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, same with other high-capacity phosphorus sulfide anodes via conversion reactions. When switching to the NaTFSI/[Emim]TFSI ionic liquid electrolyte, 96.3% of the initial discharge capacity in the 0–3 V range is retained above 0.8 V, with the suppression of low-voltage redox activity. This shift is attributed to the cointercalation of Na⁺ and Emim⁺ ions, preventing the materials from deep sodiation at lower voltage range. The incorporation of Mo₂CT_x 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. X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) analyses of postcycled 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.