Unlocking the Reversible Selenium Electrode for Non‐Aqueous and Aqueous Calcium‐Ion Batteries

Abstract Calcium‐ion batteries (CIBs) have been considered promising multivalent ion battery systems due to the natural abundance and low redox potential of calcium. The practical realization is largely hampered by the lack of reliable electrode materials. Selenium is first explored as a new conversion‐type electrode for both non‐aqueous and aqueous CIBs. The selenium provides a specific capacity of 476 mAh g^–1 with an average voltage of 2.2 V versus Ca/Ca²⁺ at a current density of 50 mA g^–1, offering a higher energy density than other reported cathode materials. Long‐term cyclic stability at a large current density of 500 mA g^–1 has been achieved in non‐aqueous electrolytes through encapsulating the selenium in mesoporous carbon. The spectroscopy analysis and density functional theory calculations suggest multi‐step conversion processes involving CaSe₄ and Ca₂Se₅ polyselenides intermediates before reaching the final CaSe phase, exhibiting a distinct reaction pathway from those in other metal‐Se batteries. Furthermore, the application of selenium is extended to the aqueous electrolyte after expanding the electrochemical window. A 1.1 V class aqueous CIB is demonstrated by coupling with a Cu‐based Prussian blue electrode. The discovery of reversible Ca–Se chemistry opens new opportunities for emerging CIBs.