Electrolyte salt and its preparation method and application

Abstract

The present invention discloses an electrolyte salt and its preparation method and application, belonging to the field of secondary battery technology. The electrolyte salt has a structure shown in formula (I), wherein M + Selected from Li + 、Na + , K + One or more of the following. The electrolyte salt of the present invention has a stable structure, is insensitive to the aqueous phase, and is not easy to react with water. Due to the presence of oxalic acid phosphate, the electrolyte salt can promote the preferential oxidation and reduction of the electrolyte on the surface of the positive and negative electrodes to form a protective film to isolate corrosion, thereby improving the safety inside the battery. The difluoroboric acid phosphate oxalate of the present invention as an electrolyte salt can maintain the excellent performance of the battery, significantly improve the high and low temperature adaptability, rate discharge performance, chemical stability, thermal stability and cycle stability of the battery, thereby improving the safety of battery use. The synthesis path of the difluoroboric acid phosphate oxalate of the present invention has simple steps, mild conditions, safe post-processing, and can be used for industrial production.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to an electrolyte salt and a preparation method and application thereof. Background Art

[0002] In recent years, the lithium-ion battery industry chain, from raw materials to finished batteries and new energy vehicles, has flourished. Researchers across various industries are striving to develop batteries with even better performance. As a result, battery life and performance have significantly improved. However, issues such as electric vehicle fires, long charging times, and reduced battery performance in harsh high and low temperature environments remain common. These issues pose safety risks to lithium-ion batteries and have become a significant constraint on their continued development.

[0003] As one of the important components of lithium-ion batteries, electrolyte plays the role of transporting lithium ions between the positive and negative electrodes of lithium-ion batteries. It is an important guarantee for lithium-ion batteries to achieve high energy density and high voltage. Under low temperature conditions, the efficiency of ion transmission within the battery decreases, and polarization is very likely to occur, thereby increasing internal resistance. In addition, metal will precipitate on the surface of the negative electrode, further damaging the internal structure of the battery, seriously affecting battery safety. High temperatures are inevitably generated when lithium-ion batteries are charged at a high rate, which places extremely high demands on the thermal stability of the positive and negative electrodes as well as the electrolyte solvent. The high and low temperature adaptability and the battery's rate charging (instant charging) are ultimately related to the battery's safety performance.

[0004] Currently, there are relatively few options for electrolyte salts in commercial electrolytes, which are insufficient to solve existing technical problems. Existing electrolyte salts generally address a single problem in the electrolyte system. For example, to protect the positive electrode material under high voltage and improve battery performance, Li3PO4-type substances with phosphate groups are generally used; to improve the chemical and thermal stability of the battery and widen the battery operating temperature range, LiBF4-type substances containing fluorine and boron functional groups are generally used; to improve the rate performance of the battery positive electrode material, LiC2O4-type substances containing oxalate ions are generally used. However, the battery's internal performance is often affected by multiple side reactions between different systems. Existing electrolyte salts cannot solve the side reactions caused by the interaction of multiple substances from the source, and cannot effectively improve the safety and stability of the battery.

[0005] In addition, common commercial electrolyte salts, such as hexafluorophosphates, inevitably use fluorinated compounds during design and production, which poses a great safety hazard during the preparation process. Moreover, these salts are extremely sensitive to water, and the moisture content needs to be strictly controlled during the electrolyte preparation process, otherwise they will react with water to generate corrosive HF, thereby affecting battery safety.

[0006] Therefore, it is crucial to research and develop new electrolyte salts to improve the safety and stability of batteries. Summary of the Invention

[0007] In view of this, the technical problem to be solved by the present invention is to provide an electrolyte salt, its preparation method, and its application. The electrolyte salt of the present invention has a stable structure and is not easily reactive with water. It can significantly improve the high and low temperature adaptability, rate discharge performance, chemical stability, thermal stability, and cycling stability of the battery, thereby improving battery safety.

