  The question of if silica nanoparticles can enhance the ionic conductivity of
a polymer electrolyte above its crystallization temperature has remained
unclear for the two decades following the first experiments on these systems.
We use Molecular Dynamics simulations to decipher the atomic scale mechanisms
affecting the properties of LiTFSI-poly(ethylene oxide) electrolytes upon the
addition of silica nanoparticles. At any ionic concentration, adding
nanoparticles significantly decreases the conductivity. Most of this reduction
can be simply accounted for by the diffusion equation, resulting from the fact
that the space occupied by the nanoparticles is made inactive and unable to
sustain ionic diffusion. We identify two distinct regimes, above and below a
concentration threshold, corresponding to very different ionic distributions
and coordination features of the various species. The lack of conductivity
enhancement observed in the simulations supports the conclusions of some recent
measurements, and disagrees with the earliest experimental reports on hybrid
silica/polyethylene-oxide electrolytes.
