  Here, using unique in-situ liquid secondary ion mass spectroscopy on
isotope-labelled solid-electrolyte-interphase (SEI), assisted by cryogenic
transmission electron microscopy and constrained ab initio molecular dynamics
simulation, for the first time we answer the question regarding Li+ transport
mechanism across SEI, and quantitatively determine the Li+-mobility therein. We
unequivocally unveil that Li+ transport in SEI follows a mechanism of
successive displacement, rather than "direct-hopping". We further reveal, in
accordance with spatial-dependence of SEI structure across the thickness, the
apparent Li+ self-diffusivity varies from 6.7*10-19 m2/s to 1.0*10-20 m2/s,
setting a quantitative gauging of ionic transport behavior of SEI layer against
the underlining electrode as well as the rate limiting step of battery
operation. This direct study on Li+ kinetics in SEI fills part of the
decade-long knowledge gap about the most important component in advanced
batteries and provides more precise guidelines to the tailoring of interphasial
chemistries for future battery chemistries.
