Structural transformation of sputtered o-LiMnO2 thin-film cathodes induced by electrochemical cycling

•Quantitative, thermodynamic modeling of the o-LiMnO2/c-LiMn2O4 phase transformation•First CV-investigations on magnetron sputtered nanocrystalline o-LiMnO2 thin films•Synthesis of o-LiMnO2 planar model systems for protective coating and SEI development

Orthorhombic LiMnO2 (o-LiMnO2) thin films were produced by non-reactive r.f. magnetron sputtering in combination with thermal post-annealing. Oxide phase formation was investigated by X-ray diffraction and Raman spectroscopy. In order to assign the X-ray signals and estimate the grain size, a simulation of the diffraction pattern was performed and compared with experimental data. The density of the films was determined to be 3.39 g/cm3 using X-ray reflectivity. Electrochemical characterization was carried out by galvanostatic cycling and cyclic voltammetry of Li/o-LiMnO2 half cells. There are distinct redox reactions at approx. 3 V and 4 V, whereas the latter splits into multiple peaks.Using ab initio calculations and thermodynamic models, Gibbs energies of o-LiMnO2 and c-LiMn2O4 were determined. The relation between these energies explains the irreversible phase transformation that has been observed during the cycling of the Li/o-LiMnO2 half cell.