A lithium hexafluorophosphate-based composite lithium salt and an in-situ preparation method thereof

Abstract

The application belongs to the technical field of lithium ion battery electrolyte lithium salt preparation, and particularly relates to a composite lithium salt based on lithium hexafluorophosphate and an in-situ preparation method thereof. The application takes high-purity LiPF6 crystal salt as the only main raw material, and through trace gas-phase water, controlled hydrolysis of the surface of LiPF6 is induced under an inert atmosphere environment. Simultaneously, an in-situ HF capturing agent is used to instantly combine the generated HF, so as to directionally generate lithium difluorophosphate and form a LiPF6-LiPO2F2 core-shell type composite lithium salt. The reaction temperature is 25-85 DEG C, and the pressure is 0.02-0.1 MPa. There is no free acid, no waste liquid, and no external phosphorus source is introduced. The application has the advantages of mild process, controllable conversion rate, high product purity (≥99.95%), and low acid value (≤5 ppm), and is suitable for high-nickel ternary, silicon-carbon negative electrode and solid-state battery electrolyte. The application solves the defects of traditional methods such as many impurities and high-temperature instability, and has significant industrialization value.

Description

Technical Field

[0001] This invention belongs to the field of lithium salt preparation technology for lithium-ion battery electrolytes, specifically relating to a composite lithium salt based on lithium hexafluorophosphate and its in-situ preparation method. Background Technology

[0002] Since the advent of commercial lithium-ion batteries, the most commonly used lithium salt is lithium hexafluorophosphate (LiPF6), which is used in various batteries due to its stable electrochemical window and good system compatibility. However, it has defects such as unstable chemical properties, limited efficiency at low temperatures, and poor thermal stability. In particular, it is extremely sensitive to moisture and will decompose to produce hydrogen fluoride (HF) during use, which will cause battery failure. As a result, it is becoming increasingly difficult to support the ever-increasing requirements for the comprehensive performance of batteries.

[0003] Lithium difluorophosphate (LiPO2F2), as a key functional additive, can optimize the SEI film, reduce interfacial impedance, and improve battery cycle and high-temperature stability. In-situ composite of LiPO2F2 and LiPF6 to form a single powder avoids problems such as uneven secondary dissolution and impurity introduction during electrolyte preparation.

[0004] Existing methods for preparing composite lithium salts primarily employ solid-phase reaction and liquid-phase synthesis. Solid-phase reaction (LiPF6 + Li3PO4) requires high temperatures (120-160°C) and introduces a large amount of external phosphorus source, inevitably generating the byproduct LiF, prone to over-reaction, and damaging the LiPF6 crystal structure. Liquid-phase synthesis requires reaction in organic solvents, easily resulting in solvent residues, the introduction of anionic impurities, complex separation and purification, high costs, and poor environmental friendliness. For example, patent document CN111924860A discloses a method for preparing a composite lithium salt concentrate. This method first places lithium carbonate in an organic solvent under an inert atmosphere to obtain a slurry, then introduces phosphorus pentafluoride gas into the slurry to react, obtaining a composite lithium salt concentrate. It uses phosphorus pentafluoride and lithium carbonate as raw materials, and prepares a composite lithium salt concentrate containing lithium difluorophosphate and lithium hexafluorophosphate in a one-step gas-solid reaction under non-aqueous solvent conditions.

[0005] Therefore, the composite lithium salt process, which is mild, free of impurities, has precise and controllable conversion rate, and is synthesized in situ in a green manner, has become a core technology requirement in the industry. Summary of the Invention

[0006] Therefore, the purpose of this invention is to provide an in-situ preparation method for lithium hexafluorophosphate-based composite lithium salts. This method has advantages such as single raw material, in-situ conversion, zero waste and environmental friendliness, and adjustable lithium salt ratio. Using LiPF6 as the sole raw material, LiPF6-LiPO2F2 composite lithium salts are generated in-situ under mild conditions through the synergistic effect of trace amounts of gas-phase water hydrolysis and a fluoride scavenger. The preparation process requires no recrystallization and generates no waste liquid, significantly reducing production costs and improving the inherent defects of lithium hexafluorophosphate.

[0007] The present invention also provides a composite lithium salt based on lithium hexafluorophosphate prepared by the above method. The composite lithium salt is a LiPF6-LiPO2F2 core-shell composite lithium salt containing 1wt%~10wt% LiPO2F2. The product has high purity and low acid value, and is suitable for high-nickel ternary, silicon-carbon anode and solid-state battery electrolytes.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides an in-situ preparation method for a lithium hexafluorophosphate-based composite lithium salt. In a closed reaction apparatus under inert gas protection, LiPF6 and a fluoride scavenger are first mixed, and then gaseous water is introduced to carry out the reaction. After the reaction is completed, a crude product is obtained. The crude product is dried, pulverized, and sieved to obtain the lithium hexafluorophosphate-based composite lithium salt.

