Aqueous Sodium‐Ion Battery using a Na3V2(PO4)3 Electrode

Abstract The NASICON‐type Na 3 V 2 (PO 4 ) 3 (NVP) cathode material is investigated in an aqueous sodium‐ion battery, which is explored by using a three‐electrode system. The battery behaviors and capacitive properties of this electrode system are critically investigated by using 1 m Li 2 SO 4 , Na 2 SO 4 , and K 2 SO 4 electrolytes, with an optimal performance found to arise in Na + ‐based electrolyte, which exhibits a capacitance of 209 Fg −1 at 8.5 C as well as enhanced ion diffusion. Larger, hydrated Li + is less able to diffuse into the network of NVP, and the low conductivity and mobility leads to near noncapacitive behavior. In the case of K + ‐based electrolyte, NVP presents asymmetric cyclic voltammograms, owing to weak solvation and the high conductivity of K + , making the ions more easily able to form electric double‐layer capacitance on the surface or pores of NVP, rather than insert into the network. The equivalent circuit based on electrochemical impedance spectroscopy result is analyzed to account for the electrochemical insertion behavior of Na + into NVP, involving ion transfer in electrolyte solution, ion diffusion from the electrolyte to the electrode surface, as well as charge transfer and ion diffusion in the electrode solid.