Electrochemically induced cation disorder and phase transformations in lithium intercalation oxides

Electrochemical cycling of lithium intercalation compounds used as energy storage electrodes often results in phase transformations that have a critical impact on charge capacity and cycle life. In this paper, the role of cation disorder and transformation microstructures on electrochemical performance is examined theoretically and experimentally. The crystallographically allowable domain formation processes for transformations in ordered-rock salt and spinel structure lithium transition metal oxides are discussed. Experimental results from orthorhombic and monoclinic phase LiMnO2 and Li(Al,Mn)O-2 materials, the exemplar of compounds that exhibit improved high capacity and stable cycling after electrochemical transformation, are presented. Electron diffraction is used to show that the transformed spinels possess approximate 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 then transform to tetragonal ferroelastic domains upon further lithiation. The respective roles of cation inversion and ferroelastic accommodation in providing cycling stability are discussed. A domain wall sliding mechanism is proposed for the ferroelastic accommodation of transformation strains in this system.