solvent and 0.5-10% of an additive; and the additive comprises a combination of a vinyl ester compound and ethylene glycol; by selecting the combination of the vinyl ester compound and ethylene glycol as the additive, the obtained electrolyte can have a low viscosity, a low melting point and a high boiling point, thereby effectively improving the compatibility between the electrode interface and the electrolyte, improving the stability of the SEI film, and thus improving the electrochemical performance of the lithium iron phosphate battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to an electrolyte for lithium iron phosphate batteries and a preparation method and application thereof. Background Art

[0002] Lithium iron phosphate batteries have become a hot topic in the field of energy storage and power due to their low cost and excellent safety performance. However, lithium iron phosphate batteries with such excellent performance are very sensitive to temperature. Studies have shown that under low temperature conditions, the voltage and capacity of lithium iron phosphate batteries will be significantly reduced, and the internal resistance will increase significantly. Therefore, improving the low temperature performance of lithium iron phosphate batteries has become a top priority for various industries.

[0003] In response to the above problems, researchers usually make improvements to lithium iron phosphate batteries from the following aspects:

[0004] (1) Improving the conductivity and ion transfer rate of the material by increasing the graphite layer spacing, controlling the particle size, and other ways through surface treatment (oxidation, fluorination), coating (carbon coating, metal coating), and doping. CN114335479A discloses a carbon-coated lithium iron phosphate positive electrode material, a preparation method thereof, and a lithium ion battery. The preparation method of the carbon-coated lithium iron phosphate positive electrode material comprises: mixing a lithium source, a ferrous source, and a first solvent for a first time under a first inert atmosphere to obtain a first raw material liquid; mixing a phosphorus source, an auxiliary agent, and a second solvent for a second time under a second inert atmosphere to obtain a second raw material liquid, wherein the auxiliary agent is selected from gentiobiose and / or gentiotriose; subjecting the first raw material liquid and the second raw material liquid to a hydrothermal synthesis reaction to obtain lithium iron phosphate containing a carbon amorphous coating layer; and sintering the lithium iron phosphate containing a carbon amorphous coating layer under a third inert atmosphere to obtain a carbon-coated lithium iron phosphate positive electrode material. The above method can greatly improve the uniformity of the carbon coating layer in the positive electrode material, thereby greatly improving its conductive performance and energy density.

[0005] (2) Ion doping (lanthanum, cerium) is performed on lithium iron phosphate to increase its conductivity. CN103367746A discloses a multi-ion doped carbon-coated lithium iron phosphate battery material and a preparation method thereof. The structure of the material can be described as Li 1-x M` x Fe 1-y My(PO4) 1-z F z / C, where 0.001≤x≤0.1, 0.001≤y≤0.1, 0.001≤z≤0.1, M` and M are metal ions doped at the lithium and iron sites, respectively, C is organic cracking carbon coated on the surface of lithium iron phosphate, M` and F come from fluoride M`F n(n=1, 2, 3 or 4). The preparation method is: preparing doped iron phosphate containing crystal water by coprecipitation method; heat treating the iron phosphate containing crystal water to remove the crystal water; mixing doped iron phosphate, lithium source, fluoride M'F n The mixture is mixed with an organic carbon source, ball-milled, dried, and calcined to obtain a multi-ion-doped carbon-coated lithium iron phosphate battery material. The lithium iron phosphate prepared by the invention has high discharge specific capacity and excellent rate performance, and has great application prospects in the field of power batteries.

