Preparation And Structure Tuning Of Three-dimensional Graphene Materials For Application Of Lithium Ion Battery

With the rapid development of electric vehicles,portable and flexible electronic devices,lithium ion batteries with high energy density and power density are increasingly demanded in the future.Owing to high electrical conductivity,surface area and mechanical intensity,Three-dimensional graphene materials?3DGMs?is suitable for the application of electrode material to enhance the conductivity and Li-ion diffusion properties for lithium ion batteries.Tuning the pore structure of Three-dimensional graphene materials?3DGMs?is the key to design high performance conductive network for the application of lithium ion battery.3DGMs with hierarchical porous structure can promote the rapid diffusion of lithium ion in the electrode.Simultaneously,graphene framework with excellent stability can improve the cycling performance of electrode.In this paper,the strategy of controlling the mass ratio of Dicyclohexylcarbodiimide?DCC?to graphene oxide?GO?,called as m_DCC:m_GO,was applied to tune the porous structure of 3DGMs by hydrothermal method.What's more,Nb₂O₅ and V₂O₅were used with the DCC tactic to synthesize composite electrode materials to study the application for lithium ion battery.Besides,the as synthesized composites were used as free-standing electrode material without any binder and conducive to realize the best electrochemical performance.The main research work and results were as follows:?1?Three-dimensional graphene architectures?TDGAs?were synthesized by one step hydrothermal method with different m_DCC:m_GO.SEM and BET shown that the morphology of TDGAs regulated from petals-like structure to corals-like structure,and the porous structure changed from narrow and blocking pore to non-rigid pore formed by partly stacking of graphene sheets.The result proved that DCC can prevent the graphene sheets from aggregation and optimize the pore structure of TDGAs.?2?Based on the DCC tactic to form hierarchical porosity,orthorhombic phase Nb₂O₅?T-Nb₂O₅?nanoparticles are loaded onto TDGAs to synthezise T-Nb₂O₅@TDGAs.SEM and BET shown that with the difference of m_DCC:m_GO,T-Nb₂O₅@TDGAs had the similar morphology transfermation of TDGAs and form herichical porous structure.The finding prove that the DCC tactic can also apply to the synthesize of composites.Besides,Without the addition of DCC,graphene stacked seriously in the as synthesized T-Nb₂O₅@G,and the pore structure mainly composed of narrow and blocking nano pore.With the m_DCC:m_GO=2:1,T-Nb₂O₅@TDGAs-2 exhibited the best electrochemical performance that possessed the reversible specific capacity of 73 m Ah g^-1 at 20C.The initial specific capacity of T-Nb₂O₅@TDGAs-2 at 1C was 230 m A g^-1 and the capacity retention after 200 cycles was 62.6%.The research demonstrated that the DCC tactic can tune the pore structure in the synthesis of graphene-based composite and form hierarchical porous structure to enhance lithium ion diffusion property and cycling stability.?3?V₂O₅@GC was synthesized by two step hydrothermal method to cover a graphene film on the surface of composite and the m_DCC:m_GO was controlled to 2:1.Simultaneously,V₂O₅@G was synthesized without graphene covering on surface.SEM found that the size of V₂O₅particle in V₂O₅@GC was smaller than V₂O₅@G.What's more,the pore size distribution of V₂O₅@GC was more various than V₂O₅@G.In the study of electrochemical performance,the initial discharged specific capacity at 0.2 A g^-1 of V₂O₅@GC and V₂O₅@G was 284 m Ah g^-1and 289 m Ah g^-1 respectively.The capacity retention of V₂O₅@GC and V₂O₅@G after 200cycles at 0.2 A g^-1 was 72%and 50%.The reversible capacity of V₂O₅@GC and V₂O₅@G at8 A g^-1 was 102 m Ah g^-1 and 71 m Ah g^-1.The result demonstrated that graphene film coated on surface of the composite can reduce the particle size of V₂O₅ so as to shorten the lithium ion transport channel and ensure the rapid difussion of lithium ion in active materials.On the other hand,graphene coated strategy can optimize the pore structure to provide excellent rate performance and enhance the cycling stability of the whole electrode.