Transition-metal redox evolution and its effect on thermal stability of LiNixCoyMnzO2 based on synchrotron soft X-ray absorption spectroscopy

Abstract Based on the synchrotron soft X-ray absorption spectroscopy experiments, the fundamental electronic structures of layered LiNixCoyMnzO2 (NCM) are investigated systematically and the data of transition-metal (TM) L- and O K-edges spectra are collected. Distribution of Ni ions under different oxidation states is evaluated according to linear combination fit. It is found that the ratio of Ni4+ expands with the increase of Ni since it dominates in charge compensation during charging, and that the existence of Ni3+ is nearly negligible in delithiated NCM. The valence state of Co also strongly depends on Ni content, the perceptible position shift of Co L3-edge absorption peak towards higher energy in Ni-rich material agrees well with the small voltage plateau at around 4.2 V. The stability of Mn is verified as no obvious spectral change with the Mn L-edge is observed. Moreover, as Ni content rises, the O 2p holes near the Femi level increases with higher oxidation state of Ni, indicating the enhanced hybridization of O 2p-TM 3d. Delithiated NCMs with higher Ni content are prior to lose electron existing in highly hybridized Ni 3d-O 2p bands upon heating, which accounts for the pronounced O2 release in phase transitions and the deterioration in thermal stability. These detailed observation of the electronic structure evolution is one of the key ingredients to improving the electrochemical and thermal performance of NCM.