Purification Method of Lithium Hexafluorophosphate Electrolyte

Abstract

The present invention provides a method for purifying lithium hexafluorophosphate electrolyte, that is, removing LiPF 6 The method of removing free acid (measured in HF) in the electrolyte improves the quality of the battery electrolyte and ensures the safety and reliability of the lithium-ion battery. It belongs to LiPF for lithium-ion batteries 6 The specific method is to immerse the fluoride salt modified glass powder (MF / glass powder) prepared by sintering the alkali metal fluoride salt (MF) of the present invention with glass powder in LiPF 6 The electrolyte is stirred thoroughly to allow LiPF to be adsorbed 6 HF in the electrolyte to achieve LiPF 6 Effective removal of free acid in electrolyte improves LiPF 6 Purity of electrolyte.

Description

Technical Field

[0001] The present invention relates to a method for improving the purity of LiPF 6 electrolyte and reducing the content of free acid (HF) in the LiPF 6 electrolyte. A fluoride-modified glass powder (MF / glass powder) obtained by sintering a fluoride salt of an alkali metal (MF) and glass powder is placed in the LiPF 6 electrolyte. By adsorbing HF in the LiPF 6 electrolyte with the MF / glass powder, free acid is effectively removed, and the purity of the LiPF 6 electrolyte is improved. Background Art

[0002] The electrolyte is a medium for ion transport in a lithium-ion battery, and its ion conduction ability plays a crucial role in the rate performance of the lithium-ion battery. In the organic electrolyte of lithium-ion batteries, the conductive salt lithium hexafluorophosphate (LiPF 6 ) is currently commonly used. LiPF 6 is an effective way to improve the ion conduction ability of the electrolyte. Due to the poor thermal stability of LiPF 6 in air, hydrogen fluoride (HF) which is prone to react with water is decomposed. Hydrofluoric acid will react with the positive and negative electrodes of the lithium-ion battery, and at the same time destroy the solid electrolyte membrane of the electrode, thereby affecting the cycle performance and safety performance of the battery and shortening the service life of the battery. Therefore, it is very necessary to control the content of free acid (calculated as HF) in the LiPF 6 electrolyte. Therefore, GB / T 19282-2014 stipulates that the index of free acid in the LiPF 6 electrolyte should be less than 0.0050%.

[0003] In the prior art, adsorption materials such as A1 2 O 3 , MgO, BaO, carbonates of lithium or calcium, activated carbon, silica gel, etc. are commonly added to the electrolyte to remove excess free acid, but the adsorption capacity is difficult to meet the requirements. The inventor found in practical applications that the adsorption effect of alumina and the like is not ideal. At present, the adsorption of HF by adsorption materials mostly relies on physical adsorption, which is difficult to achieve good adsorption in the electrolyte, and it is difficult to be regenerated for reuse or the regeneration cost is high after use.

[0004] Alkali metal fluorides (MF) have good adsorption capacity for HF gas. Especially, NaF shows excellent adsorption capacity for HF. Therefore, many electronic processing enterprises use NaF to absorb HF waste gas to achieve the purpose of reducing HF emissions. The principle of the good adsorption capacity of MF for HF is based on the reaction of MF with HF to form MHF 2 (as shown in Reaction Formula 1).

[0005] MF + HF → NaHF 2 (1)

[0006] Meanwhile, this adsorption reaction is a reversible reaction. When the temperature is above 250 °C, HF desorbs from NaHF 2 and regenerates HF and NaF, that is

[0007] MHF 2 → MF + HF (2)

[0008] In order to further improve the adsorption capacity of alkali metal fluorides for HF gas, increase their porosity, and enlarge the contact area, etc., in the prior art, in the preparation method of an adsorbent for fluorine gas purification in CN202111424526.9, by mixing an alkali fluoride salt raw material with a binder and a solvent in proportion, granulating, and sintering, the porosity and adsorption capacity of the fluorine gas purification adsorbent can be improved. In the preparation and application method of a hydrogen fluoride adsorbent in CN202211339774.8, an adsorption material, a mesoporous material, and a framework material are ground and mixed into a uniform mixture, then a solvent is added to the mixture and extruded into spheres, and finally the spherical mixture added with the solvent is calcined to obtain a hydrogen fluoride adsorbent. These materials have only been verified to be applicable to the adsorption of HF gas, but not to the adsorption of HF in the lithium hexafluorophosphate electrolyte. The inventor tried to introduce them into the LiPF 6 electrolyte to adsorb HF in the LiPF 6 electrolyte, so as to remove HF and improve the purity of the LiPF 6 electrolyte. However, since the solvent constituting the LiPF 6 electrolyte is a carbonate organic solvent, the alkali fluoride salt or modified alkali fluoride salt dissolves slightly in these solvents. Not only can it not adsorb HF and play the role of reducing free acid, but it will instead increase the F 6 content in the LiPF - electrolyte, and the HF in the electrolyte and the HF in fluorine gas have different forms, making it difficult to transfer the adsorption materials to each other.

