Increasing ionic conductivity of polymer-sodium salt complex by addition of a non-ionic plastic crystal

Ionic conductivity and other physico-chemical properties of a soft matter composite electrolyte comprising of a polymer-sodium salt complex and a non-ionic plastic crystal are discussed here. The electrolyte under discussion comprises of polyethyleneoxide (PEO)-sodium triflate (NaCF3SO3) and succinonitrile (SN). Addition of SN to PEO-NaCF3SO3 resulted in significant enhancement in ionic conductivity. At 50% SN concentration (with respect to weight of polymer), the polymer-plastic composite electrolyte room temperature (= 25 degrees C) ionic conductivity was similar to 1.1 x 10(-4) Omega(-1) cm(-1), approximately 45 times higher than PEO-NaCF3SO3. Observations from ac-impedance spectroscopy along with X-ray diffraction, differential scanning calorimetry and Fourier transform inrared spectroscopy strongly suggest the enhancement in the composite is ionicconductivity due to enhanced ion mobility via decrease in crystallinity of PEO. The free standing composite polymer-plastic electrolytes were more compliable than PEO-NaCF3SO3 thus exhibiting no detrimental effects of succinonitrile addition on the mechanical stability of PEO-NaCF3SO3. We propose that the exploratory PEO-NaCF3SO3-SN system.discussed here will eventually be developed as a prototype electrolyte.for sodium-sulfur batteries capable of operating at ambient and.sub-ambient conditions. (C) 2010 Elsevier B.V. All rights reserved.

Ionic liquids and organic ionic plastic crystals utilizing small phosphonium cations

The development of new liquid and solid state electrolytes is paramount for the advancement of electrochemical devices such as lithium batteries and solar cells. Ionic liquids have shown great promise in both these applications. Here we demonstrate the use of phosphonium cations with small alkyl chain substituents, in combination with a range of different anions, to produce a variety of new halide free ionic liquids that are fluid, conductive and with sufficient thermal stability for a range of electrochemical applications. Walden plot analysis of the new phosphonium ionic liquids shows that these can be classed as "good" ionic liquids, with low degrees of ion pairing and/or aggregation, and the lithium deposition and stripping from one of these ionic liquids has been demonstrated. Furthermore, for the first time phosphonium cations have been used to form a range of organic ionic plastic crystals. These materials can show significant ionic conductivity in the solid state and thus are of great interest as potential solid-state electrolyte materials.

Structure and dynamics in solid state sodium ion electrolytes based on organic ionic plastic crystals

 The investigation of solid state sodium ion electrolytes based on Organic Ionic Plastic Crystals were carried out for potential use in the electrochemical devices such as batteries.

Structure and transport properties of a plastic crystal ion conductor : Diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate

Understanding the ion transport behavior of organic ionic plastic crystals (OIPCs) is crucial for their potential application as solid electrolytes in various electrochemical devices such as lithium batteries. In the present work, the ion transport mechanism is elucidated by analyzing experimental data (single-crystal XRD, multinuclear solid-state NMR, DSC, ionic conductivity, and SEM) as well as the theoretical simulations (second moment-based solid static NMR line width simulations) for the OIPC diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate ([P1,2,2,4][PF6]). This material displays rich phase behavior and advantageous ionic conductivities, with three solid–solid phase transitions and a highly “plastic” and conductive final solid phase in which the conductivity reaches 10–3 S cm–1. The crystal structure shows unique channel-like packing of the cations, which may allow the anions to diffuse more easily than the cations at lower temperatures. The strongly phase-dependent static NMR line widths of the 1H, 19F, and 31P nuclei in this material have been well simulated by different levels of molecular motions in different phases. Thus, drawing together of the analytical and computational techniques has allowed the construction of a transport mechanism for [P1,2,2,4][PF6]. It is also anticipated that utilization of these techniques will allow a more detailed understanding of the transport mechanisms of other plastic crystal electrolyte materials.

Insights into the transport of alkali metal ions doped into a plastic crystal electrolyte

The application of organic ionic plastic crystals (OIPCs) as a new class of solid electrolyte for energy storage devices such as lithium batteries and, more recently, sodium batteries is attracting increasing attention. Key to this is achieving sufficient target ion transport through the material. This requires fundamental understanding of the structure and dynamics of OIPCs that have been doped with the necessary lithium or sodium salts. Here we report, for the first time, the atomic level structure and transport of both lithium and sodium ions in the plastic crystalline phases of an OIPC diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate. These molecular dynamics simulations reveal two types of coordination geometries of the alkali metal ion first solvation shells, which cooperate closely with the metal ion hopping motion. The significantly different ion migration rates between two metal ion doped systems could also be related to the differences in solvation structures.

