  Charging a Li ion battery requires Li ion transport between the cathode and
the anode. This Li ion transport is dependent upon (among other factors) the
electrostatic environment the ion encounters within the Solid Electrolyte
Interphase (SEI), which separates the anode from the surrounding electrolyte.
Previous first principles work has illuminated the reaction barriers through
likely atomistic SEI environments, but has had difficulty accurately reflecting
the larger electrostatic potential landscape that an ion encounters moving
through the SEI. In this work, we apply the recently developed Quantum
Continuum Approximation (QCA) technique to provide an equilibrium electronic
potentiostat for first-principles interface calculations. Using QCA, we
calculate the potential barrier for Li ion transport through LiF, Li$_2$O, and
Li$_2$CO$_3$ SEIs along with LiF-LiF, and LiF-Li$_2$O grain boundaries, all
paired with Li metal anodes. We demonstrate that the SEI potential barrier is
dependent on the anode electrochemical potentials in each system. Finally, we
use these techniques to estimate the change in the diffusion barrier for a Li
ion moving in a LiF SEI as a function of anode potential. We find that properly
accounting for interface and electronic voltage effects significantly lowers
reaction barriers compared to previous literature results.
