Li-ion and Na-ion intercalation in layered MnO2 cathodes enabled by using bismuth as a cation pillar

Abstract

Low-cost batteries based on Earth-abundant materials are needed for large-scale electrical storage for the grid. Cathodes based almost entirely on Mn oxides would reduce overall battery cost but cycling of Mn oxides is often not stable. In Li-ion cells, most polymorphs of MnO₂ undergo irreversible transformation to spinel LiMn₂O₄ during cycling, causing capacity loss. Doping MnO₂ with Bi is known to stabilize the structure, but previous reports have relied on low-crystallinity material making it impossible to pinpoint the Bi location in the structure or its mechanism. Herein, we report a series of hydrated Bi-doped layered MnO₂ compounds and characterize their structures as a function of Bi amount. Bi is shown to reside in the material interlayer, provoking higher long-range structural order even at a low doping level of 1.3%. Doped material improves the specific capacity and stability of cycling in both Li-ion and Na-ion cells. A high level of Bi doping, 4.3%, causes loss of the interlayer crystal water in non-aqueous electrolyte, and this reduces the interlayer distance. Crystal water is shown to be beneficial in a Na-ion system, while its loss improves Li-ion cycling. This provides fundamental insight into how pillaring by a heavy, multivalent cation stabilizes layered oxides.