AC Magnetron Sputtering: An Industrial Approach for High‐Voltage and High‐Performance Thin‐Film Cathodes for Li‐Ion Batteries

Industrial‐oriented mid‐frequency alternating current (MF‐AC) magnetron sputtering technique is used to fabricate LiNi0.5Mn1.5O4 high‐voltage thin‐film cathodes. Films are deposited on bare stainless‐steel substrate at room temperature and then annealed to induce crystallization in disordered spinel phase. In situ X‐ray diffraction is used to follow film structural evolution from room temperature to 900 °C. Scanning electron microscopy, X‐ray photoelectron spectroscopy, and Raman spectroscopy are used to study the evolution with temperature of film morphology, surface chemical composition, and crystal structure arrangement, respectively. Film structure evolves almost continuously in the studied temperature range. A pattern corresponding to spinel phase is observed after annealing at 600 °C, while poor crystallization is obtained for lower temperatures, and additional unwanted phase changes are observed for higher temperatures. Cyclic voltammetry, rate capability, and cycling performance of fabricated films are tested. Only the film annealed at 600 °C shows redox peaks corresponding to Ni oxidation from 2+ to 3+ and 3+ to 4+ oxidation states, confirming that this film crystallizes in disordered spinel phase. The thin‐film cathode shows good rate performance and outstanding cyclability, despite the impurities formed upon electrolyte decomposition at high voltage. Deposited LiNi0.5Mn1.5O4 films are amorphous and they crystallize in electroactive disordered spinel phase upon annealing at 600 °C. The obtained thin‐film electrodes are tested against metallic Li in liquid electrolyte showing outstanding cycling performance.