Decoding the Electrochemical Behavior of Redox Mediators During the Charging Process of Lithium‐Carbon Dioxide Batteries

Rechargeable lithium‐carbon dioxide (Li‐CO ₂ ) batteries present a promising solution for energy storage and CO ₂ utilization. However, the insulating and insoluble solid Li ₂ CO ₃ tightly adheres to cathodes, resulting in sluggish reaction kinetics and high charge overpotentials. Soluble redox mediators (RMs) can improve cycling reversibility by electrochemically decomposing Li ₂ CO ₃ , but the influence of RMs on the reversibility of amorphous carbon (C) during the charging process still remains unexplored. Herein, lithium bromide (LiBr) is added as an RM in Li‐CO ₂ batteries. Comparative experiments reveal distinct charge plateaus with and without LiBr. Further charging tests using electrodes with only Li ₂ CO ₃ confirm LiBr does not facilitate the decomposition of amorphous C during the charging process. In contrast, LiBr decomposes Li ₂ CO ₃ and exposes the amorphous C. The exposed C facilitates continuous decomposition of residual Li ₂ CO ₃ due to its excellent electron‐transport properties, thereby preventing rapid elevation of the charging potential. The transient behavior of the 4.0 V charging plateau in the batteries with low LiBr concentration is attributed to rapid deactivation of LiBr caused by the shuttle effect. This failure mode is identified by visualizing separator discoloration and corrosion on the lithium anode surface. This work provides critical insights for re‐evaluating the RM functions in Li‐CO ₂ batteries.