  The existence of passivating layers at the interfaces is a major factor
enabling modern lithium-ion (Li-ion) batteries. Their properties determine the
cycle life, performance, and safety of batteries. A special case is the solid
electrolyte interphase (SEI), a heterogeneous multi-component film formed due
to the instability and subsequent decomposition of the electrolyte at the
surface of the anode. The SEI acts as a passivating layer that hinders further
electrolyte disintegration, which is detrimental to the Coulombic efficiency.
In this work, we use first-principles simulations to investigate the kinetic
and electronic properties of the interface between lithium fluoride (LiF) and
lithium carbonate (Li$_2$CO$_3$), two common SEI components present in Li-ion
batteries with organic liquid electrolytes. We find a coherent interface
between these components that restricts the strain in each of them to below 3%.
We find that the interface causes a large increase in the formation energy of
the Frenkel defect, generating Li vacancies in LiF and Li interstitials in
Li$_2$CO$_3$ responsible for transport. On the other hand, the Li interstitial
hopping barrier is reduced from $0.3$ eV in bulk Li$_2$CO$_3$ to $0.10$ or
$0.22$ eV in the interfacial structure considered, demonstrating the favorable
role of the interface. Controlling these two effects in a heterogeneous SEI is
crucial for maintaining fast ion transport in the SEI. We further perform
Car-Parrinello molecular dynamics simulations to explore Li ion conduction in
our interfacial structure, which reveal an enhanced Li ion diffusion in the
vicinity of the interface. Understanding the interfacial properties of the
multiphase SEI represents an important frontier to enable next-generation
batteries.

Chemical Physics;Materials Science