Li-O₂ and Li-S cells can be further improved, if the anodic side is operated with a nonaqueous electrolyte and the cathodic side with an aqueous electrolyte. The separation of two different liquid electrolytes can be realized by using a solid Li⁺ electrolyte, because only Li⁺ ions are transported. Electronic and anionic conduction is blocked [1].

In this study the kinetic parameters of the interfacial charge transfer between a liquid electrolyte with various concentrations between 0.001 and 1 mol l^-1 LiPF₆ in ethylene carbonate/dimethyl carbonate 1:1 vol. % and a Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte is investigated. It has been shown that a multi electrode cell is a suitable experimental setup [2]. An improved symmetric liquid/solid/liquid cell with six potential probes and two lithium metal electrodes to draw a Li⁺ current through the setup has been developed to measure the electrochemical potential within the electrolytes.

The extrapolation of the potential gradients leads to the potential drop Δ_Li⁺/F at the solid/liquid electrolyte interface.

The almost symmetric S-shaped functional course of the current density i vs. Δ_Li⁺/F at LiPF₆ concentrations higher than 0.01 mol l^-1 can be described by a serial connection between a constant ohmic resistance R_SLEI (Solid-Liquid Electrolyte Interphase SLEI) and a current dependent thermally activated ion transfer process. The constant ohmic resistance R_SLEI of about 800 Ω cm² can be attributed to a surface layer on the solid electrolyte with a comparatively low conductivity as it was described by Busche et al. [3]. It may be formed by degradation reactions with the liquid electrolyte. The thermally activated ionic transfer process obeys a Butler-Volmer like behavior. The exchange current density i₀ is dependent on the Li⁺ concentration in the liquid electrolyte and has a value of 37 µA cm^-2 at a LiPF₆ concentration of 1 mol l^-1.

For very low concentrated liquid electrolytes (c_Li⁺ < 0.01 mol l^-1) the polarization curves show a strong asymmetry due to a depletion of Li⁺-ions at one of the surfaces because of the increasing diffusion zone and decreasing convection and an enlargement of the Li⁺ concentration at the other one.

The polarization resistance R_p, which can be obtained by the slope of the polarization curves at i = 0, has a characteristic dependence on the electrolyte concentration and behaves according to a power law for concentrations below 0.1 mol l^-1. For higher concentrations R_p reaches the limit given by the solid-liquid electrolyte interphase R_SLEI.

[1] Girishkumar, G., McCloskey, B., Luntz, A. C., Swanson, S. & Wilcke, W. Lithium−Air Battery: Promise and Challenges. J. Phys. Chem. Lett. 1, 2193–2203 (2010).

[2] Abe, T., Sagane, F., Ohtsuka, M., Iriyama, Y. & Ogumi, Z. Lithium-Ion Transfer at the Interface Between Lithium-Ion Conductive Ceramic Electrolyte and Liquid Electrolyte-A Key to Enhancing the Rate Capability of Lithium-Ion Batteries. J. Electrochem. Soc. 152, A2151–A2154 (2005).

[3] Busche, M.R., Drossel, T., Leichtweiss, T., Weber, D.A., Falk, M., Schneider, M., Reich, M.-L., Sommer, H., Adelhelm, P., Janek, J., 2016. Dynamic formation of a solid-liquid electrolyte interphase and its consequences for hybrid-battery concepts. Nat Chem advance online publication.