Electrochemical Na+/K+ Ion Exchange in Zeolitic Vanadium(IV) Borophosphate K1.33Na0.67[VO(B2O)(PO4)2(HPO4)]·1.63H2O as Cathode Material for Sodium-Ion Batteries.

ABSTRACT

Electrochemical ion exchange has recently been demonstrated to be a unique method for the preparation of novel cathode materials, which cannot be accessible by traditional direct synthesis routes. In this study, the vanadium borophosphate compound K1.33Na0.67[VO(B2O)(PO4)2(HPO4)]·1.63H2O (KNVBP) with zeolitic framework exhibits fast electrochemical Na+/K+ ion exchange when used as cathode material in sodium-ion batteries (SIBs). Ex situ structural analyses and electrochemical measurements confirm that most of the K+ ions in the parent KNVBP can be extracted and exchanged by Na+ ions after the first charge/discharge cycle. The in situ-generated Na-rich phase shows reversible electrochemical activity at approximately 3.9 V versus Na+/Na with a specific capacity of 52.9 mAh g-1, comparable to 96.2% of the theoretical capacity. Moreover, enhanced ionic diffusion kinetics can be achieved after the Na+/K+ exchange. This study provides a valuable insight into the electrochemical ion exchange in polyanion compounds toward application in metal-ion batteries.