[0006] (3) Use new solid electrolytes, polymer ionic liquids or other types of electrolytes with excellent conductivity to reduce the freezing point of the electrolyte, thereby reducing the impedance of the electrolyte. CN101699642A discloses a method for making a positive electrode sheet of a lithium iron phosphate battery, the method comprising the following steps: first taking lithium iron phosphate, polyvinylidene fluoride, carbon black, conductive graphite and lithium titanate to form a mixture, then baking the mixture in an inert gas atmosphere, cooling the baked mixture, ball milling the cooled mixture in a ball mill, sieving it and sealing it for storage, pouring the sieved mixture powder into an N-methylpyrrolidone solution, and stirring it under vacuum conditions to fully dissolve it to make a slurry, then taking aluminum foil, applying the slurry on the surface of the aluminum foil, and then baking the plate coated with the slurry in a vacuum baking oven, and rolling the vacuum baked plate into a plate, that is, obtaining a positive electrode sheet of a lithium iron phosphate battery. The electrode piece produced by the inventive method has the performance of low internal resistance, high rate and high cycle, and the operation method is simple and easy, which is very suitable for use in electric vehicles and electric tools driven by high-energy secondary batteries.

[0007] However, the above solutions will increase production costs, making it difficult to achieve industrial-scale production, and cannot effectively improve the low-temperature performance of lithium iron phosphate batteries.

[0008] Therefore, it is urgent to develop an electrolyte with low viscosity, low melting point and high boiling point to improve the low temperature performance of lithium iron phosphate batteries. Summary of the invention

[0009] In view of the deficiencies in the prior art, the object of the present invention is to provide an electrolyte for lithium iron phosphate batteries and a preparation method and application thereof, wherein the electrolyte comprises a combination of a lithium salt, a solvent and an additive, and by further limiting the additive to comprise a combination of a vinyl ester compound and ethylene glycol, the obtained electrolyte has the advantages of low viscosity, low melting point and high boiling point, thereby effectively improving the compatibility between the electrode interface and the electrolyte, thereby achieving the effect of improving the low temperature performance and cycle performance of the lithium iron phosphate battery.

[0010] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0011] In a first aspect, the present invention provides a low-temperature electrolyte, wherein the low-temperature electrolyte comprises the following components in terms of mass percentage:

[0012] Lithium salt 8~15%

[0013] Solvent 81~90%

[0014] Additives 0.5-10%;

[0015] The additive comprises a combination of vinyl ester compounds and ethylene glycol.

[0016] The lithium salt may be 9%, 10%, 11%, 12%, 13% or 14%, as well as specific values ​​between the above values. Due to space limitations and for the sake of simplicity, the present invention will no longer exhaustively list the specific values ​​included in the range.

[0017] The solvent may be 82%, 83%, 84%, 85%, 86%, 87%, 88% or 89%, as well as specific values ​​between the above points. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific points included in the range.

[0018] The additive may be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8% or 9%, as well as specific values ​​between the above values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific values ​​included in the range.

[0019] The electrolyte for lithium iron phosphate batteries provided by the present invention can have the advantages of low viscosity, low melting point and high boiling point by adding additives and further selecting a combination of vinyl ester compounds and ethylene glycol as additives, and can effectively improve the compatibility problem between the electrode interface and the electrolyte, thereby effectively improving the low temperature performance and cycle performance of the lithium battery, and is particularly suitable for lithium iron phosphate batteries; in addition, the additive, as an excellent film-forming additive, can also effectively reduce the gas generated in the process of forming the SEI film, improve the stability of the SEI film, thereby further improving the electrochemical performance of the lithium iron phosphate battery.

[0020] Preferably, the lithium salt includes any one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate or lithium tetrafluoroborate, or a combination of at least two thereof.

[0021] Preferably, the solvent includes any one of ethylene carbonate, diethyl carbonate, ethyl propionate or ethyl methyl carbonate, or a combination of at least two thereof.

[0022] Preferably, the solvent comprises a combination of ethylene carbonate, diethyl carbonate and ethyl propionate.

[0023] Preferably, the solvent comprises a combination of ethylene carbonate, ethyl methyl carbonate and ethyl propionate.

[0024] Preferably, the solvent comprises a combination of ethylene carbonate, diethyl carbonate, ethyl methyl carbonate and ethyl propionate.

[0025] Preferably, the vinyl ester compound includes any one of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate or vinyl sulfate, or a combination of at least two thereof, and vinylene carbonate is more preferably included.

