Vanadium Oxides-based Materials As Cathodes For Na-ion Batteries

Na-ion batteries(NIBs) have attracted more and more attention due to their low cost and natural aboundance. However, it is difficult to find suitable materials,especially cathode materials, for NIBs because of the larger ionic radius of Na+. Now,the widely studied cathode materials for NIBs are layered transition-metal oxides,polyanions and NASICON-type materials. Disappointingly, the reversible capacities of above mentioned cathodes are generally restricted to 90-130 mAh g-1. Therefore, it is urgent to seek suitable host material which can accommodate more Na+ and permit the rapid and reversible intercalation/deintercalation of Na+. Vanadium oxides and their derivative compounds have been regarded as promising cathodes for NIBs owing to their high capacity, low cost and diversely open structural frameworks.Nevertheless, vanadates suffer from poor electrical conductivity and dissolubility in organic electrolyte, which bring about low capacity, poor cycling stability and inferior rate performance. In order to solve these issues, tremendous effort has been made.The main points are listed as follows:1. Novel NaV3O8 nanosheet@polypyrrole core-shell composites have been successfully prepared for thefirst time via a chemical oxidative polymerization method. When used as a cathode for NIBs, the NaV3O8@PPy composites showed significantly enhanced electrochemical performance compared to bare NaV3O8. In particular, NaV3O8@10%PPy composite exhibited the best performance with a discharge capacity of 99 mA h g-1after 60 cycles at 80 mA g-1 and 63 mA h g-1 at a highcurrent density of 640 mA g-1. More importantly, the change in the chemicalstate of vanadium during the Na-ion insertion/extractionprocess was explored by X-ray photoelectron spectroscopy. The result illustrated that the two discharge platforms both corresponded to V5+ to V4+.2. Porous V2O5 nanofibers were synthesized by electrospinning method. The calcining conditions had been optimized. The optimal condition is: annealing temperature 500 °C, heating rate 1 °C/min and duration of action 2 h in Ar; then,annealing temperature 400 °C, heating rate 1 °C/min and duration of action 0.5 h in Air. When used as a cathode for NIBs, the porous V2O5 nanofibers revealed highly improved electrochemical performance. Under the condition of 1.5-4 V and 40 mA g-1, V2O5 nanofibers showed a capacity of 89.5 mAh g-1 after 50 cycles, which was much higher than the capacity of 32.7 mAh g-1 for bulk V2O5.