Toward Practical Li–CO2 Batteries: Mechanisms, Catalysts, and Perspectives

ConspectusAchieving the target of carbon neutrality has become a pressing global imperative in the world where the imminent threat of greenhouse gas emissions looms large. Metal–CO2 batteries, which possess dual functions of CO2 utilization and electrical energy storage, are considered as one of the promising emission reduction strategies. Among varieties of metal–CO2 batteries, Li–CO2 batteries have the highest thermodynamic equilibrium potential (∼2.80 V) and the largest theoretical specific energy (∼1880 Wh kg–1), making them the center of research efforts and potentially transformational energy storage technologies. However, the development of Li–CO2 batteries is still in its early stages. The complicated CO2 reduction and evolution mechanisms have not been fully understood. Widely accepted CO2 reduction products are Li2CO3 and carbon. These products are produced following a surface-mediated or solution-mediated discharge pathway depending on the adsorption energy of cathode catalysts to intermediates and the solubility of intermediates in electrolytes. During charging, the self-decomposition of Li2CO3 or the reversible codecomposition of Li2CO3 and carbon could occur while applying different catalysts. In addition to the selection of catalysts, the modification of electrolyte components and the control of operation conditions can also affect the reaction processes, contributing to diverse reduction products including Li2C2O4, Li2CO3 and CO, as well as Li2O and carbon. Nonetheless, the exact determining factors of controlling reaction routes have been inconclusive. Besides, owing to the intrinsic properties of CO2 reactants and reduction products as well as the sluggish reaction kinetics at multiphase interfaces, Li–CO2 batteries are confronted with large overpotentials and undesirable parasitic reactions. Further improvement in battery performance, especially the energy efficiency and cyclic life, is necessary to propel the development of practical Li–CO2 batteries. In this Account, we summarize our and the community's efforts on the investigation of Li–CO2 batteries as an attractive avenue toward carbon neutrality. We start with a brief introduction of the physicochemical properties of CO2 and an in-depth discussion about the fundamental CO2 reduction and evolution reactions across multiphase interfaces. Then, diverse reaction pathways and underlying affecting factors involving catalysts, electrolytes, and operation conditions are highlighted. Furthermore, enhancement strategies for Li–CO2 batteries from four aspects of catalyst design, electrolyte modification, anode protection, and external field assistance are presented based on our recent works. At the end of the Account, we provide some potential directions in deepening the understanding of Li–CO2 batteries, optimizing battery performance, and broadening their application toward future carbon-neutral technologies.