Electrospun nanofibrous mats as ionic liquid host for electrochromic device and lithium-ion battery
Date of Issue2016-03
School of Materials Science and Engineering
Ionic liquids (ILs) are liquid salts, composed entirely of ions, and exhibit a melting temperature generally below 100 oC. IL-based electrolytes have been widely investigated over the past decade due to their many attractive properties, such as non-flammability, non-volatility, excellent thermal and electrochemical stability. Among many IL-based electrolytes, IL-loaded polymer electrospun mats have garnered increasing interest. The random laid nanofibers generate interconnected pores, which facilitate electrolyte uptake and transport of ions. Nevertheless, the IL-loaded polymer electrospun mats also have some inherent limitations, namely, they have relatively poor mechanical strength, and their ionic conductivity and lithium-ion mobility are significantly lower than that of traditional liquid electrolytes. To address these issues, in this research, a chemical cross-linking agent and three types of ion-dissociation promoters were incorporated into/attached on poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) nanofibrous mats, to improve not only mechanical property and ionic conductivity, but also lithium-ion and proton transport property of the resultant electrolytes. The structure, morphology, thermal and mechanical properties of the electrospun mats were investigated using FTIR, Raman, 1H NMR, XRD, SEM, DSC, TGA and Instron tester. The ionic conductivities and lithium transference numbers (TLi+) were measured, and correlated to the structures and morphologies of the electrolytes. The effects of these novel electrolytes on performance of electrochromic devices and lithium-ion batteries were also demonstrated. Firstly, 1,3-diaminopropane (DAP) was used as the chemical cross-linking agent for P(VDF-HFP), and ultrathin free-standing electrospun mats with thickness of only about 2 μm were obtained. The cross-linking reaction induces inter-fiber junctions, leading to remarkable improvement in structural integrity of the electrospun mats with very small thickness. Such ultrathin electrospun mats would make IL-based polymer electrolytes more economically viable since ILs are costly chemicals. Secondly, octa(3-hydroxy-3-methylbutyldimethylsiloxy) polyhedral oligomeric silsesqui-oxane (POSS-OH) with extremely large surface area were introduced within the nanofibers, and SiO2 nanoparticles were covalently attached on ethoxysilane-functionalized surface of P(VDF-HFP) nanofibers, respectively. The hybrid mats were used as hosts for IL loaded with lithium salt. It is found that these active fillers can effectively serve as salt dissociation promoters by interacting with the anions of both ILs and lithium salts through Lewis acid-base interactions, leading to significant enhancement of ionic conductivity and TLi+. Using the IL-loaded hybrid mats as the electrolyte layer, electrochromic devices show extensively enhanced contrast, while Li/LiCoO2 batteries show significantly improved C-rate performance and cycling stability. The results demonstrate that attaching SiO2 on the nanofiber surfaces is more efficient in improving the electrochemical properties of electrolytes. Lastly, P(VDF-HFP) copolymer was grafted with sulfonic acid, a proton carrier, through the covalent attachment of taurine, which was then electrospun and used as the IL host. The sulfonic acid groups attached on the polymer chains can serve as proton sources, contribute significantly to proton conductions. Meanwhile, the negatively charged SO3- groups also coordinate with the cations, promoting the dissociation of the IL, leading to improved ionic conductivity. With this novel electrolyte, electrochromic devices show excellent switching behaviour without the need of incorporating any small cations. Furthermore, this sulfonic acid-grafted P(VDF-HFP) electrospun mat can also be lithiated, giving additional Li+ conductions, with which Li/LiCoO2 batteries display enhanced C-rate performance. In summary, the research shows that incorporating chemical cross-linking agent (DAP) and ion-dissociation promoters (POSS-OH, SiO2, -SO3H) within/on the nanofiber surface, are effective approaches to improve the mechanical property, ionic conductivity and Li+/H+ transport property of the resultant electrolytes.