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

Abstract

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.