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

Abstract

We investigated the first lithiation cycle of the positive electrode material Li_1−xNi_0.44Mn_1.56O₄ (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 3d⁸ to one with non-negligible amounts of ligand holes, i.e. Ni 3d^8−x2−x (0 < x < 2, where stands for a ligand hole) instead of a highly ionic state e.g. Ni 3d⁶. Our observations highlight the importance of studying the anionic character of redox processes in lithium ion batteries.