Is the solid electrolyte interphase in lithium-ion batteries really a solid electrolyte? Transport experiments on lithium bis(oxalato)borate-based model interphases

In order to improve the properties of the solid electrolyte interphase (SEI) on the graphite anode in lithium-ion batteries, different electrolyte additives are used, such as lithium bis(oxalate)borate (LiBOB), vinylene carbonate, and fluoroethylene carbonate. It is known that LiBOB increases the SEI resistance, but there is very little fundamental knowledge about the influence of LiBOB on the structure of the SEI as well as on ion and molecule transport mechanisms in the SEI. Here, we study SEIs grown at the interface between a planar glassy carbon electrode and battery electrolytes containing different amounts of LiBOB. The SEIs are characterized by a combination of FIB-SEM, AFM, electrochemical impedance spectroscopy and redox probe experiments. The transport of Li+ ions and of redox molecules becomes slower with increasing LiBOB concentration in the electrolyte, but like observed for a LiBOB-free electrolyte, the effective diffusion coefficients of Li+ ions and ferrocene molecules in the SEIs are virtually identical and show the same temporal evolution after voltammetric SEI formation. This gives strong indication that both Li+ ions and molecules are transported in the liquid electrolyte phase inside the pores of the SEI and thus challenges the common view of a solid-electrolyte-type Li+ transport mechanism in SEIs.