Progress in the preparation technology and industrialization exploration of sodium hexafluorophosphate

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As the global energy structure transitions toward green and low-carbon models, the importance of electrochemical energy storage technologies is becoming increasingly evident. While lithium-ion batteries hold a dominant position, the supply-demand imbalance and cost fluctuations of lithium, the key raw material for them, have prompted the industry to turn its attention to alternative systems that offer richer resources. Under these circumstances, sodium-ion batteries came into being. Thanks to their abundant reserves, wide distribution, relatively stable costs, and high inherent safety, they are regarded as a next-generation electrochemical energy storage technology with great application potential in areas such as medium and large-scale energy storage and low-speed electric vehicles. As an essential electrolyte salt in sodium-ion battery electrolytes, the performance and quality of sodium hexafluorophosphate (NaPF₆) are directly related to the overall efficiency, cycle life, and safety of the batteries. The maturity of its manufacturing technology and its capacity for large-scale production are among the key factors in the industrialization of sodium-ion batteries. Recently, domestic research teams have completed systematic process research on the development of technologies for the production of sodium hexafluorophosphate, achieving a transition from laboratory-scale tests to pilot-scale production; this provides valuable technical guidance for the domestic supply of such materials. I. Adaptive development based on mature process routes: The chemical preparation principle of sodium hexafluorophosphate is similar to that of lithium hexafluorophosphate (LiPF₆), which has been widely industrialized, providing a solid foundation for technology development. The current mainstream and reliable process routes in the industry all use anhydrous hydrogen fluoride (HF) as a solvent. The process consists of two main steps: first, phosphorus pentachloride (PCl₅) reacts with anhydrous hydrogen fluoride to produce high-purity phosphorus pentafluoride (PF₅) gas ； Subsequently, PF₅ gas reacts with sodium fluoride (NaF) in an HF solvent to synthesize sodium hexafluorophosphate. This technical development work is based on this established approach; it focuses on optimizing various processes and exploring appropriate conditions related to specific aspects such as reaction control, crystallization and separation, as well as product purification during the synthesis of sodium hexafluorophosphate. The goal is to achieve precise control over the product’s purity, impurity levels, and physical properties, so as to meet the standards required for use in battery applications. II. Research and optimization of key process steps 1. Control of the synthesis reaction process The efficiency of the synthesis reaction is the basis for determining the purity of the final product. The study focuses on the absorption reaction process between PF₅ gas and sodium fluoride in an HF solution. Efficient and steady conversion of the reactants was achieved by adjusting the initial concentration of sodium fluoride, controlling the flow rate and molar ratio of PF₅ gas, and precisely managing the temperature of the reaction system (to balance reaction activity with preventing excessive volatilization of HF). The optimized process ensures full absorption of PF₅ and complete reaction, thereby creating the conditions for obtaining sodium hexafluorophosphate with a low impurity content. At the same time, the form of the reaction equipment was examined in detail, further confirming the rationality of the design conditions for the reaction equipment. 2. Selection and comparison of crystallization separation processes Crystallization is the key step in obtaining solid products, and the method used directly affects the purity of the products, their crystal form, as well as the difficulty of subsequent processing. The research team conducted detailed experimental studies on the mainstream crystallization method—cooling crystallization (freezing crystallization). Cooling crystallization: The solute is precipitated by slowly lowering the temperature of the solution. The crystals obtained by this method are large in size and have a relatively intact crystal form. However, the experiments also revealed its shortcomings: a longer crystallization period and higher energy consumption ； Larger crystals tend to encapsulate the mother liquor, which may result in a higher level of free acid (HF) in the product ； Crystals tend to adhere to the inner walls of the equipment, affecting yield and operational continuity. Based on a comprehensive evaluation of product quality, production efficiency, and operability, the project team conducted detailed experimental studies on the cooling rate, crystallization rate, and the selection of crystallization equipment, successfully producing sodium hexafluorophosphate with low levels of free acid and high purity, thereby verifying the rationality of the chosen crystallization equipment. 3. Refinement of the product drying process: The wet cake after crystallization contains residual HF and moisture, and it must undergo deep drying to meet the stringent requirements of battery-grade materials. Since sodium hexafluorophosphate is sensitive to heat and moisture, the type of drying equipment was selected accordingly, and the impact of different drying conditions on product quality was thoroughly studied. By systematically examining the changes in product moisture and free acid under different drying temperatures and times, appropriate process conditions were determined that enable efficient removal of impurities while maintaining the chemical stability of the product. After this treatment, the moisture and acidity levels of the product can be kept at low levels stably, thereby verifying the rationality of the drying equipment chosen. III. Pilot-scale verification and product quality: To assess the feasibility of scaling up the laboratory process, multiple batches of pilot-scale experiments were conducted. The pilot-scale operation results verified the reliability of the lab-scale process parameters and indicated that this process route has good potential for scale-up. The multiple batches of sodium hexafluorophosphate produced on a pilot scale were tested, and it was found that all their key quality parameters met or exceeded the predetermined technical requirements, being on par with those of similar products available in the current market. Specifically, the main content of the product remains stable at over 99.98%, the moisture content is between 24–45 ppm, the content of free acids (expressed as HF) is controlled at 11–19 ppm, and the total amount of key metal impurities such as Na, K, Fe, etc. is below 10 ppm. The levels of anionic impurities such as chloride ions and sulfate ions are also low. These data indicate that electrolyte materials suitable for high-performance sodium-ion batteries can be stably prepared through this process. IV. Preliminary Exploration of Technical Value and Economic Viability This systematic process development effort has resulted in the accumulation of specialized technologies for the production of sodium hexafluorophosphate. The process package it verified, particularly the selection of equipment types for the reaction and crystallization stages as well as for the drying stage, along with the implementation of precise control, has enabled the production of multiple batches of qualified products. The improvement of this technology holds positive significance as a reference for promoting the large-scale and standardized production of sodium hexafluorophosphate. From the perspective of cost analysis in the early stages of industrialization, this process relies on the existing domestic fluorine chemical raw material supply system, ensuring a stable supply of key raw materials. Preliminary estimates indicate that, through appropriate equipment design, process control, and future economies of scale, the production cost of sodium hexafluorophosphate can be made competitive in the market. As the sodium-ion battery industry chain is developed at an accelerated pace, the large-scale and cost-effective supply of key upstream materials such as sodium hexafluorophosphate will become an essential foundation for the growth of the entire industry. V. Conclusion Through this systematic study on the technology for producing sodium hexafluorophosphate, moving from laboratory exploration to pilot-scale production, a relatively complete and controllable process scheme has been developed. The research results not only confirm the feasibility of developing high-performance sodium battery materials based on existing fluorine chemical technology platforms, but also provide critical data and empirical support for subsequent industrial-scale project development. The commercialization wave of sodium-ion batteries is underway, and the maturity of its industrial chain depends on technological advancements and coordinated development in every key link. As a key component of the electrolyte, the continuous improvement of the production technology for sodium hexafluorophosphate and its stable mass production will be an important step in advancing sodium-ion battery technology from a demonstration stage to widespread application. In the future, as the demands on the application side become more clear and manufacturing processes continue to improve, sodium hexafluorophosphate is likely to play an increasingly important role in the vast energy storage market.