The Effect of Ionic Liquid@UiO-66 on the Ionic Conductivity and Physicochemical Properties of Composite PEO Electrolyte

An inorganic nanofiller made from bis(trifluoromethanesulfonyl)imide-based ionic liquids (IL) and UiO-66, was incorporated into poly(ethylene oxide) (PEO) for the preparation of composite polymer electrolyte. Among various alkyl cations considered, 1-methyl-3-n-octylimidazolium bis(trifluoromethanesulfonyl)imide [OMIM][TFSI], with the longest alkyl chain, exhibited the most significant improvement in ionic conductivity, reaching 1.19 × 10-5 S cm-1 at room temperature. The impact of [OMIM][TFSI]@UiO-66 loading was further investigated, with the optimal loading achieved at 60% IL content. This loading caused a reduction in PEO crystallinity, as evidenced by the weakening of the XRD peak at the (1 1 2) plane of the PEO chain. This change resulted from coordination interactions between Li+ cations and the ether oxygen of the PEO chain. Perturbations in the LiTFSI peaks from FTIR spectrum indicated coordination interactions within the polymer matrix. These findings suggest the role of inorganic nanofiller in facilitating the segmental motion of charge carriers in the matrix. Furthermore, differential scanning calorimetry (DSC) analysis confirmed that the addition of inorganic nanofillers increased the amorphous nature of PEO, leading to a significant enhancement in polymer ionic conductivity. This was verified by the decrease in glass transition temperature (Tg) of PEO from 61.12 °C to 52.42 °C when filled with 60% IL. Further loading however increased the Tg which causes traffic jam phenomena in the ionic channels of the PEO. This study provides valuable insights into the relationship between physicochemical properties and the enhancement of ionic conductivity in composite polymer electrolytes with varying IL contents, which could direct the improved design of solid-state electrolyte to produce lithium-ion battery and energy storage device in general.