Li-ion transport and optical modulation in thin-film battery electrodes

Abstract

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.