Temperature induced reduction of the trivalent Ni ions in LiMO2 (MÂ =Â Ni, Co) thin films

We report on an in-situ photoelectron spectroscopy study of the LiMO2 (M = Ni,Co) thin film layered materials. In the stoichiometric Lix(NiCo)O2 films grown by RF-magnetron sputtering at 20-600 °C, Ni3+ ions exist in the low-spin state. The growth temperatures T > 650 °C lead to better crystallinity of the films, Li-deficiency, and reduction of Ni3+ to Ni2+ in the high-spin state. The Co3+ ions in the low spin state are stable at 400-670 °C. The missing electron at the Ni-site is compensated by a less negative charge at the oxygen site or by the oxygen deficiency in Lix(NiCo)O2.