Elucidating zinc-ion battery mechanisms in freestanding carbon electrode architectures decorated with nanocrystalline ZnMn2O4

Rechargeable zinc-ion batteries represent an emerging energy-storage technology that offers the advantages of low cost, use of abundant and nontoxic materials, and competitive energy content in lightly packaged forms. Nanoscale manganese oxides are among the most promising positive-electrode materials for zinc-ion cells, and their performance is further enhanced when these oxides are expressed as conformal deposits on porous carbon architectures, such as carbon nanfoam paper (CNF). We describe an “in-place” conversion of nanometric birnessite Na+-MnOx@CNF to crystalline spinel ZnMn2O4@CNF, a manganese oxide polymorph that nominally contains sites for Zn2+ insertion. The ZnMn2O4@CNF cathodes are electrochemically conditioned in two-terminal cells and ex situ characterized using X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy. Despite specific Zn2+ insertion sites in ZnMn2O4, we demonstrate that the predominant discharge mechanism involves coupled insertion of protons and precipitation of Zn4(OH)6SO4·xH2O; upon recharge, protons deinsert and Zn4(OH)6SO4 dissolves.