Interface and grain boundary resistance of a lithium lanthanum titanate (Li3xLa2/3−xTiO3, LLTO) solid electrolyte

Advanced experimental setups and measurement techniques are crucial for investigating and systematically improving interface characteristics. In this study we introduce an interface-cell, which allows the systematic analysis of liquid/solid electrolyte interfaces via four-point measurements. The functionality of this setup is demonstrated by analysing the impact of parameter variations on the aqueous interface of lithium lanthanum titanate (Li3xLa2/3−xTiO3, LLTO) solid electrolytes as used in Lithium–air batteries. By variation of real operating conditions their impact on cycling performance is highlighted. Examples include temperature (0 to +25 °C) and state of charge (SoC), which induces an alteration of pH (here pH ≈ 6–14) and Li+-concentration (here 0.057 mol L−1 to 10.62 mol L−1). Interestingly, a change of the polarization resistance greater than two orders of magnitude (250 Ωcm2–25,000 Ωcm2) could be identified. The proven dependency of the interface to both the pH and Li+-concentration is explained by an H+/Li+-exchange reaction at the LLTO/H2O-interface. Additionally, we were able to determine the solely grain boundary resistance (∼250 Ωcm2 at 25 °C) of the LLTO samples, without the impact of blocking electrodes. A temperature variation revealed the activation energies of the processes to be 0.4 eV (grain boundary) and 0.46 eV (interface). These results were then critically evaluated in the context of aqueous Lithium–air batteries.