Operando pH Measurements Decipher H + /Zn 2+ Intercalation Chemistry in High-Performance Aqueous Zn/δ-V 2 O 5 Batteries.

Vanadium oxides have been recognized to be among the most promising positive electrode materials for aqueous zinc metal batteries (AZMBs). However, their underlying intercalation mechanisms are still vigorously debated. To shed light on the intercalation mechanisms, high-performance δ-V 2 O 5 is investigated as a model compound. Its structural and electrochemical behaviors in the designed cells with three different electrolytes, i.e., 3 m Zn(CF 3 SO 3 ) 2 /water, 0.01 M H 2 SO 4 /water, and 1 M Zn(CF 3 SO 3 ) 2 /acetonitrile, demonstrate that the conventional structural and elemental characterization methods cannot adequately clarify the separate roles of H + and Zn 2+ intercalations in the Zn(CF 3 SO 3 ) 2 /water electrolyte. Thus, an operando pH determination method is developed and used toward Zn/δ-V 2 O 5 AZMBs. This method indicates the intercalation of both H + and Zn 2+ into δ-V 2 O 5 and uncovers an unusual H + /Zn 2+ -exchange intercalation-deintercalation mechanism. Density functional theory calculations further reveal that the H + /Zn 2+ intercalation chemistry is a consequence of the variation of the electrochemical potential of Zn 2+ and H + during the electrochemical intercalation/release.