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 °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 (≈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. Sulfur atoms intercalated Ti3C2 MXene, which serves as a high‐performance electrode for sodium‐ion storage. 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. S atoms intercalated Ti3C2 annealed at 450 °C exhibits fast Na‐ion storage kinetics and incremental storage sites after S atoms intercalation.