Transition-metal redox evolution and its effect on thermal stability of LiNi Co Mn O2 based on synchrotron soft X-ray absorption spectroscopy

Abstract

The correlations between electronic structure evolution and thermal stability of layered LiNi x Co y Mn z O 2 cathodes are investigated systematically based on the synchrotron soft X-ray absorption spectroscopy and micro-calorimetry. Based on the synchrotron soft X-ray absorption spectroscopy experiments, the fundamental electronic structures of layered LiNi x Co y Mn z O 2 (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 Ni 4+ expands with the increase of Ni since it dominates in charge compensation during charging, and that the existence of Ni 3+ is nearly negligible in delithiated NCM. The valence state of Co also strongly depends on Ni content, the perceptible position shift of Co L 3 -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 2 p holes near the Femi level increases with higher oxidation state of Ni, indicating the enhanced hybridization of O 2 p -TM 3 d . Delithiated NCMs with higher Ni content are prior to lose electron existing in highly hybridized Ni 3 d -O 2 p bands upon heating, which accounts for the pronounced O 2 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.