Synthesis And Electrochemical Performance Of Cobalt Disulfide/Graphene Composites As Anode Materials For Lithium Ion Batteries

Lithium- ion battery has been widely used due to the advantages of large energe density, long cycle life, and environmental friendliness. With the development of miniaturization and lightweight of electronic devices, the cathode and anode materials for lithium ion batteries require larger specific capacity, longer cycle life and better rate characteristics. The development of new electrode materials or the modification of tranditional electrode materials becomes the frontiers in the research field of energe storage. In this thesis, the synthesis and modification of new anode CoS₂ are investigated. Firstly, the synthesis process was studied; then the modification of CoS₂ by coating graphene was investigated; and then the synthesis of three-dimensional porous CoS₂/graphene（3DCG） and the improvement of electrochemical performance have been further investigated; finally, the synthesis and electrochemical performance of free-standing and binder-free andoe of CoS₂/3D graphene foam（C /3DGF） were investigated.The main research contents and results are as follows.（1） The synthesis and electrochemical performance of CoS₂/Graphene（CG） and CoS₂/CNTs/Graphene（CCG） powder anode materials have been investigateded. After coated by graphene, the CG composite powder anode material has better performance than CoS₂: the discharge capacity retains 237 mAh g^-1 after 50 cycles at 100 mA g^-1,much higher than that of CoS₂ without graphene coating(88.9 mAh g^-1), and the discharge capacity is 98 mAh g^-1 at the current density of 2000 mA g^-1. After the introduction of carbon nanotube, the electrochemical pefermance of CoS₂/CNTs/Graphene（CCG） powder anode material has been significantly improved: the discharge capacity retains 364 mAh g^-1 after 50 cycles at 100 mA g^-1,the discharge capacity is 212 mAh g^-1 at the current density of 2000 mA g^-1. The main reason is that the introduction of CNTs will further enhance the conductivity, enlarge the space between CoS₂ particles and prevent the restacking of graphene layers, and further suppress the volume change during charging/discharging process, resulting in further improving the electrochemical performance.（2） The synthesis and electrochemical performance of three-dimensional CoS₂/graphene（3DCG） and three-dimensional CoS₂/CNTs/graphene（3DCCG） anode materials have been investigated. The 3DCG anode shows excellent electrochemical performance: the discharge capacity retains 822 mAh g^-1 after 50 cycles at 100 mA g^-1, the discharge capacity is 428 mAh g^-1 at the current density of 2000 mA g^-1. 3DCCG also shows excellent performance: the discharge capacity retains 859 mAh g^-1 after 50 cycles at 100 mA g^-1. Compared with corresponding CG and CCG powder anode materials, the preformanc of 3DCG and 3DCCG has been greatly improved. Their excellent performance results form the three-dimensional graphene structure which can increase the specific area, provide good conductivity, restrain the volume change of CoS₂, keep the structural integrity, improve the wettability of electrode and decrease the diffusion length of lithium ions and. It indicates that graphene-based three-dimensional porous conductive network indeed enhance the electrochemical performance of lithium- ion batteries.（3） Free-standing and binder-free CoS₂/graphene foam anode with high whole-electrode capacity was developed. The results show that Free-standing and binder-free CoS₂/graphene foam anode can be successfully fabricated by in-situ growth CoS₂ nanoparticles on the CVD- grown 3D graphene foam; the biggest advantage of such Free-standing anode is that it does not require any current collector（C u foil）, conductive additive and binder, thus significantly decrease the total weight of the battery; the specific capacity based on the whole electrode is 93 mAh g^-1, which is much higher than those of other anode materials（C G, CCG, 3DCG, 3DCCG） in this thesis. The C/3DGF is of great importance and provides a new method to decrease the total weight and simplify the assembleing process of Li-ion batteries.