Frederick Nti

Organic ionic plastic crystals (OIPCs) are attractive solid electrolyte materials for advanced energy storage systems owing to their inherent advantages (e.g., high plasticity, thermal stability, and moderate ionic conductivity), which can be further improved/deteriorated by the addition of polymer or metal oxide nanoparticles. The role of the nanoparticle/OIPC combinations on the resultant interphase structure and transport properties, however, is still unclear due to the complexity within the composite structures. Herein, we demonstrate a systematic approach to specifically interrogating the interphase region by fabricating layered OIPC/polymer thin films via spin coating and correlating variation in the ionic conductivity of the OIPC with their microscopic structures. In-plane interdigitated electrodes have been employed to obtain electrochemical impedance spectroscopy (EIS) spectra on both OIPC and layered OIPC/polymer thin films. The thin-film EIS measurements were evaluated with conventional bulk EIS measurements on the OIPC pressed pellets and compared with EIS obtained from the OIPC-polymer composites. Interactions between the OIPC and polymer films as well as the morphology of the film surfaces have been characterized through multiple microscopic analysis tools, including scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy, and optical profilometry. The combination of EIS analysis with the microscopic visualization of these unique layered OIPC/polymer thin films has confirmed the impact of the OIPC-polymer interphase region on the overall ionic conductivity of bulk OIPC-polymer composites. By changing the chemistry of the polymer substrate (i.e., PMMA, PVDF, and PVDF-HFP), the importance of compatibility between the components in the interphase region is clearly observed. The methods developed here can be used to screen and further understand the interactions among composite components for enhanced compatibility and conductivity.

Organic ionic plastic crystals (OIPCs) are promising solid electrolytes because of their inherent advantages such as non-volatility, non-flammability, good thermal stability, favourable plasticity, and improved electrolyte/electrode interfacial contact. Incorporating nanoparticles into some OIPC matrixes has proven an effective strategy for further increasing conductivity and mechanical integrity. However, the nature of the interaction between the cations and anions of OIPCs with polymers requires further study. In this work, various OIPCs consisting of N-ethyl-N-methyl pyrrolidinium cations ([C₂mpyr]⁺) and three different anions bis(trifluoromethanesulfonyl)imide ([TFSI]⁻), (bis(fluorosulfonyl)imide ([FSI]⁻) and tetrafluoroborate, [BF₄]⁻) are selected. The interactions between the OIPCs and poly(vinylidene fluoride) (PVDF) are studied by incorporating different volume fractions of PVDF nanoparticles into the OIPC matrices. Differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), nuclear magnetic resonance (NMR) and synchrotron X-ray diffraction (XRD) techniques confirm that the OIPCs with different anions interact differently with PVDF. Furthermore, we propose that the degree of the interaction and ionic conductivity enhancement relates to different dipole moments of the OIPC anions. This anion originated interaction induces the formation of disordered OIPC interphases which accounts for the ionic conductivity enhancement.

Organic ionic plastic crystals (OIPCs) have been recognised as promising solid-state electrolyte materials for next-generation energy storage devices. Recently, the addition of polymer nanofillers to OIPCs has led to the design of OIPC-based solid-state electrolytes with enhanced mechanical stability and ion conductivity. However, the mechanisms of ion conductivity enhancement and the influence of different polymer surface chemistries on the ion dynamics are not yet well understood, which has hindered the further development of high-performance OIPC-based electrolytes. In this work, we selected two different polymer nanoparticles, poly(vinylidene fluoride) (PVDF) and polystyrene (PS), and investigated the effects of the polymer surfaces on the thermal behaviour and ion transport properties of the OIPC, N-ethyl N-methyl pyrrolidinium bis(fluorosulfonyl)imide ([C₂mpyr][FSI]). We found significantly different thermal behaviours, as well as ion transport properties in the OIPC/nanoparticle composites. Specifically, compared with pure [C₂mpyr][FSI], the addition of PVDF nanoparticles effectively enhanced the ion conductivity of the OIPC composite, with the optimum achieved near the percolation threshold of PVDF nanoparticles. In contrast, the addition of PS nanoparticles to the OIPC led to a slight enhancement at low concentrations and then a significant decrease in conductivity at higher concentrations. DSC, FTIR and EIS confirm that the interaction between the PVDF nanoparticles and the OIPC induces the formation of less ordered OIPC layers on the PVDF surfaces, leading to the conductivity enhancement. Finally, different structure models based on the results of this work are proposed, which provide principle guidelines for the design of future OIPC-based highly conductive electrolyte materials.