Understanding Ion Intercalation Characteristics of Layered Materials in Superconcentrated Electrolytes: Effects of Concentration, Temperature, and Anion Identity

Transition metal dichalcogenides (TMDs) are promising layered materials for energy storage applications, due to their ability to host ions within their interlayer galleries. Reducing the hydration shell of aqueous ions, which can be achieved through high-concentration electrolytes, enhances ion intercalation, with some systems reaching the "water-in-salt" regime. Understanding the interplay among ion identity, concentration, and interlayer electrochemistry is thus essential for advancing high-performance energy storage devices. In this study, exfoliated molybdenum disulfide (MoS₂) was employed as a model TMD electrode to investigate these effects. A systematic evaluation was conducted using lithium-based electrolytes (LiTFSI, LiNO₃, Li₂SO₄, and LiCl) across a wide concentration range, from conventional "salt-in-water" to superconcentrated "water-in-salt" conditions. Electrochemical performance was further analyzed under various temperatures (-5 to 60 °C). The 5 m LiTFSI system demonstrated the highest specific capacitance (205 F g^-1 at 5 mV s^-1) and optimal ion mobility under ambient conditions. Temperature-dependent behavior revealed distinct operational windows for each concentration, with the 5 m electrolyte excelling at low-to-moderate temperatures and the 20 m system becoming favorable at elevated temperatures due to enhanced ion dynamics. Among the salts, LiTFSI provided the widest electrochemical stability window and fastest relaxation behavior, while LiNO₃ emerged as a cost-effective alternative with comparable performance. These findings highlight the importance of tuning the electrolyte concentration, composition, and thermal compatibility to optimize the intercalation behavior of layered materials in supercapacitor systems.