| Summary: | 博士 === 國立清華大學 === 工程與系統科學系 === 104 === In this thesis, synchrotron and neutron techniques were performed to systematically investigate the structure evolutions and the mechanisms of enhancing the electrochemical performance of the LiFePO4 (LFP) positive electrodes in lithium-ion batteries (LIBs) by a small amount of vanadium (V) additives. The study started with the structural analysis of raw material and then evolved into the static analysis in the structure of batteries as well as kinetic investigation in the phase transition of LFP-based positive electrode materials during charge/discharge process. In the raw material studies, material characterization of the raw material reveals that V atoms substitute the Li sites in the structure and subsequently generate vacancies around Li sites, based on the results of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), X-ray absorption spectroscopy (XAS) and X-ray powder diffraction (XRPD) together with neutron powder diffraction (NPD). The V doping creates extra diffusion channels due to Li vacancies and therefore facilitates lithium ion extraction/insertion from/into positive electrode materials during charging/discharging. Furthermore, time-resolved in-situ experiments on pouch type batteries were employed in order to understand the impacts of V doping on the structure evolution of LFP, The half-cell experiments showed the irreversibility was reduced by ~5.6% in the presence of proper amounts of V doping. Results of XAS and UPS analysis indicate this improvement is attributed to the induced local chemical potential and reduction in the band gap by about 0.4 eV after V doping. Consequently, the phase transition rate and reversible specific capacity (~25% increased) at high C rate (3C) charge/discharge of LFP: V (LFPV) were substantially improved, compared to that of LFP without doping in operando LIB.
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