Pure Single-Crystalline Na1.1V3O7.9 Nanobelts as Superior Cathode Materials for Rechargeable Sodium-Ion Batteries

High performance rechargeable batteries are crucial to the development of large‐scale electric energy storage for renewable energies. Following the great success in the field of portable electronics, rechargeable lithium‐ion batteries have risen to prominence as the key energy storage technology for electric vehicle propulsion and in parallel are starting to be evaluated for stationary applications.1 However, we shall always be prepared for the exhaustion of limited and unevenly distributed Li resources in the Earth's crust. In response, room‐temperature Na‐ion batteries (NIBs) have again aroused a great deal of interest recently because Na resources are practically inexhaustible and ubiquitous, which is extremely favorable for large‐scale stationary electric energy storage applications for renewable energies and smart grids.2, 3, 4, 5 However, partially because Na ion is larger and heavier than Li ion, there is still only very limited number of potential cathode materials for NIBs, and the obtained cathode materials are still far from satisfying in terms of specific capacity, rate capability, and cycling stability.6, 7, 8 Therefore, development of advanced cathode materials for NIBs is urgently desirable but remains a great challenge. Metal vanadates have been proposed as electrode materials due to their structure flexibility, high discharge capacity, and low cost.9 On the other hand, due to their large surface to volume ratio to contact with electrolyte, continuous conducting pathways for electrons through the electrodes, facile strain relaxation during battery operation, etc., 1D structured materials could significantly improve power‐ and energy‐density over bulk electrode materials.10 In these context, nanostructure Ag–V–O and Cu–V–O have been successfully synthesized and showed excellent cathode performances in LIBs;[[qv: 9d–9g]] and single crystalline V2O5 nanobelts indicating a high cathode capacity of 231.4 mAh g−1 for NIBs due to their large interlayer spacing.[[qv: 10c]] However, although alkaline metal vanadates (AMVs) have been found to be potential Li+ and Na+ intercalated electrode materials, there is very few report on AMVs as cathode materials in NIBs, to say nothing of AMVs with uniform morphology, high purity single crystal and 1D structure, which is partially because control of morphology and phase purity of AMVs is very difficult due to their intrinsic phase structure variety and temperature sensitivity. To the best of our knowledge, as a representative example of layered AMVs, the encouraging Na1.1V3O7.9 (NVO) can only be synthesized by complicated or time‐consuming route.[[qv: 11a]],[[qv: 11b]] Even though, the obtained NVO holds irregular shape and inextirpable impurity,[[qv: 11c]] which inevitably casts shadow over its application as cathode materials in rechargeable NIBs. Thereafter, developing new strategy to synthesize pure 1D NVO and then exploring their electrochemical performance toward sodium is of great importance. Herein, pure single crystalline Na1.1V3O7.9 nanobelts (NVONBs) are successfully synthesized via a facile and low‐temperature strategy using cheap commercial bulk vanadium pentoxide (V2O5) and sodium hydroxide (NaOH) as V and Na precursor, respectively. The Rietveld refinement method shows that the NVONBs hold layered structure which could facilitate Na+ insertion/extraction. Inspired by this structure advantage, when used as cathode materials for NIBs between 1.5 and 3.8 V, the NVONBs exhibit superior electrochemical performances, including high specific capacity of 173 mAh g−1, good cycle stability and rate capability, and high coulombic efficiency. Surprisingly, even after 1000 cycles of fast charging/discharging processes, the morphology of NVONBs could keep almost unchanged. Considering the facility and effectiveness of the synthesis route as well as the very promising electrochemical performances, the results obtained here might be extended to other next generation AMVs cathode materials for NIBs.