[0009] Furthermore, the oxygen content in the sealed reaction device is ≤1ppm and the moisture content is ≤1ppm to ensure that the reaction environment is free from impurities; the inert gas includes nitrogen or argon, and the inert gas is a high-purity gas with a purity ≥99.999%.

[0010] Furthermore, the purity of the LiPF6 is ≥99.95%, and the moisture content is ≤1ppm.

[0011] Furthermore, the fluoride scavenging agent comprises fluoride salts and / or fluorophosphates, wherein the fluoride salts include at least one selected from LiF, NaF, and CaF2, and the fluorophosphates include lithium monofluorophosphate. The fluoride scavenging agent is used to capture HF generated instantaneously during the hydrolysis reaction in situ, forming a stable solid fluoride salt, thus preventing the accumulation of free acid, equipment corrosion, and product contamination.

[0012] Furthermore, the amount of the fluoride scavenger is 0.05 to 1.0% of the mass of LiPF6.

[0013] Furthermore, a trace amount of vaporized water is introduced into the closed reaction device and ensured to be uniformly distributed in the reaction atmosphere. The content of the vaporized water is 5~50ppm, which realizes directional, local, and mild hydrolysis of the surface of LiPF6 crystal particles and avoids deep hydrolysis and side reactions of LiPF6 crystal salt.

[0014] Furthermore, the reaction temperature is 25~85℃, the time is 2~12h, and the pressure is 0.02~0.1MPa.

[0015] Furthermore, the drying temperature is 85~100℃, and the sieve used for sieving has a mesh size of 100~200 mesh.

[0016] The preparation method of this invention has the core mechanism of directional trace surface hydrolysis + in-situ HF capture. Using LiPF6 as a single raw material, local hydrolysis of the LiPF6 particle surface is induced by trace amounts of vapor-phase water, while HF is in-situ bound to a fluoride scavenger to achieve in-situ generation and recombination of LiPO2F2. The reaction equation is as follows (LiF is used as an example of the in-situ scavenger): LiPF6(s)+2H2O(g)+4LiF(s)→LiPO2F2(s)+4LiF·HF(s) When preparing composite lithium salts in situ using this method, localized hydrolysis occurs only on the surface of LiPF6 particles, without damaging the main crystal structure of LiPF6, thus ensuring the purity of the main salt. The HF generated instantaneously during hydrolysis is immediately captured by the LiF scavenger, forming a stable LiF·HF solid-state composite, with no free HF, no waste acid emissions, and no equipment corrosion. The hydrolysis product LiPO2F2 adheres in situ to the surface and interstices of LiPF6 particles, forming a molecularly uniformly dispersed composite lithium salt, requiring no additional mixing. The entire reaction system is a solid-gas phase with no liquid phase involved, resulting in no waste liquid generation and an atom utilization rate close to 100%.

[0017] The present invention further provides a composite lithium salt based on lithium hexafluorophosphate, which is prepared by the above-mentioned in-situ preparation method of composite lithium salt based on lithium hexafluorophosphate; the method does not require recrystallization or washing, and the obtained composite lithium salt can be directly used for electrolyte preparation.

[0018] Furthermore, the content of LiPO2F2 in the lithium hexafluorophosphate-based composite lithium salt is 1~10wt%, and the purity of the main salt LiPF6 is not less than 99.95%.

[0019] The beneficial effects of this invention are: 1. The in-situ preparation method of lithium hexafluorophosphate-based composite lithium salt provided by this invention uses gaseous water as the reaction medium, and only a mild and uniform local transformation occurs on the crystal surface, which can avoid the violent hydrolysis and impurity generation caused by liquid water. In-situ capture is completed at the same time as the hydrolysis to generate HF, which can prevent the accumulation of free acid and eliminate the need for subsequent acid removal process. The production cost is low and the industrialization feasibility is high. LiPF6 is used as a single raw material, which eliminates the need to purchase LiPO2F2 and external phosphorus sources, and eliminates the need for recrystallization and waste liquid treatment equipment. It saves the secondary mixing and dehydration processes, and the finished product can be directly dissolved and used. The process is mild and safe, far below the thermal decomposition temperature of LiPF6, with no high temperature and high pressure risks, and is green and waste-free, with significant environmental advantages, which meet the requirements of green manufacturing of lithium battery materials.

