A method for purifying a lithium hexafluorophosphate solution

Abstract

The application belongs to the technical field of lithium battery electrolyte, and particularly relates to a lithium hexafluorophosphate solution purification method. Under the precondition of nitrogen pre-purification, perfluoro-cycloalkane is used to deeply remove water, and then trimethylsilane is used to deeply remove acid, so that the water content and the acidity of the lithium hexafluorophosphate solution are reduced to below 2 ppm, and no impurities are left, which can not only significantly improve the instant quality of the lithium hexafluorophosphate solution, but also improve the storage stability of the lithium hexafluorophosphate solution. The innovative method breaks the conventional cognition that perfluoroalkane is not suitable for water removal operation due to hydrophobicity, and unexpected technical effects are achieved.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery electrolyte technology, and particularly relates to a method for purifying lithium hexafluorophosphate solution. Background Technology

[0002] With the increasing maturity of domestic lithium hexafluorophosphate (LiPF6) production technology, product quality and supply have reached a certain level, leading to increasingly fierce competition in the industry. Reducing production costs has thus become a key focus for manufacturers. Compared to solid LiPF6, liquid LiPF6, specifically a carbonate solution of LiPF6, offers advantages such as no need for crystallization drying, stable storage, and ease of use, while also being less expensive. However, LiPF6 solutions are prone to acidity increases during storage, leading to decreased performance and even rendering them unusable. The root cause is the inability to completely remove moisture. During storage, trace amounts of moisture cause the slow decomposition of LiPF6, releasing hydrofluoric acid, which increases acidity. The reaction is LiPF6 + H2O → LiF + POF3 + 2HF. Furthermore, the generated hydrogen fluoride catalyzes further decomposition of LiPF6, creating a vicious cycle that leads to continuous degradation of the LiPF6 solution's performance.

[0003] There are two main types of water removal methods commonly used in existing technologies: one is physical methods, such as molecular sieves, resin adsorption, and nitrogen purging. These conventional methods are prone to adsorption saturation problems, have high regeneration costs, and low nitrogen purging efficiency. Moreover, water removal can generally only reach about 20 ppm, and a few can reach less than 10 ppm, which cannot meet the needs of deep water removal. The other is chemical methods, which use additives containing Si-O or Si-N structures, sulfonic anhydride additives, and phosphoramide compounds to capture water, HF, or form a stable complex with hexafluorophosphate ions to alleviate acidity rise and performance degradation. However, these additives and their reaction products will remain in the solution and form impurities. Summary of the Invention

[0004] The purpose of this invention is to provide a method for purifying lithium hexafluorophosphate solution.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for purifying lithium hexafluorophosphate solution involves first pre-purifying the lithium hexafluorophosphate solution, then bubbling perfluorocycloalkane gas into the lithium hexafluorophosphate solution to remove water, and finally removing acid.

[0006] Furthermore, the perfluoroalkane gas has 3-5 carbon atoms.

[0007] Specifically, the perfluorocycloalkanes are selected from one or more of perfluorocyclopropane, perfluorocyclobutane, and perfluorocyclopentane.

[0008] Furthermore, the solvent of the lithium hexafluorophosphate solution is a carbonate with a mass fraction of 20-40%, and the carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate, and fluoroethylene carbonate.

[0009] Furthermore, the pre-purification involves bubbling nitrogen gas into the lithium hexafluorophosphate solution before introducing perfluorocycloalkane gas; the acid removal involves bubbling a mixture of trimethylsilane and nitrogen gas.

[0010] Furthermore, the flow rate of nitrogen gas is 35-40 mL / min, the flow rate of perfluorocycloalkanes gas is 18-22 mL / min, and the flow rate of the mixed gas of trimethylsilane and nitrogen is 8-12 mL / min.

[0011] Furthermore, the system temperature is controlled at 10~35℃ when the gas is introduced, the stirring speed is 150~200rpm, and the gas introduction time for each gas is 10~30min.

[0012] Furthermore, the volume ratio of the trimethylsilane to nitrogen is 1:(5-20).

[0013] Furthermore, the purity of the nitrogen, perfluorocycloalkanes, and trimethylsilane is ≥99.99%, and the moisture content is ≤0.5ppm.

