Microstructure-Controlled Synthesis And Electrochemical Performance Of Anode Materials For Lithium Ion Batteries

In recent years, with the dramatical development of electric vehicles, hybrid vehicles and communications equipments, the demand for lithium ion batteries(LIBs) of high capacity, long life, high stability and high power density has become ever-growing urgent. As well known, the electrode is a major component in LIBs and the electrode materials play a key role in the performance of the assembled batteries. Graphite is typically used as the commercial anode material for LIBs owing to its low cost and good stability. However, its low theoretical capacity(372 mAh g-1) can not satisfy the requirements of high-efficient next-generation energy storage systems. Therefore, developing new anode materials has great significant for high-performance LIBs.In this thesis, we synthesized nanomaterials or hybrid nanomaterials with different components by adjusting the corresponding microstructure and composition. Compared to bulk ones, the nanostructured electrode materials have smaller size, higher surface area and higher reaction activity, which can shorten the transport length of Li+ and e-, increase the contact area between the electrode and the electrolyte, speed the diffusion rate, adsorb and store a large number of Li+. Then, as the electrode materials, hybrids such as the metal or metal oxides combined with carbon-based materials can exhibit the merits of each component by the synergistic effects, and thus an excellent performance can be achieved. The major research contents are as follows:1. Ge-based nanomaterilas with different components were successfully designed and synthesized by three different strategies. C/Ge nanospheres were prepared by a hydrothermal carbonization of green glucose in the presence of relatively cheap sodium germinate aqueous solution, followed by the post heat treatment. The resultant C/Ge hybrid with a small amount of Ge can achieve a high capacity of 545.1 mAh g-1; hierarchically porous C/Ge hybrid was synthesized by a facile freeze-drying method using F127 as carbon source followed by the post heat treatment. A series of C/Ge hybrids with different contents of carbon can be obtained by adjusting the usage of GeO2. Compared with bulk Ge, the resultant typical C/Ge hybrid exhibits higher capacity and better stability; hierarchically porous Ge/C-N hybrid was fabricated using a solvothermal method followed by the post heat treatment. In this hybrid, Ge nanoparticles were firmly coated with N-doped carbon and distributed in the 3D N-doped carbon network through Ge-N bond. When used as an anode in LIBs, the resultant Ge/C-N electrode delivers reversible capacities of 1240.3 mAh g-1 and 813.4 mAh g-1 after 90 cycles at 100 mA g-1 and 500 mA g-1, respectively.2. In view of the merits of transition metal oxides, we proposed low-cost methods to fabricate a series of porous materials or hybrid materials. In detailed, we demonstrated the synthesis of hierarchical spinel compounds including mesoporous Co3O4 bundles, mesoporous Co1.71Ni1.29O4 flowers and rods as well as nanoparticles, and macroporous NiCo2O4 sheets; we also reported the synthesis of metal oxides/carbon hybrids including MnO/C-N,MnO/C,TiO2/C-N,P25/GR,ZnO-Co/C-N, etc. In detailed, the as-formed Co3O4 bundles can deliver a capacity of 1667.6 mAh g-1 at 100 mA g-1 after 60 cycles, and can also exhibit capacities of 1264.8 and 603.0 mAh g-1 after 100 cycles at 1000 and 5000 mA g-1, respectively; the electrochemical performance of resultant mesoporous Co1.71Ni1.29O4 with various morphologies and macroporous NiCo2O4 sheets were also explored as anode materials for LIBs, both of which exhibit capacities above 1000 mAh g-1, showing a potential application in LIBs; MnO/C-N hybrid was successfully synthesized by a solvothermal method followed by the post heat treatment. The bonding effect between Mn and N contributed to the firm connection between MnO and N-doped carbon, which is proved to have significant effect on the high lithium-storage performance of MnO/C-N electrode; MnO/C sheets were fabricated via freeze-dying followed by the post heat treatment, which exhibit a high capacity of 1467.0 mAh g-1 at 5000 mA g-1 after 2000 cycles. The studies demonstrated that the presence of interaction between Mn and C, the improved kinetic and a reconstructed mechanism during the electrochemical process lead to its superior performance; TiO2/C-N synthesized by a hydrothermal treatment and post annealing at 500 °C achieves the amorphous state of TiO2 and the successful doping of N, which delivers a capacity of 290 mAh g-1 after 100 cycles at 1 C; P25/graphene hybrid was fabricated by a mechanical mixing route, which exhibits considerable electrochemical performance; ZnO-Co/C-N mesoporous spheres synthesized using metal organic framework(MOF) as precursor were obtained thorough a vacuum calcinations. Compared with commercial ZnO, the resultant ZnO-Co/C-N exhibits higher capacity and cycling stability.3. Generous and facile strategies were proposed to synthesize carbide-based nanomaterials and the lithium-storage performance as well as the relating mechanism was also investigated in detailed. Hierarchically porous Mo2C-C hybrid was formed using the phosphomolybdic acid and block copolymers F127 as raw materials. The resultant Mo2 C particles can be controlled to be 4–7 nm in the diameter, which combin with carbon to form a network structure. When used as an anode material for LIBs, the Mo2C-C hybrid can deliver a capacity of 1196.8 mAh g-1 after 100 cycles at 100 mA g-1. Additionally, we firstly reported the synthesis of porous core-shell Co3ZnC/N-C nanospheres and characterized their composition along with the morphology using various techniques. The lithium-storage performance and possible mechanism of Co3ZnC/N-C are explored by experimental results and theoretical calculations. These works provide efficient way and new idea for the facile production of nanoscale carbides for many of important applications.