| With the great development of portable electronic devices and increasing utilizations of electromobiles,lithium ion batteries using carbon as anode material have been unable to fully meet the market due to their relatively low lithium storage capacity.Thus,to develop lithium secondary batteries with high-energy density,friend to circumstance and safety is imminent.The metal oxides and sulfides have become competitive candidates for electrode materials of lithium second batteries owing to their high lithium storage capacity,low cost and simple preparation.However,relatively low conductivity and large volume changes during charge and discharge hinder their further commercialization.Nanostructures can alleviate volume expansion during charge;and at the same time,the conductivity of electrode materials can be greatly enhanced because high specific surface area of nanostructures can greatly enlarge the contact area between electrode material,electrolyte and current collector.Nanostructure is an effective strategy to improve electrode materials performance.The same is also true for composites.Inspired by these thoughts,the advantages of nanotechnology and composite are applied in this thesis.The TiO2-B/SnO2,MS2/SnO2,MOS2/?-MnS and FeS2/FeMoO4/Mo3S4 nanocomposites have been synthesized,and their electrochemical performances have been tested.The main research results are surmmarised as follows:Firstly,the 1D core-shell nanocomposites of TiO2-B/SnO2 have been designed.Aiming to improve the electrochemical performances of the electrode materials,the SnO2 nanoparticles are devised to implant in TiO2-B nanowires that can alleviate expansion of SnO2 during discharge.To achieve the design,the saled TiO2 is first added to alkaline solution for hydrothermal process and then TiO2-B nanowires have been obtained after the resultants are washed by acid solution.The TiO2-B/SnO2 nanocomposites are synthesized when TiO2-B nanowires are added in reaction solution of synthesizing SnO2.The capacities of TiO2-B and TiO2-B/SnO2 are 690 and 1490 mAh g-1 in the initial discharge process and the reversible capacities are 424 and 1147 mAh g-1,respectively.The nanocomposites prevail over their components in terms of lithium storage properties due to synergistic effects that TiO2-B can alleviate expansion of SnO2.And the latter can enhance the conductivity of the former in return.However,the high lithium storage capacity for TiO2-B/SnO2 nanocomposites can maintain only for 30 cycles during charge and discharge.In order to obtain electrode materials with excellent lithium storage properties,composites are prepared in the way that SnO2 is coated on hierachical MoS2 with high capacity,small volume change and layered structure.The van der Waals layer of MoS2 can hold both lithium ions and effectively buffer the volume expansion of SnO2.The MoS2/SnO2 nanocomposites are prepared by two-step hydrothermal method.First,MoS2 with different morphologies are synthesized by various methods,such as tubular and cloud,etc.Based on experience and many trial experiments,the cloudlike MoS2 is chosed to work as support materials.To obtain MoS2/SnO2 nanocomposites,the cloudlike MoS2 is added in the reaction solution of synthesizing SnO2 and then heated in 93? hydrothermal circumstance for 20 hours.The electrochemical performance of MoS2/SnO2 nanocomposites is far better than that of MoS2.The capacities of MoS2 and MoS2/SnO2 nanocomposites are 1010 and 1306 mAh g-1 in the initial discharge process and the reversible capacities are 818 and 906 mAh g-1,the discharge capacities are 400 and 949 mAh g-1 in the 80th cycle,respectively.The capacities are 142 and 462 mAh g-1 for MoS2 and MoS2/SnO2 at a 2C rate,respectively.To evaluate these advantages of MoS2/SnO2,it is necessary to conduct some experiments for understanding the reaction mechanism between MoS2/SnO2 and Li ions.As shown in optical and SEM images of electrodes photographed.after charge and discharge,the polymer film is observed from SEM images of MoS2/SnO2 electrodes and many corrosion,swell and cracks are observed from that of MoS2 electrodes.The polymer film can lessen the corrosion from electrolyte and hold back active materials falling off the current collector.The final reactions between MoS2 and Li ions can convert to that in lithium sulfur batteries.Then,inspired by the facts that MoS2 can be used as the cathode material for lithium sulfur batteries,the lithium storage capacity of metal sulfides nanocomposites working as cathode are studied.MoS2/?-MnS,FeS2/FeMoO4/Mo3S4 and NiS/Ni3S4 nanocomposites are firstly prepared by a one-pot hydrothermal method.The cauliflowerlike MoS2/?-MnS deliver capacity of 743 mAh g-1 at 200 mA g-1 in the voltage of 3-0.01 V in the initial discharge,and the corresponding coulombic efficiency is 78.1%.The reversible capacity of 743 mAh g-1 is delivered by MoS2/?-MnS nanocomposites.When they are tested in the voltage of 3-0.01 V,the capacities of 266,239,214,199 and 197 mAh g-1 are obtained by the 2nd,10th,50th,100th and 150th discharge cycles respectively,which is as much as that of commercial lithium ion batteries.The FeS2/FeMoO4/Mo3S4 nanocomposites deliver initial capacity of 951 mAh g-1 and the reversible capacity of 850 mAh g-1(89%of the initial capacity)tested at 200 mA g-1 in the voltage of 3-0.01 V.The NiS/Ni3S4 delivers capacities of 670,590 and 284 mAh g-1 in the voltage of 3-0.01 V at current density of 40,80 and 400 mA g-1 respectively.Finally,the electronic structures and elastic properties for MoS2 and SnO2 before and after insertion of lithium ions are studied by density functional theory.As the computed results shown,the energy band structures for MoS2 and SnO2 transform from semiconductor to metal type.That the band gap of SnO2 is smaller than that of MoS2 suggests better transportation of electronics for SnO2.The SnO2 behave as ductile materials before and after insertion of lithium ions.Therefore,coated MoS2 by SnO2 can improve conductivity and mechanical strtucture of electrode for MoS2/SnO2 composites in theory. |
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