Strain-Induced Stabilization of Charged State in Li-Rich Layered Transition-Metal Oxide for Lithium-Ion Batteries

Abstract

Li-rich layered oxide (LLO) is a promising cathode material for lithium-ion batteries because of its large capacity in comparison with conventional layered rock-salt structure materials. In contrast to the conventional materials, it is known that LLO of 3d transition metal has a nanodomain microstructure; however, roles of each domain and effects of strain, induced by the microstructure, on electrode properties are still unclear. In this study, the influence of the strain on an electronic structure is studied to elucidate the stabilization mechanism of LLO material Li[Li0.2Ni0.2Mn0.6]O2 in the charged state by using resonant X-ray diffraction spectroscopy (RXDS), X-ray diffraction, and X-ray absorption spectroscopy (XAS) in combination with ab initio calculation. RXDS of a superlattice peak and XAS at Mn and Ni K-edges unveil that this material has a microstructure consisting of Mn-rich and Ni-rich domains, whose structures are similar to Li2MnO3 and LiNiO2, respectively. In the Ni-rich domain, trigonal distortion in the NiO6 octahedral cluster is induced by an elastic constraint due to the microstructure. Hybridization between oxygen p- and nickel d-orbitals is enhanced by the distortion as revealed both by XAS and by ab initio calculation, accounting for stabilization of the charged state by alleviating the direct hole formation on oxygen p-orbital that usually destabilizes the charged material.