[0008] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0009] The present invention provides an electrolyte salt having a structure shown in formula (I):

[0010]

[0011] Preferably, the M + Selected from Li + 、Na + , K + One or more of .

[0012] The electrolyte salt of the present invention has a larger anionic radius and better solubility in electrolyte solvents. This significantly improves the battery's capacity, cycle, and rate performance when used in batteries. Furthermore, the electrolyte salt has a low fluorine atom ratio, minimizing corrosion to the current collector.

[0013] The electrolyte salt described in the present invention possesses oxalate, phosphate, and fluoroboron functional groups, combining the advantages of both oxalophosphate and difluorooxalatoborate, offering excellent high- and low-temperature performance stability and electrochemical stability. Furthermore, the electrolyte salt exhibits excellent film-forming properties, promoting the formation of a stable, low-impedance SEI film, thereby further improving the stability of the finished battery.

[0014] The electrolyte salt of the present invention has a stable structure, is insensitive to the aqueous phase, and has the function of inhibiting and isolating the corrosion of HF in the electrolyte, thereby greatly improving the safety performance of the battery.

[0015] The present invention also provides a method for preparing an electrolyte salt, comprising the following steps: mixing oxalophosphate, boron trifluoride and its complex and an acid binding agent, and reacting the mixture to obtain an electrolyte salt represented by formula (I).

[0016]

[0017] Preferably, the M + Selected from Li + 、Na + , K+ One or more of .

[0018] Preferably, the oxalophosphate is selected from one or more of lithium oxalophosphate, sodium oxalophosphate, and potassium oxalophosphate.

[0019] Preferably, the boron trifluoride and its complex are selected from one or more of boron trifluoride, boron trifluoride ethyl ether, boron trifluoride ethylamine, boron trifluoride acetonitrile, boron trifluoride dimethyl carbonate, boron trifluoride diethyl carbonate, boron trifluoride methyl ethyl carbonate, boron trifluoride ethylene carbonate, boron trifluoride propylene carbonate, and boron trifluoride ethyl acetate; more preferably, the boron trifluoride and its complex are selected from boron trifluoride ethyl ether, boron trifluoride ethylamine, and boron trifluoride acetonitrile. In a specific embodiment of the present invention, the boron trifluoride and its complex are boron trifluoride ethyl ether, boron trifluoride acetonitrile, and boron trifluoride ethylamine.

[0020] Preferably, the acid binding agent is selected from triethylamine or pyridine; more preferably triethylamine.

[0021] Preferably, the solvent for the reaction is selected from ester solvents.

[0022] Preferably, the ester solvent is selected from one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, and ethyl acetate; more preferably, the ester solvent is selected from diethyl carbonate or ethyl acetate.

[0023] Preferably, the molar ratio of the oxalophosphate, boron trifluoride and its complex, and the acid-binding agent is 1:(2-2.2):0.1. More preferably, the molar ratio of the oxalophosphate, boron trifluoride and its complex, and the acid-binding agent is 1:(2-2.15):0.1. In a specific embodiment of the present invention, the molar ratio of the oxalophosphate, boron trifluoride and its complex, and the acid-binding agent can be any one of 1:2:0.1, 1:2.05:0.1, 1:2.1:0.1, and 1:2.15:0.1.

[0024] Preferably, the reaction temperature is 25° C. to 60° C. More preferably, the reaction temperature is 40° C. to 60° C. In a specific embodiment of the present invention, the reaction temperature is 40° C., 50° C., or 60° C.

[0025] Preferably, the reaction time is 6 h to 24 h; more preferably, the reaction time is 6 h to 18 h; further preferably, it is 12 h to 18 h.

[0026] Preferably, the present invention further comprises a recrystallization post-treatment after the reaction is completed.

