Suppressing Dissolution of Copper Chloride Cathodes With Fluorinated Electrolytes for Rechargeable Lithium/Sodium Batteries

ABSTRACT The Li||CuCl 2 battery was proposed as a prototype for high‐energy‐density lithium batteries in the 1960s. Despite its energy density exceeded that of lithium‐ion batteries at a much lower cost, its development was hindered by the high solubility of CuCl 2 in conventional carbonate electrolytes. Here, we suppressed the dissolution of CuCl 2 by rationally fluorinating solvents, thereby maintaining it at solid state during the reversible conversion of CuCl 2 ↔ CuCl ↔ Cu. As a result, the CuCl 2 cathode delivered a lithiation capacity of 399 mAh g −1 and a reversible delithiation capacity of 356 mAh g −1 . A systematical investigation discovered distinct intermediate phases during conversion: α‐ZnS phase CuCl formed during discharging and β‐ZnS phase CuCl within charging. Besides, the redox chemistry was further extended to Na||CuCl 2 batteries using an analogous electrolyte, which realized the reversible sodiation/desodiation of CuCl 2 cathode. The electrolyte design enabled the fundamental understanding of conversion mechanism and the development of soluble chloride cathodes for rechargeable batteries.