Process for purifying battery grade sodium hexafluorophosphate and use thereof

Abstract

The present application relates to the technical field of sodium hexafluorophosphate, in particular to a refining method of battery-grade sodium hexafluorophosphate and application thereof. The present application overcomes the technical problem of poor product purity in traditional technology. The present application first prepares a mixed gas by reacting phosphorus pentachloride with anhydrous hydrogen fluoride; then, in anhydrous acetonitrile and perfluorobutyl methyl ether, 3A molecular sieves and high-purity sodium fluoride are added, the mixed gas is introduced for reaction, impurities are removed, and hexamethyldisilazane is added, and pure mother liquor is obtained by coupling separation of an alternating magnetic field and modified PTFE membrane; under the protection of inert gas, program cooling and dropwise addition of super-dry diethyl carbonate are adopted, and sweep-frequency ultrasonic waves are synchronously applied for synergistic crystallization; finally, centrifugal washing is performed, and pulse dynamic drying is performed under the protection of mixed protective gas containing phosphorus pentafluoride in a gradient temperature vacuum environment to obtain sodium hexafluorophosphate. Through multi-dimensional process synergy, the present application removes impurities and inhibits thermal decomposition, and finally obtains a high-purity battery-grade finished product.

Description

Technical Field

[0001] This invention relates to the field of sodium hexafluorophosphate technology, specifically to a method for refining battery-grade sodium hexafluorophosphate and its application. Background Technology

[0002] With the explosive growth of the global new energy industry and the increasing scarcity of lithium resources, sodium-ion batteries, due to their abundant resource reserves and significant cost advantages, are becoming a key alternative in large-scale energy storage and low-speed electric vehicles. In the material system of sodium-ion batteries, the electrolyte is the "blood" that determines the battery's high and low temperature performance, cycle life, and safety. Sodium hexafluorophosphate, as the core electrolyte salt with the best overall performance in sodium-ion batteries, directly determines the commercial ceiling of sodium-ion batteries.

[0003] Although the synthesis route of sodium hexafluorophosphate is similar to that of lithium hexafluorophosphate, the larger radius and lower charge density of sodium ions cause sodium hexafluorophosphate to exhibit extremely high water decomposition sensitivity and thermal instability during synthesis and crystallization. Existing conventional recrystallization purification techniques are insufficient to completely remove trace amounts of water, free hydrofluoric acid, and transition metal impurities such as iron, lead, and nickel from the crude product. More critically, solvent molecule encapsulation (solvent entrainment) easily occurs during traditional crystallization, resulting in poor purity of the final product and excessive levels of free acid and insoluble matter. This low-purity sodium hexafluorophosphate severely corrodes the aluminum foil current collector during battery cycling and damages the SEI film at the positive and negative electrode interfaces, causing high battery self-discharge rates and extremely rapid capacity decay, becoming a core pain point restricting the industry's development.

[0004] Therefore, a purification method for battery-grade sodium hexafluorophosphate and its application are proposed. Summary of the Invention

[0005] The purpose of this invention is to design a purification method for battery-grade sodium hexafluorophosphate and its application. The invention first prepares a mixed gas by reacting phosphorus pentachloride with anhydrous hydrogen fluoride; then, in a mixed solvent of anhydrous acetonitrile and perfluorobutyl methyl ether, type 3A molecular sieve and high-purity sodium fluoride are added, and after the mixed gas is introduced for reaction, hexamethyldisilazane is added. The mixture is then separated by coupling with a modified PTFE membrane under an alternating magnetic field to obtain a pure mother liquor; under inert gas protection, a programmed cooling process is used, and ultra-dry diethyl carbonate is added dropwise while simultaneously applying swept-frequency ultrasound for synergistic crystallization; finally, after centrifugal washing, the mixture is pulsed dynamically dried under a gradient-heating vacuum environment by introducing a mixed protective gas containing phosphorus pentafluoride to obtain sodium hexafluorophosphate. This invention, through multi-dimensional process synergy, removes impurities and inhibits thermal decomposition, ultimately obtaining a high-purity battery-grade finished product.

