Browsing by Author "Ohmer, Nils"

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In situ characterization of phase evolution in LiFePO4
(2015) Ohmer, Nils; Maier, Joachim (Prof. Dr.)
Among the candidates for electrodes in future Li-based batteries, LiFePO4 (LFP) is one of the most important and most frequently studied materials, undergoing a phase transformation upon delithiation to FePO4 (FP). In spite of the great scientific and practical interest in this material, there is still an extensive debate on the mechanism of this phase transformation and the underlying factors of influence. Within the framework of this thesis, first studies are carried out ex situ on multi-particle, full electrode LFP materials, being electrochemically cycled and analyzed at various states of charge by a combination of highly spatially resolved methods (high-resolution transmission electron microscopy and electron energy loss spectroscopy (HRTEM, EELS)) and integral measurement techniques (analyzing the X-ray diffraction and X-ray absorption near edge structure (XRD, XANES)). This combination of characterization techniques allows one to distinguish between the cycling behaviour of differently sized crystallites within the same electrode. It is found that for electrodes with hydrothermally grown LFP as active material, a particle size dependent cycling behaviour exists, with nanosized particles apparently not participating in the charging process at all. A turbostratic stacking of layers in these nanosized particles is found and identified to be responsible for sluggish lithium insertion and extraction. These higher dimensional defects prevent the small particles from participating in the charging process, most likely by disturbing the lithium diffusion along the 1-dimensional channels, as well as impair the transport along the other directions in the LFP host structure and thus blocking the lithium transport, resulting in a comparibly lower practical capacity during electrochemical cycling. To study the lithium exchange mechanism upon charging a LFP thin film cathode, an all-solid-state thin film battery cell with a lateral design concept is developed and realized by pulsed laser deposition (PLD) and thermal evaporation techniques. Using PLD and shadow masks LFP cathode, Li2O-V2O5-SiO2 (LVSO) electrolyte and LiAl anode thin films are deposited sequentially in a way that the Li transport pathway in the resulting battery is along the X-ray transparent commercial Si3N4 membrane substrate. This enables the usability of synchrotron-based energy resolved scanning transmission X-ray microscopy (STXM) with its high chemical and spatial resolution to perform in situ absorption measurements at the Fe L3 edge. Upon delithiation, a shift in the main absorption feature from 708 to 710 eV is used to fingerprint the change in the local state of charge, identifying areas containing Fe2+ (lithiated) and Fe3+ (delithiated), respectively. The initial lithiation process of a LFP thin film cathode material has been followed by in situ STXM, with a lateral resolution of 30 nm, during electrochemical charging of the thin film battery. The observed initial lithiation process does not follow the classical particle by particle mechanism, typical for multi-particle LFP cathodes, but instead a rather simultaneous, although inhomogeneous, lithiation is observed. The reason for this change in mechanism, compared to multi-particle powder electrodes, is found in mechanical interactions within the thin film upon lithiation, i.e. in the corresponding volume expansion and formation of high energy surfaces, changing the shape of the single-particle chemical potential to a monotone form upon lithiation. This has far-reaching consequences: not only the many-particle mechanism is changed to a concurrent lithiation, but also the single-particle mechanism is changed from a two-phase to a single-phase mechanism upon lithiation. Furthermore, a vanishing hysteresis loop and the disappearing of the memory effect is predicted. These findings are rather general and applicable to all kind of thin films of phase separating intercalation materials, undergoing a volume change upon lithium exchange. To fill the gap in literature on in situ observations of the (L)FP phase evolution on a single-particle level with appreciable space and time resolution, a micrometer-sized all-solid-state thin film battery is built with a defect-chemically well characterized LFP single crystal as cathode material with dimensions of 16x1x0.2 micrometer. Using STXM, the phase evolution along the fast (010) orientation is followed during in situ electrochemical (de)lithiation on a micro-meter scale with a lateral resolution of 30 nm and with minutes of time resolution. Furthermore, the STXM measurements performed on this sample are one of the few experiments ever taken on LFP materials with a well defined defect chemistry, even though fundamentally necessary for an overall understanding of the materials behaviour. This combination discloses not only the mechanism of LFP transformation on a single-particle level, but also the significance of elastic effects on the (de)lithiation process. Using a defect chemical analysis, the position of phase formation is found to be determined by the defect chemical situation, while the growth pattern of both LFP and FP is found to be dominated by elastic effects.