Unlocking High Capacity and Fast Na⁺ Diffusion of H_x CrS₂ by Proton-Exchange Pretreatment

This study presents a new material, "H_x CrS₂ " (denotes approximate composition) formed by proton-exchange of NaCrS₂ which has a measured capacity of 728 mAh g^-1 with significant improvements to capacity retention, sustaining over 700 mAh g^-1 during cycling experiments. This is the highest reported capacity for a transition metal sulfide electrode and outperforms the most promising proposed sodium anodes to date. H_x CrS₂ exhibits a biphasic structure featuring alternating crystalline and amorphous lamella on the scale of a few nanometers. This unique structural motif enables reversible access to Cr redox in the material resulting in higher capacities than seen in the parent structure which features only S redox. Pretreatment by proton-exchange offers a route to materials such as H_x CrS₂ which provide fast diffusion and high capacities for sodium-ion batteries.