Atomic Sulfur Covalently Engineered Interlayers of Ti3C2 MXene for Ultra-Fast Sodium-Ion Storage by Enhanced Pseudocapacitance

2D MXenes have been widely applied in the field of electrochemical energy storage owning to their high electrical conductivity and large redox-active surface area. However, electrodes made from multilayered MXene with small interlayer spacing exhibit sluggish kinetics with low capacity for sodium-ion storage. Herein, Ti3C2 MXene with expanded and engineered interlayer spacing for excellent storage capability is demonstrated. After cetyltrimethylammonium bromide pretreatment, S atoms are successfully intercalated into the interlayer of Ti3C2 to form a desirable interlayer-expanded structure via Ti-S bonding, while pristine Ti3C2 is hardly to be intercalated. When the annealing temperature is 450 degrees C, the S atoms intercalated Ti3C2 (CT-S@Ti3C2-450) electrode delivers the improved Na-ion capacity of 550 mAh g(-1) at 0.1 A g(-1) (approximate to 120 mAh g(-1) at 15 A g(-1), the best MXene-based Na+-storage rate performance reported so far), and excellent cycling stability over 5000 cycles at 10 A g(-1) by enhanced pseudocapacitance. The enhanced sodium-ion storage capability has also been verified by theoretical calculations and kinetic analysis. Coupling the CT-S@Ti3C2-450 anode with commercial AC cathode, the assembled Na+ capacitor delivers high energy density (263.2 Wh kg(-1)) under high power density (8240 W kg(-1)), and outstanding cycling performance.