Vanadium hexacyanoferrate with two redox active sites as cathode material for aqueous Zn-ion batteries

Aqueous rechargeable Zn-ion batteries (ARZIBs) are attractive due to their low cost, high safety and environmental friendliness. However, the development of ARZIBs still faces many challenges. One of the biggest challenges is to find suitable cathode materials for reversible Zn²⁺ ions insertion/extraction. In this work, we synthesize a Prussian blue analogue (PBA) with a new chemical composition, vanadium hexacyanoferrate (VHCF), via a simple co-precipitation method and study its performance as a cathode material for an aqueous rechargeable Zn-ion battery. The VHCF with two electrochemical redox active sites, V and Fe, delivers a three-electron redox reaction with a capacity of 187 mA h g⁻¹ at current density of 0.5 A g⁻¹. Even at a current rate of 5 A g⁻¹, VHCF can still deliver a large capacity of 122 mA h g⁻¹. The Zn//VHCF battery also exhibits a long cycle life of 1000 cycles with excellent capacity retention of 87.8% and a high Coulombic efficiency close to 100%. During the first charging process, the cubic structure of VHCF changes to a rhombohedral phase, a structure where the Zn²⁺ ions can be reversibly inserted into and extracted from in subsequent cycles. The promising performances make VHCF an attractive candidate as cathode material for ARZIBs system.