Exploring the effects of biomolecular additive on performance of aqueous zinc metal batteries

Aqueous zinc metal batteries, as a post-lithium-ion technology, exhibit exceptional performance due to their nonflammability, high theoretical anode capacity (820 mAh g⁻¹), and low redox potential (−0.762 V vs. the standard hydrogen electrode (SHE)). However, the stability of the zinc metal anode is compromised by several issues arising from the reduction of H₂O, including hydrogen generation, byproduct accumulation, and zinc dendrite formation. Herein, the low concentration of caffeine electrolyte additive is introduced to 1 M ZnSO₄ aqueous electrolyte. Despite the small quantity of additives, caffeine molecules adsorb onto the zinc metal anode, forming an unreactive layer that passivates the contact with reactive water molecules. Operando optical and X-ray imaging confirm that the caffeine passivation layer ensures homogeneous zinc deposition. Furthermore, the caffeine-enhanced electrolyte demonstrates high stability of the zinc metal anode in both symmetric and half-cell tests. Additionally, the Zn||NVO full cell with caffeine added electrolyte maintains a higher retention rate of discharge capacity compared to the blank electrolyte condition.