Doctor of Philosophy
University of Wollongong. Institute for Superconducting & Electronic Materials
Rahman, Mokhlesur MD, Advanced materials for Lithium-Ion batteries, Doctor of Philosophy thesis, University of Wollongong. Institute for Superconducting & Electronic Materials, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3380
Lithium-ion batteries are in high demand for the large spectrum of applications encompassing portable, industrial, and traction/automotive categories. High performance lithium-ion batteries must satisfy stringent requirements, including large reversible capacity, high rate capability, and long-term cycle life, with advanced materials providing the main solutions to these issues. Improved battery performance depends on the development of materials for the various battery components, with the key aspect improving the performance of the active materials used to fabricate the cathode and anode. The use of nanostructured and conductive composite materials is designed to enhance both ion transport and electron transport, and to promote liquid electrolyte diffusion into the bulk material by shortening the diffusion lengths of ions and increasing the conductivity within the whole electrode. In this doctoral work, several nanostructured and conductive composite materials were examined and characterized for possible application as electrode for lithium-ion batteries. In this respect, nanocrystalline porous α-LiFeO2 carbon composite, VO2(B)-multiwall carbon nanotube microsheet composite, carbon and iron phosphide incorporated LiFePO4 composite, amorphous carbon coated Li4Ti5O12-TiO2 composite, nanostructured Co3O4 materials, and carbon coated NiO nano composite were investigated.
Porous α-LiFeO2-C nanocomposites with high surface area were synthesized using the molten salt method, which was followed by a carbon coating process. For comparison, nanocrystalline α-LiFeO2 was also investigated. Electrochemical measurements demonstrated that the α-LiFeO2-C nanocomposites delivered a significantly higher reversible capacity compared to pure α-LiFeO2 and excellent cycling stability (230 mAh g-1 at 0.5 C after 100 cycles) when using a selected binder, sodium carboxymethyl cellulose (CMC). Even at the high rate of 3 C, the electrode showed more than 50% of its capacity at low rate (0.1 C). The key featuresof the synthesis method are very simple and easily scaled up, involving only low temperature treatment. Since this method does not require the use of high temperature, the fabrication process is also energy saving. So, it is believed that theα-LiFeO2-C nanocomposite can be used as a novel cathode material in lithium-ion batteries, with significant advantages in terms of environmental friendliness, high capacity, good cycling stability, and high-rate capability, which can lead to a future generation of lithium-ion batteries capable of satisfying the new demands on energy storage devices.
VO2(B)-multiwall carbon nanotube (MWCNT) microsheet composite was synthesized via an in situ hydrothermal process. Electrochemical tests showed that the VO2(B)-MWCNT composite cathode features cycling stability and high discharge capacity (177 mAh g−1) in the voltage range of 2.0-3.25 V at 1 C with a capacity retention of 92% after 100 cycles. The electrochemical impedance spectra(EIS) indicate that the VO2(B)-MWCNT composite electrode has very low charge transfer resistance compared with pure VO2(B), indicating the enhanced ionic conductivity of the VO2(B)-MWCNT composite. The stable cyclic retention is attributed to the fact that the MWCNTs enhance the electronic transport and reduce the resistance within the VO2(B) nanosheets. Moreover, the VO2(B)-MWCNT composite can prevent the aggregation of active materials and accommodate the large volume variation during charge/discharge processes because of the very good mechanical properties provided by the MWCNTs. This work provides a simple and feasible platform for further advances in CNT-based composites.
Carbon and iron phosphide incorporated LiFePO4 composite was achieved by using a simple ultra-fast solvent assisted manual grinding method, combined with solid state reaction, which can replace the time-consuming high-energy ball-milling method.The electrochemical performance was outstanding, especially at high C rates. The composite cathode was found to display specific capacity of 167 mAh g-1 at 0.2 Cand 146 mAh g-1 at 5 C after 100 cycles, respectively. At the high current density of 1700 mA g-1 (10 C rate), it exhibited long-term cycling stability, retaining around 96% (131 mAh g-1) of its original discharge capacity beyond 1000 cycles, which can meet the requirements of a lithium-ion battery for large-scale power applications. The results have demonstrated that the fabrication of samples with strong and extensive antiferromagnetic (AFM) and ferromagnetic (FM) interface coupling of LiFePO4/Fe2P provides a versatile strategy toward improving the electrochemical properties of LiFePO4 materials and also opens up a new window for material scientists to further study the new exchange bias phenomenon that is involved and its ability to enhance the electrochemical performance of lithium-ion battery electrode.
High grain boundary density, dual phase Li4Ti5O12-TiO2-C nanocomposite was synthesized by a simple molten salt method, followed by a carbon coating process. For comparison, Li4Ti5O12 and Li4Ti5O12-TiO2 were also investigated. The Li4Ti5O12-TiO2-C nanocomposite electrode yielded good electrochemical performance in terms of high capacity (166 mAh g-1 at a current density of 0.5 C), good cycling stability, and excellent rate capability (110 mAh g−1 at a current density of 10 C up to 100 cycles). The excellent electrochemical performance of the carbon coated nanocomposite could be related to the combined effects of the nanostructure, the carbon layering on the nanoparticles, and the grain boundary interface areas embedded in a carbon matrix, which would contribute together to enhance structural stability and improve lithium storage kinetics by reducing the traverse time of electrons and lithium ions, and also stabilizing the solid electrolyte interphase (SEI) film, which would result in improved rate and cycling performance.
High pulsed magnetic field and an aging technique were used for the synthesis of nanocrystalline Co3O4 via the hydrothermal method. The pulsed magnetic field processing produces a more compact and smooth surface composed of Co3O4 microspheres that each consist of numerous nanograins. The aging technique introduced into the Co3O4 synthesis process results in large Co3O4 hollow spheres consisting of a large quantity of nanospheres. So, both processes were proved to be effective approaches in material processing. Electrochemical measurements showed that Co3O4 materials prepared by the aging technique (Co3O4-Aging) yielded the best electrochemical performance compared with the other samples. In view of this hollow sphere structural arrangement, it is proposed that redox reactions with Licould promote more efficient and easier lithium diffusion than in the other two samples. Thus, the morphology affects not only the discharge capacity, but also the cycling stability of Li-ion batteries.
NiO-C nanocomposite, with spherical shell-like clusters of nanosized NiO particles surrounded by amorphous carbon, was synthesised by a spray pyrolysis technique.
Electrochemical tests demonstrated that the NiO-C nanocomposites exhibited better capacity retention (382 mAh g−1 for 50 cycles) than that of the pure NiO (141 mAhg−1 for 50 cycles). The enhanced capacity retention can be mainly attributed to the NiO-C composite structure, composed of NiO nanoparticles surrounded by carbon, which can accommodate the volume changes during charge-discharge and improve the electrical conductivity between the NiO nanoparticles. To verify the effects of the amorphous carbon network on the electrical conductivity of the NiO-C nanocomposites, AC impedance measurements were conducted. The diameter of the semicircle in the medium frequency region for the NiO-C electrode is much smaller than that of the NiO electrode, revealing lower charge transfer resistance. This indicates that the electronic conductivity of NiO was improved after the incorporation of carbon.