| Summary: | Calcium metal batteries are receiving growing research attention due to significant breakthroughs in recentyears that have indicatedreversible 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-dependentin general,which affects both interfacial chemistryon plated Caalong with more fundamental Ca2+/Ca redox properties. This mini-review describes recent progress towards a reversible Ca anode from the point of view of the most successful electrolyte chemistries identified to date. Thisincludes, centrally, what is currently knownabout the Ca2+solvation environment in these systems. Experimental (physico-chemical and spectroscopy) and computationalresultsare summarized forthe two major solvent classes—carbonates and ethers—that have yieldedpromising results so far. Current knowledge gaps and opportunities toimprove fundamental understandingofCa2+/Ca redox are also identified.
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