The lithium ion battery is one of the most promising electrochemical energy storage systems having great potential for a wide range of applications. However, there are certain challenges that need to be addressed such as low practical energy density, reduced lifetime and low cost-effectiveness. Comparing with the anode materials, the energy density of current cathode materials are quite low. The cathode energy density improvement could be performed by increasing the voltage or capacity. In 1999, Li₂NiPO₄F with theoretical specific capacity 287mAh g^-1 and high operating voltage (higher than 5V) was synthesized. However, the operating voltage is much higher than the commercial electrolyte working window and difficult for practical use. This thesis focuses on NaLiFePO₄F which is...[ Read more ]

The lithium ion battery is one of the most promising electrochemical energy storage systems having great potential for a wide range of applications. However, there are certain challenges that need to be addressed such as low practical energy density, reduced lifetime and low cost-effectiveness. Comparing with the anode materials, the energy density of current cathode materials are quite low. The cathode energy density improvement could be performed by increasing the voltage or capacity. In 1999, Li₂NiPO₄F with theoretical specific capacity 287mAh g^-1 and high operating voltage (higher than 5V) was synthesized. However, the operating voltage is much higher than the commercial electrolyte working window and difficult for practical use. This thesis focuses on NaLiFePO₄F which is isostructural to Li₂NiPO₄F and aims to address some challenges of this material, such as the lithium ion extraction mechanism, the operating voltage, structural evolution during lithium ion extraction, the surface properties, and surface modification by the theoretical and the experimental method.

First principle simulation, which is based on density functional theory, is employed to investigate the mechanism of lithium ion extraction from NaLiFePO₄F (Li₂FePO₄F). It is found that the electron loss not only occurred on the transition metal Fe²⁺ but also O^2- during the delithiation process by electronic structure analysis. The average redox voltages at ~3.41V, 3.51V, 4.80V, and 5.6V and structural evolution within one single phase during charging and discharging process are obtained after the calculation of lithiated phases. The morphology investigation of Li₂FePO₄F was performed by surface energy calculation. The surface (100), (111), and (010) were found appeared in the constructed Wulff shape and the delithiation voltage of these surfaces were calculated. The lithium ion extraction was found started from the surface (010) and (111) initially and then from the surface (100). With further charging, more lithium ion could be activated from the innerlayer of the bulk.

In the experimental part, the NaLiFePO₄F has been successfully synthesized. The precursor LiFePO₄ was prepared by a sol-gel method and mixed and react with NaF by solid state method. The galvanostatic measurement of NaLiFePO₄F exhibited a high capacity with 123mAh g^-1 under 0.1C and stable cyclic performance in the voltage range between 2 to 4.5V. However, the capacity obtained under larger current was low (~96mAh g^-1 at 0.2C and 70mAh g^-1 at 0.5C). The test of more than one lithium ion activation was characterized in the voltage range of 2~4.9V. The Columbic efficiency was low and capacity faded quickly at the beginning because of the electrolyte decomposition. Further redox cycling tests show that the Columbic efficiency was improved along with the curbed capacity decay presumably by the SEI layer protection. The voltage of redox reaction potential was found at 3.5V and 5.6V by the CV test. The charged electrodes were characterized by XPS and electron loss of Fe²⁺ and O^2- were found. The CV and XPS results have a good agreement with the DFT investigation.

The polypyrrole (PPy) coating of NaLiFePO₄F was performed for the improvement under large current (0.5C). The DFT calculation predicted the work function could be decreased after PPy coated on NaLiFePO₄F particles. A capacity with 85mAh g^-1 of NaLiFePO₄F/PPy composite was obtained at 0.5C with negligible capacity loss in the first 200 cycles and the cyclic performance was also improved. The EIS analysis proved that the charge transfer resistance decreased after PPy coating, consistent with the change of work function by DFT calculation. It indicates the electronic conductivity of NaLiFePO₄F was improved after PPy coating because of the extra electrons in the double bonds are free to transfer through the polymer chain. Meanwhile, the increase of charge transfer resistance of tested cell was decreased after the PPy coating indicating the surface chemistry was improved.