Role of Cr Redox and Dynamics in Electrochemical Cycling of H_<i>x</i>CrS_2−δ

HxCrS2−δ is produced by the proton exchange of NaCrS2 and features alternating layers of crystalline and amorphous lamella. It exhibits superior performance as a Na-ion battery electrode compared with its parent compound with faster Na+ diffusion, higher capacity, and better cyclability. This work explores the nature of the unique biphasic structure of HxCrS2−δ using both powder and single-crystal X-ray diffraction, as well as electron microscopy. Additionally, ex situ characterizations using X-ray absorption spectroscopy, X-ray total scattering, and magnetometry are employed to study the mechanism by which this superiority arises. These reveal that migration of Cr does not impede battery performance and may, in fact, be crucial to the observed performance improvements. These studies show that Cr redox is not only possible but abundant in HxCrS2−δ while accessing it in NaCrS2 at lower voltages results in irreversible structural transitions that limit cycling stability. Additionally, we highlight the potential of biphasic structures such as HxCrS2−δ to enable high performance in energy storage electrodes.