Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Gao, Xuanwen, Development of novel materials for rechargeable lithium batteries, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4364
In the field of electrical energy storage, lithium ion batteries (LIBs) are considered as one of the most promising technologies due to their particularly higher energy density and longer shelf life, as well as they do not suffer from the serious memory effect problems that afflict Ni-MH batteries. Graphite and LiCoO2 are currently the most common commercial anode and cathode materials for the LIB, but they still suffer from low theoretical capacities of 372 mAh g-1 and 170 mAh g-1, respectively. Such low discharge capacity would be unable to satisfy the growing demand for large-scale potential lithium ion battery applications, such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and stationary energy storage for solar and wind electrical energy generation. Therefore, the electrical performance of active electrode materials in rechargeable lithium ion batteries must continue to be improved. In this doctoral work, several promising materials for both anode and cathode electrodes were synthesized and combined with conductive polymer to further improve their electrochemical performance. These include LiV3O8-polyaniline, Germaniumpolypyrrole, and LiNi0.5Mn1.5O4-polypyrrole composites. Monodisperse porous Ni0.5Zn0.5Fe2O4 nanospheres are also successfully synthesized by the solvothermal method and their electrical performances as novel anode materials for LIB are investigated in detailed. In addition, another key aspect for the electrochemical performance of LIB is the stability of the electrolyte. The most widely used electrolyte for lithium ion batteries is LiPF6 dissolved in ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC). The battery performance may be limited, however, by the highly oxidizing conditions at high voltage (> 4.5 V). Herein, room temperature ionic liquid was used as a new type of electrolyte for the high-voltage cathode material LiNi0.5Mn1.5O4, and the relationship between the electrolyte characteristics and the performance of Li/LiNi0.5Mn1.5O4 cells at the high potential of 5.1 V was studied in more detail.
Anode materials for the LIBs
Nano-Germanium/polypyrrole composite has been synthesized by a simpe chemical reduction method in aqueous solution. The Ge nanoparticles were directly coated on the surface of the polypyrrole. The morphology and structural properties of samples were determined by X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thermogravimetric analysis was carried out to determine the polypyrrole content. The electrochemical properties of the samples have been investigated and their suitability as anode materials for the LIB was examined. The discharge capacity of the Ge nanoparticles in the Ge-polypyrrole composite was calculated as 1014 mAh g-1 after 50 cycles at the 0.2 C rate, which is much higher than that of pristine germanium (439 mAh g-1). The composite also demonstrates high specific discharge capacity at different current rates (1318, 1032, 661, and 460 mAh g-1 at 0.5, 1.0, 2.0, and 4.0 C, respectively). The superior electrochemical performance of Ge-polypyrrole composite could be attributed to the polypyrrole core, which provides an efficient transport pathway for electrons. SEM images of the electrodes have demonstrated that polypyrrole can also act as a conductive binder and alleviate the pulverization of electrode caused by the huge volume changes of the nanosized germanium particles during Li+ intercalation/deintercalation.
Monodisperse porous Ni0.5Zn0.5Fe2O4 nanospheres have been successfully synthesized by the solvothermal method. The diameter of the nanospheres can be tuned by controlling the reactant concentration. Lower reactant concentration is favoured for the synthesis of mesoporous Ni0.5Zn0.5Fe2O4 nanospheres with higher surface area. The electrochemical results show that mesoporous Ni0.5Zn0.5Fe2O4 nanospheres exhibit high reversible specific capacity (1110 mAh g-1) for Li storage and high capacity retention, with 700 mAh g-1 retained up to 50 cycles. The excellent electrochemical properties could be attributed to the large surface area and mesoporous structure. The results suggest that Ni0.5Zn0.5Fe2O4 could be a promising high capacity anode material for lithium ion batteries.
Cathode materials for the LIBs
LiV3O8-polyaniline nanocomposites have been synthesized via chemical oxidative polymerization, directed by the anionic surfactant sodium dodecyl benzene sulfate. The polyaniline particles are uniformly coated on the LiV3O8 nanorods. The composite with 12 wt. % polyaniline retains a discharge capacity of 204 Ah kg-1 after 100 cycles and has better rate capability (175 Ah kg-1 at 2 C, and 145 Ah kg-1 at 4 C) than the bare LiV3O8 reference electrode in the potential range of 1.5-4.0 V. The polyaniline coating can buffer the electrode dissolution into the LiPF6 that occurs in LiV3O8 during cycling. The charge transfer resistance of the composite electrode is much lower than that of the bare LiV3O8 electrode, indicating that the polyaniline coating significantly increases the electrical conductivity between the LiV3O8 nanorods. Conductive polyaniline is also proven as a conductive binder which buffers the dissolution of LiV3O8 into the electrolyte and reduces the contact resistance among the nanorods, so the performance of the composite is significantly improved. Conductive polypyrrole-coated LiNi0.5Mn1.5O4 (LNMO) composites have been applied as another promising cathode materials in LIB, and their electrochemical properties are explored at both room and elevated temperature. The morphology, phase evolution, and chemical properties of the as-prepared samples were analyzed by means of X-ray powder diffraction, thermogravimetric analysis, Raman spectroscopy, and scanning and transmission electronic microscopy techniques. The composite with 5 wt. % polypyrrole coating shows discharge capacity retention of 92 % after 300 cycles and better rate capability than the bare LNMO electrode in the potential range of 3.5-4.9 V vs. Li/Li+ at room temperature. At elevated temperature, the cycling performance of the electrode made from LNMO-5 wt. % polypyrrole (PPy) is also remarkably stable (~91 % capacity retention after 100 cycles). It is revealed that the polypyrrole coating can suppress the dissolution of manganese in to the electrolyte which occurs during cycling. The charge transfer resistance of the composite electrode is much lower than that of the bare LNMO electrode after cycling, indicating that the polypyrrole coating significantly increases the electrical conductivity of the LNMO electrode. Polypyrrole can also work as an effective protective layer to suppress the electrolyte decomposition arising from undesirable reactions between the cathode electrode and the electrolyte on the surface of the active material at elevated temperature, leading to high coulombic efficiency.
Ionic liquid electrolyte for the LIB
Among the high voltage cathode materials, LiNi0.5Mn1.5O4 is of particular interest, with comparable capacity (around 140 Ah kg-1) to LiCoO2 and LiFePO4, and with much higher specific energy (658 Wh kg-1). The stability of the electrolyte is still a major concern, however, for the high voltage spinel cathode materials because the potential range is beyond the decomposition potential of conventional electrolyte (~4.7 V vs. Li/Li+). In this research work, a 5 V cathode material, LiNi0.5Mn1.5O4 nanoparticles, was prepared via the sol-gel method. The room temperature ionic liquid, 1 M lithium bis(trifluoromethysulfony)imide (LiTFSI) in N-butyl-N-methylpyrrolidinium bis(trifluoromethane-sulfonyl) imide (Py14TFSI), was used as electrolyte. The electrochemical performance shows that the LiNi0.5Mn1.5O4 nanoparticles with room temperature ionic liquid as electrolyte show comparable capacity to that of conventional electrolyte (1 M LiPF6 in EC: DEC = 1:2 (v/v)), with improved coulombic efficiency at the high voltage of 5.1 V.