Hard X-ray spectroscopic methods using emitted X-ray to understand charge compensation in positive electrode materials for lithium-ion batteries

Abstract

We examined the applicabilities of hard X-ray spectroscopic methods to understand the charge-compensation (CC) behaviors of Ni in a LiNiO2-based positive electrode (PE) material for lithium-ion batteries: Ni K pre-edge high energy resolution fluorescence detection X-ray absorption spectroscopy (XAS) and Ni 1s2p resonant X-ray emission spectroscopy (RXES). Both methods exhibited sequential changes in spectral features with the change in the state of charge of the PE material, similarly to Ni L2,3-edge XAS. This elucidates that these hard X-ray spectroscopic methods can become alternatives to soft X-ray L2,3-edge XAS with the additional ability to measure reactive samples kept in a sealed pouch. Moreover, Ni 1s2p RXES can separate the electronic transition to the unoccupied Ni orbital from those to hybridized orbitals with neighboring atoms by tuning the incident X-ray energy. In this sense, it can provide spectral information reflecting the unoccupied Ni 3d state, which could be a good oxidation-state descriptor. These methods can be done in a usual hard X-ray beamline having an appropriate crystal analyzer. The ability to offer information on the unoccupied 3d state with hard X-ray and the ubiquitous feature of these methods would prompt in situ/operando studies on CC in PE materials for rational materials development.