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 (NaVO) cathode materials for AZIBs, achieving a maximum storage capacity of 450 mAh g at 0.05 A g and a capacity retention of 82% after 500 cycles at 0.4 A g. In addition to Zn and H insertion, in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) collectively provide explicit evidence on vanadyl ions (VO) conversion-intercalation at the NaVO cathode, showing the deintercalation of VO from NaVO and the consequent conversion of VO into VO 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.