Ni–deficient NiO/downsized RuO2 composite catalyst with rivalrous size evolution for rechargeable Li–CO2 batteries

Lithium–carbon dioxide (Li–CO₂) batteries utilize a lightweight and environmentally impactful CO₂ gas as a cathode and offer a high energy density (1876 Wh kg⁻¹). However, the thermodynamically stable discharge product, Li₂CO₃, necessitates the use of catalysts to facilitate reversible reaction kinetics, underscoring the importance of developing efficient catalysts to overcome this bottleneck. In this study, we fabricate nanofiber NiO–RuO₂ composite oxide catalysts (nf–NRO) to utilize the synergistic effect of the two oxides. In particular, we spotlight a critical phenomenon–rivalrous grain growth between two oxide components–as a strategy for catalyst optimization, balancing cost–effectiveness and catalytic performance. We observe rivalrous particle size changing behavior, where increasing the RuO₂ ratio in the composite oxide leads to RuO₂ downsizing and NiO coarsening. To elucidate this phenomenon, we propose expected mechanisms supported by DFT calculations; 1) Band bending between metallic oxide and p–type semiconductor, 2) Interfacial redox reactions driven by differences in the reduction potentials of the NiO and RuO₂ nanoparticles, 3) Acceleration of NiO growth due to the oxygen donor effect of RuO₂ coupled with surface energy–driven growth mechanisms. Accordingly, the catalytic property of nf–NRO₅₅ is maximized by downsizing RuO₂ and Ni³⁺–rich NiO. The Li–CO₂ battery with nf–NRO exhibits lower charge platform and superior cycle stability over 120 cycles. As a result, the correlation between the material properties of the nf–NRO and their electrochemical performance is identified.