Sulfur-functionalized binary transition metal Mo2Ti2C3 MXene for long-life hybrid sodium-ion capacitors

The binary transition metal MXene Mo 2 Ti 2 C 3 exhibits excellent structural integrity and physiochemical properties, thus becoming potential candidate for electrochemical devices. In this work, the pre-etched MXene was further surface modified with sulfur. This unique route not only modifies the MXene and its structural stability but also significantly improves its specific capacitance. Electrochemical evaluation shows that the pristine (bulk) MXene demonstrate a specific capacitance of ~169 F/g, whereas the sulfur modified MXene exhibits a much higher value around 356 F/g, clearly demonstrating the benefits of surface and structural modification. During galvanostatic charge-discharge testing, the modified MXene achieves a stable specific capacity of ~185 mAh/g and excellent durability, retaining almost 100% of its initial specific capacity over 4000 cycles at a current density of 2 A/g. Encouraged by its outstanding half-cell performance, the modified MXene was paired with an activated hard carbon acting as positive electrode to fabricate a hybrid sodium ion capacitor (HSIC). The energy and power characteristics of the hybrid device, presented in the Ragone plot, shows that the cell attains a maximum power density of 7.2 kW/kg and an energy density of 79 Wh/kg, while maintaining 73% of its initial capacity after 4000 cycles.