N-doped carbon-coated ultrasmall NbO nanocomposite with excellent long cyclability for sodium storage.

Niobium pentoxide (NbO) has drawn significant interest as a promising anode for sodium ion batteries (SIBs) due to its large interplanar lattice spacing and relatively high diffusion efficiency. However, the intrinsic drawbacks of low electrical conductivity and substantial volume change greatly impede its practical applications in large-scale energy storage systems. In this work, ultrasmall NbO nanoparticles wrapped with nitrogen-doped carbon (denoted as NbO@NC) were delicately synthesized via a facile sol-gel method and subsequent heat treatment. The unique structure of ultrasmall NbO nanoparticles in a carbonaceous matrix can not only effectively shorten the transmission distance for both ions/electrons but also relieve the strain and stress caused by volume variation during the sodiation/desodiation process. In addition, the synergistic effect of nitrogen doping and carbon coating can further improve the electronic conductivity and pseudocapacitive behavior of the active materials, thus promoting the rapid electrochemical reaction kinetics of the NbO@NC composite. The obtained 600-NbO@NC-2 anode exhibits superior rate capability and outstanding cycling stability, delivering a reversible capacity of 196 mA h g at 1 A g after 1000 cycles. Even at high current densities of 5 A g and 10 A g, the long-life cycling tests show that the reversible capacities still remain at 128.4 mA h g and 95.9 mA h g after 3000 cycles, respectively, which is the best performance of NbO-based anodes at high current densities so far. These results indicate that the feasible synthetic strategy of NbO@NC is an effective approach to develop high-performance NbO-based anodes for large-scale energy storage.