Exploring magnetron sputtering preparation of high-quality LiNi0.5Mn1.5O4 films by controlling the oxygen atmosphere at moderate temperature

•5% O2 deposition lowers the LiNi0.5Mn1.5O4 (LNMO) crystallization temperature to 400 °C.•This enables battery fabrication directly on flexible polymer substrates.•Vacuum thermal drying treatment improves the electrochemical performance of the film.•Sn from FTO doping into the LNMO film may improve electrochemical performance.•The unannealed LNMO film still has an optimal rate capability and cycling performance.