First-Order or Second-Order? Disproportionation of Lithium Superoxide in Li-O₂ Batteries

The disproportionation of LiO₂ to Li₂O₂ is a key step in Li-O₂ batteries, and it is regarded as a second-order reaction. However, its mechanism is not well addressed, and its kinetics is rarely studied due to the difficulties of quantifying the rate constants, particularly for high concentrations of superoxide (>10 mM). Here, we quantified the kinetic rate constant by a microkinetic model using a microelectrode tip with a thin diffusion layer and fast response. We report that the reaction order of LiO₂ transitions from 1 at high concentrations of superoxide (∼20 mM) to 2 at low concentrations of superoxide (∼1 mM). LiO₂ is chemically reduced by free superoxides to form Li₂O₂ and O₂, instead of reacting with another LiO₂ via a disproportionation step. This chemical-reduction mechanism explained the change of reaction order and the kinetics profile. As a rate-determining step, this step restricts the overall kinetics of the discharging process and should be the focus of future catalyst design.