[0009] To solve the problem that it is difficult for alkali fluoride salts to be used for the adsorption of HF in the LiPF 6 electrolyte, the present invention sinters glass powder with alkali metal fluorides, firmly "binds" the alkali metal fluoride MF in the glass powder body, so that the prepared MF / glass powder can exist very stably in the LiPF 6 electrolyte. At the same time, based on the MF structure in the framework of this MF / glass powder, it can strongly adsorb HF, effectively improving the purity of the LiPF 6 electrolyte. Summary of the Invention

[0010] The purpose of the present invention is to reduce LiPF 6The content of free acid in the electrolyte (calculated as HF) to achieve high-purity LiPF 6 Preparation of the electrolyte. To achieve this purpose, the present invention provides an alkali metal fluoride-modified glass powder (MF / glass powder). After sintering the alkali metal fluoride with the glass powder, the alkali metal fluoride becomes a part of the framework structure of the glass powder. It is very simple and stable to place the MF / glass powder in LiPF 6 electrolyte, which can improve the purity of LiPF 6 electrolyte.

[0011] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0012] Sinter the fluoride salt (MF) of an alkali metal with the glass powder, grind it to obtain the fluoride salt-modified glass powder (MF / glass powder), and place it in LiPF 6 electrolyte. By adsorbing the HF free acid in the LiPF 6 electrolyte by the MF / glass powder, the content of free acid in the electrolyte can be reduced.

[0013] Furthermore, M in the fluoride salt (MF) of the alkali metal is Li, Na, K; more preferably NaF.

[0014] Furthermore, the mass ratio range of the fluoride salt (MF) of the alkali metal to the glass powder is 0.1 - 0.8, and the more preferred mass ratio range is 0.3 - 0.5.

[0015] Furthermore, the sintering temperature is set at 300 - 1000 °C; more preferably 600 - 1000 °C, and it is more suitable to keep the temperature for 3 - 5 hours during sintering.

[0016] The specific application method of the alkali metal fluoride-modified glass powder in LiPF 6 electrolyte is as follows:

[0017] Place the sintered alkali metal fluoride-modified glass powder in the LiPF 6 electrolyte, perform adsorption treatment at 20 - 80 °C, stir and then filter, take the upper-layer LiPF 6 electrolyte clear liquid, and test the free acid content according to the method of GB / T 19282-2014.

[0018] Furthermore, the adsorption treatment temperature is room temperature.

[0019] Furthermore, the addition amount of the sintered alkali metal fluoride-modified glass powder in LiPF 6 electrolyte is 50 - 100 g / L.

[0020] Furthermore, the LiPF 6 electrolyte is a conventional commercially available electrolyte.

[0021] Further, the glass powder is an inorganic amorphous hard particle powder, usually used as a highly transparent and hard filling material. The raw materials used in the production of glass powder are PbO, SiO 2 , TiO 2 and other electronic-grade raw materials. After mixing evenly, a solid-phase reaction is carried out at high temperature to form a glass homogeneous body with a disordered structure. It has stable chemical properties and very high acid resistance and organic solvent resistance. The glass powder used in the present invention can be a lead-containing or lead-free glass powder. When using a lead-free glass powder, the sintering temperature can be appropriately reduced. Regardless of the type of glass powder used, the formed alkali metal fluoride-modified glass powder has little influence on the adsorption performance of free acid in the LiPF 6 electrolyte.

[0022] For example: The main components of the lead-containing glass powder are: PbO: 35-65%, B 2 O 3 : 12-20%, ZnO: 5-15%, SiO 2 : 3.8-7.6%, ZrO2: 3-7%, Al 2 O 3 : 1-4%, MgO 1-3% oxide composition.

[0023] For example: The main components of the lead-free glass powder are: Bi 2 O 3 50% - 70%, SiO 2 5% - 10%, B 2 O 3 10% - 15%, Al 2 O 3 5% - 10%, Li 2 O 1% - 5%, ZnO 1% - 5% oxide composition.

[0024] For the preparation of the alkali metal fluoride-modified glass powder, first, the alkali metal fluoride and the glass powder need to be fully mixed. To achieve this purpose, the alkali metal fluoride and the glass powder can be ground and compounded separately, which is beneficial to the sintering after their full mixing.

