The Dominant Effect of Electrolyte Concentration on Rechargeability of γ-MnO2 Cathodes in Alkaline Batteries

Achieving high cycle life rechargeable γ-MnO2 cathodes in alkaline batteries face many challenges. Chief among these is the inability of the γ-MnO2 polymorph to retain its structural integrity when cycled to high utilization of its theoretical capacity ∼300 mAh g−1. In this paper, we investigate the root cause of failure of MnO2 cathodes under deep cycling in the one-electron discharge range and establish a strong link between capacity fade and the amount of birnessite formed. We uncover the underlying cause of failure by cycling industrial scale γ-MnO2 cathodes at various levels of theoretical capacity utilization (100%, 50%, and 30%) and in different KOH concentrations (37, 25, and 10 wt%). To determine materials evolution the cycled cathodes were dissected, characterized and analyzed using SEM, XRD, FIB/SEM, EIS, and XPS. Based on our findings, we propose that one major cause of failure of MnO2 cathodes stems from the solubility of Mn+3 formed during discharge which effectively results in destruction of the γ-MnO2 phase and amorphization of the cathode. The results show that the bulk of the γ-MnO2 phase is preserved only in ∼10 wt% KOH, which indicates the attractive range of KOH concentration for cycling of rechargeable γ-MnO2 cathodes.