[0014] The advantages of this invention are as follows: Under the premise of nitrogen pre-purification, this application uses perfluorocycloalkanes for deep dehydration first, and then trimethylsilane for deep deacidification. The coordinated action of the two processes reduces the water content and acidity of the lithium hexafluorophosphate solution to below 2 ppm without leaving any impurities. This not only significantly improves the immediate quality of the lithium hexafluorophosphate solution, but also improves its storage stability. This innovative method breaks the conventional understanding that perfluoroalkanes are unsuitable for dehydration operations due to their hydrophobicity, and achieves unexpected technical results.

[0015] It is worth noting that when the water content of the lithium hexafluorophosphate solution is very low, the water is not in a free state, but in a dissolved state formed with weak hydrogen bonds with carbonate. Nitrogen gas is a nonpolar molecule and has no selectivity for carbonate and water. When bubbling, it can only remove a very small amount of free water that has not formed hydrogen bonds on the solution surface, and cannot break the hydrogen bonds between carbonate and H2O. The dissolved water cannot move at all, so the water removal is incomplete. On the other hand, perfluorocycloalkanes have a ring structure with strong molecular rigidity and dense and uniform fluorine atoms. When bubbles pass through the solution, they preferentially displace the weakly polar carbonate molecules, forming a continuous ester-repelling layer on the bubble surface. After the carbonate molecules are pushed away, the water that was originally bound to the carbonate loses its hydrogen bond binding and instantly gathers on the bubble surface (water is strongly polar and has self-polymerization properties), escaping with the bubble. This can remove more than 90% of the dissolved water. In short, perfluorocycloalkanes do not fail to remove water due to their hydrophobicity. On the contrary, because their ester-repelling property is stronger than their hydrophobicity, they facilitate the separation of water molecules from carbonate, achieving the purpose of deep water removal.

[0016] Furthermore, perfluorocycloalkanes have low intermolecular forces and low surface tension, making them easy to form fine bubbles of 50-100 μm. The bubble surface provides ample sites for water molecule enrichment. The ring structure also allows the bubbles to be subjected to uniform forces, making them less prone to breakage and merging, and allowing them to remain in the solution for a longer time. Compared with nitrogen bubbles, which are prone to merging and escaping, perfluorocycloalkanes have higher water removal efficiency. Detailed Implementation

[0017] A method for purifying lithium hexafluorophosphate solution involves bubbling perfluoroalkane gas into the lithium hexafluorophosphate solution to obtain a lithium hexafluorophosphate solution with extremely low water content. The acidity of this solution does not increase or only increases slightly after long-term storage.

[0018] One method for obtaining the lithium hexafluorophosphate solution involves pre-dispersing lithium fluoride in a carbonate solvent, then introducing phosphorus pentafluoride gas. The lithium hexafluorophosphate produced by the reaction of lithium fluoride and phosphorus pentafluoride dissolves in the carbonate solvent. The insoluble matter is then filtered off to obtain the lithium hexafluorophosphate solution. The carbonate used is generally a commonly used solvent in lithium-ion battery electrolytes, such as dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate, and fluoroethylene carbonate. One type can be used alone, or two or more types can be used in combination. The resulting lithium hexafluorophosphate solution typically has a mass concentration of 20-40%.

[0019] The preferred perfluorocycloalkanes are low-carbon perfluorocycloalkanes that are gaseous at room temperature, with 3-5 carbon atoms being the most desirable. Specifically, perfluorocyclopropane, perfluorocyclobutane, and perfluorocyclopentane are examples. These ring-shaped molecular structures are rigid, densely coated with fluorine atoms, and exhibit strong nonpolarity and chemical inertness, failing to react with lithium hexafluorophosphate or carbonates. They also possess low surface tension and hydrophobic ester repellency, enabling water removal through a combination of bubble surface enrichment and physical removal. Perfluorocyclobutane is the most preferred choice, with its high four-membered ring strain, strong molecular rigidity, dense fluorine atom layer, and gaseous state at room temperature, making it highly suitable for electrolyte purging and water removal. In contrast, perfluorocyclopropane has excessively high three-membered ring strain, resulting in molecular instability and potential decomposition; perfluorocyclopentane has relatively low five-membered ring strain, slightly weaker molecular rigidity, decreased uniformity of fluorine atom orientation on the bubble surface, insufficient fluorine atom layer density, and consequently reduced bubble ester repellency. Furthermore, its boiling point is relatively high, leading to slightly poorer overall compatibility.

