Li2O Solid Electrolyte Interphase: Probing Transport Properties at the Chemical Potential of Lithium

Lithium (Li) anodes suffer from numerous challenges arising from the chemically inhomogeneous nature of the native solid electrolyte interphase (SEI), which impedes smooth plating and leads to dendrite growth. In spite of much attention given of late to engineering Li interfaces, there is still limited understanding of the desired chemical composition of an improved Li SEI. One major challenge has been the lack of empirical data on the structure–property–performance relations in individual SEI phases, specifically those present at the metallic Li interface, where the chemical potential imposed by Li will yield different material properties than the bulk analogues typically invoked to understand the SEI behavior. Herein, we report the preparation of single-component SEIs of lithium oxide (Li2O) grown ex situ on Li foils by controlled metal–gas reactions, generating "deconstructed" model interfaces with a nanoscale thickness (20–100 nm) similar to the native, yet more complex multiphasic SEI. The model Li|Li2O electrodes serve as a platform for further chemical and electrochemical characterization. In particular, electrochemical impedance spectroscopy, combined with interface modeling, is used to extract transport properties (ionic conductivity, diffusivity, charge carrier concentration, and activation energy barriers) of Li|Li2O in symmetric cells with EC/DEC electrolytes. The Li2O SEI is further studied as a function of a synthesis condition, revealing microstructural sensitivities that can be tuned to modulate transport behaviors. Finally, results are compared with single-phase Li|LiF interfaces synthesized herein and with the native SEI to isolate chemistry- and structure-specific differences.