Electrochemically induced cation disorder and phase transformations in lithium intercalation oxides

Electrochemical cycling of lithium intercalation compounds used as energy s torage electrodes often results in phase transformations that have a critic al impact on charge capacity and cycle life. In this paper, the role of cat ion disorder and transformation microstructures on electrochemical performa nce is examined theoretically and experimentally. The crystallographically allowable domain formation processes for transformations in ordered-rock sa lt and spinel structure lithium transition metal oxides are discussed. Expe rimental results from orthorhombic and monoclinic phase LiMnO2 and Li(Al,Mn )O-2 materials, the exemplar of compounds that exhibit improved high capaci ty and stable cycling after electrochemical transformation, are presented. Electron diffraction is used to show that the transformed spinels possess a pproximate to 25% cation inversion after extensive cycling. High-resolution electron microscopy reveals that the cycling-induced spinel transformation concurrently creates antiphase domains of approximate to6-nm size, which t hen transform to tetragonal ferroelastic domains upon further lithiation. T he respective roles of cation inversion and ferroelastic accommodation in p roviding cycling stability are discussed. A domain wall sliding mechanism i s proposed for the ferroelastic accommodation of transformation strains in this system.