Chain length dependence of structural and transport properties of single lithium-ion conducting polymer electrolytes: A molecular dynamics simulation study

Abstract

In this work, the parameters affecting the performance of polymer electrolytes, for utilization in lithium-ion batteries (LiBs) are investigated. For this purpose, by using density functional theory (DFT) and molecular dynamics (MD) simulation, the structural and transport properties of single lithium-ion conducting polymer electrolytes (SLICPEs) based on boron ethylene glycol (BEG), as the anion, with the molecular structure ofBO3−−C2H4On−BO4− are evaluated. The calculated structural properties, include density, glass transition temperature, and radial distribution function (RDF). The simulated transport properties are diffusion coefficient, transference number, and ionic conductivity. In the first step, the optimization of molecular structures is obtained by DFT calculations with B3LYP functional approximation and Mullikan partial charges for the polymer chain lengths of 1, 4, 8, and 15 monomers. Then, by using MD simulations and an ten-stages compression-expansion procedure, the studied electrolyte systems are equilibrated. The results indicate that the transference numbers of these electrolytes are significantly higher than those of commonly used electrolytes based on polyethylene oxide (PEO). Inspection of the obtained results indicates that the diffusion and ionic conductivity coefficients are influenced by the interaction of Li+ with oxygen atoms in the polymer chain. Analysis of the transport properties of Li+ along the polymer chain structure reveals that the presence of a boron atom in the polymer chain backbone induces a significant non–uniform charge distribution along the polymer chain and as a result weakens the interaction between Li+ with an oxygen atom in the polymer chain. Therefore compared to the conventional electrolytes, Li+ can move more freely along the polymer chain, and as a result, the performance of LiBs is promoted significantly.