Regenerable adsorbent for removing free acid from lithium hexafluorophosphate electrolyte and its application

Abstract

The present invention provides a regenerable adsorbent for removing free acid in lithium hexafluorophosphate electrolyte and its application. The present invention sintered NaF and CaF2 in an inert gas atmosphere to form a eutectic to obtain NaF-CaF2 ceramic powder, immersed the NaF-CaF2 ceramic powder in LiPF6 electrolyte, and fully stirred to adsorb HF in the LiPF6 electrolyte, thereby achieving effective removal of free acid in the LiPF6 electrolyte and improving the quality of the LiPF6 electrolyte. The safety and reliability of lithium-ion batteries are ensured. Moreover, the NaF-CaF2 ceramic powder after adsorption can be completely regenerated by a simple heating method, can be reused, and reduces the adsorption cost.

Description

Technical Field

[0001] The present invention relates to a method for effectively improving the quality of LiPF6 electrolyte and reducing the free acid content in the LiPF6 electrolyte. NaF-CaF2 sintered ceramic powder is placed in the LiPF6 electrolyte, and the free acid in the LiPF6 electrolyte is effectively removed by the adsorption of the NaF-CaF2 ceramic powder, thereby achieving the production of high-quality LiPF6 electrolyte. Background Art

[0002] LiPF6 electrolyte has the advantages of good ionic conductivity, good electrochemical stability, safety and environmental protection, so it has become the most commercialized lithium-ion battery electrolyte. Due to its poor thermal stability in the air, the decomposed hydrofluoric acid (HF) that reacts easily with water will react with the positive and negative electrodes of the lithium-ion battery and destroy the solid electrolyte membrane of the electrode, thereby affecting the battery's cycle performance and safety performance and shortening the battery's service life. Therefore, in the production, storage, transportation of lithium-ion battery electrolytes and the production process of batteries, the free acid content (measured in HF) in the electrolyte must be accurately measured and strictly controlled.

[0003] The prior art discloses that adsorption materials such as A12O3, MgO, and BaO have the function of adsorbing HF, but in actual applications, it is found that the removal effect of hydrofluoric acid (HF) when added to LiPF6 electrolyte is very poor, and it is basically difficult to play an adsorption role. It may be that these adsorption materials mostly play a physical adsorption role, and it is difficult to effectively remove the hydrofluoric acid (HF) with a low content in LiPF6 electrolyte.

[0004] In many electronic product production processes, NaF is used to absorb the discharged HF gas because of its good properties of adsorbing HF gas. NaF is also often used to adsorb HF in electronic gas production. NaF has an extremely important application in the purification process of fluorine-containing electronic gases. The principle of NaF's good adsorption capacity for HF is based on the reaction of NaF with HF to generate NaHF2. At the same time, the adsorption reaction is a reversible reaction. When the temperature is above 250°C, HF desorbs from NaHF2 and regenerates HF and NaF.

[0005] Based on such a principle, the present invention attempts to introduce NaF into the LiPF6 electrolyte, and achieves the purpose of removing HF and reducing the content of free acid by adsorbing HF in the LiPF6 electrolyte. However, it is a well-known fact that since the solvent constituting the LiPF6 electrolyte is a carbonate organic solvent, such as polycarbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, etc., the trace dissolution of NaF in these solvents not only cannot adsorb HF, but also cannot play a role in reducing the ability of free acid, but will increase the F- content in the LiPF6 electrolyte.

