An experimental and theoretical correlation to account for the effect of LiPF6 concentration on the ionic conductivity of poly(poly (ethylene glycol) methacrylate)

Abstract

This study is intended to analyze Li+ ion conduction across amorphous poly(poly(ethylene glycol) methacrylate) (pPEGMA) electrolyte using high concentrations of lithium hexafluorophosphate (LiPF6). A monomer (PEGMA) comprised of eight units of ethylene glycol is thermally polymerized via free radicals. Physicochemical and mechanical properties of polymer electrolytes pPEGMA /LiPF6 are determined at different Li+/O atomic ratios (0.37, 0.42, 0.48 and 0.54) using FTIR, DSC, TGA, XRD, and EIS. These experimental characterizations are used to account for the dependence of the material conductivity on the LiPF6 concentration, and its enhancement at a determined ratio. pPEGMA polymer presents a glass transition temperature (Tg) of 224K and a 10% weight loss decomposition temperature (Td10%) of 453K. Excess of salt on polymer electrolyte forms different complex interactions including ion pairs, which are detected by XRD and FTIR analysis. Although the presence of these clusters is important (i.e. mechanically), it is not dominant to determine ionic conduction, since it increases monotonically with increasing salt concentration and temperature, showing values of 2.07×10−5 and 1.07×10−4Scm−1 at 298 and 323K respectively (Li+/O=0.54). This trend is opposite to that observed in typical polymer electrolytes such as PEO, and it is due to the amorphous morphology of pPEGMA which forms a weak coordination shell with Li+ ion (weak electrolyte), enhancing ionic transport. In the region of temperature exceeding, Tg (224K), the energetic conditions promote the dynamics disorder of motion chains and lithium ion hopping over Coulombic interactions, enabling diffusion motion at long time scale. DFT calculations performed to evaluate the physics and energies of the interactions of LiPF6 and PEGMA monomer reveal that mostly interactions where partial coordination crowns (two or three ether oxygens) between the polymer (i.e. central region) and one Li+ ion (also interacting with PF6−) present thermodynamically favorable energies, and the most probable bonding involves three ether oxygens, as experimentally determined.