ABSTRACT
Solvent-free polymer-based electrolytes (SPEs) have gained significant attention to realize safer and flexible lithium-ion batteries. Among all polymers used for preparing SPEs electrolytes, poly(ethylene oxide), a biocompatible and biodegradable polymer, has been the most prevalent one mainly because of its high ionic conductivity in the molten state, the capability for the dissolution of a wide range of different lithium salts as well as its potential for the environmental health and safety. However, linear PEO is highly semicrystalline at room temperature and thus exhibits weak mechanical performance. Addition of nanoparticles enhances the mechanical strength and effectively decreases the crystallization of linear PEO, yet enhancement in mechanical performance often results in decreased ionic conductivity when compared to the neat linear PEO-based electrolytes; new strategies for decoupling ionic conductivity from mechanical reinforcement are urgently needed. Herein, we used lithium bis(trifluoromethane-sulfonyl)-imide (LiTFSI) salts dissolved in various nonlinear PEO architectures, including stars (4-arms and 8-arms) and hyperbranched matrices, and SiO2 nanoparticles (approximately equal to 50 nm diameter) as fillers. Compared to the linear PEO chains, the room temperature crystallinity was eliminated in the branched PEO architectures. The electrolytes with good dispersion of the nanoparticles in the nonlinear PEOs significantly enhanced ionic conductivity, specifically by approximately equal to 40% for 8-arm star, approximately equal to 28% for 4-arms star, and approximately equal to %16 for hyperbranched matrices, with respect to the composite electrolyte with the linear matrix. Additionally, the rheological results of the SPEs with branched architectures show more than three orders of magnitude enhancement in the low-frequency moduli compared to the neat linear PEO/Li systems. The obtained results demonstrate that the solvent-free composite electrolytes made of branched PEO architectures can be quite promising especially for irregularly shaped and environmentally benign battery applications suitable for medical implants, wearable devices, and stretchable electronics, which require biodegradability and biocompatibility.
ABSTRACT
Poly(ethylene oxide) (PEO)-based polymer electrolytes are a promising class of materials for use in lithium-ion batteries due to their high ionic conductivity and flexibility. In this study, the effects of polymer architecture including linear, star, and hyperbranched and salt (lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI)) concentration on the glass transition (T g), microstructure, phase diagram, free volume, and bulk viscosity, all of which play a significant role in determining the ionic conductivity of the electrolyte, have been systematically studied for PEO-based polymer electrolytes. The branching of PEO widens the liquid phase toward lower salt concentrations, suggesting decreased crystallization and improved ion coordination. At high salt loadings, ion clustering is common for all electrolytes, yet the cluster size and distribution appear to be strongly architecture-dependent. Also, the ionic conductivity is maximized at a salt concentration of [Li/EO ≈ 0.085] for all architectures, and the highly branched polymers displayed as much as three times higher ionic conductivity (with respect to the linear analogue) for the same total molar mass. The architecture-dependent ionic conductivity is attributed to the enhanced free volume measured by positron annihilation lifetime spectroscopy. Interestingly, despite the strong architecture dependence of ionic conductivity, the salt addition in the highly branched architectures results in accelerated yet similar monomeric friction coefficients for these polymers, offering significant potential toward decoupling of conductivity from segmental dynamics of polymer electrolytes, leading to outstanding battery performance.
