Silver-Containing α-MnO₂ Nanorods: Electrochemistry in Na-Based Battery Systems

Manganese oxides are considered attractive cathode materials for rechargeable batteries due to the high abundance and environmental friendliness of manganese. In particular, cryptomelane and hollandite are desirable due to their ability to host cations within their octahedral molecular sieve (OMS-2) α-MnO₂ structure. In this work, we investigate silver containing α-MnO₂ structured materials (Ag_xMn₈O₁₆, x = 1.22, L-Ag-OMS-2 or 1.66, H-Ag-OMS-2) as host materials for Li ion and Na ion insertion/deinsertion. The results indicate a significant difference in the lithiation versus sodiation process of the OMS-2 materials. Initial reduction of Ag_1.22Mn₈O₁₆ to 1.0 V delivered ∼370 mAh/g. Cycling of Ag_1.22Mn₈O₁₆ between voltage ranges of 3.8-1.7 V and 3.8-1.3 V in a Na battery delivered initial capacities of 113 and 247 mAh/g, respectively. In contrast, Ag_1.66Mn₈O₁₆ delivered only 15 mAh/g, ∼ 0.5 electron equivalents, to 1.7 and 1.3 V. Study of the system by electrochemical impedance spectroscopy (EIS) showed a significant decrease in charge transfer resistance from 2029 Ω to 594 Ω after 1.5 electron equivalents per Ag_1.22Mn₈O₁₆ formula unit of Na ion insertion. In contrast, both Ag_1.22Mn₈O₁₆ and Ag_1.66Mn₈O₁₆ exhibited gradual impedance increases during lithiation. The formation of silver metal could be detected only in the sodiated material by X-ray diffraction (XRD). Thus, the impedance of Ag-OMS-2 decreases upon sodiation coincident with the formation of silver metal during the discharge process, consistent with the more favorable formation of silver metal during the sodiation process relative to the lithation process.