[0020] 2. The composite lithium salt based on lithium hexafluorophosphate provided by this invention has a LiPO2F2 content of 1~10wt% and a main salt LiPF6 purity of not less than 99.95%. The product has high purity and low acid value, can be used directly, and is suitable for high-nickel ternary, silicon-carbon anode and solid-state battery electrolytes. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below. All mentioned embodiments are implemented based on the technical solutions of the present invention, and detailed implementation processes are given. However, it should be stated that the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise specified, the experimental methods used in the following experimental examples are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available reagents and materials; unless otherwise specified, the room temperature or room temperature refers to 25±5℃; all operations are performed under an anhydrous and oxygen-free inert atmosphere.

[0022] Example 1

[0023] The following steps were taken to prepare a composite lithium salt with a LiPO2F2 content of 4.5 wt%: Raw material pretreatment and loading: Take 100g of anhydrous high-purity LiPF6 crystals (purity 99.96%, moisture 0.8ppm) and load them into a sealed reactor. Purge with high-purity nitrogen (purity 99.999%) for purging twice, each purging time being 30min. After purging, ensure that the oxygen content in the reactor is 0.8ppm and the moisture content is 0.9ppm by detection, and maintain a slight positive pressure (0.06MPa) in the reactor. Then slowly add 0.4g of anhydrous high-purity LiF powder and start stirring to ensure that LiF and LiPF6 are uniformly mixed without agglomeration. Trace vapor phase water directional humidity control: Turn on the bubbling humidifier and introduce high-purity nitrogen into the bubbling bottle, so that the nitrogen carries saturated water vapor to form a humidifying inert gas. By detection, the vapor phase water content entering the closed reaction vessel is stabilized at 18ppm (±1ppm). Humidifying nitrogen is continuously introduced and maintained. A mild, directional hydrolysis reaction was conducted. The reaction temperature was set at 52℃ (temperature control accuracy ±0.5℃), and the reaction time was 5.5 hours, with stirring maintained. During the reaction, the vapor phase moisture content was recorded every hour and stabilized at 18ppm (±1ppm) to ensure a smooth reaction without excessive hydrolysis. After the reaction was completed, the humidification device was turned off, and high-purity dry nitrogen was continuously introduced to replace the residual moisture and trace amounts of tail gas in the reactor. The tail gas was treated by an alkali absorption tower before being discharged to ensure no HF leakage. Finally, the obtained product was transferred to a vacuum drying oven, and the drying temperature was set to 95℃. After drying, it was cooled to room temperature, and trace agglomerated particles were removed by sieving through a sieve to obtain 100.3g of LiPF6-LiPO2F2 composite lithium salt, of which LiPF6 content was 95.2wt%, LiPO2F2 content was 4.5wt%, LiF·HF content was 0.3wt%, acidity was 2ppm, moisture content was 1ppm, and there were no other impurities.

[0024] Example 2

[0025] The following steps were taken to prepare a composite lithium salt with a LiPO2F2 content of 7.8 wt%: Raw material pretreatment and loading: Take 50g of anhydrous high-purity LiPF6 crystals (purity 99.95%, moisture 1ppm) and load them into a sealed reactor. Purge with high-purity nitrogen (purity 99.999%) for purging, repeating purging 3 times, each purging time is 20min. After purging, ensure that the oxygen content in the reactor is 1.2ppm and the moisture content is 0.9ppm by detection, and maintain a slight positive pressure (0.1MPa) in the reactor. Then slowly add 0.4g of anhydrous high-purity LiF powder and start stirring to ensure that LiF and LiPF6 are mixed uniformly without agglomeration. Trace vapor phase water directional humidity control: Turn on the bubbling humidifier and introduce high-purity nitrogen into the bubbling bottle so that the nitrogen carries saturated water vapor to form a humidifying inert gas. By detection, the vapor phase water content entering the reactor is stabilized at 32ppm (±1ppm). Humidifying nitrogen is continuously introduced and maintained. A mild, directional hydrolysis reaction was conducted. The reaction temperature was set at 68℃ (temperature control accuracy ±0.5℃), and the reaction time was 7 hours, with stirring maintained. During the reaction, the vapor phase moisture content was recorded every hour and stabilized at 32ppm (±1ppm). The temperature and moisture content were adjusted in real time to ensure a smooth reaction without excessive hydrolysis. After the reaction was completed, the humidification device was turned off, and high-purity dry nitrogen was continuously introduced to replace the residual moisture and trace amounts of tail gas in the reactor. The tail gas was treated by an alkali absorption tower before being discharged to ensure no HF leakage. Finally, the obtained product was transferred to a vacuum drying oven, and the drying temperature was set to 90℃. After drying, it was cooled to room temperature, and trace agglomerated particles were removed by sieving through a sieve to obtain 50.25g of LiPF6-LiPO2F2 composite lithium salt, of which the content of LiPF6 was 91.9wt%, the content of LiPO2F2 was 7.8wt%, the content of LiF·HF was 0.3wt%, the acidity was 1.6ppm, the moisture content was 0.6ppm, and there were no other impurities.

