Inhibiting VOPO4 ⋅x H2 O 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.