Inhibiting VOPO₄⋅x H₂O Decomposition and Dissolution in Rechargeable Aqueous Zinc Batteries to Promote Voltage and Capacity Stabilities

VOPO₄ ⋅x H₂ O has been proposed as a cathode for rechargeable aqueous zinc batteries. However, it undergoes significant voltage decay in conventional Zn(OTf)₂ electrolyte. Investigations show the decomposition of VOPO₄ ⋅x H₂ O into VOx in the electrolyte and voltage drops after losing the inductive effect from polyanions.PO₄ ³⁻ was thus added to shift the decomposition equilibrium. A high concentration of cheap, highly soluble ZnCl₂ salt in the electrolyte further prevents VOPO₄ ⋅x H₂ O dissolution. The cathode shows stable capacity and voltage retentions in 13 m ZnCl₂ /0.8 m H₃ PO₄ aqueous electrolyte, in direct contrast to that in Zn(OTf)₂ where the decomposition product VOx provides most electrochemical activity over cycling. Sequential H⁺ and Zn²⁺ intercalations into the structure are revealed, delivering a high capacity (170 mAh g⁻¹ ). This work shows the potential issue with polyanion cathodes in zinc batteries and proposes an effective solution using fundamental chemical principles.