[0006] The biggest difficulty of the present invention is that if you want to fully utilize the performance of NaF in adsorbing HF, you need to "bind" NaF very firmly. The present invention prepares NaF-CaF2 ceramic powder, that is, by sintering NaF and CaF2, to form NaF-CaF2 ceramic powder, and the NaF-CaF2 ceramic powder can exist stably in LiPF6 electrolyte. At the same time, based on the NaF structure in the skeleton of the NaF-CaF2 ceramic, it can strongly adsorb HF and effectively reduce the free acid in the LiPF6 electrolyte. Summary of the invention

[0007] The purpose of the present invention is to prepare a regenerable adsorbent for removing free acid in lithium hexafluorophosphate electrolyte and for reducing the free acid content in LiPF6 electrolyte, so as to achieve the preparation of high-quality LiPF6 electrolyte. To achieve this purpose, the present invention provides a NaF-CaF2 ceramic powder preparation, which can be very simply placed in the LiPF6 electrolyte to reduce the free acid in the LiPF6 electrolyte.

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

[0009] First, a NaF-CaF2 ceramic powder adsorbent is prepared, specifically, NaF and CaF2 that are fully ground into powder are mixed, sintered in an inert gas atmosphere to form a eutectic, and then NaF-CaF2 ceramic powder is obtained after grinding.

[0010] The NaF-CaF2 sintered ceramic powder is placed in a LiPF6 electrolyte, and the free acid content in the electrolyte is reduced by the adsorption of HF in the LiPF6 electrolyte by the NaF-CaF2 ceramic powder.

[0011] Furthermore, the mass ratio of NaF to CaF2 is in the range of 0.05 to 0.5. If the NaF ratio is too low, it will affect the adsorption effect of free acid in the LiPF6 electrolyte; on the contrary, if the NaF ratio is too high, it will affect the stability of the ceramic powder, causing the ceramic powder to be easily broken, which is also not conducive to the removal of free acid in the LiPF6 electrolyte. The preferred ratio range is 0.1 to 0.3.

[0012] Furthermore, the sintering temperature is 850-1000°C and the temperature is kept for 3-5 hours.

[0013] CaF2 belongs to equiaxed crystal system, is cube, octahedron or dodecahedron, colorless crystal, melting point is 1402 ℃, can form eutectic when adding NaF, melting point is reduced to about 800 ℃. Because NaF and CaF2 can form eutectic. So melting point is reduced to about 800 ℃, and research shows that sintering temperature is set at 850~1000 ℃, and insulation is more suitable for 3~5 hours. For temperature, being lower than 850 ℃ is unfavorable for NaF and CaF2 blending, on the contrary, temperature is too high, and there is no effect on NaF-CaF2 ceramic powder of the present invention, but 1000 ℃ is enough to realize NaF-CaF2 ceramic powder from the perspective of energy saving.

[0014] Furthermore, NaF and CaF2 are ground separately to form powders with small particle size, which is conducive to sintering after the two are fully mixed. The optimal particle size of both is less than 200 μm. The NaF-CaF2 sintered ceramic obtained after sintering needs to be further ground to less than 200 μm.

[0015] Furthermore, the sintering process of preparing NaF-CaF2 ceramic powder is carried out under an inert gas atmosphere. All inert gases can meet the sintering process of the present invention. Considering the production cost, nitrogen is the best.

[0016] CaF2 is the preferred material of the present invention. After sintering other ceramic components (such as silicon oxide, calcium oxide, etc.) with NaF, there will be problems such as high sintering temperature or difficulty in forming a ceramic phase with NaF.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: the NaF-CaF2 ceramic powder prepared by the present invention can stably exist in the LiPF6 electrolyte, can strongly adsorb HF, effectively reduce the free acid in the LiPF6 electrolyte, the recovery method is simple, and the cost of the free acid adsorbent in the electrolyte is reduced. DETAILED DESCRIPTION

[0018] The present invention is further described below with reference to the following examples: The examples provide different ratios of NaF to CaF2 and sintering temperatures when preparing ceramic powders, but the protection scope of the present invention is not limited to the following examples.

[0019] LiPF6 electrolyte is a commercially available product, and the following examples are experiments using multiple different batches of electrolyte.

[0020] 10 g of the adsorbent prepared in the examples and comparative examples was placed in 100 ml of LiPF6 electrolyte, and stirred for 10 min at room temperature. The upper layer of the clear LiPF6 electrolyte was taken and the free acid content was tested according to the GB / T 19282-2014 method.