[0006] To achieve the above objectives, the present invention provides the following technical solution: Unless otherwise specified, all the following parts are by weight.

[0007] This invention provides a method for purifying battery-grade sodium hexafluorophosphate, comprising the following steps: 95-105 parts of phosphorus pentachloride were added to a special reactor lined with polytetrafluoroethylene. The temperature inside the reactor was controlled at -20°C. Under sealed conditions and with stirring (150 rpm), 148-152 parts of liquid anhydrous hydrogen fluoride were slowly added dropwise at a rate of 2 parts / min. After the addition was complete, the temperature was raised to 15-25°C at a rate of 3°C / min for a gasification reaction. The pressure inside the reactor was controlled at 0.15 MPa. The reaction was maintained at this temperature and pressure for 2-4 hours until the pressure inside the reactor remained constant and no significant gas was generated, at which point the reaction was considered complete. The generated mixed gas was passed through a condenser at -80°C, and the condensed liquid HF was returned to the reactor. The remaining PF5 and HCl mixed gas was filtered through a sintered silicon carbide filter with a pore size of 0.1 μm to trap the liquid droplets and dust particles, and the resulting mixed gas was reserved for use. By volume, 10% of the total mixed gas was separated and stored in a buffer tank for subsequent drying processes. In a jacketed synthesis reactor, add 300 parts of anhydrous acetonitrile (water content ≤10ppm) and 95-105 parts of perfluorobutyl methyl ether, and start stirring to mix evenly. Then add 15-25 parts of type 3A molecular sieve and pretreat at -10℃ with stirring for 1 hour to remove moisture. After pretreatment, quickly filter out the molecular sieve through the built-in filter at the bottom of the reactor to obtain an ultra-dry mixed solvent. Then add 20 parts of high-purity sodium fluoride, maintain the stirring speed at 300 rpm, and slowly introduce the mixed gas into the synthesis reactor through a gas distributor. The gas introduction process takes 4 hours, and the pressure inside the reactor is controlled not to exceed 0.1 MPa. After the gas introduction is completed and aging for 1-2 hours, first remove the synthesis reactor. The system temperature was raised to 20℃, and high-purity nitrogen was continuously purged for 2 hours to remove the HCl carrier gas from the system and drive it out of the synthesis vessel. Then, 0.5-1.5 parts of hexamethyldisilazane were added to the synthesis vessel, and the reaction was continued with closed stirring for 1 hour until the pressure inside the vessel stabilized and no pressure drop occurred, which was considered as the reaction being complete. After the reaction was completed, the mixed liquid was pressurized to an external precision filtration device under a nitrogen pressure difference of 0.1 MPa. An alternating magnetic field with a frequency of 50 Hz and a magnetic field strength of 0.5 T was applied around the device. Under the action of the magnetic field, the paramagnetic metallic impurities in the liquid agglomerated and then passed through a modified PTFE filter membrane with a pore size of 0.2 μm to obtain NaPF6 mother liquor. Under the protection of high-purity argon gas, the NaPF6 mother liquor was transferred to an ultrasonic crystallizer. Mechanical stirring was started (speed set to 150 rpm), and the cooling program was initiated. The system temperature was lowered from room temperature (20-25℃) to -20℃ at a rate of 0.5℃ / min. Simultaneously, a total of 400 parts of ultra-dry diethyl carbonate (moisture content ≤10ppm, freezing point -43℃) was added dropwise to the crystallizer at a rate of 3 parts / min using a constant flow pump. At the instant the ultra-dry diethyl carbonate was added, the variable frequency ultrasonic generator was simultaneously turned on, with the frequency automatically sweeping between 20kHz and 40kHz. The ultrasonic power was set to 500W, and the ultrasonic treatment was carried out continuously for 1-3 hours. When the crystallizer system cooled to -20℃, the temperature was kept constant until all 400 parts of ultra-dry diethyl carbonate were added. After the addition was completed and the ultrasonic treatment was finished, the crystallizer was stirred and aged at -20℃ for 2-3 hours to form a NaPF6 crystal suspension. The suspension was transported through a closed pipeline to a fully automated closed centrifuge equipped with a cryogenic temperature-controlled jacket. The jacket refrigerant circulation was activated to maintain the centrifuge's internal temperature at -20°C. Under high-purity argon protection, the centrifuge was centrifuged at 2000 rpm for 15 minutes for initial dehydration. After centrifugation, the centrifuge speed was reduced to 400 rpm, and the filter cake was subjected to pulse-type rapid spray washing using 50 parts of ultra-dry diethyl carbonate pre-cooled to -20°C. During the spray washing process, forced heat exchange through the cryogenic jacket controlled the local center temperature of the filter cake layer to not exceed -10°C. After washing, the speed was increased back to 2000 rpm, and centrifugation was continued for 10 minutes to further remove residual impurities and mother liquor from the surface, collecting the wet NaPF6. The wet NaPF6 was then sent to a... The drying oven employs a gradient heating curve, with the heating rate controlled at 2℃ / min in each stage: the first stage is held at 40℃ for 1-3 hours (to remove most of the diethyl carbonate and acetonitrile from the surface); the second stage is held at 80℃ for 3-5 hours (to deeply remove residual solvent from the micropores); throughout the drying process, the drying oven maintains a high vacuum of -0.09MPa, and every 30 minutes, a mixed protective gas (composed of a reserved mixed gas and high-purity argon, with a volume ratio of 1:99) is pulsed into the drying oven to pressurize to -0.02MPa, held for 5 minutes, and then re-evacuated to a high vacuum; after drying, the oven is naturally cooled to room temperature (20-25℃) under vacuum, and discharged under atmospheric pressure high-purity argon to obtain battery-grade sodium hexafluorophosphate.

