Charge transport mechanisms in potassium superoxide

Rechargeable metal-air batteries based on superoxide discharge products are attractive due to the facile one-electron redox process of O-2/O-2(-). Recently, a K-O-2 battery has been reported that showed a significantly lower discharge/charge potential gap than the Li-O-2 battery systems. Here, we perform first-principles calculations on potassium superoxide (KO2) to unravel the charge transport mechanism in this discharge product. The concentration and mobility of intrinsic carriers are calculated. The results show that hole polarons and negatively charged potassium ion vacancies are the main charge carriers. The conductivity associated with polaron hopping (2 x 10(-12) S cm(-1)) is 8 orders of magnitude higher than that of Li2O2, and the ionic conductivity has a comparable value (1 x 10(-13) S cm(-1)). Our calculation results can rationalize the experimental findings and provide a theoretical basis for the understanding of superoxide discharge products in metal-air batteries.