In this presentation, we report the breakthrough of reversibly accessing the 2nd electron capacity of MnO2 by using its layered polymorph called birnessite mixed with bismuth oxide (Bi-birnessite) and intercalating the layers with Cu ions (5). Bi-birnessite undergoes conversion reactions in alkaline electrolyte and ultimately forms electro-inactive hausmannite (Mn3O4) because of its poor charge transfer characteristics. Intercalating the layers of Bi-birnessite with Cu ions is shown to improve its charge transfer characteristics dramatically and regenerate its layered structure reversibly for thousands of cycles. We also present a case of Cu-intercalated Bi-birnessite’s applicability in practical batteries by cycling the material at high areal capacities (10-29mAh/cm2) for thousands of cycles at C-rates that are of interest in the battery community. Finally, a Cu-intercalated Bi-birnessite/Zn battery is shown to reversibly cycle at 140 Wh/L for over 90 cycles.
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