[0026] Preferably, the mass ratio of the vinyl ester compound to ethylene glycol is 1:(0.5-1.5), for example, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, 1:1.1, 1:1.2, 1:1.3 or 1:1.4, etc.

[0027] As a preferred technical solution of the present invention, the mass ratio of vinyl ester compounds to ethylene glycol is limited to 1:(0.5-1.5), so that the comprehensive performance of the obtained electrolyte can be optimized, and the lithium iron phosphate battery prepared using the electrolyte can have better low temperature performance and cycle performance.

[0028] In a second aspect, the present invention provides a method for preparing the electrolyte as described in the first aspect, the preparation method comprising: mixing a solvent and an additive, adding a lithium salt, to obtain the electrolyte.

[0029] In a third aspect, the present invention provides a lithium iron phosphate battery, comprising a positive electrode sheet, a negative electrode sheet, and the electrolyte and the separator as described in the first aspect.

[0030] Preferably, the positive electrode sheet is prepared by the following method, which comprises: coating the positive electrode slurry on aluminum foil, and drying to obtain the positive electrode sheet.

[0031] Preferably, the positive electrode slurry comprises a combination of lithium iron phosphate, a binder, a conductive agent and a solvent.

[0032] Preferably, the negative electrode sheet is prepared by the following method, which comprises: coating a negative electrode slurry on a copper foil, and drying the copper foil to obtain the negative electrode sheet.

[0033] Preferably, the negative electrode slurry comprises a combination of graphite, a conductive agent, a binder and a solvent.

[0034] Preferably, the separator comprises any one of a polyethylene single-layer film, a polypropylene single-layer film or a polypropylene / polyethylene / polypropylene three-layer composite film.

[0035] In a fourth aspect, the present invention provides a use of the lithium iron phosphate battery as described in the third aspect in an automobile.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] The lithium iron phosphate battery provided by the present invention comprises, by mass percentage, 8-15% of lithium salt, 81-90% of solvent and 0.5-10% of additive; and the additive comprises a combination of vinyl ester compounds and ethylene glycol; by selecting the combination of vinyl ester compounds and ethylene glycol as the additive, the obtained electrolyte can have low viscosity, low melting point and high boiling point, can effectively improve the compatibility between the electrode interface and the electrolyte, improve the stability of the SEI film, and thus improve the electrochemical performance of the lithium iron phosphate battery. Specifically, the lithium iron phosphate battery obtained by using the electrolyte provided by the present invention has a -10°C@0.1C discharge capacity ratio of 86.1-94.5%, a 10°C@0.33C discharge capacity ratio of 74.2-87.6%, a -20°C@0.1C discharge capacity ratio of 66.2-75.9%, a -20°C@0.33C discharge capacity ratio of 62.4-73.6%, and a capacity after 2000 cycles / first cycle capacity of 88.6-91.5%, and has excellent low temperature resistance and cycle performance. DETAILED DESCRIPTION

[0038] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0039] Example 1

[0040] An electrolyte for a lithium iron phosphate battery, the electrolyte comprising the following components in percentage by mass:

[0041]

[0042] The preparation method of the electrolyte provided in this embodiment includes: uniformly mixing a solvent (obtained by mixing ethylene carbonate, diethyl carbonate and ethyl propionate in a volume ratio of 1:1:1), vinylene carbonate and ethylene glycol, and adding lithium hexafluorophosphate and mixing uniformly to obtain the electrolyte.

[0043] Example 2

[0044] An electrolyte for a lithium iron phosphate battery, the electrolyte comprising the following components in percentage by mass:

[0045]

[0046] The preparation method of the electrolyte provided in this embodiment includes: uniformly mixing a solvent (obtained by mixing ethylene carbonate, diethyl carbonate, ethyl methyl carbonate and ethyl propionate in a volume ratio of 1:1:1:1), vinylene carbonate and ethylene glycol, adding lithium hexafluorophosphate and mixing uniformly to obtain the electrolyte.