[0020] Furthermore, it is not recommended to use non-cyclic perfluoroalkanes, such as straight-chain perfluorobutane, which lacks cyclic structural support, has insufficient molecular rigidity, uneven fluorine atom arrangement, poor fluorine layer density, and a water removal limit of only 3-4 ppm; branched perfluoroneopterane, affected by steric hindrance of the branches, has a disordered molecular arrangement and a loose fluorine layer with gaps, with a water removal limit of 6-8 ppm. Neither of these can meet the deep water removal requirement of ≤2 ppm.

[0021] Furthermore, the use of perfluoroolefins, such as hexafluoropropylene, is also not recommended. It contains unsaturated C=C double bonds, and its chemical stability is much lower than that of saturated perfluorocycloalkanes. When it is introduced into a lithium hexafluorophosphate solution, the double bonds are prone to breakage, resulting in addition or polymerization reactions and the formation of impurities. Moreover, its fluorine atom layer is incomplete due to the presence of double bonds, resulting in low ester rejection and hydrogen bond breaking ability, and insufficient water removal depth.

[0022] Unfortunately, perfluorocycloalkanes cannot simultaneously remove HF during dehydration because HF in lithium hexafluorophosphate solution mainly exists as free H+. + and F - In its existing form, the fluorine layer of perfluorocycloalkanes cannot capture H. + or F - The removal of HF is almost zero because only a very small amount of HF molecules can be physically carried away by bubbles. A special HF removal step is required afterward.

[0023] Generally, the initial product of lithium hexafluorophosphate solution has an acid and water content of 80-200 ppm. If perfluorocycloalkanes are used directly for water removal, it will increase the amount of perfluorocycloalkanes used and the cost. A better option is to bubble nitrogen gas before bubbling in perfluorocycloalkanes to reduce the acid and water content to about 20 ppm, and then use perfluorocycloalkanes for deep water removal to below 2 ppm.

[0024] After thorough dehydration, further deep acid removal using trimethylsilane can reduce the HF content to below 2 ppm. Specifically, after bubbling through perfluorocycloalkanes, a mixture of trimethylsilane and nitrogen is bubbled through, with a volume ratio of trimethylsilane to nitrogen of 1:(5-20). It is crucial to emphasize that this "dehydration first, then acid removal" approach is essential to achieve this effect; otherwise, incomplete dehydration will result in new acid formation after acid removal, making deep acid removal impossible.

[0025] Trimethylsilane reacts very sensitively with HF in lithium hexafluorophosphate solution. Its Si-H bond exhibits excellent reactivity, allowing for irreversible substitution with HF. Even at HF ​​concentrations as low as 1 ppm, it can rapidly and accurately capture and completely react with the HF, exhibiting a fast reaction rate without hysteresis. Furthermore, trimethylsilane does not react with the main components of the solution, such as lithium hexafluorophosphate or carbonates, resulting in no ineffective consumption. The reaction products, trimethylfluorosilane and hydrogen, are both gaseous and can be removed along with nitrogen, leaving no impurities. The reaction equation is (CH3)3SiH + HF → (CH3)3SiF + H2.

[0026] More specifically, the preferred flow rate of nitrogen is 35-40 mL / min, the preferred flow rate of perfluorocycloalkanes is 18-22 mL / min, the preferred flow rate of the mixed gas of trimethylsilane and nitrogen is 8-12 mL / min, the temperature is controlled at 10-25℃ during gas flow, the preferred stirring speed is 150-200 rpm, and the preferred gas flow time for each gas is 10-30 min. These parameters are selected to balance the acid and water removal effect with the production cost and achieve the best overall benefits.

[0027] In the following examples, the nitrogen used had a purity ≥99.999% and a moisture content ≤0.5ppm; the perfluorocycloalkanes used had a purity ≥99.999% and a moisture content ≤0.1ppm; the trimethylsilane used had a purity ≥99.99% and a moisture content ≤0.3ppm; and chlorine impurities ≤0.1ppm. The reaction was carried out in a closed reactor, with aeration via microporous aeration at the bottom of the reactor (pore size 5-10μm). The temperature was controlled at 10-35℃, the stirring speed at 150-200rpm, and a slight positive pressure of 0.1-0.11MPa maintained throughout the process. The oxygen content in the atmosphere was ≤3ppm. In the product testing, acidity was measured using hydrofluoric acid via potentiometric titration, and moisture content was measured using the Karl Fischer method.

[0028] Example 1 A method for purifying lithium hexafluorophosphate solution: Take 2000g of lithium hexafluorophosphate solution with a mass fraction of 30%, using dimethyl carbonate as the solvent, and starting with an initial water content of 95ppm and an initial HF content of 76ppm.

