Princeton researchers have demonstrated that acid pre-treatment of NaCrS2 to form a new phase (named HxCrS2) results in significant improvements to the material’s performance as a sodium battery electrode. Acid exchange to remove sodium from NaCrS2 is accompanied by a phase transformation to a biphasic structure with alternating amorphous and crystalline layers. This unique structure allows for faster diffusion of Na+ and results in higher capacities that can be sustained over longer cycling lifetimes. Specifically, their new material features a capacity of 728 mAh/g that is maintained throughout cycling and is nearly double the reversible capacity of NaCrS2 in the same potential region. HxCrS2 also features a Na+ diffusion constant on the order of 10,000 times faster than in NaCrS2.