A High-Energy and Long-Life Aqueous Zn/Birnessite Battery via Reversible Water and Zn²⁺ Coinsertion

Aqueous rechargeable Zn/birnessite batteries have recently attracted extensive attention for energy storage system because of their low cost and high safety. However, the reaction mechanism of the birnessite cathode in aqueous electrolytes and the cathode structure degradation mechanics still remain elusive and controversial. In this work, it is found that solvation water molecules coordinated to Zn²⁺ are coinserted into birnessite lattice structure contributing to Zn²⁺ diffusion. However, the birnessite will suffer from hydroxylation and Mn dissolution with too much solvated water coinsertion. Through engineering Zn²⁺ primary solvation sheath with strong-field ligand in aqueous electrolyte, highly reversible [Zn(H₂ O)₂ ]²⁺ complex intercalation/extraction into/from birnessite cathode is obtained. Cathode-electrolyte interface suppressing the Mn dissolution also forms. The Zn metal anode also shows high reversibility without formation of "death-zinc" and detrimental dendrite. A full cell coupled with birnessite cathode and Zn metal anode delivers a discharge capacity of 270 mAh g^-1 , a high energy density of 280 Wh kg^-1 (based on total mass of cathode and anode active materials), and capacity retention of 90% over 5000 cycles.