Probing a solid electrolyte interphase layer with sub-nanometer pores using redox mediators

The solid electrolyte interphase (SEI) layer is crucial for lithium-ion batteries and has a significant impact on the electrochemical performance of negative electrodes, particularly for conversion-type materials with large volume changes and metallic lithium anode. However, the SEI layer has not yet been well understood. In this work, we used redox mediators of various sizes to probe the SEI layer that formed in carbonate-based electrolytes. The SEI layer has diffusion channels that allow the mediators smaller than benzoquinone (5.7 Å) to pass, suggesting that lithium ions have to partially de-solvate to pass through. Additionally, due to partial desolvation, the diffusion coefficient in the diffusion channels was higher than that in the bulk electrolytes. Both lithium salts and solvents influenced the size and areal density of channels. Herein, we aim to enhance comprehension of SEI structure and provide a method to study porous SEI layers using mediators, which can be extended to other electrochemical systems. • The size of diffusion channels (pores) in SEI layer was identified by probing the channel dimensions of the SEI using redox mediators of various sizes. The influence of different lithium salts, solvents, and additives on the diffusion channels SEI layer has been also discussed. • The diffusion channels exhibit size-selectivity, in the case of SEI formed in LiPF 6 -EC/DEC, the channel size is approximately 5 Å, only allowing partially solvated lithium ions, such as Li(EC) 2 + , to pass through, whereas Li(DEC) 2 + is too large to pass. • The mass transport behavior within these channels can be studied using the diffusion of RM, and this method can be extended to other electrochemical systems.