Synthesis And Electrochemical Properties Of Tungsten Bronze Mo₃Nb₂O₁₄ Complex Oxides

In the face of global energy and environmental issues,it is urgent to develop new energy materials for energy storage and conversion.Lithium-ion batteries with high energy density,safety,good cycle performance,and no pollution are among the most promising energy storage devices in the future energy storage field.The carbon anode materials that have been commercialized can no longer meet the requirements of new energy storage devices.Therefore,the development of high safety and high capacity anode materials that can replace carbon materials is a key step in the development of new high performance energy storage devices.There are large numbers of three-sides,four-sides and five-sides channels in the structure of tungsten bronze and tungsten bronze-like structure oxides.These channels if open are favaorable to storage and transport of lithium and sodium ion.At the same time,these materials have safe operating voltages.It has potential applications in anode materials for lithium ion batteries.In this thesis,Mo₅O₁₄-type tetragonal tungsten bronze-like?TTB?oxide is selected as the research object from the material crystal structure feature of open channels.Considering the problem of poor electrical conductivity of these TTB-like transition metal oxide,we investigated the effects of oxygen vacancy defects and material morphology on electronic conductivity and electrochemical performance.The main research results are described as the following four points:1.A variety of Mo₅O₁₄-type TTB-like complex oxides(Mo_1-xM_x)₅O₁₄?M=V,Nb,Ti,W?,were synthesized using different preparation methods,and their electrochemical performance was studied.Owing to their high theoretical capacity,these TTB-like oxides(Mo_1-xNb_x)₅O₁₄,(Mo_1-xV_x)₅O₁₄,Mo₃VNbO₁₄,(Mo_0.68V_0.23W_0.09)₅O₁₄ were used as the anode material of lithium ion batteries.Its first charge and discharge capacity can reach about 350 mAh g^-1.However,it exhibits poor rate performance resulting from the inherent poor conductivity of these transition metal oxides.2.The micro-scale Mo₃Nb₂O_14-x with oxygen vacancies was synthesized in a vacuum environment by traditional solid-phase method.Its electronic conductivity was improved by 6 orders of magnitude compared to Mo₃Nb₂O₁₄,which can reach7.02×10^-5 S cm^-1.Compared with Mo₃Nb₂O₁₄,Mo₃Nb₂O_14-x possessed better cycle and rate performances.At a high current density of 400 mA g^-1,its reversible capacity remained at 105.7 mAh g^-1 after 400 cycles.In contrast,the reversible capacity of Mo₃Nb₂O₁₄4 is only 44.6 mAh g^-1.In addition,the reversible capacity of the LiCoO₂//V-MNO full battery remained at 145.4 mAh g^-1 after 100 cycles at 100 mA g^-1.These results indicate that the presence of oxygen vacancies can improve the electrochemical performance of micron-scale Mo₃Nb₂O₁₄4 anode materials.3.Solvothermal method was used to synthesize porous nanorod microsphere Mo₃Nb₂O₁₄,and the effect of material morphology on the electrochemical performance of lithium ion battery was studied.The microsphere Mo₃Nb₂O₁₄ shows an excllent reversible specific capacity and rate and cycle performances.For 500cycles at a current density of 0.2 A g^-1,its reversible capacity can be maintained above150 mAh g^-1,even at 1 A g^-11 for 500 cycles,the capacity can still be maintained at 110mAh g^-1.The results show that the nanorods can shorten the diffusion distance of lithium ions,and porous structure increased the contact area with the electrolyte.Therefore,it showed a good reversible specific capacity and excellent rate and cycle performances.4.In addition to lithium ions,the four-membered ring and five-membered ring channels in Mo₃Nb₂O₁₄ may also facilitate the transport of Na ions.Thus Mo₃Nb₂O₁₄was also applied to Na-ion batteries as anode material.After introducing oxygen vacancies using CaH₂ from topological reduction,Mo₃Nb₂O_14-x material exhibited excellent sodium storage performance.At a current density of 200 mA g^-1,its capacity can be maintained at 105 mA g^-1 even after 450 cycles;while Mo₃Nb₂O₁₄4 is 32.03mAh g^-1 after 400 cycles.The results confirm that Mo₃Nb₂O_14-x with oxygen vacancies has better cycle performance.Compared with other tungsten bronze compounds reported,Mo₃Nb₂O₁₄ materials with oxygen vacancies exhibit more excellent electrochemical performance.