Pristine-state structure of lithium-ion-battery cathode material Li1.2Mn0.4Co0.4O2 derived from NMR bond pathway analysis

Abstract

Layered lithium ion battery cathode materials have been extensively investigated, of which layered–layered composites xLi₂MnO₃·(1 − x)LiMO₂ (M = Mn, Co, Ni) are of particular interest, owing to their high energy density. Before the structural transformations that occur in these materials with cycling can be understood, the structure of the pristine material must be established. In this work, NMR spectra are measured for the model layered–layered system xLi₂MnO₃·(1 − x)LiCoO₂ and Bond-Pathway-model analysis is applied to elucidate the atomic arrangement and domain structure of this material in its pristine state, before electrochemical cycling. The simplest structural element of an Li₂MnO₃ domain consists of a stripe of composition LiMn₂ parallel to a crystallographic axis in a metal layer of the composite. A simple model of the composite structure may be constructed by a superposition of such stripes in an LiCoO₂ background. We show that such a model can account for most of the features of the observed NMR spectra.