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

Abstract 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/Ca ²⁺ ) 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.