Unlocking High Capacity and Fast Na Diffusion of H CrS by Proton-Exchange Pretreatment.

This study presents a new material, "H CrS " (denotes approximate composition) formed by proton-exchange of NaCrS which has a measured capacity of 728 mAh g with significant improvements to capacity retention, sustaining over 700 mAh g 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 CrS 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 CrS which provide fast diffusion and high capacities for sodium-ion batteries.