[0027] Preferably, the solvent for recrystallization is selected from one or more of dichloromethane, 1,2-dichloroethane, toluene, xylene, petroleum ether, and n-hexane; more preferably, the solvent for recrystallization is selected from n-hexane or dichloromethane.

[0028] The preparation method of the electrolyte salt of the present invention specifically includes the following steps:

[0029]

[0030] 1) Dissolve the reactants oxalic acid phosphate and boron trifluoride and its complex in sufficient organic solvents respectively, and then mix the two solutions evenly to obtain a mixed system S1;

[0031] 2) Slowly drop the acid-binding agent into the above-mentioned mixed system S1, heat the reaction system to a certain temperature, and react to obtain a mixed system S2;

[0032] 3) Filter and remove impurities from the above-mentioned mixed system S2, collect the filtrate, and obtain oxalic acid phosphate difluoroborate through the method of recrystallization with a mixed solvent.

[0033] The BF3-R is the above-mentioned boron trifluoride and its complex.

[0034] The synthesis route of the present invention uses oxalic acid phosphate and boron trifluoride and its complex as raw materials, and uses an acid-binding agent to absorb the impurity acid generated in the reaction system, thereby promoting the forward progress of the reaction and finally obtaining oxalic acid phosphate difluoroborate.

[0035] The synthesis route of the oxalic acid phosphate difluoroborate of the present invention has simple steps, mild conditions, and safe post-treatment, and can be used for industrial production.

[0036] The present invention also provides a secondary battery electrolyte, including the above-mentioned electrolyte salt or the electrolyte salt prepared by the above-mentioned preparation method.

[0037] The above-mentioned electrolyte can also include other electrolyte salts, including but not limited to tetrafluoroborate, hexafluorophosphate, bis(fluorosulfonyl)imide salt, trifluoromethylsulfonylimide salt, bis(oxalato)borate, etc.

[0038] Preferably, the solvent of the electrolyte is selected from ethylene carbonate and ethyl methyl carbonate.

[0039] Preferably, the volume ratio of ethylene carbonate to ethyl methyl carbonate is V1:V2, and V1 and V2 satisfy the following conditions: V1 + V2 = 10 and V1 < V2; including but not limited to 1:9, 2:8, 3:7, 4:6, and more preferably, the volume ratio of ethylene carbonate to ethyl methyl carbonate is 3:7.

[0040] The present invention also provides a secondary battery, and the electrolyte uses the above-mentioned secondary battery electrolyte.

[0041] Compared with the prior art, the electrolyte salt provided by the present invention has a structure shown in formula (I), wherein the M + Selected from Li + 、Na + , K + One or more of the following. The electrolyte salt of the present invention has a stable structure, is insensitive to the aqueous phase, and does not readily react with water. Due to the presence of oxalate phosphate, the electrolyte salt can promote preferential oxidation and reduction of the electrolyte on the positive and negative electrode surfaces to form a protective film that isolates corrosion, thereby improving the safety of the battery. The difluoroboric acid phosphate oxalate of the present invention as an electrolyte salt can maintain the excellent performance of the battery, significantly improving the battery's high and low temperature adaptability, rate discharge performance, chemical stability, thermal stability, and cycling stability, thereby improving the safety of the battery. DETAILED DESCRIPTION

[0042] In order to further illustrate the present invention, the electrolyte salt provided by the present invention, its preparation method and application are described in detail below with reference to the examples.

[0043] The following reaction raw materials and solvents used are all common commercially available products.

