Organic ionic plastic crystal/PVDF composites prepared by solution casting

Abstract

Solid-state electrolytes have been considered promising candidates to address the safety issues for next-generation lithium batteries. Organic ionic plastic crystals (OIPCs) are attracting increasing interest as solid electrolyte materials due to their unique advantages. In this study, an OIPC-based composite electrolyte consisting of the OIPC 1-ethyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr12TFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and the polymer polyvinylidene fluoride (PVDF) has been developed by a facile solution casting strategy. Free-standing and flexible OIPC/polymer composite membranes were fabricated by the solution casting method, which not only provides flexibility and better electrode/electrolyte contact but also is compatible with current battery processing methods. The thermal behavior and ionic conductivity of the OIPC-based composites with different molar proportions (10 mol % to 67 mol %) of LiTFSI in LiTFSI/Pyr12TFSI, as well as different weight fractions (20 wt % to 50 wt %) of PVDF, were studied to understand the effect on transport properties. Among all the compositions studied, the Li0.33[Pyr12]0.67TFSI/30 wt % PVDF composite exhibited high ionic conductivity (e.g., 1.2 × 10–4 S cm–1 at 30 °C). The Li0.33[Pyr12]0.67TFSI/30 wt % PVDF composite membrane was evaluated in a Li/Li symmetric cell and was cycled stably over 900 h at a current density of 0.1 mA cm–2 at 50 °C, demonstrating that this OIPC/polymer composite electrolyte enabled the reversible and stable lithium plating and stripping behaviors. Further tests of the Li0.33[Pyr12]0.67TFSI/30 wt % PVDF composite membrane as a solid electrolyte in a LiFePO4/Li cell presented a high specific discharge capacity of 149 mAh g–1 at 0.1 C and a long cycle life of over 440 cycles with a capacity retention of 89% at 0.5 C at 50 °C, which showed improved rate capability and cycling stability in comparison with the composites with similar compositions but obtained by the powder pressing method. This study demonstrated the potential of the OIPC/polymer composite solid electrolyte prepared by the solution casting method and will promote the development of high-performance OIPC-based composite electrolytes for solid-state batteries.