Conformational Dynamics in an Organic Ionic Plastic Crystal

Surface and Conductivity Characterization of Layered Organic Ionic Plastic Crystal (OIPC)-Polymer Films

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.

Development of New Plastic-Crystal Based Electrolytes using Pyrrolidinium- Bis(fluorosulfonyl)imide Dicationic Salts

Dicationic organic salts are an interesting class of solid-state electrolyte materials due to their unique structure. Here we present, for the first time, the synthesis and characterization of three dicationic-FSI salts, 1,2-bis(N-methylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C₂ -Pyrr1][FSI]₂ ), 1,2-bis(N-ethylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C₂ -Pyrr2][FSI]₂ ) and 1,2-bis(N-n-propylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C₂ -Pyrr3][FSI]₂ ). The structure and dynamics of the organic salts were probed using variable temperature solid-state NMR and were compared with the thermal and transport properties. The investigation revealed that [C₂ -Pyrr1][FSI]₂ , with shorter alkyl-side chains on the dication, displayed increased transport properties compared to [C₂ -Pyrr2][FSI]₂ and [C₂ -Pyrr3][FSI]₂ . To determine the proficiency of these dicationic-FSI salts as electrolyte materials for battery applications, 10 mol% and 50 mol% lithium bis(fluorosulfonyl)imide (LiFSI) was mixed with [C₂ -Pyrr1][FSI]₂ and [C₂ -Pyrr2][FSI]₂ . Increased transport properties were observed for [C₂ -Pyrr1][FSI]₂ /10 mol % LiFSI in comparison to [C₂ -Pyrr2][FSI]₂ /10 % LiFSI, while pulse field gradient NMR analysis revealed the highest Li⁺ self-diffusion ratio for [C₂ -Pyrr1][FSI]₂ /50 % LiFSI out of the four Li-salt-containing mixtures.

Exploration of phase diagram, structural and dynamic behavior of [HMG][FSI] mixtures with NaFSI across an extended composition range

Hexamethylguanidinium bis(fluorosulfonyl)imide ([HMG][FSI]) has recently been shown to be a promising solid state organic ionic plastic crystal with potential application in advanced alkali metal batteries. This study provides a detailed exploration of the structural and dynamic behavior of [HMG][FSI] mixtures with the sodium salt NaFSI across the whole composition range from 0 to 100 mol%. All mixtures are solids at room temperature. A combination of differential scanning calorimetry (DSC), synchrotron X-ray diffraction (SXRD) and multinuclear solid state NMR spectroscopy is employed to identify a partial phase diagram. The 25 mol% NaFSI/75 mol% [HMG][FSI] composition presents as the eutectic composition with the eutectic transition temperature at 44 °C. Both DSC and SXRD strongly support the formation of a new compound near 50 mol% NaFSI. Interestingly, the 53 mol% NaFSI [HMG][FSI] composition was consistently found to display features of a pure compound whereas the 50 mol% materials always showed a second phase. Many of the compositions examined showed unusual metastable behaviour. Moreover, the ion dynamics as determined by NMR, indicate that the Na⁺ and FSI^- anions are signifcantly more mobile than the HMG cation in the liquid state (including the metastable state) for these materials.

Plastic crystal-based electrolytes using novel dicationic salts

The unique structures of dications increase the number of possible combinations of cations and anions that can be used to obtain new materials with a wide range of physicochemical properties. However, structure-property relationships related to dicationic organic salts are seldom explored. Here, we report the synthesis and characterization of two new dicationic salts, 1,2-bis(N-ethylpyrrolidinium)ethane bis(trifluoromethanesulfonyl)imide ([C₂-Pyrr2][TFSI]₂) and 1,2-bis(N-n-propylpyrrolidinium)ethane bis(trifluoromethanesulfonyl)imide ([C₂-Pyrr3][TFSI]₂). To investigate the physicochemical properties of the organic salts, local structure and dynamics were investigated by variable temperature solid-state NMR and correlated with the thermal analysis and ionic conductivity. These studies revealed that [C₂-Pyrr3][TFSI]₂, with the longer alkyl-side chain on the dication, showed improved transport properties compared to [C₂-Pyrr2][TFSI]₂. Further exploration of the organic salts as potential electrolyte materials was conducted by mixing with 10 mol% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). This study demonstrates the effect that lithium salt addition has on thermal and ionic conductivity properties, where the largest increase in conductivity was found for [C₂-Pyrr3][TFSI]₂/LiTFSI (10 mol% LiTFSI). Solid-state NMR analysis revealed that Li⁺ and [TFSI]^- ions acted as the major contributors to ionic conductivity while the dications in the bulk structure showed lower mobility.