[0047] Example 3

[0048] An electrolyte for a lithium iron phosphate battery, the electrolyte comprising the following components in percentage by mass:

[0049]

[0050] The preparation method of the electrolyte provided in this embodiment includes: uniformly mixing a solvent (obtained by mixing ethylene carbonate, ethyl methyl carbonate and ethyl propionate in a volume ratio of 1:1:1), vinylene carbonate and ethylene glycol, and adding lithium hexafluorophosphate and mixing uniformly to obtain the electrolyte.

[0051] Example 4

[0052] An electrolyte for a lithium iron phosphate battery, wherein the added amount of vinylene carbonate is 2%, the added amount of ethylene glycol is 3%, and the other components, amounts and preparation methods are the same as those in Example 1.

[0053] Example 5

[0054] An electrolyte for a lithium iron phosphate battery, wherein the added amount of vinylene carbonate is 3%, the added amount of ethylene glycol is 2%, and the other components, amounts and preparation methods are the same as those in Example 1.

[0055] Example 6

[0056] An electrolyte for a lithium iron phosphate battery, wherein the addition amount of vinylene carbonate is 1%, the addition amount of ethylene glycol is 4%, and the other components, amounts and preparation methods are the same as those in Example 1.

[0057] Example 7

[0058] An electrolyte for a lithium iron phosphate battery, wherein the addition amount of vinylene carbonate is 4%, the addition amount of ethylene glycol is 1%, and other components, amounts and preparation methods are the same as those in Example 1.

[0059] Example 8

[0060] An electrolyte for a lithium iron phosphate battery, which differs from Example 1 only in that fluoroethylene carbonate is used to replace vinylene carbonate, and other components, amounts and preparation methods are the same as those in Example 1.

[0061] Example 9

[0062] An electrolyte for a lithium iron phosphate battery, which differs from Example 1 only in that vinyl ethylene carbonate is used to replace vinylene carbonate, and other components, amounts and preparation methods are the same as those in Example 1.

[0063] Example 10

[0064] An electrolyte for a lithium iron phosphate battery, which differs from Example 1 only in that vinyl sulfate is used to replace vinylene carbonate, and other components, amounts and preparation methods are the same as those in Example 1.

[0065] Comparative Example 1

[0066] An electrolyte for a lithium iron phosphate battery, which differs from Example 1 in that vinylene carbonate is not added, the added amount of ethylene glycol is 5%, and other components, amounts and preparation methods are the same as those in Example 1.

[0067] Comparative Example 2

[0068] An electrolyte for a lithium iron phosphate battery, which differs from Example 1 in that ethylene glycol is not added, the added amount of vinylene carbonate is 5%, and other components, amounts and preparation methods are the same as those in Example 1.

[0069] Comparative Example 3

[0070] An electrolyte for a lithium iron phosphate battery, the low-temperature electrolyte comprising the following components by mass percentage:

[0071]

[0072]

[0073] The preparation method of the electrolyte provided in this embodiment includes: uniformly mixing a solvent (obtained by mixing ethylene carbonate, diethyl carbonate and ethyl propionate in a volume ratio of 1:1:1), vinylene carbonate and ethylene glycol, and adding lithium hexafluorophosphate and mixing uniformly to obtain the electrolyte.

[0074] Comparative Example 4

[0075] A low-temperature electrolyte for a lithium iron phosphate battery, the low-temperature electrolyte comprising the following components in terms of mass percentage:

[0076]

[0077] The preparation method of the electrolyte provided in this embodiment includes: uniformly mixing a solvent (obtained by mixing ethylene carbonate, diethyl carbonate and ethyl propionate in a volume ratio of 1:1:1), vinylene carbonate and ethylene glycol, and adding lithium hexafluorophosphate and mixing uniformly to obtain the electrolyte.

