Browsing by Author "Joshi, Yug"

Filter results by typing the first few letters
Now showing 1 - 3 of 3
  • Thumbnail Image
    ItemOpen Access
    Li-ion transport and optical modulation in thin-film battery electrodes
    (2021) Joshi, Yug; Schmitz, Guido (Prof. Dr.)
    The optical modulation of lithium manganese oxide (LiMn2O4, LMO) and lithium titanate (Li4Ti5O12, LTO) due to Li-ion insertion is quantitatively characterized. Ion beam sputtering is used to deposit the layers of respective materials on top of already sputtered platinum which acts as the current collector/reflector. The structure and morphology of the layers were probed using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Well-defined intercalation states were prepared electrochemically and investigated by optical spectrometry in reflectance geometry. The obtained dispersion curves were then modeled using the Clausius-Mossotti dispersion equation to obtain the complex refractive index as a function of wavelength at various intercalation states. A continuous change in the effective resonant wavelength with lithium intercalation was observed. This was found to be consistent with the evolution of the band structure upon ion insertion. In LMO, two significant resonances were identified in the visible region of the spectrum, which shifts with the degree of intercalation. By associating this shift with the evolving band structure, the resonances were attributed to electronic transitions between the O-2p band and the split Mn-d band. In the case of LTO, the mechanism and effect of the phase transformation (from spinel structured Li4Ti5O12 to rock-salt type Li7Ti5O12 upon lithium insertion) on the optical response is studied. The same model (using Clausius-Mossotti dispersion) unveils the presence of one and two major resonant wavelengths/frequencies in the case of Li4Ti5O12 and Li7Ti5O12, respectively, in the UV/visible/NIR region of light. The single resonance in the case of Li4Ti5O12 is allocated to a transition from O-2p to Ti-t2g i.e., across the band-gap. Whereas for the Li7Ti5O12 phase, the two resonances were characterized for the electronic transitions from O-2p to empty Ti-t2g and from filled Ti t2g to empty Ti-eg. The concentration dependence of the derived dielectric constants indicates a fast lithium-ion transport through the grain boundaries. This helps in nucleating the grain boundaries with a conductive lithium-rich phase. This increases the electronic conductivity of the thin films in the initial stages of intercalation and explains the debated understanding of the fast dis-/charge capability of Li4Ti5O12 electrodes on a nanoscale. On a micrometer scale, the diffusion is controlled by the bulk diffusion. To investigate the kinetics of lithium migration at this length scale, an innovative technique is developed that employs optical microscopy in a constrained region of the sputtered thin-film sample. At this constrained region, lithium is blocked from entering the LTO structure directly from the electrolyte. Therefore, the technique enables the observation of the lateral transport of lithium through the electrode due to the optical contrast generated in this material during the ion insertion and subsequent phase transformation. The poor diffusivity of lithium in its end phases (or Li4Ti5O12 and Li7Ti5O12) is confirmed but, this poor diffusivity challenges the notion of high dis-/charging performance reported in this material. Surprisingly, the movement of the phase boundary is hindered which has been refuted in prior reports. However, this hindrance is confirmed here by the slow, linear growth kinetics of the Li-rich phase in the initial stages of the lithium transport. Interestingly, the partial solubility of lithium in the spinel structured Li4+δTi5O12 phase increases the diffusivity of lithium in this spinel phase drastically. This drastic increase in diffusivity along with the reduction in the size of the electrode seems to be compensating for the kinetic hindrance experienced by the phase boundary.
  • Thumbnail Image
    ItemOpen Access
    Optical modulation and phase distribution in LiCoO2 upon Li-ion de/intercalation
    (2022) Banifarsi, Sanaz; Joshi, Yug; Lawitzki, Robert; Csiszár, Gábor; Schmitz, Guido
    Modulation of reflectance resulting from the change in optical constants in LixCoO2 during lithium de/intercalation is studied and quantified by in-operando and ex situ optical spectroscopy. To this aim, the LiCoO2 (LCO) thin films are sputter deposited using radio-frequency ion-beam sputtering. The films are structurally characterized by X-ray diffraction and transmission electron microscopy. The reversible electrochemical and electrochromic performance is determined by in-operando optical reflectance. Ex-situ reflectance, at particular charge states, is used to determine the optical constants by modeling the optical spectrum using the Clausius-Mossotti relation. The model reveals a dominant resonant wavelength at 646 nm for the fully intercalated state of LCO. For the delithiated state or Li0.5CoO2, a much broader and significantly larger absorption peak is obtained by the model description. This significantly broad and intense absorption peak can be associated with the conducting nature of the films upon lithium removal. Furthermore, the observed complex refractive index (CRI), evolving with the lithium content, is justified by the prior reported density of states calculations. With the CRI, the corresponding variation of the real and imaginary part of the dielectric function reveals that the intercalation of lithium and the consequent phase propagation follows a layer-like reaction.
  • Thumbnail Image
    ItemOpen Access
    Slow‐moving phase boundary in Li4/3+xTi5/3O4
    (2021) Joshi, Yug; Lawitzki, Robert; Schmitz, Guido
    Lithium titanate is one of the most promising anode materials for high‐power demands but such applications desire a complete understanding of the kinetics of lithium transport. The poor diffusivity of lithium in the completely lithiated and delithiated (pseudo spinel) phases challenges to explain the high‐rate performance. This study aims at clearing the kinetics of lithium transport using an innovative technique that employs optical microscopy in a constrained region of sputter‐deposited thin‐film samples. It enables the in situ observation of the transport of lithium through the electrode. Furthermore, with a thermostatically controlled cell, the Arrhenius‐like temperature dependence is revealed. The quantitative findings demonstrate that indeed the end phases have poor diffusivity which is, however, accelerated at intermediate Li concentrations in the spinel structured Li4/3+δTi5/3O4 phase. Surprisingly, the slow migration of the phase boundary hinders the formation of the Li‐rich (rock‐salt) phase in the initial stages. Such kinetic control by the phase boundary stands in obvious contrast to a prior (theoretical) study postulating almost “liquid” behavior of the interface. Only after the Li diffusion into the Li‐poor (spinel) phase has faded, when approaching the solubility limit, the further growth of the rock‐salt phase becomes diffusion controlled.