Unlocking High Capacity and Fast Na + Diffusion of H x CrS 2 by Proton-Exchange Pretreatment.

This study presents a new material, "H _x CrS ₂ " (denotes approximate composition) formed by proton-exchange of NaCrS ₂ 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 CrS ₂ 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 CrS ₂ which provide fast diffusion and high capacities for sodium-ion batteries.
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