Mechanochemical Large-Scale Rapid Synthesis of Ultrapure Sodium Hexafluorophosphate

Abstract

Among the sodium battery electrolytes, sodium hexafluorophosphate (NaPF₆) exhibits superior conductivity, anodic stability, and stable cathode electrolyte interface compared to other electrolytes. Therefore, the synthesis of pure NaPF₆ through a simple process is very important. Usually, NaPF₆ is synthesized using HF. In our approach, NaPF₆ is synthesized by grinding dry ammonium hexafluorophosphate (NH₄PF₆) and sodium metal. Sodium injects an electron into the ammonium ion, which results in the formation of ammonia and hydrogen. The gram scale synthesis is completed in about 30 min. Purification of the product is not needed. The product purity is confirmed by various spectroscopic and electrochemical techniques. Usually, NaPF₆ comprises NaF, HF, and solvents as impurities that affect the performance of SIBs. It has been confirmed that the NaPF₆ synthesized by our mechanochemical approach in the absence of solvent is devoid of impurities despite the absence of product purification step. Furthermore, the synthesis of pure NaPF₆ (250 g) is demonstrated using a grinder used as household item in cooking Indian pancakes, which costs about 300 USD. The duration of the synthesis of 250 g pure NaPF₆ is 1 h. The purity of this sample is comparable to that of NaPF₆ (5 g) synthesized using mortar and pestle.