Pathways for sustainable reaction kinetics in Li-CO₂ batteries

Lithium-carbon dioxide batteries hold great promise for high-energy-density storage applications. However, advancing this technology as a sustainable alternative to Li-ion systems requires a deeper understanding of the underlying reaction mechanisms, which remain elusive. A key challenge stems from the added complexity introduced by the presence of oxygen in CO₂ environment. In this study, we employ a stable Cu₃(VBi)_0.5Se₄ mid-entropy catalyst and conduct comprehensive investigation to uncover the underlying reaction mechanisms in Li-CO₂ batteries under varying CO₂/O₂ ratios. Under pure CO₂ conditions, the battery shows extended rechargeability, sustaining up to 1200 cycles at a current density of 0.2 mA/cm² and capacity of 0.1 mAh/cm². However, at high current densities, the discharge potential drops significantly (below 2.0 V), primarily due to sluggish reaction kinetics caused by solid carbon formation. Interestingly, introducing O₂ mitigates this limitation, leading to a 58% increase of the discharge potential (from 1.7 V to 2.7 V) at the current density of 0.8 mA/cm², signifying a substantial boost in energy output. Our results reveal that the reactions follow distinct pathways, shifting from surface- to solution-based mechanism, and even exhibit coexistence of both mechanisms, depending on the CO₂/O₂ ratio. These findings offer useful insights for designing sustainable Li-gas batteries utilizing CO₂ and O₂ mixtures.