On the interfacial charge transfer between solid and liquid Li⁺ electrolytes

The Li⁺ ion transfer between a solid and a liquid Li⁺ electrolyte has been investigated by DC polarisation techniques. The current density i is measured as a function of the electrochemical potential drop Δ[small mu, Greek, tilde]_Li at the interface, using a liquid electrolyte with different Li⁺ concentrations. The subject of this experimental study is the interface between the solid electrolyte Ta-substituted lithium lanthanum zirconate (Li_6.6La₃Zr_1.6Ta_0.4O₁₂) and a liquid electrolyte consisting of LiPF₆ dissolved in ethylene carbonate/dimethyl carbonate (1 : 1). The functional course of i vs. Δ[small mu, Greek, tilde]_Li can be described by a serial connection between a constant ohmic resistance R_slei and a current dependent thermally activated ion transfer process. For the present solid-liquid electrolyte interface the areal resistance R_slei of the surface layer is independent of the Li⁺ concentration in the liquid electrolyte. At room temperature a value of about 300 Ω cm² is found. The constant ohmic resistance R_slei can be attributed to a surface layer on the solid electrolyte with a (relatively) low conductivity (solid-liquid electrolyte interphase). The low conducting surface layer is formed by degradation reactions with the liquid electrolyte. R_slei is considerably increased if a small amount (ppm) of water is added to the liquid electrolyte. The thermally activated ionic transfer process obeys a Butler-Volmer like behaviour, resulting in an exchange current density i₀ depending on the Li⁺ concentration in the liquid electrolyte by a power-law. At a Li⁺ concentration of 1 mol l^-1 a value of 53.1 μA cm^-2 is found. A charge transfer coefficient α of ∼0.44 is measured. The finding of a superposed constant ohmic resistance due to a solid-liquid electrolyte interphase and a current dependent thermally activated ion transfer process is confirmed by the results of two former experimental studies from the literature, performing AC measurements/EIS.