Design And Fabrication Of Graphene Based Materials And Their Applications In Anode Of Lithium Ion Batteries For High Performance

Rechargeable lithium ion batteries?LIBs?have been considered the most promising power sources due to their high energy density,high operating voltage,long cycle life,high coulombic efficiency,and excellent safety,etc.However,to make them applicable in hybrid electric vehicles and plug-in hybrid electric vehicles,energy and power densities of LIBs must be improved significantly.The energy and power densities of LIBs are largely determined by physical and chemical properties of the electrode materials.The traditional LIBs use graphite as the anode material due to its high coulombic efficiency and good cycle performance.However,the applications of graphite in LIBs with high energy and power densities have been severely limited by its low theoretical specific capacity(372 mAh g^-1).In the present thesis,we employ graphene-based materials as their anodes of LIBs with aims at improving their energy and power densities.The following aspects of research have been carried out:1)Fabrication of a 3-dimensional porous graphene material?PGM?and its application in LIBs: A simple two-step procedure has been employed for the synthesis of the PGM: hydrothermal reaction and calcination.Hydrothermal reaction of graphene oxide?GO?in the presence of resorcinol and glutaraldehyde leads to covalent grafting of partially reduced GO with glutaraldehyde and the deposition of phenolic resin.The subsequent calcination of the product consisting of phenolic resin deposited on partially reduced GO in the presence of KOH generates structurally stable,highly porous graphene material with a specific surface area of -1066 ± 2 m² g^-1.When evaluated as an active electrode material in a lithium ion battery,the PGM exhibits an initial discharge capacity of -1538 mAh g^-1,which is much higher than those of graphite and other carbonaceous materials reported previously.Additionally,when cycled at higher discharge/charge rates,the PGM could still deliver large capacities and excellent cycling performance,greatly demonstrating its potential for high-performance lithium-ion batteries.The attractive electrochemical performance of the PGM could be attributed to its unique porous structure with large specific surface area and the presence of a large amount of disordered carbon atoms produced by the KOH activation.2)Design of composites consisting of transition metal oxide?TMO?and graphene composite with a core-void-shell structure and their applications for the Li+ storage: The encapsulation of transition metal oxide?TMO?particles in a graphene hollow shell to form a core-void-shell structure could be an attractive way to improve the electrochemical performance of graphene-based electrodes for lithium ion batteries?LIBs?.First,the continuous graphene shell could enhance the electrical conductivity of the electrodes and thereby facilitate current collection and charge transfer associated with lithium storage.Second,the unique shell structure could suppress the aggregation of the core TMO particles while the void space between the core and shell may accommodate the large volume changes of the core during charge–discharge cycling,which enhances electrode stability against cycling.In the present work,we took Fe₃O₄ as an example and fabricated Fe₃O₄/graphene composite with a core-void-shell structure.When tested in a LIB,the Fe₃O₄@graphene composite exhibits an initial reversible capacity of 1236.6 mAh g^-1,much higher than that of an electrode based on bare Fe₃O₄,a physical mixture of Fe₃O₄ and graphene,or other forms of Fe₃O₄ reported in the literature.In addition,the cycling performance and rate capacity are also much better.These results clearly indicate that this unique core-void-shell architecture is ideally suitable for LIBs and other electrochemical energy storage and conversion devices.3)Morphology evolution of graphene supported TMOs and its influence on the electrochemical performance of LIBs:It has been widely reported that nanostrctured materials could exhibt excellent electrochemical performance as the electrode for LIBs.Their nanostructured morphological features have been considered as the main causes leading to their high electrochemical performance.In our work,graphene supported MnO₂ nanorods?MnO₂-NR/ graphene?have been employed as the anode for LIBs.The results show that the MnO₂-NR/graphene could exhibit excellent electrochemical performance when used in LIBs,especially after discharged/charged for 300 cycles.Characterization of the microscopic features suggests that the morphology and crystal structure of the MnO₂ nanorods evolve gradually during cycling,transforming the product of the MnO₂-NR/graphene into a unique electrode architecture consisting of well separated graphene coated with well-crystallized ?-MnO₂ after 300 cycles.The significantly enhanced electrochemical performance of the MnO₂-NR/graphene electrode after 300 cycles could be attributed mainly to the resulting electrode architecture,which enhances the interaction between MnO₂ and graphene and reduces the charge transfer resistance across the MnO₂/graphene interface,while not to the morphological feature of MnO₂ nanorods.These results would be greatly helpful for designing and fabricating the active electrode materials for LIBs.In summary,using graphene-based materials as the anode is an effective way to improve the energy density and power density of LIBs.The fascinating features of graphene,such as the high electrical conductivity and large special surface area,could improve the electrical conductivity of the electrode materials,and reduce the diffusion paths of lithium ion,which could thereby enhance the electrochemical performance of LIBs.In the present thesis,graphene-based anodes have been successfully designed and synthesized,and their potential applications and the mechanism for Li⁺-storage have been systematically investigated.These results would be of great significance for the design and fabrication of the electrode materials with enhanced performance for LIBs,and would also be helpful to promote the commercialization of LIBs.