Thin films as model system for understanding the electrochemical reaction mechanisms in conversion reaction of MgH2 with lithium

Metal hydrides are promising high-capacity anode materials for Li-ion batteries but their conversion reaction with lithium suffers from low reversibility at room temperature (RT). Irreversibility issues in magnesium hydride MgH2 thin films are investigated, as well-defined model system. Films are deposited over Cu current collectors by means of microwave plasma-assisted sputtering and coated with aluminum to minimize formation of passivating MgO native oxide. Structural and chemical properties of the electrodes have been analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS). Galvanostatic cycling reversibility at RT and C/50 regime is limited to 25% in the first cycle for 1 mu m thick films. The lithiation of the thin film is complete and doubles its thickness. Despite drastic volume changes, neither cracks, voids, nor detachment of the thin film from the substrate are noticed. Moreover, electronic resistivity decreases upon lithiation due to the formation of metallic Mg. The origin of irreversibility phenomena in MgH2 films is attributed to sluggish mass transport of species within the electrode at RT.