Cation Inversion and Microstructural Transformations in Electrochemically Cycled Lithium Manganese Oxide Cathodes for Rechargeable Batteries

LiMnO2 and Li(Al,Mn)O2 crystallizing in the monoclinic and orthorhombic ordered-rocksalt polymorphs have been shown to be stable upon cycling over a wide voltage range despite the occurrence of a spinel transformation. Repeated lithiation to an Li/Mn ratio of unity is possible, yielding high charge capacity (>200 mAh/g) and energy density (>600 Wh/kg) superior to that of conventional LiMn2O4 spinels. The purpose of this work is to understand the mechanism(s) by which cycling stability is achieved. Direct observations using HREM show that electrochemically cycled materials exhibit both cation and microstructural disorder. Stability can be correlated with the formation of local spinel ordering in a nanodomain configuration that accom-modates local Jahn-Teller deformation. Two stages of transformation have been ident-ified. In the first, antiphase domains are formed as the Mn ions are disordered and re-order as spinel. This spinel is partially inverse, in contrast to conventional LiMn2O4. In the second stage, increased lithiation causes a transformation from cubic to tetragonal spinel. The transformation strains appear to be accommodated by the for-mation of ferroelastic domains from the pre-existing antiphase domains. The tetragonality varies significantly at the nanodomain level in the final transformed material. Thus the stability of this family of cathodes appears to have both crystal chemical and microstructural origins. Current work is aimed at separating the effects of cation inversion and the ferroelastic nanodomains.