A calcium oxalate protective interphase for regulated zinc deposition in aqueous batteries

Interfacial instability of Zn metal anodes, including nonuniform deposition, parasitic reactions, hydrogen evolution, corrosion, and by-product accumulation, remains a major challenge for aqueous zinc-ion batteries. Herein, we report a facile calcium oxalate (CaC₂O₄) protective coating to regulate the Zn/electrolyte interface. Owing to its low solubility and inorganic stability, the CaC₂O₄ layer serves as a persistent ion-regulating interphase that reduces direct water contact and promotes more uniform Zn²⁺ transport and deposition. Structural characterization confirms a continuous coating, while XPS indicates interfacial electronic interaction. In situ Raman spectroscopy shows that the perturbed oxalate-related ν_s(COO^-) band remains detectable and evolves in intensity during initial Zn plating, supporting both persistence of the oxalate-containing framework and perturbation of its local interfacial environment. Electrochemical kinetic analyses show that the apparent activation energy for interfacial Zn²⁺ transport decreases from 17.58 to 10.04 kJ mol^-1 after CaC₂O₄ modification, accompanied by reduced charge-transfer resistance and nucleation overpotential. These improvements suppress corrosion, hydrogen evolution, and zinc hydroxide sulfate by-product formation, enabling approximately 1450 h of Zn∥Zn cycling at 5 mA cm^-2 and 1 mAh cm^-2. Moreover, CaC₂O₄@Zn∥MnO₂ full cells retain 60% capacity after 600 cycles at 1 A g^-1. This work demonstrates a low-cost inorganic interphase for stabilizing Zn anodes through persistent protection and interfacial Zn²⁺ transport regulation.