Conductivity and NMR properties of plasticized polyethers complexed with lithium salts

NMR provides a tool whereby the dynamic properties of specific nuclei can be investigated. In the present study, a poly(ethylene oxide-co-propylene oxide) network has been used as the polymer host to prepare solid polymer electrolytes (SPE) containing either LiClO₄ or LiCF₃SO₃. In addition, a low molecular weight plasticizer [propylene carbonate (PC), dimethyl formamide (DMF) or tetraglyme] has been added to several of the samples to enhance the mobility of the polymer and, thus, of the ionic species. The effects of plasticizer and salt concentration on the ionic structure and mobility in these SPEs, as measured by NMR relaxation times, and correlation to the conductivity behaviour in these systems are discussed. Temperature dependent triflate diffusion coefficients, as measured by Pulsed Field Gradient ¹⁹F-NMR, in plasticized SPEs are also reported.

Novel high salt content polymer electrolytes based on high Tg polymers

Conductivities greater than or equal to 10⁻⁸ S cm⁻¹ at T_g are reported in polymer electrolytes based on lithium triflate salt and a series of polymers whose T_g is greater than 90°C. The highest conductivities were observed for poly(acrylonitrile) based systems with salt concentrations greater than 60 wt.%. The conductivity in all cases investigated increases with increasing salt concentration. ¹H-NMR T₂ relaxation measurements suggest that T_g decreases with increasing salt content and confirms that these materials are glassy at room temperature and hence that the conductivity is significantly decoupled from the structural relaxations. It appears that the nature of the polymer is important in determining the level of ionic conductivity, possibly due to differences in polymer coordinating ability or differences in T_g. Polymer-in-salt mixtures based on a tetra-alkyl ammonium imide molten salt and several high T_g polymers are also reported. The conductivities of these mixtures appear to be independent of the polymer type.

Glass transition and free volume behaviour of poly(acrylonitrile)/LiCF3SO3 polymer-in-salt electrolytes compared to poly(ether urethane)/LiClO4 solid polymer electrolytes

Measurements of the glass transition temperature (T_g) and free volume behaviour of poly(acrylonitrile) (PAN) and PAN/lithium triflate (LiTf), with varying salt composition from 10 to 66 wt% LiTf, were made by positron annihilation lifetime spectroscopy (PALS). Addition of salt from 10 to 45 wt% LiTf resulted in an increase in the mean free volume cavity size at room temperature (r.t.) as measured by the orthoPositronium (oPs) pickoff lifetime, τ₃, with little change in relative concentration of free volume sites as measured by oPs pickoff intensity, I₃. The region from 45 to 66 wt% salt displayed no variation in relative free volume cavity size and concentration. This salt concentration range (45 wt%<[LiTf]<66 wt%) corresponds to a region of high ionic conductivity of order 10⁻⁵ to 10⁻⁶ S cm⁻¹ at T_g as measured by PALS. A percolation phenomenon is postulated to describe conduction in this composition region. Salt addition was shown to lower the T_g as measured by PALS; T_g was 115°C for PAN and 85°C for PAN/66 wt% LiTf. The T_g and free volume behaviour of this polymer-in-salt electrolyte (PISE) was compared to a poly(ether urethane)/LiClO₄ where the polymer is the major component, i.e. traditional solid polymer electrolyte (SPE). In contrast to the PISE, the T_g of the SPE was shown to increase with increasing salt concentration from 5.3 to 15.9 wt%. The relative free volume cavity size and concentration at r.t. were shown to decrease with increasing salt concentration. Ionic conductivity in this SPE was of order 10⁻⁵ S cm⁻¹ at r.t., which is over 60°C above T_g, 10⁻⁸ S cm⁻¹ at 25°C above T_g, and conductivity was not measurable at T_g.

Poly methacrylate-plasticiser-salt blends as solid polymer electrolytes

Methoxy-ethylene glycol methacrylates, CH₂=CMeCOO(CH₂CH₂O)_nMe (n = 1, 2, 3), ethoxy-triethylene glycol methacrylate, CH₂=CMeCOO(CH₂CH₂O)₃Et, and N,N-dimethylaminoethyl methacrylate, CH₂=CMeCOOCH₂CH₂NMe₂, were used to synthesise the corresponding polymers. Conductivities of these polymers complexed with lithium perchlorate were investigated. Tetraethylene glycol dimethyl ether was used as plasticiser to increase the conductivity of the materials. A conductivity of 10⁻⁵ S cm⁻¹ was obtained at room temperature for the plasticised polymer samples. Effects of polymer structure, plasticiser, salt concentration and temperature on conductivity and glass transition temperature of the polymer electrolytes are discussed.

