Achieving ultrahigh gravimetric and volumetric supercapacitors with nitrogen-doped MXene nanomesh

2D MXenes have attracted great attention in portable flexible supercapacitors owing to their high volumetric capacitances, excellent electrical conductivity, tunable surface terminations, and good flexibility. However, the low gravimetric capacitances due to the tortuous ion transport in restacked nanosheets may limit the possible applications. Herein, we propose an atomic tailoring strategy to produce nitrogen-doped porous films assembled by Ti3C2Tz MXene nanomesh. The introduced Ti-N-O/Ti-N-OH terminations greatly improve the electrochemical activity, and the rich in-plane pores in Ti3C2Tz nanomesh provide abundant pathways and new active sites for ion transfer/adsorption. Benefiting these surface modifications and structural engineering, the obtained freestanding films achieve ultrahigh gravimetric and volumetric capacitances (700F g- 1 or 2023F cm- 3 at 1 A g- 1 ) and superior rate performance (550F g- 1 or 1860F cm- 3 at 100 A g- 1 ). When assembling into symmetric super- capacitors, high energy densities, and good cycling stability are achieved at room temperature and-40 degrees C, respectively, demonstrating great promise for practical applications.