Non-Aqueous Rechargeable Mg–O ₂ /Mg-CO ₂ Batteries: Mechanisms, Catalysts, Redox Mediators, and Future Perspectives

High Resolution Image Download MS PowerPoint Slide Nonaqueous rechargeable magnesium–oxygen (Mg–O 2 ) and magnesium–carbon dioxide (Mg–CO 2 ) batteries are emerging as highly competitive candidates for next-generation energy storage, by virtue of their ultrahigh volumetric energy density, inherent dendrite-free safety, low cost, and unique capability for reversible CO 2 utilization. Nevertheless, their practical implementation is still plagued by unresolved key issues, including ambiguous reaction mechanisms, sluggish cathode redox kinetics, severe passivation caused by insulating discharge products, and poor interfacial compatibility among key components. This review focuses on the mechanistic understanding and rational design of nonaqueous Mg–O 2 /Mg–CO 2 batteries, with particular emphasis on the comparative analysis of the two Mg-based metal–gas systems, the regulation of discharge product pathways, and the synergistic roles of catalysts, redox mediators (RMs), and electrolytes. We first elaborate the fundamental reaction pathways and core bottlenecks of Mg–O 2 and Mg–CO 2 batteries, emphasizing the critical differences in discharge product evolution (MgO, MgCO 3, MgC 2 O 4 ) and kinetic constraints. We then comprehensively overview recent advances in catalyst development (covering structural design, heteroatom doping, composite construction, and interface engineering) and RM strategies (including monofunctional OER mediation, bifunctional ORR/OER synergy, shuttle effect suppression, and discharge product pathway induction). Distinct from existing reviews that focus on single metal–gas systems, this review highlights the importance of product pathway regulation in improving battery reversibility and reaction kinetics, and emphasizes the importance of catalyst–RM–electrolyte co-optimization in overcoming electrode passivation and kinetic limitations. Finally, we outline the remaining critical challenges and forward-looking perspectives for bridging the gap between laboratory research and practical applications of Mg–O 2 /Mg–CO 2 batteries. This review aims to fill the knowledge gap of integrated understanding for Mg-based metal-gas batteries and provide targeted guidance for the rational design of high-performance and practical Mg–O 2 /Mg–CO 2 battery systems.