Tuning the Cation Ordering with the Deposition Pressure in Sputtered LiMn1.5Ni0.5O4 Thin Film Deposited on Functional Current Collectors for Li-Ion Microbattery Applications

The development of high voltage spinel LiMn1.5Ni0.5O4 (LMNO) sputtered thin films on functional current collector was reported within the framework of this study. We have first solved the technological issue due to the PtSi phase which originates from the interdiffusion between silicon wafer and chromium/platinum current collector to form PtSi phase under annealing treatment. By substituting the Cr layer with a very dense and pinhole free Al2O3 thin film deposited by ALD acting as a barrier diffusion between the silicon substrate and the LMNO layer, the synthesis process (sputtered thin films annealed under air at 700 °C) has been validated. In the second part of this study, the behavior of the sputtered LMNO thin films as a function of the deposition pressure and the film thickness has been investigated. The deposition pressure has been found to play a key role on the Mn–Ni cations ordering in spinel-like structures (P4332 ordered vs Fd3̅m disordered spinel) and consequently on the electrochemical performance. We have thus shown that LMNO thin films deposited at 10–2 mbar and annealed at 700 °C deliver a normalized capacity of 65 μAh·cm–2·μm–1 with good capacity retention upon cycling thanks to its disordered spinel structure. Moreover, the Coulombic efficiency has been shown to be highly dependent on the film thickness for high voltage LMNO thin film electrodes. To the best of the authors’ knowledge, this work is one of the most complete study combining structural and electrochemical characterizations of the LMNO thin film obtained by sputtering deposition on Si/Al2O3/Pt functional current collectors.