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.