[0025] For the composition of the alkali metal fluoride-modified glass powder, the composition ratio range of MF: glass powder is 0.1 - 0.8; if the MF ratio is too low, it will affect the adsorption effect of free acid in the LiPF 6 electrolyte; on the contrary, if the MF ratio is too high, it will affect the stability of the alkali metal fluoride-modified glass powder and is also not conducive to LiPF 6The removal of free acid in the electrolyte, the optimal proportion range is 0.3 - 0.5. Research shows that the sintering temperature is set at 600 - 1000 °C, and it is more appropriate to hold the temperature for 3 - 5 hours. For the temperature, too low temperature is not conducive to the blending of alkali metal fluorides and glass powder, and it is difficult for alkali metal fluorides to enter the framework structure of glass powder. Similarly, too high temperature is likely to cause the formation of oxidation products of alkali metal fluorides. At a temperature of 600 - 1000 °C, various alkali metal fluorides and glass powder can be guaranteed to be in a molten state, so the temperature should not be higher than 1000 °C.

[0026] The sintering process for preparing the alkali metal fluoride modified glass powder is realized in an inert gas atmosphere. All inert gases can meet the sintering process of the present invention. Considering the production cost, nitrogen is the best.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] In the present invention, through the mixed sintering of alkali metal fluorides and glass powder, the alkali metal fluoride MF is firmly "constrained" in the glass body, so that the prepared MF / glass powder can exist very stably in LiPF 6 electrolyte. Based on the fact that the MF structure exists in the framework of the MF / glass powder, it can strongly chemically adsorb HF. MF reacts with HF to generate MHF 2 , realizing the effective removal of free acid in LiPF 6 electrolyte, and improving the purity of LiPF 6 electrolyte. Ensure the safety and reliability of lithium-ion batteries, and for the used alkali metal fluoride modified glass powder, it can be regenerated and reused only by heating, with low recycling cost. Description of the Drawings

[0029] Figure 1 It is the content analysis of the fluoride ions precipitated from the electrolyte solvent of the NaF / glass powder prepared in Example 3.

[0030] Figure 2 It is the content analysis of the fluoride ions precipitated from the electrolyte solvent of the NaF / glass powder sintered at 500 °C. Detailed Embodiments

[0031] The following further describes the present invention in conjunction with embodiments. The embodiments give different proportions of alkali metal fluorides and glass powder during the preparation of the alkali metal fluoride modified glass powder, but the protection scope of the present invention is not limited to the following embodiments.

[0032] In the embodiments, the main components of the lead-free glass powder used are: Bi 2 O 3 50% - 70%, SiO 2 5% - 10%, B2 O 3 10% to 15%, Al 2 O 3 5% to 10%, Li 2 O 1% to 5%, ZnO 1% to 5%.

[0033] (excluding lead oxide composition)

[0034] The electrolyte source in the examples is a commercially available electrolyte. The following examples are experiments conducted on electrolytes from multiple different batches.

[0035] The method for adsorbing LiPF 6 by alkali metal fluoride modified glass powder in the electrolyte is as follows:

[0036] Place 10 g of the alkali metal fluoride modified glass powder sintered in the examples and comparative examples into 100 ml of LiPF 6 electrolyte. Under room temperature conditions, after stirring for 10 min, take the upper layer of LiPF 6 electrolyte supernatant, and test the free acid content according to the method of GB / T19282 - 2014. The analysis results are listed in Table 1.

[0037] Regeneration and evaluation of alkali metal fluoride modified glass powder:

[0038] Heat the used alkali metal fluoride modified glass powder to above 300 °C to achieve the regeneration of the alkali metal fluoride modified glass powder. To verify whether the regeneration of the alkali metal fluoride modified glass powder is complete, place 5 g of the regenerated alkali metal fluoride modified glass powder into 10 ml of deionized water, stir for 30 min, take the upper layer of the supernatant, and detect the F ions by ion chromatography. The regenerated alkali metal fluoride modified glass powder of the present invention, detected by the above analysis method, does not detect fluoride ions. It can be proved that the used alkali metal fluoride modified glass powder can be regenerated by heating.

[0039] The specific experimental process of Example 1 is as described above. The specific preparation conditions of the alkali metal fluoride modified glass powder are as follows: Take the ground NaF and glass powder respectively, mix them in a mass ratio of NaF:glass powder of 0.1:1. Take 50 g of the mixture and place it in a graphite crucible. Pass nitrogen to fully displace the air to make the sintering process of the ceramic in an oxygen-free atmosphere. Heat to 1000 °C and hold for 3 hours. After cooling, put the prepared sintered sodium fluoride modified glass powder into a planetary ball mill for grinding to obtain the sodium fluoride modified glass powder. Adsorb and analyze the free acid in the LiPF 6 electrolyte according to the above analysis method. The analysis results are listed in Table 1.

