Photoassisted Li-CO₂ Batteries with Ultrahigh Energy Efficiency and Cycle Stability by a Redox Mediator

Li-CO₂ batteries are considered promising energy storage systems for implementation in space applications. However, unsatisfactory overpotentials and poor cycling stability caused by the sluggish reaction kinetics of CO₂ reduction and evolution have greatly limited the practical application of Li-CO₂ batteries. Herein, we introduce a redox mediator LiI to stabilize the Bi₅O₇I photocathode by the successive separation of electrons and holes to accelerate the sluggish kinetics. Under illumination, LiI allows photogenerated holes to preferentially react with I^- in the solid-liquid interface, thereby inhibiting the release of I^- from Bi₅O₇I and effectively improving the lifespan of the photocathode. Thus, a high discharging platform of 3.05 V and an ultralow charging platform of 3.09 V are achieved to exhibit an energy efficiency of 98.8%. Moreover, a significantly enhanced reversibility is attained, attributed to the decomposition of Li₂CO₃, which further extends the cycle life to 250 h. This double-phase catalytic strategy demonstrates effectiveness in the development of high-performance Li-CO₂ batteries and other energy storage devices.