Dataset of "Ionic Liquid Electrolyte Suppresses Deep Sodiation in Nb4P2S21/Mo2CTx Enabling Transition from Mixed-Voltage to Pure High-Voltage Operation for Sodium-Ion Battery Cathodes"

Elemental sulfur has garnered significant attention due to its low cost and high theoretical capacity; however, its reliance on ether electrolytes leads to the formation of soluble polysulfides, thereby limiting its application. Sulfur-rich transition metal polysulfides demonstrate potential as sulfur-equivalent cathodes to replace conventional sulfur in alkali metal-sulfur batteries; however, adequate research in this area remains unrevealed. In this study, we investigate the Nb4P2S21 in carbonate, ether, and ionic liquid electrolytes for sodium-ion battery testing.

The material exhibits a high discharge capacity exceeding 1000 mAh/g and a prolonged discharge plateau at low potentials in both ether and carbonate electrolytes, similar to other phosphorus sulfide anodes. When switching to the NaTFSI/[EMIM]TFSI ionic liquid electrolyte, 96.3% of the discharge capacity in the 0–3 V range is retained above 0.8 V, with no redox peaks observed at lower potentials. The incorporation of Mo2CTx MXene into the material further reduces electrochemical polarization and enhances cycle stability. 

During 100 cycles, a self-activation phenomenon occurs, resulting in a maximum capacity of 384 mAh/g, while the median voltage remains above 1.5 V, predominantly governed by a pair of reversible redox peaks. XPS and HRTEM analyses of post-cycled material confirm the structural and compositional stability of the material during cycling. This study advances the understanding of sulfur-rich materials in sodium-ion batteries across various electrolytes, particularly ionic liquids.