[0044] Example 1

[0045] To a reaction flask equipped with a thermometer, 10 g (0.0633 mol) of 99% pure lithium oxalophosphate was added. 50 g of diethyl carbonate was weighed and added to the flask, stirring was initiated to mix the mixture thoroughly. A total of 17.98 g (0.1266 mol) of boron trifluoride etherate was weighed and added to the reaction system. Using a constant pressure dropping funnel, 1 mL of triethylamine (0.0063 mol) was added dropwise to the reaction system while stirring. The temperature of the reaction system was controlled to be below 35°C during addition. After the addition was complete, the system was heated to 40°C and stirred for a total of 18 hours. After the reaction was complete, the system was filtered, the filter cake was washed twice with diethyl carbonate, and the filtrate was collected. The crude product was vacuum distilled until a solid precipitated. With stirring, 20 g of n-hexane was slowly added to the filtrate, gradually precipitating a white solid. After the addition of n-hexane was complete, the mixture was stirred at room temperature for 0.5 h, then filtered to obtain a solid. After drying, the product, lithium difluoroborate phosphate oxalate, was obtained with a yield of 68% and a purity of 99.5%. TOF-MS (ESI) m / z calculated for C2BF2LiO6P: 206.74; [M+H] + found:206.5.

[0046] Example 2

[0047] To a reaction flask equipped with a thermometer, 50 g (0.317 mol) of 99% pure lithium oxalophosphate was added. 150 g of diethyl carbonate was weighed and added to the reaction flask, and stirring was started to mix the system thoroughly. A total of 92 g (0.649 mol) of boron trifluoride acetonitrile complex was weighed and added to the reaction system. Using a constant pressure dropping funnel, 4.5 mL of triethylamine (0.0317 mol) was added dropwise to the reaction system while stirring. The system temperature was controlled to not exceed 35°C during the addition of the ingredients to the reaction flask. After the addition was complete, the system was heated to 50°C and stirred for a total of 15 hours. After the reaction was complete, the system was filtered, the filter cake was washed twice with diethyl carbonate, and the filtrate was collected. The crude product was distilled under reduced pressure until a solid precipitated. A total of 40 g of n-hexane was slowly added to the filtrate while stirring, and a white solid gradually precipitated from the solution. After the addition of n-hexane was complete, the mixture was stirred at room temperature for 0.5 h, then filtered to obtain a solid. After drying, the product, lithium difluoroborate phosphate oxalate, was obtained with a yield of 72% and a purity of 99.5%. TOF-MS (ESI) m / z calculated for C2BF2LiO6P: 206.74; [M+H] + found:206.5.

[0048] Example 3

[0049] To a reaction flask equipped with a thermometer, 50 g (0.317 mol) of 99% pure lithium oxalophosphate was added. 200 g of ethyl acetate was weighed and added to the flask, and stirring was initiated to mix the mixture thoroughly. A total of 72.4 g (0.665 mol) of boron trifluoride acetonitrile complex was weighed and added to the reaction system. 2.5 mL of pyridine (0.0317 mol) was added dropwise to the reaction using a constant pressure dropping funnel while stirring. The temperature of the reaction flask was controlled to be below 35°C during addition. After the addition was complete, the system was heated to 60°C and stirred for a total of 12 hours. After completion of the reaction, the system was filtered, the filter cake was washed twice with ethyl acetate, and the filtrate was collected. The crude product was distilled under reduced pressure until a solid precipitated. A total of 40 g of n-hexane was slowly added to the filtrate while stirring, and a white solid gradually precipitated from the solution. After the addition of n-hexane was complete, the mixture was stirred at room temperature for 0.5 h, and then filtered to obtain a solid. After drying, the product, lithium difluoroborate phosphate oxalate, was obtained with a yield of 73% and a purity of 99.5%. TOF-MS (ESI) m / z calculated for C2BF2LiO6P: 206.74; [M+H] + found:206.5.