[0008] Another aspect of the present invention provides an application of battery-grade sodium hexafluorophosphate, which can be used in the preparation of electrolytes for sodium-ion batteries.

[0009] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention, through the synergistic effect of a "polar-passivation" mixed solvent system and a dual-effect chemical impurity remover, removes moisture, free acid, and metal impurities, significantly improving the initial purity of the product. The highly polar anhydrous acetonitrile and the weakly polar perfluorobutyl methyl ether are mixed to efficiently dissolve NaPF6 while simultaneously utilizing the microscopic phase separation properties of fluoroethers to force metal impurities to become free. Combined with the relay dehydration-acid removal reaction of type 3A molecular sieve and hexamethyldisilazane, residual trace amounts of moisture and free acid in the system are completely consumed. Furthermore, the physical synergy of an alternating magnetic field and a modified PTFE filter membrane causes paramagnetic heavy metal impurities to aggregate into large particles for efficient retention, effectively solving the problem of poor purity in traditional processes.

[0010] 2. This invention introduces a crystallization process coupled with acoustic field and anti-solvent, effectively avoiding solvent encapsulation (solvent trapping) that easily occurs during traditional cooling crystallization, thus ensuring the purity of the crystal interior. During programmed cooling and the addition of ultra-dry diethyl carbonate to disrupt the solubility equilibrium, a variable-frequency ultrasonic wave of 20kHz to 40kHz is simultaneously applied. The ultrasonic cavitation effect generates a micro-jets that forcibly break down and reassemble the primary crystal nuclei, forcibly expelling solvent molecules such as acetonitrile trapped within the crystal lattice. Combined with a constant-temperature crystal growth (aging) process at -20℃, the resulting NaPF6 crystals are dense and uniform, overcoming the technical bottleneck of insufficient purity caused by microscopic encapsulation of solvent impurities within the crystal.

