Novel Composite Polymer Electrolytes of PVdF-HFP Derived by Electrospinning with Enhanced Li-Ion Conductivities for Rechargeable Lithium–Sulfur Batteries

Abstract

Composites of poly­(vinylidene fluoride-co-hexafluoro propylene) (PVdF-HFP) incorporating 10 wt % bis­(trifluoromethane)­sulfonimide lithium salt (LiTFSI) and 10 wt % particles of nanoparticulate silica (nm-SiO₂), nanoparticulate titania (nm-TiO₂), and fumed silica (f-SiO₂) were prepared by electrospinning. These membranes served as host matrix for the preparation of composite polymer electrolytes (CPEs) following activation with lithium sulfur battery electrolyte comprising 50/50 (vol %) dioxolane/dimethoxyethane with 1 M LiTFSI and 0.1 M LiNO₃. The membranes consist of layers of fibers with average fiber diameter of 0.1–0.2 μm. CPEs with f-SiO₂ exhibited higher ionic conductivity with a maximum of 1.3 × 10^–3 S cm^–1 at 25 °C obtained with 10 wt % filler compositions. The optimum CPE based on PVdF-HFP with 10 wt % f-SiO₂ exhibited enhanced charge–discharge performance in Li-S cells at room-temperature eliminating polysulfide migration, delivering initial specific capacity of 895 mAh g^–1 at 0.1 C-rate and a very low electrolyte/sulfur (E/S) ratios between 3:1 to 4:1 mL.g^–1. The CPEs also exhibited very stable cycling behavior well over 100 cycles (fade rate ∼ 0.056%/cycle), demonstrating their suitability for Li-S battery applications. In addition, the interconnected morphological features of PVdF-HFP result in superior mechanical properties (200–350% higher tensile strength). Higher Li-ion conductivity, higher liquid electrolyte uptake (>250%) with dimensional stability, lower interfacial resistance, and higher electrochemical stability are some of the attractive attributes witnessed with these CPEs. With these improved performance characteristics, the PVdF-HFP system is projected herein as suitable polymer electrolytes system for high-performance Li–S rechargeable batteries