Structural Characterization of the Delithiated Noncrystalline Phase in a Li-Rich Li2VO2F Cathode Material

Abstract

As a cathode material for lithium-ion batteries, the disordered rock-salt-type Li2VO2F has a high theoretical capacity of 462 mA h/g; however, this capacity fades rapidly upon cycling. Here, we performed X-ray total scattering measurements of the material to gain insights into the battery properties of Li2VO2F. The atomic pair distribution function obtained from these measurements indicates that the disordered rock-salt crystal partially transforms to an amorphous phase on delithiation. Structural information of the amorphous phase was extracted by precisely evaluating the structural parameters of the disordered rock-salt crystal. Using the structural information, we modeled an atomic three-dimensional structure by the reverse Monte Carlo method. The structural model reproduced the tetrahedral or pyramidal structure with the anions at the vertices and a V ion at the center. These polyhedral structures are likely to involve the anion transfer between the crystalline and amorphous phases. It induces the concentration fluctuation of anions in the crystalline phase and might be a factor for capacity fading of the Li2VO2F cathode material.