Electrochemical oxidation of maricite NaFePO4 in mild aqueous solutions as a way to boost its charge storage capacity

Abstract

Lithium has a low abundance in the Earth's crust, which in a few years will lead to difficult lithium production, and therefore difficult production of lithium-ion batteries. Sodium-ion batteries, on the other hand, have been proven to be a good replacement. The material obtained from iron combined with the phosphate and pyrophosphate compounds of sodium has attracted attention as a possible cathode material for sodium-ion batteries. NaFePO4 exists in two polymorphic structures (triphylite and maricite). Maricite NaFePO4 is a more thermodynamically stable structure than triphylite NaFePO4 but doesn’t have channels for Na+ movement and electrochemical performance of this structure is low. In comparison to maricite NaFePO4, triphylite NaFePO4 (structural analogue to LiFePO4) has one-dimensional channels for Na+-ions movement and better electrochemical activity but it is not stable and is difficult to synthesize. Herein, the maricite NaFePO4 can be obtained by sintering a polyanionic compound, Na4Fe3(PO4)2P2O7, at temperatures above 600 °C, as shown by XRD. Na4Fe3(PO4)2P2O7 is synthesized by the glycine-nitrate process after which it was sintered at temperatures above 500 °C. The glycine-nitrate process was found to catalyze the decomposition of the sintered Na4Fe3(PO4)2P2O7 to the NaFePO4 maricite. The electrochemical characterization of the sintered material, evaluated in aqueous NaNO3 and LiNO3 electrolyte by cyclic voltammetry, showed poor electrochemical activity of maricite NaFePO4. By exposing the sintered material to high anodic potentials, the electrochemical activity and specific capacity of the material were increased by 50% in case of NaNO3 and 80% in case of LiNO3 relative to the pristine with low activity. After electrochemical measurements, residual powder was characterized by FTIR and Raman spectroscopy. It was shown that high anodic polarization of the material tested in LiNO3 causes the formation of triphylite LiFePO4. Similarly, it is assumed that the electrochemical activity obtained by deep anodic polarization of the material in NaNO3 electrolyte originates from the formed triphylite NaFePO4. The obtained results open novel directions regarding the use of NaFePO4 in metal-ion rechargeable batteries.