A multi-edge study: investigating Co oxidation states of pristine LiNixMnyCo1−x−yO2 cathode materials by high energy-resolution X-ray spectrometry

Abstract

The investigation of Co oxidation states in pristine LiNi_xMn_yCo_1−x−yO₂ (NMC) cathodes (NMC111, NMC622, NMC811) has been a subject of ongoing debate, with conflicting findings in the literature. In this study, we present a novel and comprehensive approach to address and clarify this issue using a variety of high energy-resolution X-ray spectroscopy techniques. To shed light on the Co oxidation states in NMC cathodes, we employed independent measurements including X-ray absorption spectrometry in both soft and hard X-ray ranges, as well as resonant X-ray emission spectrometry in the soft X-ray range. The investigation centered on the transition metal (TM) K and L edges, providing a thorough exploration of the electronic structure transitions. The study identified minor shifts in Co oxidation states, and theoretical calculations quantified the ratio of Co atoms undergoing oxidation state changes, which were approximately 2.05% (NMC111 to NMC622) and 3.75% (NMC111 to NMC811). Independent measurements that targeted electronic structure transitions using K-edge and L-edge absorption and emission spectrometry were strategically combined to enhance the reliability of the results. The diverse methodological approach aimed to contribute to a comprehensive understanding of Co oxidation states in NMC cathodes. This study highlights the importance of combining complementary techniques to address intricate scientific debates effectively.