Computational Investigation on the Role of Plasticizers on Ion Conductivity in Poly(ethylene oxide) LiTFSI Electrolytes

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A combination of molecular dynamics and connectivity-altering Monte Carlo simulations was carried out to understand on the molecular level the effect of the addition of plasticizers on lithium ion transport in poly(ethylene oxide) with LiTFSI. The simulations were performed using a moderately high molecular weight polymer (Mn = 10 000 g/mol) mixed with 10 wt % plasticizers at 320 and 348 K at an EO:Li ratio of 15. Comparisons with experiment showed slight underestimation of the ionic conductivity with an array of experimental values, but within a factor of 2 of most. With the addition of ethylene carbonate and propylene carbonate plasticizers, the ionic conductivity increased a moderate degree. However, the lithium diffusion did not show a significant increase with the addition of plasticizers, and most of the conductivity increase was due to faster TFSI− motion. It was found that propylene carbonate formed complexes with the TFSI−, in which lithium was an intermediary, creating moderate sized clusters. This allowed enhanced diffusion of lithium ions bound with TFSI− ions, but this was offset by slower diffusion for lithium ions bound with ethylene oxide oxygens. Ethylene carbonate, on the other hand, showed no significant complexing with TFSI−. The formation of these clusters may be an avenue for increasing lithium diffusion but would likely require a plasticizer with stronger interactions with lithium than the carbonates studied.