Synthesis,Characterization And Electrochemical Property Of Nb₂O₅ And TiNb₂O₇

In order to deal with the energy crisis caused by the large consumption of the traditional energy as well as the deteriorating environmental problems,to develop and utilize clean energy become more and more urgent.Possessing a lot of advantages,electrical energy has received a lot of attention because it can be obtained from some kinds of clean energies,such as wind energy as well as solar energy,and conversed and stored in batteries.At present,the voltage platform of the commercial anode material,graphite is close to the deposition voltage of lithium,which is leading to the dendrite accumulation,and this could bring the safe issue for lithium ion batteries.Therefor avoiding the formation of lithium dendrite on the surface of anode material would solve the safe issue from the dendrite aspect.Li4Ti5O12 is considered to be a very promising anode material concerning to this idea because it exhibits a relatively high lithium insertion/extraction potential,which circumvents the potential range where SEI formation occurs.However,the low storage capacity of Li4Ti5O12 has prevented its widespread use.Nb2O5 and TiNb2O7 possess similar operating voltages to Li4Ti5O12,but the theoretical capacities are much higher than Li4Ti5O12,therefor Nb2O5 and TiNb2O7 are considered to be promising candidates for next generation safe lithium ion anode materials.As most researches about Nb2O5 are based on pseudocapacitors,and as a new material,TiNb2O7 has not been widely studied.Hence,in this research taking Nb2O5 and TiNb2O7 anodes as objects,and adopting sol-gel method to synthesize Nb2O5 nanoparticles and TiNb2O7 nanoparticles.Some modification researches related to Nb2O5 and TiNb2O7 anodes were did to overcome the poor rate and cycling performance of pure metal oxides.Thus,their overall electrochemical properties have been improved.These possess both theoretical and realistic significance.Besides,in this research the electrochemical properties of TiNb2O7 as sodium ion electrode were investigated for the first time.With good properties of sodium insertion/extraction,TiNb2O7 can be a promising anode for sodium ion batteries.The main results are summarized as following.Amorphous,pseudohexagonal and orthorhombic Nb2O5 nanoparticles were synthesized using a facile and green sol-gel process followed by a thermal treatment.The resulting material has been subjected to a detailed experimental study on and comparison of structural,morphology and electrochemical properties.The experiments have demonstrated that the pseudohexagonal Nb205(TT-Nb2O5)exhibited higher storage capacity,largely due to its high specific surface area and small crystallites,and better cycling performance than both amorphous Nb2O5(a-Nb2O5)and orthorhombic Nb2O5(T-Nb2O5);such experimental findings were found to be associated,and thus ascribed to possess the lower charge transfer resistance and higher lithium ion diffusion coefficient of TT-Nb2O5 than that of a-Nb2O5 and T-Nb2O5.Macroporous Nb2O5(MP-Nb2O5)has been synthesized using dispersed polystyrene microspheres(PS)as template followed by annealing in air.The structural characterization showed that the diameter of the macroporous is around 200 nm and the average particle size of the composition is 20?50 nm.XPS revealed the presence of low valence Nb4+ and oxygen vacancies on the surface of the resulting product introduced during the pyrolysis of PS.Such a unique combination of macroporous nanostructure and tetravalent niobium ions enables the electrode with superior lithium ion insertion properties,such as high specific capacity(?190 mAh/g at 0.5 C)and rate capability.Even at a current density of 1.6 A/g,an average capacity of 129 mAh/g can still be obtained.TiNb2O7,TiNb1,9O7 and Ti0.9Nb2O7 were synthesized through a sol-gel method.The resulting materials have been subjected to a detailed experimental study of their structure and electrochemical properties.The capacity of TiNb2O7 electrode was found to be around 280 mAh/g at the current rate of 0.1 C.Compared with TiNb2O7,the capacity,rate performance and cycling performance of TiNb1.9O7 electrode have been much enhanced.The EIS results showed that the reason for its higher capacity and better rate performance is because of the good conductivity of TiNb1.9O7 electrode.Besides,the partly relief of the increasing charge transfer resistance of TiNb1.9O7 electrode as the cycling accumulated is beneficial for the good cycling stability.Layered TiM2O7/Graphene composites(TNO/G)were synthesized through a simple direct dispersion and blending in aqueous solvent followed by freeze drying process and an annealing treatment under Ar.The graphene sheets are well separated by TiNb2O7(TNO)nanoparticles,and simultaneously,the TNO particles are uniformly anchored between the graphene sheets.The combined advantages of graphene sheets and TNO,such a weight ratio and layered structure,exhibit tremendous benefits for high rate Li ion capability reaching around 180 mAh/g under a current density of 38.7 A/g(100 C rate).Through the synergistic effects of their combination the total specific capacity of TNO/G is higher than the sum of specific capacity of pure TNO and graphene in their relative ratios.For the study of Na ion capability,as graphene is an active material during the de-sodiation process,a reversible capacity of 340 mAh/g was obtained from this TNO/G at the current density of 25 mA/g.Even at a higher current density of 200 mA/g,a stable capacity of 200 mAh/g can still be obtained.