Anion‐Blocking and Multipath‐Conducting Interfaces Enable Long‐Life Room‐Temperature Ester‐Based Ca‐Metal Batteries

Ester-based lithium storage has been reliably validated for tens of years. However, calcium-metal (Ca-metal) batteries, as one of the most promising alternatives to lithium ion batteries, are restrained to several non-ester or mixed-cations electrolytes, while those utilizing pure calcium salt ester electrolytes have so far been considered impossible to durably work at room temperature due to easily passivated Ca-metal anodes by anion corrosion, urgently necessitating stable and fast kinetics interfaces. Paired with a biomass-derived carbon cathode, this work demonstrated the first case of long-life pure calcium salt ester-based Ca-metal batteries of >500 cycles and 85.4% capacity retention at 25 mA g^-1 by engineered Ca-metal interfaces. Notably, the Ca-metal electrodes also achieved 0.53 V (vs Ca/Ca²⁺) deposition overpotential, 6 mAh cm^-2 interface-stable deposit capacity, and >950 h deposition/stripping stability at 0.02 mA cm^-2 and 0.02 mAh cm^-2. The enhanced interface kinetics and reversibility is attributed to the engineered Ca-metal interfaces with not only diverse interphases of calcium-(Ca-)/iron-(Fe-)based inorganic salts and core-shell iron (Fe) nanocrystals affording abundant multiple ion/electron transportation pathways, but also compact carbon-nitrogen (C─N organics as an effective anion-blocking medium. This work opens an interface engineering avenue for ester-based Ca-metal batteries.