The role of anionic processes in Li1-xNi0.44Mn1.56O4 studied by resonant inelastic X-ray scattering

e investigated the first lithiation cycle of the positive electrode material Li1-xNi0.44Mn1.56O4 (LNMO) using soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the transition metal L- and oxygen K-edges. Our XAS results show that charge compensation in LNMO takes place mostly within the Ni-O bonds, which is consistent with previous similar studies. O K- and Ni L-RIXS reveals how the holes that are created by removal of electrons during delithiation are distributed between the Ni- and O-ions. Non-trivial anionic activity is revealed by O K-RIXS features such as the appearance of low-energy intra-band excitations and re-hybridization with Ni 3d-states forming a new intense band close to the top of the oxygen valence band. At the same time, Ni L-RIXS compares more favorably with covalently than with ionically bonded Ni-oxide based compounds. Thus, a picture emerges where delithiation leads to a gradual transition of the ground state of LNMO from Ni 3d(8) to one with non-negligible amounts of ligand holes, i.e. Ni 3d(8-x)(2-x) (0 < x < 2, where stands for a ligand hole) instead of a highly ionic state e.g. Ni 3d(6). Our observations highlight the importance of studying the anionic character of redox processes in lithium ion batteries.

Automated summary

This study investigated the first lithiation cycle of the positive electrode material LNMO using XAS and RIXS techniques. The results revealed that charge compensation in LNMO primarily occurs within the Ni-O bonds. RIXS analysis provided insights into the distribution of holes between Ni and O ions during delithiation. Additionally, the study highlighted the presence of non-trivial anionic activity.