Molecular dynamics simulation of the effect of nanoparticle fillers on ion motion in a polymer host

There is some empirical evidence to show that proton conductivity in a polymer host (typically Nafion©) can be enhanced by the addition of nanoparticle “fillers”. The possible underlying mechanism(s) involved in such an effect is here probed by Molecular Dynamics (MD) simulation not for proton conductivity, but for lithium-ion conductivity in the system LiX.(PEO)n, where “PEO”=amorphous poly(ethylene oxide); X=Cl, Br, I or BF4; and n lies in the range 10–50. The filler used is a ca. 14-Å diameter quasi-spherical nanoparticle of Al2O3; simulations are made in the range 290–360 K. Possible conductivity enhancement is assessed by comparing with salt- and/or “particle-free” reference calculations. The most general structural effect observed is that the PEO forms an immobilised “coordination sphere” around the particle, and that this has a crucial effect on all parameters monitored. Typically; Li-ion mobility decreases on the addition of the particle, and is consistently least near the particle surface. Importantly, unpaired BF4− anions are found attached to the particle within this region of immobilised PEO, leaving free Li-ions in the regions away from the particle. At lower concentration (Li:EO ratio 1:50), Li-ion conductivity is found to increase on the addition of nanoparticles at 330 K, but decreases or remains unchanged at lower temperatures and higher concentrations. Significant LiX pairing/clustering (for X=Cl, Br, I) is also observed away from the particle surface, and is greatest for LiBr and least for LiCl.