Role of Cr Redox and Dynamics in Electrochemical Cycling of H_xCrS_2−δ

H_xCrS_2−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 H_xCrS_2−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 H_xCrS_2−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 H_xCrS_2−δ to enable high performance in energy storage electrodes.