Research On Three-dimensional Graphene-based Composites As Anode Materials For Lithium-ion Batteries

Graphene materials of various morphologies, dimensions and chemistry can provide considerable opportunities for multifunctional application requirements. Espicially, self-assembling 2D graphene sheets into 3D porous architectures can overcome their re-stacking and thus realize their individual functions in real applications. Furthermore, functionalized 3D graphene has proved promising for developing high-performance composites for Lithium ion batteries, which typically has synergistic effects between graphene and particles. To develop 3D graphene-based materials for enhanced Li+ storage capacities, we have devoted efforts in two main aspects. For LIBs, a novel graphene based Co2(OH)3Cl composite with potential to efficiently store and deliver Li+ was proposed and synthesized. And also we developed a facial, high effective, one-step, in situ growth, solvothermal approach for constructing 3D graphene monoliths. The research contents could be seen as follows.(1) A new graphene sheets-wrapped Cobalt hydroxychloride composite(Co2(OH)3Cl@GS) was proposed and synthesized by a facile one-pot hydrothermal method. In the as-prepared composites, Co2(OH)3Cl particles were evenly and tightly anchored into the closely-interwined graphene matrix. Here, Co2(OH)3Cl particles as spacers can effectively inhibite the restacking of GS. Meanwhile, the GS with excellent flexibility can buffer volume expansion and improve the electronic conductivity of the composite. The Co2(OH)3Cl particles exhibits high initial charge capacity with 1080 m A h g-1. However, it suffered from severe capacity degradation along prolonged cyclings with only 407 m A h g-1 retained after 50 cycles at 200 m A g-1. While, the Co2(OH)3Cl@GS composite possesses outstanding cycling stability with a reversible capacity of 753 m A h g-1 after 50 cycles at 200 m A g-1. The Co2(OH)3Cl@GS composite also possesses extraordinary high-rate cycling performance, which can still deliver a reversible capacity of 414 m A h g-1 after 300 cycles even at a high current density of 1600 m A g-1. Those excellent cycling and rate performance can be ascribed to the graphene wrapped structure and the synergistic effect between Co2(OH)3Cl and GS.(2) To further overcome the re-stacking of graphene sheets in closelyinterwined structure, three-dimensional(3D) Co2(OH)3Cl/GS composite with porous architecture was obtained by one-pot in situ hydrothermal method without any additive agents. The as-prepared 3D porous Co2(OH)3Cl/GS exhibits dramatically improved electrochemical performance, owing to the unique self-assembled porous structure, which can not only overcome the stacking problems of graphene sheets but also provide multidimensional pathways for electron transport and Li+ diffusion. It can deliver a high reversible capacity of 690 m A h g-1 with excellent cycling performance at the current density of 1600 m A g-1, which is much higher than that of Co2(OH)3Cl@GS composite(about 400 m A h g-1). In addition, The initial coulombic efficiency(CE)(76.5 %) of 3D porous Co2(OH)3Cl/GS is 10 % higher than that of Co2(OH)3Cl@GS(66%), indicates more reversible reactions occured between 3D porous Co2(OH)3Cl/GS and Li+. Furthermore, the lithium storage mechanism of Co2(OH)3Cl/GS was explored. Results show that the reversible reactions of Co2(OH)3Cl/GS with Li are mainly associated with the reversible formation of Li OH.(3) In order to overcome the drawbacks of commonly used methods, a one-step, in situ growth, solvothermal approach has been developed to synthesize three-dimensional(3D) porous graphene-based monoliths. Here, as a proof-of-concept experiment, self-assembled 3D Co O/graphene sheets(Co O/GS) composites with porous structures have been successfully fabricated in ethanol medium by this solvothermal method. During the process, the in situ nucleation and growth of Co O particles on GS were tuned by the formation of a 3D GS network. Owing to the unique self-assembled 3D structure and the superior synergistic effect between the two components, the 3D Co O/GS composites as Li-ion battery anodes show excellent high-rate cycling performance with stable reversible capacities of about 706, 503 and 434 m A h g-1 after 50 cycles at high current densities of 1600, 4800 and 6400 m A g-1, respectively. Such a synthesis strategy can be a promising route to produce diverse 3D graphene-based monoliths in various solvents.(4) To verify the advantages of in situ solvothermal techniques in constructing functional 3D graphene-based composites, we presented in this dissertation by comparing their application on the preparation of 3D Fe2O3/graphene sheets(Fe2O3/GS) composites, which are used as anode materials for lithium-ion batteries(LIBs) and served as a probe here for exploring the merits from in situ solvothermal techniques. The ex situ solvothermal and in situ hydrothermal methods were adopted for comparison purpose. Physically, it was found that the in situ solvothermal process facilitates forming attractive interfacial interaction between GS and Fe2O3 resulting from the well wrapping and homogeneous distribution of nano Fe2O3 particles into the 3D GS matrix and the forceful Fe-O-C bonds between nano Fe2O3 and few-layer GS. We also confirmed electrochemically that the asprepared 3D Fe2O3/graphene prepared by in situ solvothermal technique showed better performance than those obtained via ex situ solvothermal and in situ hydrothermal methods. The reversible capacity of IS-Fe2O3/GS even gradually rises to 1071 m A h g-1 at a current density of 1600 m A g-1 after 80 cycles, while those of IH-Fe2O3/GS and ES-Fe2O3/GS drop progressively to 697 m A h g-1 and 550 m A h g-1, respectively. Even at a high current density of 10 A g-1, IS-Fe2O3/GS can still maintain a stable reversible capacity of above 550 m A h g-1 after 50 cycles. In addition, LFP/IS-Fe2O3/GS full cell with commercially available Li Fe PO4(LFP) as cathode material and IS-Fe2O3/GS as anode material also possesses high-rate cycling performance, which shows a stable reversible capacity of over 800 m A h g-1 after 40 cycles at a high current density of 1600 m A g-1.