Browsing by Author "Zhu, Changbao"

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    Size effects on lithium storage and phase transition in LiFePO4/FePO4 system
    (2013) Zhu, Changbao; Maier, Joachim (Prof. Dr.)
    LiFePO4 is one of the most promising cathode materials, especially for its great potential to be applied in electric vehicles (EVs) and hybrid electric vehicles (HEVs), and has attracted great interest due to its appealing advantages, such as high theoretical capacity (170 mAhg-1), high safety, environmental benignity and low cost. Although a great improvement has already been reached in terms of electrochemical performance of LiFePO4 by doping, size-reduction, and network formation, several intrinsic properties of LiFePO4 are still not clear and need further investigation. One of the most important unresolved issues is the effect of size on lithium storage and phase transition in the LiFePO4/FePO4 system, which is not only crucial for fundamental understanding of LiFePO4 behavior, but also relevant to the application of such materials. In this thesis, morphology and size controlled synthesis of LiFePO4 and related electrochemical performance are discussed at first. Afterwards, size effects on miscibility gap, lithium potential variations and phase transition process are investigated systematically. The main results of this thesis are the following: Carbon-coated single-crystalline LiFePO4 thin nanowires are successfully prepared by the electrospinning method, which show good rate performance and excellent cycling stability due to the unique morphology. Small LiFePO4 nanoparticles (the thickness only around 10 nm) can be prepared by the oleylamine-assisted polyol method and the particle sizes can be controlled by adjusting experimental parameters, such as the ratio of oleyamine to tetraethylene glycol (TEG), the precursor concentration, the reaction time and the addition of carbon nanotubes. After sintering at 700 °C for 2 hours the material displays excellent electrochemical performance. The shrinking of the miscibility gap with reduction of the particle size is observed by the potentiostatic intermittent titration technique (PITT). Lithium potential variations for nanocrystalline and amorphous LiFePO4 are investigated thermodynamically and experimentally by considering the lithium intercalation regime (single phase regime and two phase regime). For nanocrystalline LiFePO4, the reversible open-circuit voltage (OCV) values decrease with reduction of particle sizes. Surface chemistry (γ) plays a crucial role in the OCV variations. For amorphous LiFePO4, compared with crystalline LixFePO4, the excess OCV can be either negative or positive, which can be explained by the signs of the ionic part and the electronic part of the excess chemical potential of lithium. Phase transition of large LiFePO4 single crystal is investigated by chemical delithiation. FePO4 layers with high porosity and cracks are observed at the surface of LiFePO4. The kinetics is governed by a parabolic growth law that indicates diffusion limitation. The pore/crack network provides fast diffusion channels and enhances the kinetics pronouncedly. With the help of the advanced scanning transmission electron microscopy with annular bright field imaging (STEM-ABF) performed in Sendai (Japan), a first order lithium staging structure is directly observed in the partially delithiated Li1-xFePO4 (x~0.5) nanowires for the first time. Size-dependent staging structure is also found. For large crystals, staging structures form an intermediate phase between LiFePO4 and FePO4, and the staging area narrows with increasing size. For small crystals, the staging structure appears throughout the whole particle.