Charge Transport in Alkali-Metal Superoxides: A Systematic First-Principles Study

Abstract

Charge-transport mechanisms within the discharge products of alkali-metal/O2 batteries can strongly influence the performance of these systems. To date, discharge products comprising alkali peroxides (Li2O2 and Na2O2) and superoxides (LiO2, NaO2, and KO2) have been observed. In general, cells that discharge to a superoxide exhibit lower overpotentials than those that form the corresponding peroxide. These lower overpotentials have been hypothesized to originate from more efficient charge transport within the superoxides. While the transport mechanisms in the peroxides have been well studied, consensus regarding the intrinsic conductivity across the alkali-metal superoxides is lacking. Here, we present a systematic first-principles study of charge-transport mechanisms across the alkali-metal (Li, Na, K) superoxides. Our study draws on our prior investigations of the alkali peroxides and of sodium superoxide, while adding new analyses for lithium and potassium superoxides (LiO2 and KO2). In the case of KO2, a nonsymmetrized room-temperature structure is proposed to account for a dynamic Jahn–Teller effect. Band gaps, equilibrium (charged) defect concentrations, mobilities, and intrinsic conductivities are estimated for both LiO2 and KO2. Overall, the alkali superoxides are predicted to be wide band gap insulators, with gaps exceeding 4 eV. These large band gaps imply that negligible transport occurs via band conduction. Compared to the alkali peroxides, ionic conductivities in the superoxides are predicted to be 8–11 orders of magnitude larger, ranging from 4 × 10–9 to 5 × 10–12 S/cm at room temperature. Regarding electronic conductivities, transport in NaO2 and KO2 is predicted to occur via polaron hopping, with low conductivities on the order of 10–19 to 10–20 S/cm. These values are similar to what has been previously reported for the peroxides. Importantly, the present calculations indicate that LiO2 has a much larger electronic conductivity (also arising from polaron hopping) than its Na and K analogues, 9 × 10–12 S/cm at 300 K. These data suggest that the low overpotentials observed for Na- and KO2-based batteries cannot be explained by high intrinsic electronic conductivities. In contrast, the much larger conductivities predicted for LiO2 imply that the superior performance observed in LiO2-based cells may reflect the ability of LiO2 to support higher charge-transport rates.