Artificially coated NaFePO4 for aqueous rechargeable sodium-ion batteries

Aqueous rechargeable sodium-ion batteries are attractive as alternative materials to replace conventional lithium-ion batteries for the development of next-generation devices due to the abundance of sodium resources. Among various positive electrode materials for sodium-ion batteries, olivine NaFePO4 is attractive due to its high theoretical capacity and large electrochemical stability window. In this work, AlF3-coated olivine NaFePO4 is obtained by a facile electrochemical ion-exchange process in an aqueous electrolyte that is more environmentally friendly than conventional carbonate electrolytes. The robust AlF3-coated NaFePO4 displays better cycling stability and surface stability in the electrochemically fragile aqueous electrolyte. Physicochemical and electrochemical measurements confirm that the optimal improvement in the electrochemical performance of NaFePO4 is obtained using 0.5 wt% AlF3 in an aqueous electrolyte solution. Furthermore, the AlF3 coating on the surface of olivine NaFePO4 reduces surface failure by stabilizing the NaFePO4 surface in an aqueous electrolyte solution and facilitates lithium ion diffusion, leading to a NaFePO4 material with lower surface resistance and better surface stability.