Mapping of stoichiometric inhomogeneities in field-stressed Li(0.29±δ)La0.57TiO3 electrolytes

Abstract

The intermittent nature of renewable electricity production as well as electro-mobility requires electrochemical storage of electrical energy. Lithium-ion batteries (LIBs) are the dominant technology due to their high energy densities and low self-discharge. However, liquid electrolytes, which are used in commercial LIBs, are flammable and extremely sensitive towards humidity. A promising alternative is the perovskite-type solid electrolyte Li(0.29±δ)La0.57TiO3 (LLTO) that can be applied to all-solid-state LIBs. The exact voltage stability limit and decomposition kinetics of this material are, however, unknown and the mapping of stoichiometric inhomogeneities in field-stressed LLTO is a powerful tool for a better understanding of these mechanisms.In this thesis, the stoichiometry of non-polarized and polarized LLTO electrolytes was investigated by laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS). Therefore, qualitative measurements of LLTO were performed and elemental distribution images were created. For the study of stochiometric changes during polarization, quantification of lithium (Li) is necessary, so matrix-matched standards were prepared. Quantitative LA-ICP-MS measurements of under different pressures polarized LLTO substrates were conducted. Moreover, various voltages (2.5 V – 8 V) were applied to LLTO substrates, subsequently investigating the effects using quantitative LA-ICP-MS analysis.The conducted measurements showed an excess of Li at the surface-near region of the investigated LLTO samples, irrespective of the applied field stress. The enrichment in this area could be removed by pre-ablation or leaching in an ultrasonic bath. In qualitative measurements of polarized LLTO, a local Li depletion beneath the anode is observed. Furthermore, pressed pellets with various quantities of lithium oxide (Li2O), lanthanum oxide (La2O3) and titanium oxide (TiO2) were found to be suitable for quantification. Quantitative measurements indicated that atmospheric pressure polarization results in a confined lithium accumulation at the cathodic terminal. In comparison, polarization in a high vacuum (10-5 mbar) led to a non-detectable lithium enrichment in a larger area surrounding the cathode. Furthermore, it was found that there is a direct correlation between the height of the applied voltage and the depth of the propagation in the depletion zone.The obtained results help to increase the knowledge about ion conductivity which is decisive for the application of LLTO as a solid electrolyte.