[0029] Step 1, nitrogen pre-dehydration and acid removal: introduce electronic-grade dry nitrogen at a flow rate of 38 mL / min, a temperature of 22℃, stir at 180 rpm, and aeration time of 22 min; the endpoint test showed that the water content was 19 ppm and the HF content was 18 ppm.

[0030] Step 2, deep dehydration with perfluorocyclobutane: Electronic grade perfluorocyclobutane is introduced at a flow rate of 20 mL / min, a temperature of 22℃, and stirred at 180 rpm for 25 min. The water content at the endpoint is 0.8 ppm.

[0031] Step 3, deep acid removal with trimethylsilane + nitrogen mixture: a mixture of trimethylsilane and nitrogen is introduced at a flow rate of 10 mL / min, a volume ratio of trimethylsilane to nitrogen of 1:10, a temperature of 22℃, stirring at 180 rpm, and a gas introduction time of 28 min; the endpoint detection shows that the HF content is 1.5 ppm.

[0032] Finished product specifications: water content 0.8ppm, HF content 1.5ppm, solution is clear and transparent.

[0033] Example 2 A method for purifying lithium hexafluorophosphate solution: Take 2000g of lithium hexafluorophosphate solution with a mass fraction of 20%, the solvent is ethylene carbonate + propylene carbonate (volume ratio 1:1), the initial water content is 88ppm, and the initial HF content is 72ppm.

[0034] Step 1, nitrogen pre-dehydration and acid removal: introduce electronic-grade dry nitrogen at a flow rate of 35 mL / min, a temperature of 15℃, stir at 150 rpm, and aeration time of 30 min; the endpoint test showed that the water content was 18 ppm and the HF content was 17 ppm.

[0035] Step 2, deep dehydration with perfluorocyclobutane: Electronic grade perfluorocyclobutane is introduced at a flow rate of 18 mL / min, a temperature of 15℃, stirring at 150 rpm, and aeration time of 30 min. The water content at the endpoint is 0.9 ppm.

[0036] Step 3, deep acid removal with trimethylsilane + nitrogen mixture: a mixture of trimethylsilane and nitrogen is introduced at a flow rate of 8 mL / min, a volume ratio of trimethylsilane to nitrogen of 1:5, a temperature of 15℃, stirring at 150 rpm, and a gas introduction time of 30 min; the endpoint detection showed that the HF content was 1.3 ppm.

[0037] Finished product specifications: water content 0.9ppm, HF content 1.3ppm, solution is clear and transparent.

[0038] Example 3 A method for purifying lithium hexafluorophosphate solution: Take 2000g of lithium hexafluorophosphate solution with a mass fraction of 40%, the solvent is methyl ethyl carbonate + fluoroethylene carbonate (volume ratio 9:1), the initial water content is 100ppm, and the initial HF content is 80ppm.

[0039] Step 1, nitrogen pre-dehydration and acid removal: introduce electronic-grade dry nitrogen at a flow rate of 40 mL / min, a temperature of 25℃, stir at 200 rpm, and aeration time of 18 min. The endpoint test showed that the water content was 20 ppm and the HF content was 19 ppm.

[0040] Step 2, deep dehydration with perfluorocyclobutane: Electronic grade perfluorocyclobutane is introduced at a flow rate of 22 mL / min, a temperature of 25℃, stirring at 200 rpm, and aeration time of 20 min. The water content at the endpoint is 1.1 ppm.

[0041] Step 3, deep acid removal with trimethylsilane + nitrogen mixture: a mixture of trimethylsilane and nitrogen is introduced at a flow rate of 12 mL / min, a volume ratio of trimethylsilane to nitrogen of 1:20, a temperature of 25℃, stirring at 200 rpm, and a gas introduction time of 25 min; the endpoint detection showed that the HF content was 1.8 ppm.

[0042] Finished product specifications: water content 1.1ppm, HF content 1.8ppm, solution is clear and transparent.

[0043] Example 4 A method for purifying lithium hexafluorophosphate solution, which differs from Example 1 in that perfluorocyclobutane is replaced with perfluorocyclopropane.

[0044] Take 2000g of lithium hexafluorophosphate solution with a mass fraction of 30%, using dimethyl carbonate as the solvent, and starting with 95ppm water and 76ppm HF.

[0045] Step 1, nitrogen pre-dehydration and acid removal: nitrogen flow rate 38 mL / min, 22℃, 180 rpm, ventilation for 22 min; endpoint detection: water 19 ppm, HF 18 ppm.

