Theoretical Studies of Zincate Contamination of $\gamma$ -MnO$_2$ in Deep-Cycled Rechargeable Zn/MnO$_2$ Batteries

Rechargeable alkaline Zn/MnO$_2$ batteries are attractive for large-scale energy storage because of their high energy density, non-toxicity, and low cost. However, efforts to develop rechargeable Zn/MnO$_2$ batteries have been hindered by a short cycle life due to the accumulation of irreversible redox reaction products in the $\gamma$-MnO$_2$ cathode. The penetration of Zn ions into the $\gamma$-MnO$_2$ electrode leads to the formation of hetaerolite (ZnMn$_2$O$_4$). The contamination of the $\gamma$-MnO$_2$ cathode material with hetaerolite reduces the battery capacity and eventually leads to the failure of the battery. We apply {\it ab initio} computational methods based on density functional theory to calculate the structure and formation energy of ZnMn$_2$O$_4$ using several different gradient corrected exchange-correlation functionals, including PBE, PBESol, and PBE+U. Our calculations show that the PBE and PBEsol functionals tend to underestimate the formation energy of hetaerolite, whereas the PBE+U functional significantly improves agreement with experiment. Using the results of our calculations, we analyze the influence of hetaerolite on the performance and cycle life of rechargeable Zn/MnO$_2$ batteries.