Probing Mechanisms for Inverse Correlation between Rate Performance and Capacity in K-O₂ Batteries

Owing to the formation of potassium superoxide (K⁺ + O₂ + e^- = KO₂), K-O₂ batteries exhibit superior round-trip efficiency and considerable energy density in the absence of any electrocatalysts. For further improving the practical performance of K-O₂ batteries, it is important to carry out a systematic study on parameters that control rate performance and capacity to comprehensively understand the limiting factors in superoxide-based metal-oxygen batteries. Herein, we investigate the influence of current density and oxygen diffusion on the nucleation, growth, and distribution of potassium superoxide (KO₂) during the discharge process. It is observed that higher current results in smaller average sizes of KO₂ crystals but a larger surface coverage on the carbon fiber electrode. As KO₂ grows and covers the cathode surface, the discharge will eventually end due to depletion of the oxygen-approachable electrode surface. Additionally, higher current also induces a greater gradient of oxygen concentration in the porous carbon electrode, resulting in less efficient loading of the discharge product. These two factors explain the observed inverse correlation between current and capacity of K-O₂ batteries. Lastly, we demonstrate a reduced graphene oxide-based K-O₂ battery with a large specific capacity (up to 8400 mAh/g_carbon at a discharge rate of 1000 mA/g_carbon) and a long cycle life (over 200 cycles).