Electrospun poly(vinylidene fluoride) copolymer/octahydroxy-polyhedral oligomeric silsesquioxane nanofibrous mats as ionic liquid host: Enhanced salt dissociation and its function in electrochromic device

Electrospun polymer nanofibrous mats loaded with ionic liquids (ILs) and lithium salts are promising non-volatile electrolytes owing to their high ionic conductivities. However, the large cations of ILs are difficult to diffuse into solid electrodes, whereas the lithium ions in ILs tend to form anionic complexes with the IL anions, reducing the number of free lithium ions. To address these issues, octa(3-hydroxy-3-methylbutyldimethylsiloxy) polyhedral oligomeric silsesquioxane (POSS-OH), which has large specific surface area and functionality number, is incorporated into electrospun poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) nanofibrous mats, and the mats are used to host LiClO₄/1-butyl-3-methylimidazolium tetrafluoroborate (BMIM⁺BF₄^-). It is found that POSS-OH can significantly increase both ionic conductivity and lithium transference number of the electrolytes owing to the Lewis acid-base interactions of POSS-OH with ClO₄^- and BF₄^-. The electrochromic device using the hybrid mat (with 5 wt% POSS-OH) loaded with LiClO₄/BMIM⁺BF₄^- as the electrolyte shows significantly improved transmittance contrast and switching time, as a result of increased number of free lithium ions.