[0011] 3. This invention employs a synergistic technology of dynamic reactive vacuum thermodynamics and common ion effect in the drying section to suppress the thermal decomposition of NaPF6 during the high-temperature solvent removal process, ensuring high purity and extremely low insoluble content in the final product. By controlling the temperature gradient increase at 2℃ / min, residual solvent on the surface and in micropores is smoothly removed, and a mixed gas containing the gas produced in the previous section is pulsedly introduced under high vacuum. This operation utilizes the principle of chemical equilibrium shift, suppressing the reverse reaction of NaPF6 decomposition to NaF by forming a high PF5 partial pressure on the crystal surface. This allows the process to dry the residual solvent at a higher temperature, ultimately obtaining high-purity battery-grade sodium hexafluorophosphate free of insoluble impurities. Attached Figure Description

[0012] Figure 1 The purity diagrams are for sodium hexafluorophosphate in Example 1 and Comparative Examples 1-5 of this invention. Detailed Implementation

[0013] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0014] For details, please refer to [link / reference]. Figure 1 This invention provides a method for refining battery-grade sodium hexafluorophosphate and its application. The technical solution is as follows: Example 1 100 parts of phosphorus pentachloride were added to a special reactor lined with polytetrafluoroethylene. The temperature inside the reactor was controlled at -20°C. Under sealed conditions and with stirring (150 rpm), 150 parts of liquid anhydrous hydrogen fluoride were slowly added dropwise at a rate of 2 parts / min. After the addition was complete, the temperature was raised to 20°C at a rate of 3°C / min for a gasification reaction. The pressure inside the reactor was controlled at 0.15 MPa. The reaction was maintained at this temperature and pressure for 3 hours until the pressure inside the reactor remained constant and no significant gas was generated, at which point the reaction was considered complete. The generated mixed gas was passed through a condenser at -80°C, and the condensed liquid HF was returned to the reactor. The remaining PF5 and HCl mixed gas was filtered through a sintered silicon carbide filter with a pore size of 0.1 μm to trap the liquid droplets and dust particles, and the resulting mixed gas was reserved for use. By volume, 10% of the total mixed gas was separated and stored in a buffer tank for subsequent drying processes. In a jacketed synthesis reactor, 300 parts of anhydrous acetonitrile and 100 parts of perfluorobutyl methyl ether were added and stirred until homogeneous. Then, 20 parts of type 3A molecular sieve were added, and the mixture was pretreated at -10℃ with stirring for 1 hour to remove moisture. After pretreatment, the molecular sieve was rapidly filtered out through a filter screen at the bottom of the reactor to obtain an ultra-dry mixed solvent. Then, 20 parts of high-purity sodium fluoride were added, and the stirring speed was maintained at 300 rpm. The mixed gas was slowly introduced into the synthesis reactor through a gas distributor over a period of 4 hours, with the pressure inside the reactor controlled to not exceed 0.1 MPa. After the gasification was completed and the mixture was aged for 1.5 hours, the temperature of the synthesis reactor system was first raised to... At 20℃, high-purity nitrogen was continuously purged for 2 hours to remove the HCl carrier gas from the system and drive it