Synthesis Of Carbon-based Nanocomposites An Their Electrochemical Applications As Anodes Fo Lithium-ion Batteries

Lithium-ion batteries（LIBs） have been widely used in our daily life. They can not only power mobile phone, notebook computer and other small electronic devices, but also power electric vehicles and even large community. Lithium-ion batteries are mainly composed of shell, cathode materials, anode materials, the electrolyte and the separator. Anodes are an important component in the LIBs.Anode materials of graphite have been successfully commercialized, but there are some weaknesses difficult to overcome. In order to further improve the performance of LIBs, both the preparation of carbon-based nanocomposites and the electrochemical behaviors of them served as anodes for LIBs were investigated in this thesis. The main task was to improve the specific capacity, the initial charge-discharge efficiency and cycle performance of LIBs and to reduce the manufacturing costs of LIBs. At present, it was known that there were three different mechanisms performed in anodes for LIBs during the discharge/charge process, which were the intercalation-deintercalation mechanism, the “conversion”（redox） reaction with Li and the Li alloying-dealloying reaction. Since the mechanism of carbon material in the composite was the intercalation-deintercalation mechanism, the other electroactive materials in the composites were selectively designed to react as the conversion reaction or alloying-dealloying reaction. The influence of experimental condition on the physical characteristics of composites was investigated via various techniques, and the differences of their electrochemical behaviors were also compared to disclose how the above differences affect the batteries performance when the composite served as anodes for LIBs. The investigation was focused on the following four aspects:In the first part, porous carbon spheres doped with Fe₃C were prepared and used as an anode for high-rate lithium-ion batteries. The property of the electroactive material Fe₃C to the reversible capacity was estimated by embedding it in the porous carbon spheres. Firstly, cross-link polymer resorcinol-formaldehyde（RF） nanospheres were synthesized via the St?ber method using resorcinol, ethanol, ammonia solution and formaldehyde solution. Then the polymer nanospheres, FeCl₃?6H₂O and CH₃ COONa were dispersed in the solution mixed by ethanol and H₂ O. After hydrothermal reaction, the RF nanospheres which embedded some compounds containing iron element were pyrolyzed in N₂ atmosphere and generated porous carbon spheres doped Fe₃ C nanoparticles. It was found that the different volume ratio of H₂ O to ethanol in the hydrothermal reaction would affect the batteries performance of the final products. When the ratio of H₂ O to ethanol was 17:3, the anode made by corresponding final product had a reversible capacity of 533.6 mAhg^-1 at the current rate of 100 mA g^-1 after 250 times discharge-charge cycles. It also had a reversible capacity of 181.7 mAh g^-1 at the current rate of 2000 mA g^-1 after 1000 times discharge-charge cycles. Cyclic voltammetry and electrochemical impedance spectrum showed that the embedded Fe₃ C nanoparticles in the carbon spheres had pseudocapacity behavior, which resulted in the high reversible capacity and fast rate performance of the anode for LIBs. The phenomenon of the stepwise increase of capacity during the cycling was related to the self-reactivation of the anode material.In the second part, hollow cubic-like carbon-based composites containing iron element were prepared base on Prussian blue（PB） nanoparticles and used as an anode for high-rate lithium-ion batteries. The influence of pyrolysis temperature on both the outside carbon layer and the inner electroactive material was studied by growing a dense and even carbon layer on the hollow electroactive PB material’s surface. PB nanoparticles about 400⁶00 nm were firstly synthesized by polyvinylpyrrolidone（PVP） and K₄Fe（CN）₆. And then they were dispersed in a solution mixed with resorcinol, ethanol, ammonia solution and formaldehyde solution, and followed a hydrothermal reaction in which a layer of resorcinol-formaldehyde resin grew on the surface of PB. At last, hollow cubic-like carbon-based composites containing iron element were prepared by pyrolyzing them at the temperature of 400, 600 and 800 °C. It was found that pyrolysis made the composites are cubic-like shape inherited from PB, their interior hollow room was protected by the outside carbon shell. With the increasing of pyrolysis temperature, PB was reduced to Fe₃O₄ and then to metallic Fe nanoparticles. The hollow Fe₃O₄/carbon cubic-like composite generated at 600 °C had the best battery performance. Its reversible capacity was 1126 mAh g^-1 after 100 times discharge-charge cycles at the current rate of 100 mA g^-1.In the third part, nanomaterials of Sn/C-ZnO core/shell structure were prepared based on metal organic frameworks, ZIF-8. The electroactive material of SnO₂ was covered by a layer of carbon materials, and the effect of the thickness of carbon layer on the battery performance was investigated. At first, porous SnO₂ nanospheres was prepared by pyrolyzing the gel of SnCl₂/RF. And then, a material of metal organic frameworks, ZIF-8, grew on SnO₂ nanospheres with the aid of PVP. The thickness of ZIF-8 layer was controlled by the reaction time. Finally, materials of Sn/C-ZnO core/shell structure were prepared via pyrolysis. Study showed that BET surface area, total pore volume and the contents of metallic Sn of the composite with a thinner carbon layer were 297 m² g^-1, 0.4251 cm³ g^-1 and 55.1%, respectively; while those of the composite with a thinner carbon layer were 32.9 m² g^-1, 0.0543 cm³ g^-1 and 11.2%. When discharge-charge cycled for 50 times at the current rate of 100 mA g^-1, the reversible capacity of the composite with a thinner carbon layer was only 456.2 mAh g^-1, while that of the composite with a thicker carbon layer was 515.6 mAh g^-1. And the rate performance of the composite with a thicker carbon layer was superior to that with a thinner carbon layer. The results showed that the nanostructure of the composite with a thinner carbon layer had been changed during the discharge-charge cycling, the Sn nanoparticles inside it might aggregate or the porous structure might collapse, which resulted in a lower battery capacity.In the fourth part, an electroactive material of NiCo₂O₄ nanoarray layer directly grew on a carbon material was acted as the current collector, and the effect of the thickness of nanoarray layer on the battery performance was discussed. The precursor of NiCo₂O₄ was Co-Ni carbonate hydroxyl salt, which would release H₂ O and CO₂ when pyrolyzed at 300 °C in air and the final product, NiCo₂O₄, presented mesoporous structure. When the hydrothermal reaction time was 6, 12 and 18 hours, respectively, three different anodes were finally obtained with different mass of NiCo₂O₄ on carbon fiber cloth（CFC）. Correspondingly, the thickness of NiCo₂O₄ layer of the above three electrode was 1.8, 3.3 and 4.4 μm, respectively, and the loading mass of NiCo₂O₄ on CFC was 1.09, 2.25 and 5.09 mg cm^-2, respectively. When tested the above three anodes at the current density of 100 mA g^-1, their calculated gravimetric capacity of NiCo₂O₄ in NiCo₂O₄/CFC at the 80 th cycle was 1186, 767 and 354 mA g^-1, respectively. The fact that lower loading mass led to a higher capacity of NiCo₂O₄ indicated that a lower loading mass of NiCo₂O₄ on CFC, or a thinner NiCo₂O₄ layer on carbon fiber was beneficial to improve the capacity of NiCo₂O₄. However, study showed that the areal capacity of the above three anodes was 2.59, 3.04 and 3.10 mAh cm^-2, respectively. In practice, a higher areal capacity was more valuable than a higher gravimetric capacity. When all things considered, a thicker NiCo₂O₄ layer would improve the areal capacity of the whole anode in some degree in this paper.