Plastic Crystal Materials Research Articles

Development of Organic Ionic Plastic Crystals As Solid-State Electrolytes

The advantages of ionic liquids as electrolytes for electrochemical applications such as lithium batteries and dye-sensitized solar cells are now well recognized. Organic Ionic Plastic Crystals (OIPCs) are crystalline phases found in many of the same organic salt families as ILs but these materials have elevated melting points and exhibit various forms of disorder, which is the origin of their plastic mechanical properties. These plastic crystal materials are showing increasing promise as solid state electrolytes for various electrochemical applications.[1] We have also recently discovered that this disorder allows the use of OIPC-based membranes for light gas separation.[2] OIPCs display one or more solid-solid phase transition before melting, which is associated with the onset of rotational or translational motions of the ions and thus a progressive transformation from an ordered crystalline phase to an increasingly disordered structure. This disorder is fundamentally responsible for the fast ion conduction of target ions, such as lithium or sodium, and core to their efficient performance as solid-state electrolytes. Key to the development of OIPCs is expanding the range of cations and anions available, and understanding the resulting structure and dynamics. Here we discuss our development of new OIPC-based materials for energy applications, in particular lithium metal batteries, and insights into the mechanism of ion transport using techniques such as variable temperature solid state NMR. The recent development of new OIPCs using the N,N-diethylpyrrolidinium ([C2epyr]) cation will be discussed, in addition to the use of very high lithium content [C2epyr][FSI] / Li[FSI] ionic liquids and OIPCs as battery electrolytes.

Plastic crystals could be solid-state refrigerants

Regulators and manufacturers started phasing out chlorofluorocarbon gas refrigerants like Freon in the late 1970s after scientists discovered that they could deplete the ozone layer. But their replacements, hydrochlorofluorocarbons and hydrofluorocarbons, are greenhouse gases. Some scientists and engineers have suggested using solid-state refrigerants to avoid those environmental issues, but their use has yet to be realized. Now researchers propose that so-called plastic crystal materials could be more effective refrigerants than previously studied solid-state materials (Nature 2019, DOI: 10.1038/s41586-019-1042-5). However, experts caution that major engineering challenges stand between these crystals and your fridge. In conventional refrigerators, standard refrigerants change from liquid to gas, absorbing energy from the air inside the refrigerator and cooling it. A compressor then increases the pressure and temperature of the gas, dumping that absorbed energy outside the refrigerator as hot air as the ...

Atomistic simulation of structure and dynamics of the plastic crystal diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate

Molecular dynamics simulations have been performed to investigate the interrelations between structures, transport mechanisms, and phase transitions of an organic ionic plastic crystal material, diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate ([P1,2,2,4][PF6]), in both solid and liquid phases. Examination of the temperature dependence of supercell parameters and radial distribution functions provides evidence of plastic phase transitions. Nonlinear increments of cell size within the temperature range 123-413 K are consistent with the plastic phase transitions identified from experimental analysis. The time- and temperature-dependent microstructure and dynamics have been intensively studied through analysis of trajectory files. The rotational motion and diffusion of the matrix ions are quantitatively analysed via rotational correlation functions and mean square displacements. We present new information on the evolution of molecular motions in different phases, and compare and contrast our findings with previously reported hypotheses based on nuclear magnetic resonance results. This work provides valuable information at an atomistic level to explain the experimental observations, which helps further understanding of the molecular motions underlying the plastic phase transitions.

A Comparative Study on the Thermophysical Properties for Two Bis[(trifluoromethyl)sulfonyl]imide-Based Ionic Liquids Containing the Trimethyl-Sulfonium or the Trimethyl-Ammonium Cation in Molecular Solvents

Herein, we present a comparative study of the thermophysical properties of two homologous ionic liquids, namely, trimethyl-sulfonium bis[(trifluoromethyl)sulfonyl]imide, [S(111)][TFSI], and trimethyl-ammonium bis[(trifluoromethyl)sulfonyl]imide, [HN(111)][TFSI], and their mixtures with propylene carbonate, acetonitrile, or gamma butyrolactone as a function of temperature and composition. The influence of solvent addition on the viscosity, conductivity, and thermal properties of IL solutions was studied as a function of the solvent mole fraction from the maximum solubility of IL, x(s), in each solvent to the pure solvent. In this case, x(s) is the composition corresponding to the maximum salt solubility in each liquid solvent at a given temperature from 258.15 to 353.15 K. The effect of temperature on the transport properties of each binary mixture was then investigated by fitting the experimental data using Arrhenius' law and the Vogel-Tamman-Fulcher (VTF) equation. The experimental data shows that the residual conductivity at low temperature, e.g., 263.15 K, of each binary mixture is exceptionally high. For example, conductivity values up to 35 and 42 mS·cm(-1) were observed in the case of the [S(111)][TFSI] + ACN and [HN(111)][TFSI] + ACN binary mixtures, respectively. Subsequently, a theoretical approach based on the conductivity and on the viscosity of electrolytes was formulated by treating the migration of ions as a dynamical process governed by ion-ion and solvent-ion interactions. Within this model, viscosity data sets were first analyzed using the Jones-Dole equation. Using this theoretical approach, excellent agreement was obtained between the experimental and calculated conductivities for the binary mixtures investigated at 298.15 K as a function of the composition up to the maximum solubility of the IL. Finally, the thermal characterization of the IL solutions, using DSC measurements, showed a number of features corresponding to different solid-solid phase transitions, T(S-S), with extremely low melting entropies, indicating a strong organizational structure by easy rotation of methyl group. These ILs can be classified as plastic crystal materials and are promising as ambient-temperature solid electrolytes.

