Studies On Mesoporous Structure;Graphene Composite For Lithium-ion Battery Anode Materials

Lithium-ion batteries have the advantages of large energy density, long cycle life, no memory effect, and so on, which are the most widely used secondary batteries at present. In order to meet the needs of the development of power batteries, lithium-ion batteries with more excellent performance are required. Mesoporou-or nano-structured materials are beneficial to the contact between electrode and electrolyte. As the surface current density remains unchanged, the output power of the battery will exponentially increase. Mesoporous or nano-structure shorten the diffusion distance of lithium-ion from electrode surface to the interior of the electrode, and the lithium-ion can sequently insert into or extract form electrode much more easily. Furthermore, pore volume in mesoporous materials are able to relax the swell of the electrode during charging/discharging. Graphene is a kind of nanostructured carbon with the advantages of large specific surface area, excellent electrical conductivity, super strong flexibility and mechanical strength, which is an excellent matrix for electrode materials. The loading of active electrode materials on graphene surface would effectively improve the conductivity of electrode materials, significantly enhance the rate capability, and improve the stability of electrode structure. In this thesis, lithium titanate, titanium dioxide, and iron oxide were used as anode materials. To improve rate capability and cycle life, mesoporous or nanostructured anode materials were deposited in the three-dimensional network of thermally exfoliated graphene. Both half cells and full cells prepared with those materials as anodes demonstrate excellent properties of lithium-ion battery. The specific research works are as follows:1. The synthesis and performance of lithium titanate nanoparticales/thermally exfoliated graphene composite structure (LTO/TEG):Thermally exfoliated graphene (TEG) was prepared by thermally exfoliating of graphite oxide, and LTO nanoparticles were in situ deposited inside the nanopore of TEG networks by solid-state reaction. Compared with pure LTO, LTO/TEG has higher rate capability and cycle performance. At50C rate current, LTO/TEG battery retains72%of initial capacity. At10C rate current, the battery capacity only faded6%after5000cycles. During the electrochemical reaction, the three-dimensional continuous networks of TEG significantly enhance the electrical conductivity of electrode and effectively increase the structural stability of the materials. The full cell with LTO/TEG as anode material and lithium iron phosphate nanoparctiles/carbon (LFP/C) as cathode material also showed good rate capability and cycle life.2. The synthesis and performance of mesoporous titanium dioxide/amorphous carbon/thermally exfoliated graphene (mesoporous TiO2/amorphous carbon/TEG) ternary composite:Triblock copolymer P123was used as the soft template. TiO2and P123micelles were in situ self-assembly within the nanospace of TEG by evaporation-induced self-assembly (EISA). After the heat treatment in Ar, P123carbonized into amorphous carbon and in situ formed on the mesopore wall. In this ternary composite, mesoporous TiO2interweaved with two carbon networks at different scales, where TEG serves as a three-dimensional continuous conductive network for mesoporous TiO2. The continuous composite structure is able to ensure that all mesoporous TiO2particles have electrical contact with the current collector during the electrochemical reaction. Furthermore, the structure prevents the TiO2particles from detaching from the graphene during the long cycling, which makes the long-term cycling performance of the sample. In addition, the carbon coating derived from PI23not only enhances the conductivity of TiO2but also stabilizes its structure. So the stabiliy of the structure and the electrical conductivity of the materials are significantly enhanced. The half cell and the full cell using the composite as anode presented good rate capability and cycle life.3. The effect of pore structure parameters on the lithium battery performance of mesoporous TiO2microspheres:Mesoporous TiO2microspheres were synthesized using triblock copolymer F108as the soft template via sol-gel method. The synthesized samples were sinteringed at various temperatures to adjust different BET surface areas and pore sizes. The effect of pore structure parameters on the lithium battery performance was systematically investigated. The experimental results show that, as increasing the annealing temperature, the BET surface area of mesoporous TiO2microspheres decreases, and the pore size increases. Large specific surface area is beneficial to the contact between electrode and electrolyte and large pores size is in favour of the electrolyte transport. There is a optimum structure parameters for the battery performance.4. Investigations on full cells using mesoporous Fe2O3/graphene (MIO/TEG) as anode materials and LFP/C, commercial LiFePO4, LiCoO2, LiMn2O4as cathode materials:We systematically discussed and analyzed the capacity match and voltage match at different capacities, irreversible capacities and curve shapes. The results showed that full cell with LFP/C as cathode materials had good cycle performance. In addition, the condition of lithium precipitated on the anode surface after battery overcharged was analyzed. The precipitation of lithium would affect the cycle performance of batteries.