[0078] Application Example 1

[0079] A lithium iron phosphate battery, comprising a positive electrode sheet, a negative electrode sheet, a low-temperature electrolyte and a diaphragm;

[0080] The preparation method of the lithium iron phosphate battery provided in this application example includes the following steps:

[0081] (1) Preparation of positive electrode: lithium iron phosphate (LFP), carbon black (SP), and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 95:1:4, mixed evenly in a stirring tank at a certain speed, and N-methylpyrrolidone (NMP) is added to prepare a positive electrode slurry. The viscosity of the positive electrode slurry is controlled to be 12000mPa·s and the solid content is about 60%. Then, it is evenly coated on aluminum foil, cold pressed and baked to obtain a positive electrode sheet;

[0082] (2) Preparation of negative electrode: Graphite, carbon nanotubes and SBR are mixed in a mass ratio of 90:2:8, mixed evenly in a stirring tank at a certain speed, and deionized water is added to prepare a negative electrode slurry. The viscosity of the negative electrode slurry is controlled to be 5000 mPa·s and the solid content is 50%. Then, the slurry is evenly coated on a copper foil, cold pressed and baked to obtain a negative electrode sheet;

[0083] (3) The positive electrode sheet and the separator obtained in step (1) and the negative electrode sheet obtained in step (2) are assembled in order, and the battery cell is baked at 45° C. for 24 hours before injection. When the battery cell satisfies the moisture content of ≤200 ppm, the electrolyte (Example 1) is injected and maintained at a negative pressure of -80 kPa. According to a certain injection coefficient, the injection coefficient can be 4 g / Ah, and the lithium iron phosphate battery is obtained by formation and separation.

[0084] Application Examples 2 to 10

[0085] A lithium iron phosphate battery, which differs from Application Example 1 only in that the electrolyte obtained in Example 1 is replaced by the electrolytes obtained in Examples 2 to 10, and other components and preparation methods are the same as those in Application Example 1.

[0086] Comparative Application Examples 1 to 4

[0087] A lithium iron phosphate battery, which differs from Application Example 1 only in that the electrolyte obtained in Example 1 is replaced by the electrolytes obtained in Comparative Examples 1 to 4, and the other components and preparation methods are the same as those in Application Example 1.

[0088] Performance Test:

[0089] (1) Low temperature rate performance:

[0090] ① Charge the battery to 3.65V at 0.5C constant current and constant voltage at room temperature, with a cut-off current of 0.05C. Then, discharge it to 2.5V at 0.5C constant current, cycle 3 times, calibrate its room temperature discharge capacity Q1, and finally fully charge it at room temperature with constant current and constant voltage; ② Place the battery at -10℃ for 2h, discharge it at 0.1C, and record the discharge capacity Q2 under the temperature rate condition; ③ Place the battery at -10℃ for 2h, discharge it at 0.33C, and record the discharge capacity Q3 under the temperature rate condition; ③ The measurement of -20℃@0.1C and 0.33C can also be carried out by analogy with the above steps; ④ Calculate the discharge capacity ratio and energy ratio as required, such as: the calculation formula for the discharge capacity ratio at -10℃@0.1C is Q2 / Q1×100%. (2) Cycle performance: Charging: 1C constant current constant voltage charging to 3.65V, cut-off current: 0.05C; Discharging: 1C constant current discharge to 2.5V; The above steps are used for cyclic charging and discharging up to 2000 cycles; Capacity retention rate = capacity after 2000 cycles / first cycle capacity × 100%.

[0091] According to the above test method, the lithium iron phosphate batteries provided in the corresponding application examples 1 to 10 and the comparative application examples 1 to 4 were tested, and the test results are shown in Table 1:

[0092] Table 1

[0093]

[0094]

[0095] According to the data in Table 1, we can see that:

[0096] The lithium iron phosphate battery prepared by using the electrolyte provided by the present invention has excellent electrochemical properties. Specifically, the lithium iron phosphate batteries obtained in Application Examples 1 to 10 have a -10°C@0.1C discharge capacity ratio of 86.1 to 94.5%, a 10°C@0.33C discharge capacity ratio of 74.2 to 87.6%, a -20°C@0.1C discharge capacity ratio of 66.2 to 75.9%, a -20°C@0.33C discharge capacity ratio of 62.4 to 73.6%, and a capacity after 2000 cycles / first cycle capacity of 88.6 to 91.5%.

