Role of Cr\nRedox and Dynamics in Electrochemical Cycling\nof H<sub><i>x</i></sub>CrS<sub>2−δ</sub>

H<sub><i>x</i></sub>CrS<sub>2−δ</sub> is\nproduced by the proton exchange of NaCrS<sub>2</sub> and features\nalternating layers of crystalline and amorphous lamella. It exhibits\nsuperior performance as a Na-ion battery electrode compared with its\nparent compound with faster Na<sup>+</sup> diffusion, higher capacity,\nand better cyclability. This work explores the nature of the unique\nbiphasic structure of H<sub><i>x</i></sub>CrS<sub>2−δ</sub> using both powder and single-crystal X-ray diffraction, as well\nas electron microscopy. Additionally, <i>ex situ</i> characterizations\nusing X-ray absorption spectroscopy, X-ray total scattering, and magnetometry\nare employed to study the mechanism by which this superiority arises.\nThese reveal that migration of Cr does not impede battery performance\nand may, in fact, be crucial to the observed performance improvements.\nThese studies show that Cr redox is not only possible but abundant\nin H<sub><i>x</i></sub>CrS<sub>2−δ</sub> while\naccessing it in NaCrS<sub>2</sub> at lower voltages results in irreversible\nstructural transitions that limit cycling stability. Additionally,\nwe highlight the potential of biphasic structures such as H<sub><i>x</i></sub>CrS<sub>2−δ</sub> to enable high performance\nin energy storage electrodes.