Realisation of an all solid state lithium battery using solid high temperature plastic crystal electrolytes exhibiting liquid like conductivity

Replacement of volatile and combustible electrolytes in conventional lithium batteries is desirable for two reasons: safety concerns and increase in specific energy. In this work we consider the use of an ionic organic plastic crystal material (IOPC), N-ethyl-N-methylpyrrolidinium tetrafluoroborate, [C2mpyr][BF(4)], as a solid-state electrolyte for lithium battery applications. The effect of inclusion of 1 to 33 mol% lithium tetrafluoroborate, LiBF(4), into [C2mpyr][BF(4)] has been investigated over a wide temperature range by differential scanning calorimetry (DSC), impedance spectroscopy, cyclic voltammetry and cycling of full Li|LiFePO(4) batteries. The increases in ionic conductivity by orders of magnitude observed at higher temperature are most likely associated with an increase in Li ion mobility in the highest plastic phase. At concentrations >5 mol% LiBF(4) the ionic conductivity of these solid-state composites is comparable to the ionic conductivity of room temperature ionic liquids. Galvanostatic cycling of Li|Li symmetrical cells showed that the reversibility of the lithium metal redox reaction at the interface of this plastic crystal electrolyte is sufficient for lithium battery applications. For the first time we demonstrate an all solid state lithium battery incorporating solid electrolytes based on IOPC as opposed to conventional flammable organic solvents.

Lithium doped N,N-dimethyl pyrrolidinium tetrafluoroborate organic ionic plastic crystal electrolytes for solid state lithium batteries

The organic ionic plastic crystal material N,N-dimethyl pyrrolidinium tetrafluoroborate ([C1mpyr][BF4]) has been mixed with LiBF4 from 0 to 8 wt% and shown to exhibit enhanced ionic conductivity, especially in the higher temperature plastic crystal phases (phases II and I). The materials retain their solid state well above 100 °C with the melt not being observed up to 300 °C. Interestingly the conductivity enhancement is highest with the lowest level of LiBF4 addition in phase II, but then the order of enhancement is reversed in phase I. In all cases, a conductivity drop is observed at the II → I phase transition (105 °C) which is associated with increased order in the pure matrix, as previously reported, although the conductivity drop is least for the highest LiBF4 amount (8 wt%). The 8 wt% sample displays different conductivity behaviours compared to the lower LiBF4 concentrations, with a sharp increase above 50 °C, which is apparently not related to the formation of an amorphous phase, based on XRD data up to 120 °C. Symmetric cells, Li/OIPC/Li, were prepared and cycled at 50 °C and showed evidence of significant preconditioning with continued cycling, leading to a lower over-potential and a concomitant decrease in the cell resistivity as measured by EIS. An SEM investigation of the Li/OIPC interfaces before and after cycling suggested significant grain refinement was responsible for the decrease in cell resistance upon cycling, possibly as a result of an increased grain boundary phase.

Organic ionic plastic crystal electrolytes; a new class of electrolyte for high efficiency solid state dye-sensitized solar cells

Dye-sensitized solar cells are an increasingly promising alternative to conventional silicon solar cells as a method of converting solar energy to electricity and thus providing an effectively inexhaustible energy source. However, the most efficient of these devices currently utilize liquid electrolytes, which suffer from the associated problems of leakage and evaporation. Hence, significant research is currently focused on the development of solid state alternatives. Here we report a new class of solid state electrolyte for these devices, organic ionic plastic crystal electrolytes, that allow relatively rapid diffusion of the redox couple through the matrix, which is critical to the cell performance. A range of different organic ionic plastic crystal materials, utilizing different cation and anion structures, have been investigated and the conductivities, diffusion rates and photovoltaic performance of the electrolytes are reported. The best material, utilizing the dicyanamide anion, achieves efficiencies of more than 5%.

