Organic Ionic Plastic Crystal/PVDF Composites Prepared by Solution Casting

Abstract

Solid-state electrolytes have been considered as a promising candidate 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 more 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 Li0.33Pyr0.67TFSI/30wt%PVDF composite exhibited high ionic conductivity (e.g. 1.2 × 10−4 S cm-1 at 30 °C). The Li0.33Pyr0.67TFSI/30wt%PVDF composite membrane was evaluated in 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 Li0.33Pyr0.67TFSI/30wt%PVDF composite membrane as solid electrolyte in LiFePO4/Li cell presented a high specific capacity of 149 mAh g−1 at 0.1 C and a long cycle life of over 440 cycles with 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 powder pressing method. This study demonstrated the potential of the OIPC/polymer composite solid electrolyte prepared by solution casting method and will promote the development of high-performance OIPC-based composite electrolytes for solid-state batteries.