The Behavior of the Interfacial Charge Transfer between Solid and Liquid Li+-Electrolytes
Authors: Schleutker Marco , Korte Carsten , Stolten Detlef 
Published: February 2020

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 ΔLi
+/F at the solid/liquid electrolyte interface.

The almost symmetric S-shaped functional course of the current density i vs. ΔLi
+/F at LiPF6 concentrations higher than 0.01 mol l-1 can be described by a serial connection between a constant ohmic resistance RSLEI
(Solid-Liquid Electrolyte Interphase SLEI) and a current dependent thermally activated ion transfer process. The constant ohmic resistance RSLEI
 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 (cLi
+
< 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 Rp
, 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 Rp
reaches the limit given by the solid-liquid electrolyte interphase RSLEI
.

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[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.