N-doped carbon-coated ultrasmall Nb₂O₅ nanocomposite with excellent long cyclability for sodium storage

Niobium pentoxide (Nb₂O₅) 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 Nb₂O₅ nanoparticles wrapped with nitrogen-doped carbon (denoted as Nb₂O₅@NC) were delicately synthesized via a facile sol-gel method and subsequent heat treatment. The unique structure of ultrasmall Nb₂O₅ 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 Nb₂O₅@NC composite. The obtained 600-Nb₂O₅@NC-2 anode exhibits superior rate capability and outstanding cycling stability, delivering a reversible capacity of 196 mA h g^-1 at 1 A g^-1 after 1000 cycles. Even at high current densities of 5 A g^-1 and 10 A g^-1, the long-life cycling tests show that the reversible capacities still remain at 128.4 mA h g^-1 and 95.9 mA h g^-1 after 3000 cycles, respectively, which is the best performance of Nb₂O₅-based anodes at high current densities so far. These results indicate that the feasible synthetic strategy of Nb₂O₅@NC is an effective approach to develop high-performance Nb₂O₅-based anodes for large-scale energy storage.