[0040] The specific experimental process of Example 2 is as described above. Ground NaF and glass powder were taken respectively, and mixed in a ratio of NaF:glass powder mass ratio of 0.3:1. 50 g of the mixture was placed in a graphite crucible, and nitrogen was introduced to fully displace the air, so that the sintering process of the ceramic was in an oxygen-free atmosphere. It was heated to 1000 °C and held for 3 hours. After cooling, the prepared sintered sodium fluoride modified glass powder was put into a planetary ball mill for grinding, and the sodium fluoride modified glass powder could be obtained. According to the above analysis method, LiPF 6 The free acid in the electrolyte was adsorbed and analyzed, and the analysis results are listed in Table 1.

[0041] The specific experimental process of Example 3 is as described above. Ground NaF and glass powder were taken respectively, and mixed in a ratio of NaF:glass powder mass ratio of 0.5:1. 50 g of the mixture was placed in a graphite crucible, and nitrogen was introduced to fully displace the air, so that the sintering process of the ceramic was in an oxygen-free atmosphere. It was heated to 1000 °C and held for 3 hours. After cooling, the prepared sintered sodium fluoride modified glass was put into a planetary ball mill for grinding, and the sodium fluoride modified glass powder (NaF / glass powder) could be obtained. According to the above analysis method, LiPF 6 The free acid in the electrolyte was adsorbed and analyzed, and the analysis results are listed in Table 1.

[0042] 1 g of the NaF / glass powder prepared in Example 3 was placed in 50 ml of dimethyl carbonate solvent. After shaking for 1 h, 10 ml of the supernatant was taken. After slowly evaporating dimethyl carbonate at 50 °C, the residue after evaporation was accurately dissolved in 10 ml of its ionized water. This aqueous solution was analyzed by ion chromatography, and no fluoride ions were found ( Figure 1 ). Thus, it was proved that NaF / glass powder could stably exist in the electrolyte.

[0043] If the ratio of NaF:glass powder = 0.5 was sintered at 300 °C, no integration was seen from the appearance. If sintered at 500 °C, although it was observed from the appearance that the alkali metal fluoride entered the framework structure of the glass powder, according to the above analysis results, a large fluoride ion peak appeared on the ion chromatography ( Figure 2 ), and its concentration was 178.46 ppm. It can be seen that at the sintering temperature of 500 °C, the alkali metal fluoride did not effectively enter the framework structure of the glass powder and was difficult to be used for the removal of the free acid in LiPF 6 electrolyte.

[0044] The specific experimental procedures of Example 4 are as described above. Ground NaF and glass powder were respectively taken and mixed at a mass ratio of NaF:glass powder of 0.8:1. 50 g of the mixture was placed in a graphite crucible, and nitrogen was introduced to fully displace the air so that the ceramic sintering process was in an oxygen-free atmosphere. It was heated to 1000 °C and held for 3 hours. After cooling, the prepared sintered sodium fluoride modified glass powder was put into a planetary ball mill for grinding to obtain sodium fluoride modified glass powder. According to the above analysis method, the free acid of the LiPF 6 electrolyte was analyzed, and the analysis results are listed in Table 1.

[0045] The specific experimental procedures of Example 5 are as described above. Ground LiF and glass powder were respectively taken and mixed at a mass ratio of LiF:glass powder of 0.5:1. 50 g of the mixture was placed in a graphite crucible, and nitrogen was introduced to fully displace the air so that the ceramic sintering process was in an oxygen-free atmosphere. It was heated to 1000 °C and held for 3 hours. After cooling, the prepared sintered lithium fluoride modified glass powder was put into a planetary ball mill for grinding to obtain lithium fluoride modified glass powder. According to the above analysis method, the free acid of the LiPF 6 electrolyte was analyzed, and the analysis results are listed in Table 1.

[0046] The specific experimental procedures of Example 6 are as described above. Ground KF and glass powder were respectively taken and mixed at a mass ratio of KF:glass powder of 0.5:1. 50 g of the mixture was placed in a graphite crucible, and nitrogen was introduced to fully displace the air so that the ceramic sintering process was in an oxygen-free atmosphere. It was heated to 1000 °C and held for 3 hours. After cooling, the prepared sintered potassium fluoride modified glass powder was put into a planetary ball mill for grinding to obtain potassium fluoride modified glass powder. According to the above analysis method, the free acid of the LiPF 6 electrolyte was analyzed, and the analysis results are listed in Table 1.

