Spectro-nanoscopic investigation of oxidation-state changes of Ni, Co, and Mn ions in LixNi0.88Co0.08Mn0.04O2 primary particles by annealing

Abstract

Submicron-size primary particles of LixNi0.88Co0.08Mn0.04O2 cathode material for next-generation lithium-ion-battery (LIB) application were prepared and annealed in-situ from room temperature (RT) to 200 and 400 ̊C, respectively, and their space-resolved Ni L3-, Co L3-, Mn L3-and O K-edge x-ray absorption spectra were acquired by scanning transmission x-ray microscopy (STXM). The aim was to investigate the local phase changes and oxidation-state changes of the cations within each of the primary particles, in order to understand the annealing-induced degradation mechanism of the material. Compared with the Co L3-and Mn L3-edge spectra, the Ni L3-and O K-edge spectra showed clear changes manifesting as a reduction of Ni ions when annealed, especially from 200 to 400 ̊C. The average oxidation numbers of the Co and Mn ions showed negligible changes. On the other hand, the Ni ions showed clear changes: when averaged over the particle areas, the number was +3 at RT and 200 ̊C, and +2.6 at 400 ̊C. For smaller particles (~100 nm in diameter), the values were smaller: +2.8 at RT, +2.7 at 200 ̊C, and +2.2 at 400 ̊C. In the O K-edge spectrum, the intensity of the O 2p – Ni 3d hybridization peaks decreased and a NiO-related peak appeared at 400 ̊C. These observations imply that at 400 ̊C, Ni ions are predominantly reduced, to a lower average oxidation number, as the annealing temperature reaches 400 ̊C, and that the reduction of Ni ions occurs mainly in the edge area and in smaller particles. The overall results suggest that preparation of larger-size primary particles and proper capping of each primary particle can improve the lifetime and stability of the LIB.