out of the synthesis vessel. Then, 1 part of hexamethyldisilazane was added to the synthesis vessel, and the reaction was continued with closed stirring for 1 hour until the pressure inside the vessel stabilized and no pressure drop occurred, which was considered as the reaction being complete. After the reaction was completed, the mixed liquid was pressurized to an external precision filtration device under a nitrogen pressure difference of 0.1 MPa. An alternating magnetic field with a frequency of 50 Hz and a magnetic field strength of 0.5 T was applied around the device. Under the action of the magnetic field, the paramagnetic metallic impurities in the liquid agglomerated and then passed through a modified PTFE filter membrane with a pore size of 0.2 μm to obtain NaPF6 mother liquor. Under the protection of high-purity argon gas, the NaPF6 mother liquor was transferred to an ultrasonic crystallizer. Mechanical stirring was started (speed set to 150 rpm) and the cooling program was initiated, reducing the system temperature from room temperature (20-25℃) to -20℃ at a rate of 0.5℃ / min. Simultaneously, a total of 400 parts of ultra-dry diethyl carbonate was added dropwise to the crystallizer at a rate of 3 parts / min using a constant flow pump. At the instant the ultra-dry diethyl carbonate was added, a variable frequency ultrasonic generator was simultaneously turned on, with the frequency automatically sweeping between 20kHz and 40kHz. The ultrasonic power was set to 500W, and the ultrasonic treatment was carried out continuously for 2 hours. When the crystallizer system cooled to -20℃, the temperature was kept constant until all 400 parts of ultra-dry diethyl carbonate were added. After the addition was completed and the ultrasonic treatment was finished, the crystallizer was stirred and aged at -20℃ for 2.5 hours to form a NaPF6 crystal suspension. The suspension was transported through a closed pipeline to a fully automated closed centrifuge equipped with a cryogenic temperature-controlled jacket. The jacket refrigerant circulation was activated to maintain the centrifuge's internal temperature at -20°C. Under high-purity argon protection, the centrifuge was centrifuged at 2000 rpm for 15 minutes for initial dehydration. After centrifugation, the centrifuge speed was reduced to 400 rpm, and the filter cake was subjected to pulse-type rapid spray washing using 50 parts of ultra-dry diethyl carbonate pre-cooled to -20°C. During the spray washing process, forced heat exchange through the cryogenic jacket controlled the local center temperature of the filter cake layer to not exceed -10°C. After washing, the speed was increased back to 2000 rpm, and centrifugation was continued for 10 minutes to further remove residual impurities and mother liquor from the surface. The wet product was then collected. NaPF6; Wet NaPF6 was placed in a dynamic vacuum drying oven, which adopted a gradient heating curve, and the heating rate of each stage was controlled at 2℃ / min: the first stage was held at 40℃ for 2h; the second stage was held at 80℃ for 4h; throughout the drying process, the drying oven maintained a high vacuum of -0.09MPa. Every 30min, a mixed protective gas (a reserved mixed gas + high-purity argon, with a volume ratio of 1:99) was pulsed into the drying oven to pressurize to -0.02MPa, held for 5min, and then re-evacuated to a high vacuum; after drying, it was naturally cooled to room temperature (20-25℃) under vacuum, and discharged under normal pressure high-purity argon to obtain battery-grade sodium hexafluorophosphate.