Unusual phase behaviour of the organic ionic plastic crystal N,N-dimethylpyrrolidinium tetrafluoroborate

Analysis of N,N-dimethylpyrrolidinium tetrafluoroborate by (1)H and (11)B NMR, Raman spectroscopy and powder XRD shows that this organic ionic plastic crystal material exhibits unusual phase behaviour. (1)H NMR analysis indicates that the mobility of the pyrrolidinium cation decreases when the material is heated into phase I, while the X-ray diffraction pattern changes from a simple, one peak structure in phase II to a more complex pattern in phase I. The possible origins of these unusual transitions are discussed.

Proton transport in choline dihydrogen phosphate/H3PO4 mixtures

Mixtures of the plastic crystal material choline dihydrogen phosphate [Choline][DHP] and phosphoric acid, from 4.5 mol% to 18 mol% H(3)PO(4), were investigated and shown to have significantly higher proton conductivity compared to the pure [Choline][DHP]. This was particularly evident from the electrochemical hydrogen reduction reaction and the proton NMR diffusion measurements, in addition to ionic conductivity measured from the impedance spectroscopy. The ionic conductivity was observed to increase by more than an order of magnitude in phase I (i.e. the highest temperature solid phase in [Choline][DHP]) reaching up to 10(-2) S cm(-1). The multinuclear NMR spectroscopy data suggest that, at least on the timescale of the NMR measurement, the H(+) cations and [DHP] anions are equivalent in both phases. The pulsed field gradient NMR diffusion measurements of the 18 mol% acid sample indicate that all three ions are mobile, however the H(+) diffusion coefficient is an order of magnitude higher than for the [Choline] cation or the [DHP] anion, and therefore conduction in these materials is dominated by proton conductivity. The thermal stability, as measured by TGA, is unaffected with increasing acid additions and remains high; i.e. no significant mass loss below 200 °C.

Surprising effect of nanoparticle inclusion on ion conductivity in a lithium doped organic ionic plastic crystal

Doping lithium bis(trifluoromethanesulfonyl)amide (Li[NTf2]) into the N-ethyl,N′-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C2mpyr][NTf2]) plastic crystal material has previously indicated order of magnitude enhancements in ion transport and conductivity over pure [C2mpyr][NTf2]. Recently, conductivity enhancements in this ionic plastic crystal induced by SiO2nanoparticles have also been reported. In this work the inclusion of SiO2nanoparticles in Li ion doped [C2mpyr][NTf2] has been investigated over a wide temperature range by differential scanning calorimetry (DSC), impedance spectroscopy, positron annihilation lifetime spectroscopy (PALS), Raman spectroscopy, NMR spectroscopy and scanning electron microscopy (SEM). Solid state 1H NMR indicates that the addition of the nanoparticles increases the mobility of the [C2mpyr] cation and positron lifetime spectroscopy (PALS) measurements indicate an increase in mean defect size and defect concentration as a result of nanoparticle inclusion, especially with 10 wt% SiO2. Thus, the substantial drop in ion conductivity observed for this doped nanocomposite material was surprising. This decrease is most likely due to the decrease in mobility of the [NTf2] anion, possibly by its adsorption at the SiO2/grain boundary interface and concomitant decrease in mobility of the Li ion.

All-solid-state polymer electrolyte with plastic crystal materials for rechargeable lithium-ion battery

In this paper, we report the synthesis and properties of a series of composite polymer electrolytes formed by dispersion of a non-ionic organic plastic material succinonitrile (SN) into poly(vinylidene fluoride- co-hexafluoropropylene) (P(VDF-HFP)) complexed with four kinds of lithium salts. The addition of SN greatly enhances ionic conductivities of the electrolytes which is due to the high polarity and diffusivity of SN. The integrity of the plastic-crystalline phase is kept at low temperature range in polymer electrolytes. The high solubility of SN on Li salts is observed by Fourier transform infrared (FTIR) spectra measurement, thus could lead to enhanced lithium-ion transference number. The battery performances of composite polymer electrolytes at different temperatures are tested and the results are comparable to that of the pure plastic crystal electrolytes.

Plastic Crystal Electrolyte Materials: New Perspectives on Solid State Ionics

Plastic crystal materials have long been known but have only relatively recently become of interest as solid–state ion conductors. Their properties are often associated with dynamic orientational disorder or rotator motions in the crystalline lattice. This paper describes recent work in the field including the range of organic ionic compounds that exhibit ion conduction at room temperature. Conductivity in some cases is high enough to render the compounds of interest as electrolyte materials in all solid state electrochemical devices. Doping of the plastic crystal phase with a small ion such as Li+ in some cases produces an even higher conductivity. In this case the plastic crystal acts as a solid state “solvent” for the doped ion and supports the conductive motion of the dopant via motions of the matrix ions. These doped materials are also described in detail.