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 Ca_0.67V₈O₂₀·3.5H₂O (CaVO) nanobelts as a type of ZIB cathode with a discharge capacity of 466 mAh g^-1 (equivalent to an energy density of 345.6 Wh kg^-1) at 0.1 A g^-1 and a capacity retention rate of 100%, 95%, and 74% at 5.0 A g^-1 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 Zn²⁺-transport friendly, bilayer ρ-type V₂O₅ structure, and the structure-derived reversibility in single-phase Zn²⁺-intercalation/deintercalation process. We also uncover that Ca²⁺ as a structural stabilizer in CaVO undergoes a fast, performance-harmless ion-exchange with Zn²⁺ in the electrolyte and the entire Zn²⁺-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 Ca²⁺/Zn²⁺ and structural water.