Electrochemical behavior of electrolytic manganese dioxide in aqueous KOH and LiOH solutions: A comparative study

As an inexpensive and high capacity oxidant, electrolytic manganese dioxide (γ-MnO₂) is of interest as a cathode for secondary aqueous batteries. Electrochemical behavior of γ-MnO₂ was characterized in aqueous 5.0 M KOH and LiOH solutions, and found to depend strongly upon cation identity. In LiOH and mixed LiOH / KOH solutions, Li-ion intercalation appeared to operate in competition with proton intercalation, being favored at higher [Li⁺] and, for mixed electrolytes, lower sweep rates. Electrochemical and in situ X-ray diffraction data indicated that γ-MnO₂ underwent a chemically irreversible transformation upon the first reduction in LiOH solution, while in KOH solution, structure was largely unchanged after the first cycle. These experiments with γ-MnO₂ as well as with a closely-related, ramsdellite-like sample, suggest that depending on sample morphology/rate capability, the irreversible process proceeds either through a solid-solution reaction or a two-phase reaction followed by a solid-solution reaction. While discharge capacity and capacity retention during galvanostatic cycling of γ-MnO₂ were worse in LiOH than in KOH solution, some improvement was noted in a mixed LiOH/KOH solution.