Effect of PVDF Particles on Magnesium-Ion Transport Properties in Organic Ionic Plastic Crystal-Based Solid Electrolytes

As the demand for safer and more sustainable energy storage technologies continues to grow, magnesium-ion secondary batteries (MIBs) have attracted considerable attention as next-generation candidates, offering both high volumetric capacity and the advantage of abundant raw materials compared with lithium-based systems. To develop solid electrolytes for MIBs, we have focused on organic ionic plastic crystals (OIPCs), which exhibit molecular rotational freedom and combine relatively high ionic conductivity with nonflammability and excellent electrochemical stability. Meanwhile, the incorporation of poly(vinylidene fluoride) (PVDF) into OIPCs has been proposed as a practical approach to create ion-transport pathways at the interface, thereby improving ionic conductivity. In this study, we systematically evaluated the thermal behavior, ionic conductivity, cation transference number, and structural properties of magnesium-ion conductors based on N,N -diethylpyrrolidinium bis(fluorosulfonyl)amide ([C 2 epyr][FSA]) pyrrolidinium-based OIPC with relatively high intrinsic conductivity─composited with Mg(FSA) 2 and PVDF. Differential scanning calorimetry (DSC) revealed that the addition of PVDF suppressed both the OIPC-derived solid–solid phase transition ( T s–s ) and melting point ( T m ), and pronounced amorphization was observed, particularly at 60 wt % PVDF. The ionic conductivity reached its maximum at 60 wt % PVDF, confirming a synergistic enhancement arising from interfacial interactions between OIPC and PVDF. Although the Mg 2+ transference number ( t Mg2+ ) decreased upon PVDF addition, the observed interfacial structural changes provide evidence for the presence of alternative ion-transport pathways. X-ray diffraction (XRD) analyses indicated that the amorphous component became dominant with increasing PVDF content. These findings demonstrate the critical role of OIPC/PVDF interfacial structures in determining ion-transport characteristics, providing design insights for solid electrolytes in magnesium-ion batteries.