Structure Design And Energy Storage Mechanism Of Graphene-based Hybrids

Graphene, due to its unique structure and excellent physical and chemical properties,has broad application prospects in the field of energy storage and conversion.Currently, the main bottlenecks for realizing the engineering of graphene in such fieldare the uniform dispersion and mass-production, meanwhile the main researchdirection is focusing on the preparation of graphene-based high-performance materials.In this thesis, with a purpose to promote the solution to the basic problem forengineering of graphene in the field of electrochemical energy storage and conversion,a stable graphene dispersion was efficiently obtained; a mass-production method ofthin-layered graphite was developed and the comparative study of its lithium storagebehavior was conducted; The effects of different forms of carbon coating for iontransport in lithium ion battery were studied and a graded carbon coated cathode withhigh capacity and rate performance was prepared; a high-performancegraphene-carbon nanotube hybrid as an all-carbon oxygen reduction catalyst wasdeveloped and its related catalysis mechanism was investigated.For the dispersing of graphene, the efficient and stable dispersing of graphene wasachieved in both aqueous solution and organic solvent, and the results demonstratedthat polyvinylpyrrolidone is an excellent dispersing agent and the concentration ofgraphene suspension can reach1.3mg mL-1. We also found that improving thewettability of graphene could improve the dispersing efficiency in the aqueoussolution. In this thesis, the method for graphene partial wrapping was developed,which is a good example of dispersing graphene in the active material. We haveprepared graphene-carbon nanotube hybrid which is a macroform of graphene, andthe good dispersing of graphene in the electrode level could be achieved by using thismacroform.For the large-scale application of graphene, we have developed a method forpreparing thin-layered graphite which is easy to be scaled up, and the thin-layeredgraphite can be used as a low-cost alternative to graphene in some application. Wehave investigated the effect of graphene on ion transport, which is a problem of usinggraphene as the conductive additive in high capacity commercial lithium ion battery.For the area of developing high-performance graphene-based materials and exploring the related mechanism, we have developed thin-layered graphite whichcould be used as the anode material with high energy and power density in lithium ionbattery. By the comparison of lithium storage behavior of graphene and graphite withdifferent layer number, we have found that reducing the layer number could improvethe capacity and accelerate the process of mass and charge transfer. We have proposedthat the ideal carbon coating for the cathode should balance the electron and iontransport process, and prepared a graded carbon coated LiFePO4which delivered highenergy and power density. To overcome the theoretical capacity limit of the lithiumion battery, we subtly engineered the interfaces between planar graphene sheets andcurved carbon nanotubes and gained remarkable quasi-4e selectivity and excellentdurability for oxygen reduction. We further demonstrated the application of thishybrid as a high-performance cathode catalyst for lithium oxygen batteries. Wespeculate that the high selectivity of this all-carbon catalyst stems from the localizedcharge separation at the interface of the graphene and carbon nanotube, which resultsfrom the tunneling electron transfer due to the Fermi level mismatch on the planar andcurved sp2surfaces.