High-Capacity Aqueous Storage in Vanadate Cathodes Promoted by the Zn-Ion and Proton Intercalation and Conversion-Intercalation of Vanadyl Ions.

Aqueous Zn-ion batteries (AZIBs) are promising alternatives to lithium-ion batteries in stationary storage. However, limited storage capacity and cyclic life impede their large-scale implementation. We report reversible electrochemical insertion of multi-ions into sodium vanadate (NaV3O8) cathode materials for AZIBs, achieving a maximum storage capacity of 450 mAh g-1 at 0.05 A g-1 and a capacity retention of 82% after 500 cycles at 0.4 A g-1. In addition to Zn2+ and H+ insertion, in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) collectively provide explicit evidence on vanadyl ions (VO2+) conversion-intercalation at the NaV3O8 cathode, showing the deintercalation of VO2+ from NaV3O8 and the consequent conversion of VO2+ into V2O5 on charging, and vice versa on discharging. Our study is the first to report on the cation conversion-intercalation mechanism in AZIBs. This reversible multi-ion storage mechanism provides a design principle for developing high-capacity aqueous electrode materials by engaging both the intercalation and conversion of charge carriers.