Ni and oxygen K-edge XAS investigation into the chemical bonding for lithiation of Li yNi 1−xAl xO 2 cathode material

Abstract

The local structure refinement for the chemically Li-ion extracted LiNi 1− x Al x O 2 system has been intensively carried out with Ni K-edge and O K-edge X-ray absorption spectroscopy. The Li-ion extraction gives rise to the substantial variation of unit cell dimension, which presents the systematic decrease of lattice parameter a and the increase of lattice parameter c. For Ni K-edge XANES spectra, the strong absorption peak by ligand to metal charge transfer (LMCT) process represents that the electronic configuration of Ni ion in the ground state is located in the equilibrium between Ni 3+(t 2g 6e g 1) + O 2− (2p 6) ↔ Ni 2+(t 2g 6e g 2) + O − (2p 5). The Li-ion extraction leads to the peak shift towards higher energy region and particularly the systematic decrease of peak intensity for the LMCT process, which is due to the dominant formation of Ni 3+ O 2− ion pair by the transfer of hole state ( L) in the oxygen 2p band to the Ni atomic site. For EXAFS refinement, the separated FT peak by Jahn–Teller distorted NiO 6 octahedra becomes a symmetric feature of regular oxygen distribution on the extraction. The average interatomic distances of Ni O and Ni Ni bonds decrease systematically, meaning the formation of more covalent Ni 3+ O 2− character. From oxygen K-edge spectra, furthermore, it is found out that the charge compensation for the electron exchange can be achieved at oxygen site as well as Ni site.