[0021] The following is a description of specific embodiments.

[0022] Example 1 The specific experimental process is as described above. Take the ground NaF and CaF2 respectively, mix them at a mass ratio of NaF:CaF2 of 0.05, take 50g of the mixture and place it in a graphite crucible, introduce nitrogen to fully replace the air, so that the sintering process of the ceramic is in an oxygen-free atmosphere, heat to 1000°C, keep warm for 3 hours, and after cooling, put the prepared NaF-CaF2 ceramic into a planetary ball mill and grind it to less than 200μm to obtain NaF-CaF2 ceramic powder.

[0023] According to the above analysis method, the free acid of LiPF6 electrolyte was analyzed, and the analysis results are listed in Table 1. Example 2 Specific experimental process As described above, take the ground NaF and CaF2 respectively, mix them at a mass ratio of NaF:CaF2 of 0.1, take 50g of the mixture and place it in a graphite crucible, introduce nitrogen to fully replace the air, so that the sintering process of the ceramic is in an oxygen-free atmosphere, heat to 950℃, keep warm for 3 hours, and after cooling, put the prepared NaF-CaF2 ceramic into a planetary ball mill and grind it to less than 200μm to obtain NaF-CaF2 ceramic powder. According to the above analysis method, the free acid of LiPF6 electrolyte was analyzed, and the analysis results are listed in Table 1.

[0024] Embodiment 3 Specific experimental process As described above, take the ground NaF and CaF2 respectively, mix them at a mass ratio of NaF:CaF2 of 0.3, take 50g of the mixture and place it in a graphite crucible, introduce nitrogen to fully replace the air, so that the sintering process of the ceramic is in an oxygen-free atmosphere, heat to 900°C, keep warm for 3 hours, and after cooling, put the prepared NaF-CaF2 ceramic into a planetary ball mill and grind it to less than 200μm to obtain NaF-CaF2 ceramic powder. According to the above-mentioned analysis method, the free acid of LiPF6 electrolyte is analyzed, and the analysis results are listed in Table 1.

[0025] Embodiment 4 Specific experimental process As described above, take ground NaF and CaF2 respectively, mix them at a mass ratio of 0.5 for NaF:CaF2, take 50g of the mixture and place it in a graphite crucible, introduce nitrogen to fully replace the air, so that the sintering process of the ceramic is in an oxygen-free atmosphere, heat to 850°C, keep warm for 3 hours, and after cooling, put the prepared NaF-CaF2 ceramic into a planetary ball mill and grind it to less than 200μm to obtain NaF-CaF2 ceramic powder. According to the above-mentioned analysis method, the free acid of LiPF6 electrolyte is analyzed, and the analysis results are listed in Table 1.

[0026] If sintering is performed at 650°C with a mass ratio of NaF:CaF2 of 0.5 as in Example 4, the experimental results show that it cannot become a ceramic body after sintering at this temperature and cannot exist stably in the electrolyte.

[0027] Embodiment 5 Specific experimental process As described above, take the ground NaF and CaF2 respectively, mix them at a mass ratio of NaF:CaF2 of 0.7, take 50g of the mixture and place it in a graphite crucible, introduce nitrogen to fully replace the air, so that the sintering process of the ceramic is in an oxygen-free atmosphere, heat to 850℃, keep warm for 3 hours, and after cooling, put the prepared NaF-CaF2 ceramic into a planetary ball mill and grind it to less than 200μm to obtain NaF-CaF2 ceramic powder. According to the above-mentioned analysis method, the free acid of LiPF6 electrolyte is analyzed, and the analysis results are listed in Table 1.

