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 (similar to 20 mM) to 2 at low concentrations of superoxide (similar to 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.

Automated summary

The disproportionation of LiO2 to Li2O2 in Li-O2 batteries is a key step, transitioning from a first-order reaction at high concentrations of superoxide to a second-order reaction at low concentrations. LiO2 is chemically reduced by free superoxides to form Li2O2 and O2, rather than reacting with another LiO2 via disproportionation. This chemical-reduction mechanism explains the change in reaction order and kinetics profile.