Nuclear magnetic resonance characterisation of ionic liquids and organic ionic plastic crystals: common approaches and recent advances

Ionic liquids, and their solid-state equivalents organic ionic plastic crystals, show many useful and tailorable properties that make them interesting for a wide range of applications including as electrolytes for energy storage devices. Nuclear magnetic resonance spectroscopy and related techniques offer a powerful and versatile toolkit for the characterisation of structure, interactions and dynamics in these materials. This article summarises both commonly used methods and some recent advances in this area, including solution- and solid-state methods, dynamic nuclear polarisation, imaging, diffusion and relaxation measurements, and example applications of some less commonly studied nuclei.

A Promising Phase Change Material with Record High Ionic Conductivity over a Wide Temperature Range of a Plastic Crystal Phase and Magnetic Thermal Memory Effect

The emerging organic ion plastic crystals (OIPCs) are the most promising candidates used as solid-state electrolytes in a range of ionic devices. To endow an OIPC with additional functionality may create a new type of material for multifunctional devices. Herein, we present an ion plastic crystal, [EMIm][Ni(mnt)₂] (1; [EMIm]⁺ = 1-ethyl-3-methylimidazolium and mnt^2- = maleonitriledithiolate), and its crystal consists of twin dimeric chains of [Ni(mnt)₂]^- anions, embraced by [EMIm]⁺ cations. A crystal-to-plastic crystal transformation with a large latent heat that occurred at ∼367/337 K on heating/cooling is confirmed by the differential scanning calorimetry (DSC) technique. The plastic crystal phase in 1, characterized by variable temperature powder X-ray diffraction (PXRD) and optical microscopy images, spans a broad temperature range with ΔT ∼123/153 K on heating/cooling (DSC measurement), and the wide ΔT is relevant to an extra stable anion chain owing to the strong antiferromagnetic (AFM) interactions protecting the chain from collapse in the plastic crystal state. 1 is a single-component ion plastic crystal with a record high ion conductivity, 0.21 S·cm^-1, at 453 K. The crystal-to-plastic crystal transformation in 1 is coupled to a bistable magnetic transition to give a multi-in-one multifunctional material. This study provides a creative thought for the design of OIPCs with striking thermal, electrical, and magnetic multifunctionality.

Macrophase-Separated Organic Ionic Plastic Crystals/PAMPS-Based Ionomer Electrolyte: A New Design Perspective for Flexible and Highly Conductive Solid-State Electrolytes

A material design approach was taken for the preparation of an organic ionic plastic crystal (OIPC)-polymer electrolyte material that exhibited both good mechanical and transport properties. Previous attempts to form this type of electrolyte material resulted in the solvation of the OIPC by the ionomer and loss of the plastic crystal component. Here, we prepared, in situ, a macrophase-separated OIPC-polymer electrolyte system by adding lithium bis(fluorosulfonyl)imide (LiFSI) to a (PAMPS-N₁₂₂₂) ionomer. It was found that an optimal compositional window of 40-50 mol % LiFSI exists whereby the electrolyte conductivity suddenly increased 4 orders of magnitude while exhibiting elastic and flexible mechanical properties. The phase behavior and transport properties were studied using differential scanning calorimetry and ⁷Li and ¹⁹F solid-state nuclear magnetic resonance spectroscopy. This is the first example of a fabrication principle that lends itself to a wide range of promising OIPC and ionomeric materials. Subsequent studies are required to characterize and understand the morphology and conductive nature of these systems and their application as electrolyte materials.

Synthesis of pyrrolidinium-based plastic crystals exhibiting high ionic conductivity at ambient temperature

Pyrrolidinium-based plastic crystals were synthesised with various anions to investigate the effect of the anion structure on the properties.

Structure and Ion Dynamics in Imidazolium-Based Protic Organic Ionic Plastic Crystals

A fundamental understanding of the structure and dynamics of organic ionic plastic crystal (OIPC) materials allows for a more rational design of molecular chemistry toward improved mechanical and electrochemical performances. This Letter investigates the solid-state structure and ion dynamics of two imidazolium-based protic organic ionic plastic crystals as well as the ion-transport properties in both compounds. A combination of DSC, conductivity, NMR, and synchrotron X-ray studies revealed that a subtle change in cation chemistry results in substantial differences in the thermal phase behavior, crystalline structures, as well as the ion conduction mechanisms in the protic plastic crystal compounds. Whereas most of the research nowadays has been focused on the optimization of chemistry of cations and anions, this work highlights the importance of microstructures on the ion transport rate and pathways of the OIPC materials.