Binary solvents assisting the long-term stability of aqueous K/Zn hybrid batteries

Aqueous Zn-based batteries with intrinsic safety and cost-effectiveness possess attractive prospects for grid-scale electric energy storage applications. However, the sluggish intercalation kinetics of divalent Zn ions and their strong electrostatic interaction with the cathode materials restrict the development of aqueous Zn-based batteries. In this work, we fabricate K/Zn hybrid batteries with Berlin green (FeHCF) cathode and aqueous KCF3SO3/Zn(CF3SO3)2 electrolyte using fire-retardant triethyl phosphate (TEP) as co-solvent. The addition of TEP suppresses the water decomposition and widens the electrochemical stability window, increasing the Coulombic efficiency. The TEP co-solvent prevents the Zn displacement of FeHCF but also inhibits its surface reduction and decomposition by reducing water activity. Moreover, the TEP suppresses the dendrite growth and parasitic reactions on the Zn anode. Through these positive effects, significant improvement is achieved in the cycle performance of the hybrid batteries. The battery with a TEP-H2O ratio of 9-1 delivers a high specific capacity of 142 mAh·g−1 at 0.1 A·g−1 and excellent cyclability with a capacity retention of 80.5% over 3000 cycles at 1 A·g−1. Our results suggest that tuning the solvent of the electrolyte can effectively optimize the electrochemical cyclability of aqueous Zn-based batteries.