Highly Reversible Zn Metal Anode with Low Voltage Hysteresis Enabled by Tannic Acid Chemistry

The zinc dendrites and side reactions formed on the zinc anode have greatly hindered the development of aqueous zinc-ion batteries (ZIBs). Herein, we introduce tannic acid (TA) as an additive in the ZnSO₄ (ZSO) electrolyte to enhance the reversible Zn plating/stripping behavior. TA molecules are found to adsorb onto the zinc surface, forming a passivation layer and replacing some of the H₂O molecules in the Zn²⁺ solvation sheath to form the [Zn(H₂O)_6-xTA_x]²⁺ complex; this process effectively prevents side reactions. Moreover, the lower desolvation energy barrier of the [Zn(H₂O)_6-xTA_x]²⁺ structure facilitates uniform Zn metal deposition and enables a stable plating/stripping lifespan of 2500 h with low voltage hysteresis (53 mV at 0.5 mA cm^-2) as compared to the ZSO electrolyte (167 h and 104 mV). Additionally, the incorporation of the MnO₂ cathode in the TA + ZSO electrolyte shows improved cycling capacity retention, from 64% (ZSO) to 85% (TA + ZSO), after 250 cycles at 1 A g^-1, demonstrating the effectiveness of the TA additive in enhancing the performance of ZIBs.