Electronic structure of the transition metal ions in LiCoO2, LiNiO2 and LiCo0.5Ni0.5O2

We studied the electronic structure of the transition metal ions in LiCoO2, LiNiO2 and LiCo0.5Ni0.5O2. The experimental technique used in the study was Co 2p and Ni 2p X-ray absorption spectroscopy. The experimental results were analyzed using atomic multiplet plus crystal field calculations. The Co ions in LiCo0.5Ni0.5O2 are in a trivalent Co3+ low-spin state given by t2g6 (1A1), as in LiCoO2. The Ni ions in LiCo0.5Ni0.5O2 are in a divalent Ni2+ high-spin state given by t2g6eg2 (3A2), as in LiNiO2. The missing charge at the Ni site is compensated by a less negative O ion; this means that the charge state in LiNiO2, for instance, is closer to Li+Ni2+O21.5− than to the usually quoted Li+Ni3+O22−. This is important for understanding the oxidation–reduction processes upon Li deintercalation–reintercalation reactions.