[0047] Examples 7 - 10

[0048] The ratios of Examples 7 - 10 and Example 3 are different in that the sintering temperature of 1000 °C was replaced with 800 °C, 850 °C, 900 °C, and 950 °C, and other operations were the same as those in Example 3. The removal effect of the sodium fluoride modified glass powder obtained in Examples 7 - 10 on the free acid of the LiPF 6 electrolyte was the same as the result of Example 3.

[0049] Example 11

[0050] The ratio of Example 11 and Example 3 is different in that the sintering temperature of 1000 °C was replaced with 750 °C, and other operations were the same as those in Example 3. According to the above analysis method, the free acid of the LiPF 6 electrolyte was analyzed, and the analysis results are listed in Table 1.

[0051] The specific experimental procedures of Comparative Example 1 are as described above. Ground MgF 2 and glass powder were taken respectively, and mixed in a ratio of MgF 2 : glass powder mass of 0.5:1. 50 g of the mixture was placed in a graphite crucible, and nitrogen was introduced to fully displace the air, so that the sintering process of the ceramic was in an oxygen-free atmosphere. It was heated to 1000 °C and held for 3 hours. After cooling, the prepared sintered magnesium fluoride modified glass powder was put into a planetary ball mill for grinding, and the magnesium fluoride modified glass powder body could be obtained. According to the above analysis method, the free acid of the LiPF 6 electrolyte was analyzed, and the analysis results are listed in Table 1.

[0052] The specific experimental procedures of Comparative Example 2 are as described above. Ground NaF and glass powder were taken respectively, and mixed in a ratio of NaF: glass powder of 0.01:1. 50 g of the mixture was placed in a graphite crucible, and nitrogen was introduced to fully displace the air, so that the sintering process of the ceramic was in an oxygen-free atmosphere. It was heated to 1000 °C and held for 3 hours. After cooling, the prepared sintered sodium fluoride modified glass powder was put into a planetary ball mill for grinding, and the sodium fluoride modified glass powder body could be obtained. According to the above analysis method, the free acid of the LiPF 6 electrolyte was analyzed, and the analysis results are listed in Table 1.

[0053] Comparative Example 3

[0054] Compared with Example 3, NaF was not added, and only the glass powder was sintered. The obtained glass powder body was analyzed for the free acid of the LiPF 6 electrolyte according to the above analysis method, and the analysis results are listed in Table 1.

[0055] Table 1 Free acid content (%) in the LiPF 6 electrolyte before and after adsorption by MF / glass powder

[0056]

[0057] As can be seen from the results in Table 1, the magnesium fluoride modified glass powder prepared in Comparative Example 1 had very limited adsorption of the free acid in the LiPF 6 electrolyte, and even did not exceed the modification of the glass powder by trace sodium fluoride (Comparative Example 2). Treating the LiPF 6 electrolyte with the glass powder modified by an appropriate proportion of sodium fluoride reduced its free acid content by almost half. It can be seen from this that the alkali metal fluoride modified glass powder has a good adsorption effect on HF in the LiPF 6 electrolyte and can be used to improve the purity of the LiPF 6 electrolyte.

Claims

1. A purification method for lithium hexafluorophosphate electrolyte, characterized in that: The method is as follows: A fluoride salt-modified glass powder obtained by sintering a fluoride salt MF of an alkali metal and glass powder under an inert gas is denoted as MF / glass powder. The MF / glass powder is placed in a LiPF 6 electrolyte, and the free acid HF in the LiPF 6 electrolyte is adsorbed by the MF / glass powder to reduce the content of free acid in the electrolyte; in the fluoride salt MF of the alkali metal, M is one or more of Li, Na, and K in mixture; the sintering temperature is 600 - 1000 °C; the mass ratio range of the fluorinated salt MF of alkali metal to glass powder is 0.1 - 0.

8.

2. The purification method for lithium hexafluorophosphate electrolyte according to claim 1, characterized in that: the fluorinated salt of alkali metal is sodium fluoride.

3. The purification method for lithium hexafluorophosphate electrolyte according to claim 1, characterized in that: keep warm for 3 - 5 hours at the sintering temperature.

4. The purification method for lithium hexafluorophosphate electrolyte according to claim 1, characterized in that: the inert gas used for sintering is nitrogen.

5. The purification method for lithium hexafluorophosphate electrolyte according to claim 1, characterized in that: MF / Glass powder on LiPF 6 The adsorption temperature of free acid in the electrolyte is 20 - 80 °C.

6. The purification method for lithium hexafluorophosphate electrolyte according to claim 1, characterized in that: the glass powder is lead-containing glass powder or lead-free glass powder.

Images