A High Capacity Bilayer Cathode for Aqueous Zn-Ion Batteries.

Aqueous Zn-ion batteries (ZIBs) are promising candidates for grid-scale energy storage because they are intrinsically safe, cost competitive, and energy intense. However, the development of ZIBs is currently challenged by the performance of cathode materials. Herein, we report on CaVO·3.5HO (CaVO) nanobelts as a type of ZIB cathode with a discharge capacity of 466 mAh g (equivalent to an energy density of 345.6 Wh kg) at 0.1 A g and a capacity retention rate of 100%, 95%, and 74% at 5.0 A g for 500, 1000, and 2000 cycles, respectively. Through a combined theoretical and experimental study, we reveal that the outstanding energy and power performances of CaVO are deeply rooted in its Zn-transport friendly, bilayer ρ-type VO structure, and the structure-derived reversibility in single-phase Zn-intercalation/deintercalation process. We also uncover that Ca as a structural stabilizer in CaVO undergoes a fast, performance-harmless ion-exchange with Zn in the electrolyte and the entire Zn-intercalation/deintercalation process is accompanied by a counter migration of solvent water. Last, we show that a successful synthesis of CaVO depends critically on pH value of the precursor solution and the structural stability of CaVO is controlled by the co-presence of Ca/Zn and structural water.