[0015] Examples 2-5 refer to the parameter conditions in Example 1, with specific differences shown in Table 1.

[0016]

[0017] Comparative Example 1: The parameters and conditions in Example 1 were the same, except that perfluorobutyl methyl ether was not added and the amount of anhydrous acetonitrile, the main solvent, was increased to 400 parts.

[0018] Comparative Example 2 follows the same parameters and conditions as in Example 1, except that type 3A molecular sieve and hexamethyldisilazane are not added.

[0019] Comparative Example 3 uses the same parameters as in Example 1, except that in the precision filtration stage, the external alternating magnetic field is turned off, and the feed liquid is passed through the 0.2μm modified PTFE filter membrane by relying solely on the nitrogen pressure difference of 0.1MPa.

[0020] Comparative Example 4 follows the same parameters and conditions as in Example 1, except that the ultrasonic generator is not turned on, and conventional dripping and cooling crystallization is carried out by mechanical stirring at 150 rpm.

[0021] Comparative Example 5 follows the same parameters and conditions as in Example 1, except that during the drying stage, the protective gas pulsed into the drying chamber every 30 minutes is replaced with 100% high-purity argon.

[0022] Experiment Example 1: Purity Test According to GB / T 6283-2008, the purity of Examples 1-5 and Comparative Examples 1-5 was tested using ion chromatography and inductively coupled plasma atomic emission spectrometry. The results are shown in Tables 2-3 (referencing T / CAAMTB 188—2024 standard). The purity of sodium hexafluorophosphate in Examples 1 and Comparative Examples 1-5 is as follows: Figure 1 As shown.

[0023]

[0024] As shown in Table 3 and Figure 1As shown, in Comparative Example 1, since no perfluorobutyl methyl ether was added, only highly polar anhydrous acetonitrile existed in the system. This caused the transition metal impurities to lose the microscopic phase separation and encapsulation protection provided by the fluoroether, and they completely dissolved in the main solvent. The test results showed that the content of metal impurities such as iron (Fe), aluminum (Al), and nickel (Ni) increased sharply, with the iron ion content reaching 8.5 mg / kg, and aluminum and nickel both exceeding the standard red line of 1 mg / kg. This proves the key role of the mixed solvent system in inhibiting the dissolution of metal impurities and improving the initial purity. Comparative Example 2 removed type 3A molecular sieve and hexamethyldisilazane. The data showed that conventional physical dehydration and protective gas alone could not completely remove trace water and free acid in the reaction system. Its water content soared to 38 mg / kg, and the free acid (calculated as HF) reached as high as 145 mg / kg, which seriously exceeded the upper limit requirement of the battery grade standard. The high content of water and acid also caused slight hydrolysis of the product, resulting in the overall purity dropping to 99.85%. Comparative Example 3 had the alternating magnetic field turned off during the filtration stage. The results showed that, in the absence of a magnetic field to cause paramagnetic impurities to agglomerate, a large number of nanoscale heavy metal particles (such as iron and nickel) easily penetrated the 0.2 μm filter membrane pores. The iron ion content rose to 4.8 mg / kg, and the nickel ion content exceeded the standard by 1.2 mg / kg. This indicates that relying solely on physical interception by the filter membrane cannot achieve deep removal of metal impurities; the introduction of a magnetic field significantly improved filtration accuracy. Comparative Example 4 did not turn on the ultrasonic generator during the crystallization drop-addition stage. Tests showed that although the levels of conventional metallic and non-metallic impurities were not severely exceeded, the purity of its main component, sodium hexafluorophosphate, declined significantly (to only 99.75%). This was because during the cooling crystallization process with pure mechanical stirring, the crystals grew too quickly and generated numerous lattice defects, resulting in a large number of solvent molecules (acetonitrile and diethyl carbonate) being tightly encapsulated inside the crystals. Conventional testing could not directly identify these encapsulated solvents, but they accounted for a significant portion of the mass, causing the overall purity to fail to meet the standard. This confirms the advantage of ultrasonic cavitation in eliminating solvent entrainment. Comparative Example 5 used 100% high-purity argon gas during the high-temperature vacuum drying stage, eliminating the pulse protection of phosphorus pentafluoride gas. Data showed that the dimethyl carbonate insoluble content index increased significantly, soaring to 1500 mg / kg, far exceeding the national standard. The core reason for this was the lack of the common ion suppression effect of the external phosphorus pentafluoride partial pressure. Sodium hexafluorophosphate underwent significant irreversible thermal decomposition during the high-temperature desolventization process at 80℃, generating a large amount of sodium fluoride solid impurities. This data strongly demonstrates that the pulse protection gas process makes a significant and substantial contribution to ensuring the thermal stability of the product.

