Lithium silicate gel/metal interfaces under an electric field: Low temperature/high voltage.

摘要

Glass/metal interfaces are common in batteries, fuel cells, and oxygen sensors. An electric field imposed on a glass/metal interface can lead to degradation, which can result in failure. Defects at the interface can accelerate the degradation through ion migration, metal migration, and gas evolution. In this study of glass/metal interfaces under the influence of an electric field, lithium silicate gels between aluminum substrates were used to simulate ion migration, and characterize the effects with dielectric measurements and impedance spectroscopy.; Sol-gel prepared lithium silicates were used for easy control of composition and microstructure. Samples contained 5, 10 and 15 weight percent lithium oxide (Li₂O), called LS5, LS10 and LS15, respectively. Samples generally were disc shaped, 15 mm in diameter and 10 mm thick. The samples were exposed to an AC frequency generator that produced an electric field of about 20 kV/cm. Samples were exposed for different periods of time, 1, 2, 3, 4, and 5 minutes.; Dielectric measurements were used as an indication of the effect of high voltage, where lithium ions respond to the high voltage, but cannot relax at room temperature. Capacitance, dissipation factor and series resistance were measured and the dielectric constant was calculated. The dissipation factor showed an increase with increasing lithium content, indicating that energy is dissipated when there are a large number of mobile ions. Similarly, the resistance increased with increasing exposure time in LS15. The dielectric measurements showed that the samples were affected by exposure to high field in times less than 5 minutes, with the largest effects occurring in the LS15 samples.; Impedance spectroscopy was used to study the samples under alternating current conditions. The sample impedance was measured over a range of frequencies (5 Hz to 65 KHz), and conductivities, mobility, and relaxation times were calculated. The mobility in LS15 was 4 orders of magnitude higher than the mobility in LS5 or LS10, especially at low frequency. This corresponds to a higher conductivity in LS15, which contains more mobile ions. The relaxation time in LS15 is 2 orders of magnitude longer than for LS5 and LS10. At the same time, this means the LS15 gel has higher conductivity that reduces the polarization of the material.; In conclusion, the impedance spectroscopy and the dielectric measurements both show a difference in response between LS15 and the compositions with less lithium oxide. In addition, the high field caused measurable differences within 5 minutes of exposure. The effects of exposure were more severe in LS15 because of the higher number of mobile ions.