Preparation and Electrochemical Properties of Mesoporous Carbon/Transition Metal Compound Composites

Author:Zhou Ze Ping

Supervisor:zhong ming qiang chen feng


Degree Year:2019





As the most promising important energy storage device,lithium-ion batteries have been widely used in a variety of portable equipment,fixed energy storage equipment and hybrid vehicles,which is essential to alleviate the energy crisis and global environmental problems.In the past few years,lithium-ion batteries have been used in a large number of applications,however,it also has a lot of problems such as low specific capacity,poor cycling stability and rate capability.In these researches,MoS2 and NiO are selected as active materials,polymer materials are used as carbon sources and templates to fabricate micro-nano-composite anode materials.The electron chemical properties of hierarchical porous structures and impurity-doping materials have been systematically studied.The main research contents and results are as follows:(1)For the first time,molybdenum disulfides/lignin composite microspheres were prepared by a simple hydrothermal reaction.The results showed that the use of isopropanol as a solvent played a crucial role in the self-assembly of lignin segments into microspheres.At the same time,the presence of microspheres adsorbed the free nano-sheet MoS2 to the surface to self-assembly,eliminated some dangling bonds,reduced the surface energy of nano-sheet molybdenum disulfide,inhibited the agglomeration and self-growth of its sheet structure.The formation mechanism of composite microspheres was obtained after the measuring of composite microspheres structure.The whisker-like MoS2 was firstly produced by the hydrothermal reaction between Na2MoO4 and NH2CSNH2,then partially entered into the sparse core position of the microspheres,which was highly dispersed.In the in-situ colloidal carbonaceous material produced by hydrothermal carbonization of lignin,the remainder formed a layered shell of molybdenum disulfide on the surface.This unique structure had many advantages:firstly,the specific surface area of the material was increased(462.8m2·g-1);secondly,the material had a good conductivity;thirdly,the agglomeration of the active material and the charge and discharge were greatly suppressed.Volume expansion in the process.Therefore,the composite microsphere had a high reversible capacity,excellent stable cycling performance(1049mAh·g-1(200 cycles))and good rate capability.(2)For the first time,lignin carbon microspheres were prepared by self-assembly.NiO/lignin composite microspheres(NiO/HPCM NSs)were prepared by simple physical adsorption and chemical precipitation methods.The microstructure of the composite microspheres was studied by a series of testing methods.The results showed that NiO had an uniformly distribution on the surface of lignin carbon spheres.At the same time,the microspheres also had a large specific surface area(851.8m2·g-1),rich pore structure and high degree of graphitization(ID/IG=0.84).As lithium ion battery electrode material,it had the following advantages:firstly,the nano-sized structure was conducive to the rapid diffusion of lithium ions;secondly,the carbon sphere formed by lignin as a carbon source it had high crystallinity and greatly improves the electrical conductivity of the material.The amorphous carbon matrix greatly improved the structural stability of the material.During the charging and discharging process,the composites can not only transfer electrons quickly but also inhibit the volume expansion and pulverization of the active material,so that the material had high electron chemical activity and stability;Thirdly,the rich pore structure and the large specific surface area were favorable for increasing area between the material and the electrolyte,improving the electron chemical performance of the materia and preventing the powder material from pulverizing and falling off.As an electrode material for supercapacitors,NiO/HMPC composites had the characteristics of carbon-shell electric double-layer capacitors and Faraday tantalum capacitors.Due to the uniform distribution of NiO and stable carbon structure network,these composites have a good capacitance performance(508F·g-1)and good cycling performance.(3)Hierarchical MoS2 nanoflakes were grown directly on polyphosphazene microspheres by using a simple hydrothermal method.The effects of reaction temperature and reaction time on the composite microsphere morphology were investigated.The results showed that the use of uniform polyphosphazene microspheres as a template for the growth of two-dimensional MoS2 nanosheets,due to the rich and uniform hydroxyl structure of the microspheres,these carbon spheres had a great appeal as a template for the growth of nanostructured shells.Uniform adsorption of the initial molybdenum disulfide nanosheets induced further uniform growth,which provides a greater possibility for obtaining more stable electron chemical performance.It was found by a series of measuring methods and other characterization methods that the polyphosphazene microspheres played a role as a template in the experiment and the internal oligomer acted as an alternative pore-expanding agent.Without the addition of the P/S/N elements source,multi-stage porous hollow carbon microspheres were also doped with N、P and S elements.In addition,the obtained hollow carbon microsphere structure not only had high electrical conductivity,but also had strong structural stability,so that these composites had a high reversible capacity(1421 mAh·g-1),excellent stable cycling performance and better rate capability.(4)By a simple precipitation polymerization method to prepare a special structure that molybdenum disulfides used as a core,polyphosphazene material uesd as a shell layer and the 3D carbon network used as a link without any auxiliary template.Through a series of microstructure tests,it showed that molybdenum disulfide is connected to the outer carbon shell through a conductive carbon network,which has excellent structural stability,high specific surface area(573.9m2·g-1)and porosity.Electron chemical tests showed that the composite microspheres used a flexible conductive material to seal the active material,retain part of the pores to promote expansion and contraction,fully released the expansion stress of the active material,and had stable electrical contact,thereby greatly improving the electrical properties of the material.At the same time,simple first-principles calculations showed that N,P,S doped modified carbon structure can improve the binding energy of materials and lithium ion/atoms,and change the local charge density of materials to produce more active sites.What’s more,the defects caused by doping can increase the electron chemical activity and electron transport rate of the material.