Temperature and concentration dependence of the ionic charge transfer between solid and liquid Li+ electrolytes – the systems LLZO:Ta/LiPF6–EC–DMC, LATP/LiPF6–EC–DMC and LLZO:Ta/LiBOB–DME–THF

Abstract

The kinetics of the electrochemically driven lithium ion (Li⁺) transfer from a liquid Li⁺ electrolyte to a solid (ceramic) Li⁺ electrolyte is investigated. A DC polarisation is applied to measure the current density i vs. the drop in the electrochemical potential Δ_Li⁺ of Li⁺ ions at the interface. LLZO:Ta and LATP were chosen in this study as the two most promising oxide-ceramic electrolytes and combined with LiPF₆ in EC/DMC (1 : 1) and LiBOB in THF/DME (1 : 1) as the most relevant liquid electrolytes. To determine the rate-limiting step of the Li⁺ transfer across the interface, the results were modelled using a combination of a constant ohmic resistance and a current-dependent, thermally activated Butler–Volmer-like ion transfer process. At low Li⁺ concentrations in the liquid electrolyte, the Butler–Volmer-like transfer process is rate limiting, while at high Li⁺ concentrations, the low-conductive surface layer on the solid electrolyte is rate limiting. The areal resistance of the low-conductivity surface layer is in the order of 600 Ω cm² (25 °C) for LLZO:Ta, and thus about three times higher compared to that for LATP. The activation energy of the ionic transport in the low-conductivity surface layer is about twice that of the solid electrolytes LLZO:Ta and LATP. The exchange current density of the Butler–Volmer-like transfer process is in the order of 100–300 μA cm⁻² (25 °C, 1 mol l⁻¹ Li⁺). There is a symmetric transition state (α ≈ 1/2).