[0046] Step 2, deep dehydration with perfluorocyclopropane: Electronic grade perfluorocyclopropane is introduced at a flow rate of 20 mL / min, at 22℃ and 180 rpm, and aeration is carried out for 25 min; endpoint detection: water content 2.3 ppm.

[0047] Step 3, deep acid removal with trimethylsilane + nitrogen mixture: gas flow rate of 10 mL / min, volume ratio of 1:10, 22℃, 180 rpm, 28 min; endpoint detection showed HF content of 1.6 ppm.

[0048] Finished product specifications: water content 2.3ppm, HF content 1.6ppm, solution is clear.

[0049] Example 5 A method for purifying lithium hexafluorophosphate solution, which differs from Example 1 in that perfluorocyclobutane is replaced with perfluorocyclopentane.

[0050] Lithium hexafluorophosphate solution parameters: mass to be purified 2.0 kg, mass fraction 30%, solvent dimethyl carbonate, initial water content 95 ppm, initial HF content 76 ppm.

[0051] Step 1, nitrogen pre-dehydration and acid removal: nitrogen flow rate 38 mL / min, 35℃, 180 rpm, aeration for 22 min; endpoint test: water 19 ppm, HF 18 ppm.

[0052] Step 2, deep dehydration of perfluorocyclopentane: electronic grade perfluorocyclopentane is first vaporized (35℃), and then lithium hexafluorophosphate solution is introduced at a flow rate of 20 mL / min, 22℃, 180 rpm, for 25 min; the water content at the endpoint is 3.5 ppm.

[0053] Step 3, deep acid removal with trimethylsilane + nitrogen mixture: gas flow rate of mixture 10 mL / min, volume ratio 1:10, 35℃, 180 rpm, gas flow for 28 min; endpoint detection HF content 1.7 ppm.

[0054] Finished product specifications: water content 3.5ppm, HF content 1.7ppm, electrolyte clear.

[0055] Comparative Example 1 A method for purifying lithium hexafluorophosphate solution, based on Example 1, eliminates perfluorocyclobutane.

[0056] Take 2000g of lithium hexafluorophosphate solution (30% by mass), dimethyl carbonate as solvent, with an initial water content of 95ppm and an initial HF content of 76ppm.

[0057] Step 1, nitrogen pre-dehydration and acid removal: nitrogen flow rate 38 mL / min, 22℃, 180 rpm, aeration for 22 min; endpoint test: water 19 ppm, HF 18 ppm.

[0058] Step 2, deep acid removal with trimethylsilane + nitrogen mixture: gas flow rate of mixture 10 mL / min, volume ratio 1:10, 22℃, 180 rpm, 28 min.

[0059] Test results: Water content 18.5 ppm, HF content 5.2 ppm, electrolyte slightly turbid.

[0060] Comparative conclusion: Without removing perfluorocyclobutane for dehydration, the water in the system was not removed, trimethylsilane reacted with water to introduce impurities, and water continued to hydrolyze to produce HF, making it impossible to achieve deep impurity removal.

[0061] Comparative Example 2 A method for purifying lithium hexafluorophosphate solution is basically the same as that in Example 1, except that perfluorocyclobutane is replaced with perfluoron-butane.

[0062] The test results of the obtained product were: water content 3.8ppm, HF content 1.6ppm, and electrolyte clear.

[0063] Comparative conclusion: Straight-chain perfluorobutane has no cyclic rigid structure, the fluorine layer is unevenly distributed, and the water removal capacity is insufficient, failing to meet the deep water removal requirement of ≤2ppm of this invention.

[0064] Comparative Example 3 A method for purifying lithium hexafluorophosphate solution is basically the same as that in Example 1, except that perfluorocyclobutane is replaced with perfluoroneopentane.

[0065] The test results of the obtained product were: water content 7.2 ppm, HF content 2.1 ppm, and clear electrolyte.

[0066] Comparative conclusion: Branched perfluoroneopteranes are affected by steric hindrance of the branches, resulting in a loose fluorine layer and poor water removal effect.

[0067] Comparative Example 4 A method for purifying lithium hexafluorophosphate solution is basically the same as that in Example 1, except that perfluorocyclobutane is replaced with perfluoropropylene.

[0068] The test results of the obtained product were: water content 9.5ppm, HF content 4.8ppm, and electrolyte slightly turbid.

