Synthesis of Sponge-Like Nitrogen-Doped Ti₃C₂T_<i>x</i> for High-Rate Sodium-Ion Batteries

MXene has remained a research hotspot since its discovery in 2011, owing to its excellent electrical conductivity, abundant functional groups, and outstanding mechanical flexibility. It is widely utilized as an electrode material in energy storage applications. However, its electrochemical performance is limited by self-stacking driven by van der Waals forces and hydrogen bonding, restricting ion diffusion and rate capability. Although the introduction of one-dimensional pore channels via interlayer spacers can alleviate stacking to some extent, they retain compact layers that hinder electrolyte access. Herein, sponge-like N-doped MXene (N–Ti3C2Tx) with interconnected three-dimensional pores was synthesized by precisely regulating the NH4HCO3 concentration, enabling multidirectional and rapid Na+ diffusion. At the concentration of 3 M, N–Ti3C2Tx exhibited the highest surface area (68.8 m2 g–1), excellent Na+ storage performance (203.6 mAh g–1 at 0.1 A g–1), and superior rate capability (170.5 mAh g–1 at 1 A g–1 and 146.8 mAh g–1 at 5 A g–1). Furthermore, sponge-like N–Ti3C2Tx displayed remarkable cycle stability, retaining a specific capacity of 206.9 mAh g–1 after 100 cycles at 0.1 A g–1 and 147.1 mAh g–1 after 1000 cycles at 5 A g–1. The formation mechanism of sponge-like N–Ti3C2Tx was discussed through theoretical calculations.