[0050] Example 4

[0051] To a reaction flask equipped with a thermometer, 100 g (0.633 mol) of 99% pure lithium oxalophosphate was added. 400 g of ethyl acetate was weighed and added to the reaction flask, and stirring was started to mix the system evenly. A total of 74.4 g (1.36 mol) of boron trifluoride ethylamine complex was weighed and added to the reaction system. Using a constant pressure dropping funnel, 9 mL of triethylamine (0.0633 mol) was added dropwise to the reaction system while stirring. The system temperature was controlled not to exceed 35°C during the addition of the reaction flask. After the addition was complete, the system was heated to 60°C and stirred for a total of 12 hours. After completion of the reaction, the system was filtered, the filter cake was washed twice with ethyl acetate, and the filtrate was collected. The crude product was distilled under reduced pressure until a solid was about to precipitate. A total of 50 g of n-hexane was slowly added to the filtrate while stirring, and a white solid gradually precipitated out of the solution. After the addition of n-hexane was complete, the mixture was stirred at room temperature for 0.5 h, then filtered to obtain a solid. After drying, the product, lithium difluoroborate phosphate oxalate, was obtained with a yield of 71% and a purity of 99.5%. TOF-MS (ESI) m / z calculated for C2BF2LiO6P: 206.74; [M+H] + found:206.5.

[0052] Example 5

[0053] DFT theoretical calculations were performed on the designed and synthesized molecule lithium difluoroborate phosphate oxalate and the solvent (EC / DEC). The calculated HOMO and LUMO energy levels are shown in Table 1.

[0054] Table 1 DFT theoretical calculation energy level table

[0055] energy level <![CDATA[EC-Li + ]]> <![CDATA[DEC-Li + ]]> lithium difluoroborate phosphate oxalate LUMO energy level (eV) -0.659 -0.861 -1.076 HOMO energy level (eV) -9.163 -8.879 -8.456 Energy level difference (eV) 8.468 8.018 7.38

[0056] Note: EC-Li + With DEC-Li + It is a solvation structure layer molecule formed by solvent molecules and lithium ions in lithium salts.

[0057] As shown in Table 1, lithium difluoroborate phosphate oxalate has a higher HOMO energy level, which facilitates its preferential oxidation on the surface of the battery's positive electrode when used as an electrolyte solution, forming a CEI film (catholyte intercalation), thereby more effectively isolating the positive electrode material from HF-related substances. Furthermore, lithium difluoroborate phosphate oxalate has a lower LUMO energy level, which further facilitates its preferential reduction on the negative electrode surface, forming a SEI film (solid electrolyte intercalation), thereby protecting the battery's negative electrode.

[0058] Example 6

[0059] Preparation of button battery: weigh the active material LiNi 0.5 Mn 1.5O4, carbon black, and PVDF (a binder) in a ratio of 8:1:1 were dissolved in NMP (a 5% PVDF solution prepared with NMP as the solvent), mixed and stirred for 5 hours, and then evenly coated on the current collector. The mixture was vacuum-dried for 12 hours and pressed using a punching machine to produce the positive electrode sheet, which was then brought into a glove box for assembly of the assembled battery. The assembled 2032-type button cell used the electrolyte of a solution of lithium difluoroborate phosphate oxalate (1 mol / L) prepared in the present invention in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (3:7 by volume). The battery's negative electrode used a metal lithium sheet, and the separator used a PE film.

[0060] The prepared battery was tested for room temperature (25°C) cycle (300 cycles) performance, low temperature (-20°C) discharge performance, high temperature (60°C) cycle (150 cycles) performance, and rate (5C) discharge performance. The experimental data are shown in Table 2.

[0061] Comparative Example 1

[0062] The button battery was prepared by the same method as in Example 6, and the electrolyte used in the preparation of the battery was replaced with lithium hexafluorophosphate (LiPF6). The battery performance data was tested and collected. The experimental data are shown in Table 2.

[0063] Table 2 Button battery performance data

[0064]

[0065] The data in Table 2 clearly show that the battery using lithium difluoroborate phosphate oxalate solution as the electrolyte has better room temperature cycle performance, low temperature discharge performance, high temperature cycle performance, and rate discharge performance. This shows that lithium difluoroborate phosphate oxalate can significantly improve the thermal and chemical stability of the battery. It can still maintain excellent battery performance under harsh conditions such as high and low temperatures, and has a high capacity retention rate after cycling. It also performs well at a 5C rate discharge, providing conditions for the subsequent development of fast-charging electrolytes for lithium batteries.