[0069] Comparative conclusion: Hexafluoropropylene is a perfluoroolefin containing unsaturated C=C double bonds. Its structure is unstable and it reacts to introduce impurities. Its incomplete fluorine layer results in poor water removal ability.

[0070] Comparative Example 5 A method for purifying lithium hexafluorophosphate solution combines steps 2 and 3 into one step, based on Example 1.

[0071] Take 2000g of lithium hexafluorophosphate solution, with a mass fraction of 30%, using dimethyl carbonate as the solvent, and an initial water content of 95ppm and an initial HF content of 76ppm.

[0072] Step 1, nitrogen pre-dehydration and acid removal: nitrogen flow rate 38 mL / min, 22℃, 180 rpm, ventilation for 22 min; endpoint detection: water 19 ppm, HF 18 ppm.

[0073] Step 2: A mixture of perfluorocyclobutane, trimethylsilane, and nitrogen is introduced: the flow rate of perfluorocyclobutane is 20 mL / min, the volume ratio of trimethylsilane to nitrogen is 1:10, the flow rate of the mixture is 10 mL / min, and the mixture is introduced at 22°C and 180 rpm for 25 min. Test results: Water content 5.6 ppm, HF content 3.9 ppm, electrolyte clear.

[0074] Comparative conclusions: When perfluorocyclobutane and trimethylsilane are introduced simultaneously, the surface of the perfluorocyclobutane bubbles is covered by trimethylsilane, which inhibits the ester-repellent hydrogen bond breaking effect and results in incomplete water removal; the system does not remove water first, leading to the hydrolysis of trimethylsilane and the continuous production of HF from water, resulting in incomplete acid removal.

[0075] Verification Example The purified products from the above examples and comparative examples were subjected to storage tests. The method was to store them in a sealed environment at a constant temperature of 45°C for 7 days, and then test the acidity. The results are summarized in the table below.

[0076] The data comparison in the table shows that Examples 1-3 used perfluorocyclobutane, which removed water to below 1.1 ppm, and the acidity increased by only 0.2 ppm after high-temperature storage. Examples 4-5 used other cycloalkanes, which had a slightly worse water removal effect, and the acidity increased slightly more than that of Examples 1-3 after high-temperature storage. Comparative Examples 1-5 used non-preferred solutions, and the acidity increased significantly.

Claims

1. A method for purifying lithium hexafluorophosphate solution, characterized in that: First, the lithium hexafluorophosphate solution is pre-purified, then perfluorocycloalkane gas is bubbled into the lithium hexafluorophosphate solution to remove water, and then acid is removed.

2. The method for purifying lithium hexafluorophosphate solution as described in claim 1, characterized in that: The perfluoroalkane gas has 3-5 carbon atoms.

3. The method for purifying lithium hexafluorophosphate solution as described in claim 1, characterized in that: The perfluorocycloalkanes are selected from one or more of perfluorocyclopropane, perfluorocyclobutane, and perfluorocyclopentane.

4. The method for purifying lithium hexafluorophosphate solution as described in claim 1, characterized in that: The solvent of the lithium hexafluorophosphate solution is a carbonate with a mass fraction of 20-40%, and the carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate, and fluoroethylene carbonate.

5. The method for purifying lithium hexafluorophosphate solution as described in claim 1, characterized in that: The pre-purification involves bubbling nitrogen gas into the lithium hexafluorophosphate solution before introducing perfluorocycloalkane gas; the acid removal involves bubbling a mixture of trimethylsilane and nitrogen gas.

6. The method for purifying lithium hexafluorophosphate solution as described in claim 5, characterized in that: The flow rate of nitrogen gas is 35-40 mL / min, the flow rate of perfluorocycloalkanes gas is 18-22 mL / min, and the flow rate of the mixed gas of trimethylsilane and nitrogen is 8-12 mL / min.

7. The method for purifying lithium hexafluorophosphate solution as described in claim 5, characterized in that: The temperature of the gas introduced is 10~35℃, the stirring speed is 150~200rpm, and the gas introduction time for each gas is 10~30min.

8. The method for purifying lithium hexafluorophosphate solution as described in claim 5, characterized in that: The volume ratio of trimethylsilane to nitrogen is 1:(5-20).

9. The method for purifying lithium hexafluorophosphate solution as described in claim 5, characterized in that: The purity of the nitrogen, perfluorocycloalkanes, and trimethylsilane is ≥99.99%, and the moisture content is ≤0.5ppm.

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