[0025] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for purifying battery-grade sodium hexafluorophosphate, characterized in that, Includes the following steps: A mixed gas was synthesized and purified using phosphorus pentachloride and anhydrous hydrogen fluoride as raw materials. The mixed gas was then passed into a system containing sodium fluoride, a mixed solvent, and a type 3A molecular sieve, and hexamethyldisilazane was added to carry out a synthesis reaction. After deep filtration to remove impurities, NaPF6 mother liquor was obtained. The NaPF6 mother liquor was subjected to ultrasonic-assisted cooling and the introduction of an antisolvent to complete the crystallization, resulting in a NaPF6 crystal suspension. The NaPF6 crystal suspension was centrifuged and washed, and then subjected to two-stage drying to obtain the battery-grade sodium hexafluorophosphate.

2. The method for refining battery-grade sodium hexafluorophosphate according to claim 1, characterized in that, The preparation method of the phosphorus pentafluoride gas is as follows: phosphorus pentachloride is added into a reaction vessel, the temperature inside the vessel is controlled at -20℃, and anhydrous hydrogen fluoride is added dropwise at a rate of 2 parts / min under closed stirring conditions. After the addition is completed, the temperature is raised to 15-25℃ at a rate of 3℃ / min for reaction, and the reaction is maintained at this temperature for 2-4 hours. The generated mixed gas is refluxed back into the reaction vessel through a condenser tower. The remaining gas is filtered through a sintered silicon carbide filter element to obtain the mixed gas.

3. The purification method for battery-grade sodium hexafluorophosphate according to claim 1, characterized in that, The preparation method of the NaPF6 mother liquor is as follows: anhydrous acetonitrile and perfluorobutyl methyl ether are added to a synthesis reactor, mixed evenly, and then the 3A type molecular sieve is added. After treatment and filtration, the sodium fluoride is added, and the mixed gas is introduced into the synthesis reactor. After sealing and stirring for 1-2 hours, nitrogen gas is introduced for purging. Then, the hexamethyldisilazane is added. After the reaction is completed by sealing and stirring, the mixture is hydraulically sent to a filtration device. An alternating magnetic field is applied around the device, and then the mixture passes through a modified PTFE filter membrane to obtain the NaPF6 mother liquor. The mixed solvent is composed of anhydrous acetonitrile and perfluorobutyl methyl ether.

4. The purification method for battery-grade sodium hexafluorophosphate according to claim 1, characterized in that, The preparation method of the NaPF6 crystal suspension is as follows: the NaPF6 mother liquor is transferred to an ultrasonic crystallization vessel under argon protection, and the system temperature is lowered from room temperature to -20℃ at a rate of 0.5℃ / min. Ultra-dry diethyl carbonate is added dropwise to the crystallization vessel by a constant flow pump. At the same time, a variable frequency ultrasonic generator is turned on, and the frequency is automatically swept between 20kHz and 40kHz. The ultrasonic treatment is carried out continuously for 1-3 hours. After the dropwise addition is completed and the ultrasonic treatment is ended, stirring is continued for 2-3 hours to grow crystals, thus forming the NaPF6 crystal suspension.

5. The method for refining battery-grade sodium hexafluorophosphate according to claim 1, characterized in that, The preparation method of the battery-grade sodium hexafluorophosphate is as follows: the NaPF6 crystal suspension is transported to a centrifuge through a closed pipeline, maintained at -20°C under argon protection, and centrifuged. After the filter cake is washed with ultra-dry diethyl carbonate pre-cooled to -20°C, centrifugation is continued to collect wet NaPF6. The wet NaPF6 is sent to a vacuum drying oven: the first stage is kept at 40°C for 1-3 hours; the second stage is kept at 80°C for 3-5 hours. During the entire drying process, a mixed protective gas is pulsed into the drying oven every 30 minutes. After drying, the product is naturally cooled to room temperature under vacuum and discharged under atmospheric pressure argon to obtain the battery-grade sodium hexafluorophosphate.

6. An application of battery-grade sodium hexafluorophosphate, characterized in that, The battery-grade sodium hexafluorophosphate can be used in the preparation of electrolytes for sodium-ion batteries; the battery-grade sodium hexafluorophosphate is prepared by the purification method described in any one of claims 1-5.

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