Modelling ion-pair geometries and dynamics in a 1-ethyl-1-methylpyrrolidinium-based ion-conductive crystal

Full conformational and energy explorations are conducted on an organic ionic plastic crystal, 1-ethyl-1-methylpyrrolidium tetrafluoroborate [C2 mpyr][BF4 ]. The onsets of various stages of dynamic behaviour, which appear to account for low-temperature solid-solid phase transitions, are investigated by using quantum-chemical simulations. It is suggested that pseudorotation of the pyrrolidine ring occurs in the first instance; the partial rotation of the entire cation subsequently occurs and may be accompanied by reorientation of the ethyl chain as the temperature increases further. A cation-anion configuration, whereby BF4 (-) interacts with the C2 mpy cation from the side of the ring, is the most likely structure in the low-temperature phase IV region. These interpretations are supported by (13) C nuclear magnetic resonance chemical-shift analysis.

Li-Metal symmetrical cell studies using ionic organic plastic crystal electrolyte

A low current density preconditioning process, which produces an improved lithium transport mechanism is created by the action of charge flow through a plastic crystal electrolyte (figure). A reduction in cell polarisation at high applied current density is demonstrated which approaches the rates required for these electrolytes to be used in practical devices.

A new class of proton-conducting ionic plastic crystals based on organic cations and dihydrogen phosphate

Choline dihydrogen phosphate ([N_1.1.1.2OH]DHP) and 1-butyl-3-methylimidazolium dihydrogen phosphate ([C₄mim]DHP) were synthesized as a new class of proton-conducting ionic plastic crystals. Both [N_1.1.1.2OH]DHP and [C₄mim]DHP showed solid–solid phase transition(s) and showed a final entropy of fusion lower than 20 J K⁻¹ mol⁻¹ which is consistent with Timmerman’s criterion for molecular plastic crystals. The ionic conductivity of [N_1.1.1.2OH]DHP was in the range of 10⁻⁶ S cm⁻¹–10⁻³ S cm⁻¹ in the plastic crystalline phase. On the other hand, the ionic conductivity of [C₄mim]DHP showed about 10⁻⁵ S cm⁻¹ in the plastic crystalline phase. [N_1.1.1.2OH]DHP showed one order of magnitude higher ionic conductivity than [C₄mim]DHP in the temperature range where the plastic phase is stable.

Structure and transport properties in an N,N-substituted pyrrolidinium tetrafluoroborate plastic crystal system

The structure and transport of N-propyl-N-methylpyrrolidinium tetrafluoroborate (P₁₃BF₄) has been investigated over a wide temperature range in consequence to exhibiting properties suitable for potential solid-state superionic electrolyte applications. Prior to melting, the organic salt, P₁₃BF₄, transforms into a plastic crystal phase. Intrinsic conductivity in this solid, phase I (45–65 °C), is comparable to that in the melt (_~10⁻³ S cm⁻¹). Ionic motion and transport properties were investigated by ¹H and ¹¹B nuclear magnetic resonance (NMR) spectroscopy. Pressure-induced plastic flow in this system may accommodate volume changes in device application and to this extent, X-ray diffraction (XRD) has been used. Scanning electron microscopy (SEM) revealed complex surface morphology and lattice imperfections associated with the strong orientational disorder of the plastic state.

N-methyl-N-alkylpyrrolidinium nonafluoro-1-butanesulfonate salts : Ionic liquid properties and plastic crystal behaviour

A series of N-methyl-N-alkylpyrrolidinium nonafluoro-1-butanesulfonate salts were synthesised and characterised. The thermophysical characteristics of this family of salts have been investigated with respect to potential use as ionic liquids and solid electrolytes. N-Methyl-N-butylpyrrolidinium nonafluoro-1-butanesulfonate (p_1,4NfO) has the lowest melting point of the family, at 94 °C. Electrochemical analysis of p_1,4 NfO in the liquid state shows an electrochemical window of ~6 V. All compounds exhibit one or more solid–solid transitions at sub-ambient temperatures, indicating the existence of plastic crystal phases.

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.