  Lithium superionic conductors are of major importance as solid electrolytes
for next-generation all-solid-state lithium-ion batteries. While $ab$ $initio$
molecular dynamics have been extensively applied to study these materials,
there are often large discrepancies between predicted and experimentally
measured ionic conductivities and activation energies due to the high
temperatures and short time scales of such simulations. Here, we present a
strategy to bridge this gap using moment tensor potentials (MTPs). We show that
MTPs trained on energies and forces computed using the van der Waals optB88
functional yield much more accurate lattice parameters, which in turn leads to
accurate prediction of ionic conductivities and activation energies for the
Li$_{0.33}$La$_{0.56}$TiO$_3$, Li$_3$YCl$_6$ and Li$_7$P$_3$S$_{11}$ lithium
superionic conductors. NPT MD simulations using the optB88 MTPs also reveal
that all three lithium superionic conductors undergo a transition between two
quasi-linear Arrhenius regimes at relatively low temperatures. This transition
can be traced to an expansion in the number and diversity of diffusion
pathways, in some cases with a change in the dimensionality of diffusion. This
work presents not only an approach to develop high accuracy MTPs, but also
outlines potential design strategies for lithium superionic conductors.