[0097] By comparing Example 1 with Comparative Examples 1 and 2, it can be seen that when the total amount of additives is 5%, but only vinylene carbonate or ethylene glycol is used as an additive, the low-temperature discharge capacity ratio of the obtained lithium iron phosphate battery will be significantly attenuated; in addition, it can be found that vinylene carbonate has a greater effect on low-temperature performance than ethylene glycol.

[0098] By comparing Example 1 with Comparative Examples 3 and 4, it can be seen that too high or too low content of the additive is not good for the electrical performance of the lithium iron phosphate battery.

[0099] Further comparison of Example 1 and Examples 4 to 7 shows that when the total amount of additives is 5% (5% for vinylene carbonate and ethylene glycol), as the content of vinylene carbonate increases, the discharge capacity ratio of the lithium iron phosphate battery shows an obvious trend of first increasing and then significantly decreasing, so the preferred mass ratio of vinylene carbonate to ethylene glycol is 1:(0.5 to 1.5).

[0100] The applicant declares that the present invention illustrates an electrolyte for lithium iron phosphate battery and its preparation method and application through the above-mentioned embodiments, but the present invention is not limited to the above-mentioned process steps, that is, it does not mean that the present invention must rely on the above-mentioned process steps to be implemented. Those skilled in the art should understand that any improvement of the present invention, equivalent replacement of the raw materials selected by the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. An electrolyte for a lithium iron phosphate battery, characterized in that: The electrolyte comprises the following components in terms of mass percentage: Lithium salt 8~15% Solvent 81~90% Additives 0.5~9%; The additive includes a combination of vinyl ester compounds and ethylene glycol; The solvent is any one of ethylene carbonate, diethyl carbonate, ethyl propionate or ethyl methyl carbonate, or a combination of at least two thereof; The mass ratio of the vinyl ester compound to ethylene glycol is 1:(0.5-1.5); The vinyl ester compound is vinylene carbonate.

2. The electrolyte according to claim 1, characterized in that The lithium salt includes any one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate or lithium tetrafluoroborate, or a combination of at least two thereof.

3. The electrolyte according to claim 1, characterized in that The solvent is a combination of ethylene carbonate, diethyl carbonate and ethyl propionate.

4. The electrolyte according to claim 1, characterized in that The solvent is a combination of ethylene carbonate, ethyl methyl carbonate and ethyl propionate.

5. The electrolyte according to claim 1, characterized in that The solvent is a combination of ethylene carbonate, diethyl carbonate, ethyl methyl carbonate and ethyl propionate.

6. A method for preparing the electrolyte according to any one of claims 1 to 5, characterized in that: The preparation method comprises: mixing a solvent and an additive, and adding a lithium salt to obtain the electrolyte.

7. A lithium iron phosphate battery, characterized in that: The lithium iron phosphate battery comprises a positive electrode sheet, a negative electrode sheet, the electrolyte according to any one of claims 1 to 5, and a separator.

8. The lithium iron phosphate battery according to claim 7, characterized in that: The positive electrode sheet is prepared by the following method, which includes: coating the positive electrode slurry on aluminum foil and drying to obtain the positive electrode sheet.

9. The lithium iron phosphate battery according to claim 8, characterized in that: The positive electrode slurry includes a combination of lithium iron phosphate, a binder, a conductive agent and a solvent.

10. The lithium iron phosphate battery according to claim 7, characterized in that: The negative electrode sheet is prepared by the following method, which includes: coating the negative electrode slurry on copper foil and drying to obtain the negative electrode sheet.

11. The lithium iron phosphate battery according to claim 10, characterized in that: The negative electrode slurry includes a combination of graphite, a conductive agent, a binder and a solvent.

12. The lithium iron phosphate battery according to any one of claims 7 to 11, characterized in that: The diaphragm includes any one of a polyethylene single-layer film, a polypropylene single-layer film, or a polypropylene / polyethylene / polypropylene three-layer composite film.

13. Use of the lithium iron phosphate battery according to any one of claims 7 to 11 in an automobile.

Images