[0026] Implementation effect analysis

[0027] Electrolytes were prepared by adding the LiPF6-LiPO2F2 composite lithium salt obtained in Examples 1 and 2, and pure LiPF6, respectively, to solvents. The lithium ion concentration in the electrolytes was uniformly 1 mol / L. Electrolyte A used commercially available LiPF6 as the lithium salt, electrolyte B used the lithium salt obtained in Example 1, and electrolyte C used the lithium salt obtained in Example 2. All three electrolytes used a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1. The additives were 1 wt% vinylene carbonate (VC) and 2 wt% fluoroethylene carbonate (FEC). Subsequently, the three electrolytes were assembled into batteries with a ternary cathode, a graphite anode, and a ceramic-coated separator. After assembly, high and low temperature charge-discharge, high rate charge-discharge, and high-temperature cycling were performed to test the free HF content of the electrolytes. The test results are summarized in Table 1 below. Table 1 Summary of Battery Performance Tests Containing Different Lithium Salt Electrolytes

[0028] As can be seen from the test results in Table 1, electrolytes containing only LiPF6 have defects such as poor high and low temperature and rate performance, and continuous increase in acidity during cycling. Electrolytes B and C prepared using composite lithium salts can dissociate simultaneously after dissolution and participate in interfacial film formation, effectively inhibiting lithium salt hydrolysis, metal dissolution and electrolyte decomposition, and significantly improving battery cycle life, low temperature discharge and high rate charge and discharge performance. Among them, composite lithium salt electrolyte B with a LiPO2F2 mass fraction of 4.5 wt% has the best comprehensive electrochemical performance, while taking into account lithium salt dissolution stability and electrochemical performance under all operating conditions.

[0029] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An in-situ preparation method of a composite lithium salt based on lithium hexafluorophosphate, characterized in that, In a closed reaction apparatus, under the protection of an inert gas, LiPF6 and a fluoride scavenger are first mixed, and then gaseous water is introduced to carry out the reaction. After the reaction is completed, a crude product is obtained. The crude product is dried, pulverized, and sieved to obtain the composite lithium salt based on lithium hexafluorophosphate.

2. The in-situ preparation method of lithium hexafluorophosphate-based composite lithium salt according to claim 1, characterized in that, The closed reaction device contains oxygen content ≤1ppm and moisture content ≤1ppm, and the inert gas includes nitrogen or argon.

3. The in-situ preparation method of lithium hexafluorophosphate-based composite lithium salt according to claim 1, characterized in that, The purity of the LiPF6 is ≥99.95%, and the moisture content is ≤1ppm.

4. The in-situ preparation method of lithium hexafluorophosphate-based composite lithium salt according to claim 1, characterized in that, The fluoride scavenger includes fluoride salts and / or fluorophosphates, wherein the fluoride salts include at least one of LiF, NaF, and CaF2, and the fluorophosphates include lithium monofluorophosphate.

5. The in-situ preparation method of lithium hexafluorophosphate-based composite lithium salt according to claim 1, characterized in that, The amount of the fluoride scavenger is 0.05 to 1.0% of the mass of LiPF6.

6. The in-situ preparation method of lithium hexafluorophosphate-based composite lithium salt according to claim 1, characterized in that, The content of the vapor phase water is 5~50ppm.

7. The in-situ preparation method of lithium hexafluorophosphate-based composite lithium salt according to claim 1, characterized in that, The reaction is carried out at a temperature of 25~85℃ for 2~12 hours and at a pressure of 0.02~0.1MPa.

8. The in-situ preparation method of lithium hexafluorophosphate-based composite lithium salt according to claim 1, characterized in that, The drying temperature is 85~100℃, and the sieving process uses a sieve mesh of 100~200 mesh.

9. A composite lithium salt based on lithium hexafluorophosphate, characterized in that, It is prepared using the in-situ preparation method of lithium hexafluorophosphate-based composite lithium salt according to any one of claims 1 to 8.

10. The composite lithium salt based on lithium hexafluorophosphate according to claim 9, characterized in that, The content of LiPO2F2 in the composite lithium salt is 1~10wt%.

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