Electrochemical Na + /K + Ion Exchange in Zeolitic Vanadium(IV) Borophosphate K 1.33 Na 0.67 [VO(B 2 O)(PO 4 ) 2 (HPO 4 )]·1.63H 2 O as Cathode Material for Sodium-Ion Batteries.

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 K 1.33 Na 0.67 [VO(B 2 O)(PO 4 ) 2 (HPO 4 )]·1.63H 2 O (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.