[0066] In summary, the difluoroboric acid phosphate oxalate of the present invention has a stable structure, is insensitive to aqueous phases, and is not prone to reaction with water. Furthermore, due to the presence of oxalophosphate, the electrolyte preferentially oxidizes and reduces on the positive and negative electrode surfaces, forming a protective film to isolate corrosion, thereby improving the safety of the battery. Using difluoroboric acid phosphate oxalate as a battery electrolyte or additive can maintain the battery's excellent performance, significantly improve the battery's high and low temperature adaptability and cycling stability, and enhance battery safety.

[0067] The above embodiments are only intended to help understand the method and core concept of the present invention. It should be noted that, without departing from the principles of the present invention, a number of improvements and modifications may be made to the present invention by those skilled in the art, and such improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. An electrolyte salt, characterized in that It has the structure shown in formula (I): The M + Selected from Li + 、Na + , K + One or more of .

2. A method for preparing an electrolyte salt, characterized in that: The method comprises the following steps: mixing oxalophosphate, boron trifluoride and its complex and an acid binding agent and reacting the mixture to obtain an electrolyte salt represented by formula (I); The M + Selected from Li + 、Na + , K + One or more of .

3. The method for preparing an electrolyte salt according to claim 2, wherein: The oxalophosphate is selected from one or more of lithium oxalophosphate, sodium oxalophosphate, and potassium oxalophosphate.

4. The method for preparing an electrolyte salt according to claim 2, wherein: The boron trifluoride and its complex are selected from one or more of boron trifluoride, boron trifluoride ethyl ether, boron trifluoride ethylamine, boron trifluoride acetonitrile, boron trifluoride dimethyl carbonate, boron trifluoride diethyl carbonate, boron trifluoride methyl ethyl carbonate, boron trifluoride ethylene carbonate, boron trifluoride propylene carbonate, and boron trifluoride ethyl acetate.

5. The method for preparing an electrolyte salt according to claim 2, wherein: The acid binding agent is selected from triethylamine or pyridine.

6. The method for preparing an electrolyte salt according to claim 2, wherein: The solvent for the reaction is selected from ester solvents.

7. The method for preparing an electrolyte salt according to claim 6, wherein: The ester solvent is selected from one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, and ethyl acetate.

8. The method for preparing an electrolyte salt according to claim 2, wherein: The molar ratio of the oxalophosphate, boron trifluoride and its complex and the acid binding agent is 1:(2-2.2):0.

1.

9. The method for preparing an electrolyte salt according to claim 2, wherein: The reaction temperature is 25°C to 60°C.

10. The method for preparing an electrolyte salt according to claim 2, wherein: The reaction time is 6 hours to 24 hours.

11. The method for preparing an electrolyte salt according to claim 2, wherein: After the reaction is completed, the process also includes recrystallization; The recrystallization solvent is selected from one or more of dichloromethane, 1,2-dichloroethane, toluene, xylene, petroleum ether, and n-hexane.

12. A secondary battery electrolyte, characterized in that: The electrolyte salt comprises the electrolyte salt according to claim 1 or the electrolyte salt prepared by the preparation method according to any one of claims 2 to 11.

13. The secondary battery electrolyte according to claim 12, characterized in that The solvent of the electrolyte is selected from ethylene carbonate and ethyl methyl carbonate.

14. The secondary battery electrolyte according to claim 13, characterized in that The volume ratio of ethylene carbonate to ethyl methyl carbonate is V1:V2, and V1 and V2 satisfy the following conditions: V1+V2=10 and V1 <V2。 15. A secondary battery, characterized in that: The electrolyte solution is the secondary battery electrolyte solution according to any one of claims 12 to 14.

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