Synthesis And Electrochemical Performance Of NaV₆O₁₅ Electrode Material For Sodium-ion Batteries

Sodium-ion battery (NIB) has been widely studied as an alternative candidate for rechargeable lithium-ion battery (LIB). It is emerging as a good choice due to the abundance and relatively low price of raw materials used to fabricate the electrodes and electrolytes, compared with the lithium-containing precursors. As one of the cathode materials, the sodium vanadium oxide - β-MxV2O5 bronzes - is of special interest to researchers in view of their outstanding transport and magnetic properties. To further improve the electrochemical performance of NaV6O15, hydro thermal and self-combustion methods were employed to synthesize the NaVeO15 with smaller and more uniform particle size. What’s more, the preparing condition was discussed and optimized. The main research results are as follows:(1) NaNO3, NH4VO3 and citric acid monohydrate were used to fabricate the NaV^O15 nanoplates through self-combustion method, which followed by a calcining process to improve the crystallinity. The products synthesized at 450℃ exhibited the best electrochemical performance. The obtained NaV6O15 nanoplate had uniform particle sizes and a smooth and clean surface, with an average length of 400 nm and width of 100 nm. The initial discharge capacities of NaV6O15 nanoplates at the current densities of 20 and 200 mA g-1 were 149 and 82 mAh g-1, with the potential range of 1.5-3.8 V. What’s more, the NaV6O15 nanoplates exhibited excellent rate and cycling performance. Graphene obtained by thermal exfoliation method was ball milled with NaV6O15 active material to prepare the NaV6O15/graphene composite. The results showed that the graphene effectively improved the electrochemical performance of NaV6O15. The coulombic efficiency of the first cycle increased from 68% to 80%, and finally reached 95% after 30 cycles, which implies the capacity fading is effectively restrained.(2) NaCl and NH4VO3 were used to synthesize NaV6O15 nanorods by hydrothermal method. The results showed the samples with pH value of 2, calcined at 300℃ exhibited the best electrochemical performance. The samples delivered the initial discharge capacity of 146 mAh g-1 at the current density of 20 mA g-1 and remained 87 mAh g-1 after 30 cycles. The effect of different types and amounts of surfactants on the morphology of products were also discussed in this study. The samples with 10 wt% PVP as the surfactant delivered better performance. The obtained NaV6O15 nanorods had uniform particle sizes, with an average length of 1μm and width of 10 nm. The initial discharge capacities at the current densities of 20 and 200 mA g-1 were 157 and 121 mAh g-1. The samples added with surfactants showed excellent rate and cycling performance.(3) To understand the diffusion performance of sodium ion in NaV6O15 crystalline lattice, the diffusion coefficients of sodium ion were investigated by the electrochemical impedance spectroscopy (EIS) method and the first-principles calculation. The DNa obtained by EIS method were 3.33×10-12 (before the 1st cycle) and 1.32×10-12 cm2 s-1 (after the 1st cycle). Further first-principles calculation proposed a quasi-2D energy favorable trajectory along b-axis for sodium ion, with desirable activation energy of 0.481 eV, which indicated an calculated DNa of 1.2×10-12 cm2 s-1. The DNa obtained by experimental work and theoretical simulation indicated high diffusion coefficient of sodium ions. Simulation results showed that the volume changed from 518.29 A33 of the fully charged state V6O15 to 551.25 A3 of the fully discharged state Na2V6O15 and the volume change was 6.4%. During the charge-discharge process, the tunnel-like structure is not damaged, thus ensuring a good cycling performance of NaV6O15 cathode material.