Revealing Hidden Structural Anisotropy in Cation-Disordered Rock Salts

Cation disordered rock salts (DRX), a new class of cathode materials for Li ion batteries, have attracted lots of attention in recent years, due to their fascinatingly simple cubic structure, highly diverse composition, and great electrochemical performance. As cations in DRX are randomly distributed in a long-range, how the cations (Li and transition metal) are arranged in a shorter range is an intriguing question for the community of cathode materials research. In this work, we study the vibrational structure of a series of DRXs with well controlled compositions and revealed significant anisotropy of cation arrangements. Based on the results, we propose a scheme that describes how the structural anisotropy could exist in rock salt structures but shows an overall cubic Fm-3m diffraction pattern. Furthermore, we raise a model of Li transport based on the scheme we proposed, which complements the theory of Li percolation in DRX. The electrochemical behavior of the cathodes used in the study supports the model.