[0028] Comparative Example 1 Specific experimental process As described above, take ground LiF and CaF2 respectively, mix them at a ratio of LiF:CaF2 of 0.3, take 50g of the mixture and place it in a graphite crucible, introduce nitrogen to fully replace the air, so that the sintering process of the ceramic is in an oxygen-free atmosphere, heat to 1000°C, keep warm for 3 hours, and after cooling, put the prepared LiF-CaF2 ceramic into a planetary ball mill and grind it to less than 200μm to obtain LiF-CaF2 ceramic powder. According to the above-mentioned analysis method, the free acid of the LiPF6 electrolyte is analyzed, and the analysis results are listed in Table 1.

[0029] Comparative Example 2 Specific experimental process As described above, take the ground KF and CaF2 respectively, mix them at a ratio of KF:CaF2 of 0.3, take 50g of the mixture and place it in a graphite crucible, introduce nitrogen to fully replace the air, so that the sintering process of the ceramic is in an oxygen-free atmosphere, heat to 1000℃, keep warm for 3 hours, and after cooling, put the prepared KF-CaF2 ceramic into a planetary ball mill and grind it to less than 200μm to obtain KF-CaF2 ceramic powder. The free acid of the LiPF6 electrolyte is analyzed according to the above-mentioned analysis method, and the analysis results are listed in Table 1.

[0030] Comparative Example 3 Specific experimental process As described above, 50g of ground CaF2 was placed in a graphite crucible, nitrogen was introduced to fully replace the air, so that the sintering process of the ceramic was in an oxygen-free atmosphere, heated to 1000°C, kept warm for 3 hours, and after cooling, the prepared CaF2 ceramic was placed in a planetary ball mill and ground to less than 200μm to obtain CaF2 ceramic powder. The free acid of the LiPF6 electrolyte was analyzed according to the above-mentioned analysis method, and the analysis results are listed in Table 1.

[0031] Table 1 Free acid content in LiPF6 electrolyte before and after adsorption of NaF-CaF2 ceramic powder (%)

[0032]

[0033] From the results in Table 1, it can be seen that the LiF-CaF2 and KF-CaF2 ceramic powders prepared in the comparative example hardly adsorb the free acid in the LiPF6 electrolyte, but after using the NaF-CaF2 ceramic powder, the free acid content in the LiPF6 electrolyte is almost reduced by half. It can be seen that the NaF-CaF2 ceramic powder has a good adsorption effect on HF in the LiPF6 electrolyte.

[0034] Regeneration method of used NaF-CaF2 ceramic powder: Take 10g of used NaF-CaF2 ceramic powder, heat it at 250℃ for 1h, then place 5g of NaF-CaF2 ceramic powder in 10ml of deionized water and stir for 30min, take the supernatant, and use ion chromatography to detect the presence of F ions, and the result is that no F ions are detected. This proves that used NaF-CaF2 ceramic powder can be regenerated by heating.

Claims

1. An application of a regenerable adsorbent for removing free acid in lithium hexafluorophosphate electrolyte, characterized in that: The NaF-CaF2 sintered ceramic powder is placed in a LiPF6 electrolyte, and the free acid content in the electrolyte is reduced by the adsorption of the NaF-CaF2 ceramic powder on the LiPF6 electrolyte; The preparation method of NaF-CaF2 ceramic powder is as follows: NaF and CaF2 are mixed and sintered in an inert gas atmosphere to form a eutectic, the sintering temperature is 850-1000°C, and the mixture is kept warm for 3-5 hours, and the NaF-CaF2 ceramic powder is obtained after grinding; the mass ratio of NaF to CaF2 is in the range of 0.05-0.

5.

2. The use according to claim 1, characterized in that: The adsorption temperature of NaF-CaF2 ceramic powder on free acid in LiPF6 electrolyte is 20-60℃.

3. A method for regenerating a regenerable adsorbent for removing free acid in a lithium hexafluorophosphate electrolyte according to claim 1, characterized in that: The NaF-CaF2 sintered ceramic powder which has adsorbed free acid is placed at a temperature above 250°C to remove the adsorbed free acid and achieve regeneration of the NaF-CaF2 sintered ceramic powder.

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