Fabrication Of Vanadium Nitride Based Nanostrucutral Electrode Materials And Their Energy Storage Performance

Developing electrochemical energy storage devices with high power and energy density energy plays a significant role in improving the energy ulitization effiency. Lithium ion batteries and supercapacitors are two common electrochemical energy stroage devices, which usually consist of electrode, separator and electrolyte. Electrode material is a critical part in energy storage device. Compared to carbon and transitional metal oxide, transitional metal nitrides have recently emerged as promising candidates for next-generation energy storage devices owing to their high electrical conductivity and large specific pseudocapacitance. Moreover, the high density and high specific capacitance renders its wide application in all solid supercapacitor devices. However, some disadvantages of transitional metal nitrides should be overcomed to meet the high requirement of further application. Firstly, specific charge transfer events and charge storage mechanism of transitional metal nitrides for supercapacitor is still an open question. Secondly, transitional metal nitrides are easily oxidized in an aqueous acidic or alkaline electrolyte causing capacitance decay. Thirdly, transitional metal nitrides are inorganic nonmetallic ceramic materials being brittle and having poor flexibility.To overcome above problems, the controllable fabrication, optimizing the component, insight of the energy storage mechanism have been investigated to obtain high performance energy storage devices, such as supercapacitors and lithium sulfur batteries. The main research results are described in the following:(1) VN nanowires(NWs) with different N/O ratio have been fabricated by nitriding V2O5 NWs at different temperature. The pseudocapacitance of the VN NWs is proportional to the content of the VNOx in VN NWs. The structural changes and the charge storage mechanism of the VN NWs during charge/discharge process were investigated by ion isolation methods, XPS spectroscopy and in situ Raman spectra, which indicates that the pseudocapacitive behavior is mainly attributed to the reversible reaction of VNOx chain in VN. The electron transfer number of electrochemical reaction was also measured. Our findings provide a significant theoretical guide for improving the energy storage performance of VN-based electrode materials and can be extended to other transition metal nitrides.(2) Vanadium nitride/carbon(VN/C) composite film was successfully prepared by hydrothermal treatment of ammonium metavanadate and bacterial cellulose and further annealing in NH3 atmosphere. The VN/C composite electrode boasts a high specific capacitance of 185 F/g at a scan rate of 10 mV/s and outstanding rate performance with a high specific capacitance of 96 F/g when the scan rate increases 50 times to 500 m V/s. Moreover, the cycle stability can be effectively improved by anchoring the mesoporous VN nanobelts in the three dimensional carbon framework. The VN/C film shows a high capacitance retention of 78 % after 4,000 cycles, which is superior to that of VN electrode.(3) Three dimensional intertwined nitrogen-doped carbon encapsulated mesoporous vanadium nitride NWs(MVN@NC NWs) have been fabricated by dopamine polymerization and further annealing of V2O5 NWs. The MVN NWs have abundant active sites for charge storage and NC shell suppresses electrochemical dissolution of the inner MVN NWs in an alkaline electrolyte, leading to excellent capacitive properties. The flexible MVN@NC NWs film electrode delivers a high areal capacitance of 282 mF/cm2 and exhibits excellent long-term stability with 91.8 % capacitance retention after 12,000 cycles in a KOH electrolyte. All-solid-state flexible supercapacitors assembled by sandwiching two flexible MVN@NC NWs film electrodes boast a volumetric capacitance of 10.9 F/cm3, energy density of 0.97 mWh/cm3 and power density of 2.72 W/cm3 at a current density of 0.051 A/cm3 based on the entire cell. By virtue of the excellent mechanical flexibility, high capacitance and large energy/power density, the self-supported MVN@NC NWs paper-like electrodes have large potential in portable and wearable flexible electronics.(4) Lithium sulfur(Li-S) batteries have drawn special attentions due to superior capacity. However, the poor stability owning to the solubility and shuttle effect of the intermediate polysulfide and inferior rate performance arising insulativity of the sulfur hinders further application. Considering the high conductivity and porosity of VN in MVN@NC NWs, the sulfur nanoparticles are inserted and distributed uniformly in the VN NWs of MVN@NC for the first time. The core-shell structure composite NWs with a polar vanadium oxide on the surface of vanadium nitride core shows chemical interactions with polysulphides and carbon nanotube shell exhibits physical immobilization for confining the polysulphides. Thus, the composite cathode with a high sulfur mass loading(57.2 %), high capacity(0.2 C?1302 mAh/g), superior rate performance(543.4 m Ah/g at 10 C) and remarkable cycle stability has been designed, which extends the potential application of the transitional nitrides in Li-S batteries.(5) We also extent above method to investigate other transitional metal nitrides. In this chapter, mesoporous niobium nitride nanobelt arrays(Nb4N5 NBAs) are fabricated directly on Nb foil by a hydrothermal reaction in KOH, protonation treatment in HNO3, and calcination in an NH3 ambient. The morphology, composition and pore structure of the Nb4N5 NBAs are characterized. In addition, the mesoporous Nb4N5 NBAs electrode has good specific capacitance(37.4 mF/cm2) and delivers excellent rate performance due to the high porosity and good electron conductivity boding well for application to next-generation energy storage systems.