Routes to Bidirectional Cathodes for Reversible Aprotic Alkali Metal-CO₂ Batteries

Aprotic alkali metal-CO₂ batteries (AAMCBs) have garnered significant interest owing to fixing CO₂ and providing large energy storage capacity. The practical implementation of AAMCBs is constrained by the sluggish kinetics of the CO₂ reduction reaction (CO₂RR) and the CO₂ evolution reaction (CO₂ER). Because the CO₂ER and CO₂RR take place on the cathode, which connects the internal catalyst with the external environment. Building a bidirectional cathode with excellent CO₂ER and CO₂RR kinetics by optimizing the cathode's internal catalyst and environment has attracted most of the attention to improving the electrochemical performance of AAMCBs. However, there remains a lack of comprehensive understanding. This review aims to give a route to bidirectional cathodes for reversible AAMCBs, by systematically discussing engineering strategies of both the internal catalyst (atomic, nanoscopic, and macroscopic levels) and the external environment (photo, photo-thermal, and force field). The CO₂ER and CO₂RR mechanisms and the "engineering strategies from internal catalyst to the external environment-cathode properties-CO₂RR and CO₂ER kinetics and mechanisms-batteries performance" relationship are elucidated by combining computational and experimental approaches. This review establishes a fundamental understanding for designing bidirectional cathodes and gives a route for developing reversible AAMCBs and similar metal-gas battery systems.