Local oxidation states of Ni, Co, and Mn atoms within pristine and charged LixNi0.88Co0.08Mn0.04O2 primary particles

Abstract

Pristine and charged (4.35 V at 1st cycle charge) primary particle samples of LixNi0.88Co0.08Mn0.04O2 lithium-ion battery (LIB) cathode material have been prepared by a sonication method. The local oxidation states of Ni, Co, and Mn atoms within these samples have been estimated from the local Ni L3-edge, Co L3-edge, Mn L3-edge, and O K-edge spectra measured by scanning transmission X-ray microscopy (STXM) with around 30 nm lateral space resolution. The average oxidation state of the Mn atoms has been estimated as about 4+, and that of the Co atoms as about 3+, over the whole particle area for both the pristine and charged samples. For the Ni atoms, with space integration over the particles, the average oxidation state was estimated as about 2.5+ for the pristine sample and 3.0–3.2+ for the charged sample. The space-resolved spectral features indicate that, for the pristine sample, the average oxidation state of the Ni atoms is 2.2–2.5+ in the outside (surface) area, and 2.9–3.0+ in the inside (bulk) area of the particles. For the charged particles, the average oxidation state of the Ni atoms ranged from about 2.4+ to 3.4+, although in most areas it was 3.0+ or higher. The O K-edge spectra, correspondingly, showed different features according to the sample and the location within the particles. The O K-edge spectral feature changes have been interpreted with reference to the hybridization between the 2p orbitals of O and the 3d orbitals of the Ni4+–Ni2+, Co3+, and Mn4+ cations.