High-Performance\nAqueous Calcium Ion Batteries Enabled\nby Zn Metal Anodes with Stable Ion-Conducting Interphases

Aqueous\ncalcium ion batteries, promising for energy storage, are\nstill challenged by very limited anode choices. Although a Zn metal\nanode is popular in aqueous batteries, interface instability due to\nincessant corrosion and severe Zn dendrites hinders its development.\nHere, an interphase layer with densely packed nanocrystals of Ca<sub>3</sub>(CO<sub>3</sub>)<sub>2</sub>(OH)<sub>2</sub>·1.5H<sub>2</sub>O and ZnF<sub>2</sub>, and amorphous organic species, is demonstrated\nfor a Zn metal anode with 1 M calcium trifluoromethyl sulfonate aqueous\nelectrolyte. The hybrid interface fully avoids direct Zn–H<sub>2</sub>O contact, maintains fast ion conductivity, and effectively\nprevents corrosion and dendrite growth. Therefore, the symmetric cell\nstably lasts for 1600 h at 0.5 mA cm<sup>–2</sup> and 2.5 mAh\ncm<sup>–2</sup>, far superior to 150 h for the control cell.\nFurthermore, the device maintains 80% capacity retention after 700\ncycles at 1 A g<sup>–1</sup>, outperforming 13% retention after\n200 cycles for the control device. This work indicates that interface\nand interphase engineering is also crucial for aqueous batteries.