Insight into the structural evolution and thermal behavior of LiNi0.8Co0.1Mn0.1O2 cathode under deep charge

By virtue of the crucial effect of the crystal structure and transition metal (TM) redox evolution on the performance of LiNixCoyMnzO2 (NCM) cathode, systematical investigation is carried out to better understand the charge mechanism upon deep charging. Based on the results of X-ray diffraction and high-resolution transmission electron microscope, phase transformations existing on particle surface are promoted by high potential because of the deeper lithium vacancies, accompanied by more substantial structure instability. Soft X-ray absorption spectroscopy indicates that Ni acts as the major contributor to charge compensation while Co displays a remarkable redox activity over the deep charge range. The elevated integrated intensity of pre-edge in O K-edge spectra reveals the extensive amount of holes formed in O 2p orbitals and the enhanced hybridization of TM 3d - O 2p orbitals. Considering the close relationship between thermal behavior and structural evolution, the tendency of phase transitions and O2 release upon heating is accelerated by voltage rise, demonstrating the aggravated instability due to deeper Li utilization. Remaining Li contents in NCM are employed to estimate the amount of oxygen released in structural transformation and its detrimental effect on stability declares Li content-dependent characteristics. Furthermore, the extended Li vacancies, higher proportion of Ni4+ and stronger orbital hybridization are considered as three factors impeding the thermal stability of the highly-delithiated NCM.