Synthesis And Performance Of Electrode Materials For Li/Na-ion Batteries And Supercapacitor

In this thesis,the research content contains the exploration,synthesis,structural and electrochemical properties of electrodes for Li-ion,Na-ion batteries and supercapacitors.The concerning material systems include polyanionic phosphate,transition metal oxides,transition metal sulfide and biological carbon materials.The details are as follows:Firstly,we for the first time synthesized the whole solid solutions Li3V2-xFex(PO4)3(0?x?2)via a two-step synthesis method and gave the experimental evidence on structural and morphological characteristics of these mixing transition-metal phosphates from the standpoint of cathode materials for Li-ion batteries.Then,we did a whole comparison on the electrochemical performance of Li3V2-xFex(PO4)3(0?x?2)cathodes,and explained the electrochemical differences caused by the different proportions of V3+ and Fe3+ ions.Although our results did not show Fe3+/Fe2+ electrochemical activity contrary to our expectation,this preliminary study on Li3V2-x Fex(PO4)3(0?x?2)have provided a prerequisite for further improving the electrochemical properties by nanosizing and carbon-coating to better use these mixing transition-metal phosphate to achieve a new capacity record of more than 200 mAh g-1 for a phosphate cathode in the future.Secondly,one dimensional Li2MoO4 nanostructures including nanorods,nanotubes and two dimensional carbon-coated ?-Na2MoO4 nanoplate have been successfully fabricated for the first time via a simple sol-gel method with citric acid as an assistant agent and carbon source.Our experimental results show that the formation of these special morphologies are evaporation and crystallization process with self-adjusting into a rod-like hexagonal cross-section or a plate-like structure,while the citric acid played an important role during the formation of these nanomorphologies.When evaluated as an anode firstly,the results demonstrate the important influence on electrochemical performance caused by the different morphology and carbon-coating on the surface.Furthermore,the structure change of Li2MoO4,?-Na2MoO4 and their Li storage mechanism upon lithiation and delithiation process are studied by ex-situ XRD and TEM.Thirdly,single-phase NaVO3 sample with the valence of vanadium in +5 have been synthesized by a facile sol-gel method on a large-scale respectively.Our experimental results show that sintering temperature has an important influence on crystal growth and grain size,and the grain size affects the electrochemical performance directly.When evaluated as an anode material for Li-ion battery firstly,whether the first discharge and recharge capacities or the cycle performance,the nanosize NaVO3 electrode shows a better electrochemical performance compared to the microsize electrode.It is believed that the nanostructure enhances the specific surface area of the material,decreases lithium-ion diffusion distances and increases the contact area between the electrode and the electrolyte.Furthermore,the structure change of NaVO3 and its Li storage mechanism upon lithiation and delithiation process are studied by ex-situ XRD and TEM.Fourthly,cheese-like bulk carbon with nanoholes has been successfully fabricated from egg white via a simple clean and environment friendly method.It was found that the decomposition products of boiled egg white after annealing are bulk carbon containing NaCl and KCl with trace amount of nitrogen doping before ultrasonic washing.The distilled water removed the uniformly distributed nanocrystal NaCl and KCl from the bulk carbon completely which is playing a role of green clean “corrosive agent” during the washing process.These porous structures increase the specific surface area of the carbon enormously,and improve the ionic conductivity by increasing the contact area between the electrode and electrolyte which greatly contribute to the improvement of electrochemical performance when evaluated as anode material for Li-ion and Na-ion batteries.Most importantly,because of good electronic conductivity and structural stability of this cheese-like bulk carbon,it also has the hope to be used in the other material field.Fifthly,Co9S8/C nanocmposite with a particle size less than 100 nm has been successfully synthesized via solid state reaction and then a facile mechanical ball milling treatment with sucrose as carbon source.When evaluated as anode material for SIBs,the Co9S8/C nanocmposite electrode shows the excellent electrochemical performance whether in the discharge-recharge capacity or the cycle stability.The small particle size is beneficial to the quick penetration of electrolyte and short distance of lithium ions diffusion while the carbon coating film favors the electrons transporting and accommodates the volume expansion/contraction of the composite during the discharge-charge process.The ex-situ XRD and TEM tests demonstrate that Co9S8 undergoes conversion type sodium storage mechanism.Furthermore,the carbon-coated Co9S8 nanocmposite with particle size less than 100 nm exhibits excellent electrochemical performance when used as electrode for supercapacitor.Our results indicate that nanosizing the particle size by a facile mechanical ball-milling treatment and carbon coating on the active material surface are also effective approach to fabricating metal sulfide materials for supercapacitors with excellent electrochemical performance.