Homogeneous coating of carbon nanotubes with tailored N-doped carbon layers for improved electrochemical energy storage.

The combination of activity-enriched heteroatoms and highly-conductive networks is a powerful strategy to craft carbon-based electrodes for high-efficiency electrochemical energy storage. Herein, N-doped carbon (N-C) coated carbon nanotubes (N-CNTs) were fabricated via a facile in situ synthesis of polyimide in the presence of carbon nanotubes (CNTs), followed by carbonization. The polyimide-divided N-C layers were uniformly covered on the surface of CNTs with a tailored layer thickness. The as-fabricated N-CNTs were further used as electrode active materials for energy storage. When employed as the electrodes for supercapacitors, the N-CNTs exhibited a specific capacitance of 63 F g -1 at 0.1 A g -1 (an energy density of 1.4 W h kg -1 at a power density of 20 W kg -1 ), which was much higher than that of pure N-C (5 F g -1 ) and CNTs (13 F g -1 ). The supercapacitor also retained 66.7% of its initial capacitance (42 F g -1 at 10 A g -1 ) after a 100-fold increase in the current density and nearly 100% of its initial capacitance after running 10 000 cycles. Furthermore, functioning as an anode material for a Li-ion battery, the N-CNTs also delivered a larger reversible capacity (432 mA h g -1 at 50 mA g -1 ), higher rate capability, and better cycling stability compared to pure CNTs. The electrochemical performances of the N-CNTs were improved overall due to the synergistic effects of interconnected 3D networks and core-shell structures capable of facilitating electrolyte percolation and charge transportation, enhancing conductivity and surface/interface wettability, and contributing additional pseudocapacitance.