Current Understanding of Nonaqueous Electrolytes for Calcium‐Based Batteries

Calcium metal batteries are receiving growing research attention due to significant breakthroughs in recent years that have indicated reversible Ca plating/stripping with attractive Coulombic efficiencies (90–95 %), once thought to be out of reach. While the Ca anode is often described as being surface film‐controlled, the ability to access reversible Ca electrochemistry is highly electrolyte‐dependent in general, which affects both interfacial chemistry on plated Ca along with more fundamental Ca 2+ /Ca redox properties. This minireview describes recent progress towards a reversible Ca anode from the point of view of the most successful electrolyte chemistries identified to date. This includes, centrally, what is currently known about the Ca 2+ solvation environment in these systems. Experimental (physico‐chemical and spectroscopy) and computational results are summarized for the two major solvent classes – carbonates and ethers – that have yielded promising results so far. Current knowledge gaps and opportunities to improve fundamental understanding of Ca 2+ /Ca redox are also identified.