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 ivs. the drop of the electrochemical potential Δμ~_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 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 ion Butler-Volmer like transfer process. At low Li⁺ concentration in the liquid electrolyte the Butler-Volmer like transfer process is rate limiting, while at high Li⁺ concentration the low conducting surface layer on the solid electrolyte. The areal resistance of the low conducting surface layer is in the order of 600 Ω cm² (25 °C) for LLZO:Ta, and thus about three times higher compared to LATP. The activation energy of the ionic transport in the low conducting surface layer is about twice compared to the solid electrolytes LLZO:Ta and LATP. The exchange current density 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).