Enhanced ionic conductivity and mechanical strength in nanocomposite electrolytes with nonlinear polymer architectures

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 SiO(2) 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.