Mechanical properties of polyether-plasticiser-salt systems as polymer electrolytes

The mechanical properties of urethane crosslinked poly(ethylene oxide-co-propylene oxide) glyceryl ether-plasticiser (tetraethylene glycol dimethyl ether, or methylformamide)-salt (LiClO₄)-based polymer electrolytes have been studied. It was found that, with increasing concentration of salt, the elastic modulus and tensile strength of the materials unexpectedly decrease. This is interpreted in terms of a predominance of intramolecular coordination of the Li⁺ ions by the polymer.

Poly(triazine imide) with intercalation of Lithium and ChlorideiIons [(C3N3)2(NHxLi1−x)3⋅LiCl]: A Crystalline 2D Carbon Nitride network

Poly(triazine imide) with intercalation of lithium and chloride ions (PTI/Li+Cl−) was synthesized by temperature-induced condensation of dicyandiamide in a eutectic mixture of lithium chloride and potassium chloride as solvent. By using this ionothermal approach the well-known problem of insufficient crystallinity of carbon nitride (CN) condensation products could be overcome. The structural characterization of PTI/Li+Cl− resulted from a complementary approach using spectroscopic methods as well as different diffraction techniques. Due to the high crystallinity of PTI/Li+Cl− a structure solution from both powder X-ray and electron diffraction patterns using direct methods was possible; this yielded a triazine-based structure model, in contrast to the proposed fully condensed heptazine-based structure that has been reported recently. Further information from solid-state NMR and FTIR spectroscopy as well as high-resolution TEM investigations was used for Rietveld refinement with a goodness-of-fit (χ2) of 5.035 and wRp=0.05937. PTI/Li+Cl− (P63cm (no. 185); a=846.82(10), c=675.02(9) pm) is a 2D network composed of essentially planar layers made up from imide-bridged triazine units. Voids in these layers are stacked upon each other forming channels running parallel to [001], filled with Li+ and Cl− ions. The presence of salt ions in the nanocrystallites as well as the existence of sp2-hybridized carbon and nitrogen atoms typical of graphitic structures was confirmed by electron energy-loss spectroscopy (EELS) measurements. Solid-state NMR spectroscopy investigations using 15N-labeled PTI/Li+Cl− proved the absence of heptazine building blocks and NH2 groups and corroborated the highly condensed, triazine-based structure model.

Investigation of the lithium ion mobility in cyclic model compounds and their ion conducting properties

Efficient energy storage and conversion is playing a key role in overcoming the present and future challenges in energy supply. Batteries provide portable, electrochemical storage of green energy sources and potentially allow for a reduction of the dependence on fossil fuels, which is of great importance with respect to the issue of global warming. In view of both, energy density and energy drain, rechargeable lithium ion batteries outperform other present accumulator systems. However, despite great efforts over the last decades, the ideal electrolyte in terms of key characteristics such as capacity, cycle life, and most important reliable safety, has not yet been identified. rnrnSteps ahead in lithium ion battery technology require a fundamental understanding of lithium ion transport, salt association, and ion solvation within the electrolyte. Indeed, well-defined model compounds allow for systematic studies of molecular ion transport. Thus, in the present work, based on the concept of ‘immobilizing’ ion solvents, three main series with a cyclotriphosphazene (CTP), hexaphenylbenzene (HBP), and tetramethylcyclotetrasiloxane (TMS) scaffold were prepared. Lithium ion solvents, among others ethylene carbonate (EC), which has proven to fulfill together with pro-pylene carbonate safety and market concerns in commercial lithium ion batteries, were attached to the different cores via alkyl spacers of variable length.rnrnAll model compounds were fully characterized, pure and thermally stable up to at least 235 °C, covering the requested broad range of glass transition temperatures from -78.1 °C up to +6.2 °C. While the CTP models tend to rearrange at elevated temperatures over time, which questions the general stability of alkoxide related (poly)phosphazenes, both, the HPB and CTP based models show no evidence of core stacking. In particular the CTP derivatives represent good solvents for various lithium salts, exhibiting no significant differences in the ionic conductivity σ_dc and thus indicating comparable salt dissociation and rather independent motion of cations and ions.rnrnIn general, temperature-dependent bulk ionic conductivities investigated via impedance spectroscopy follow a William-Landel-Ferry (WLF) type behavior. Modifications of the alkyl spacer length were shown to influence ionic conductivities only in combination to changes in glass transition temperatures. Though the glass transition temperatures of the blends are low, their conductivities are only in the range of typical polymer electrolytes. The highest σ_dc obtained at ambient temperatures was 6.0 x 10-6 S•cm-1, strongly suggesting a rather tight coordination of the lithium ions to the solvating 2-oxo-1,3-dioxolane moieties, supported by the increased σ_dc values for the oligo(ethylene oxide) based analogues.rnrnFurther insights into the mechanism of lithium ion dynamics were derived from 7Li and 13C Solid- State NMR investigations. While localized ion motion was probed by i.e. 7Li spin-lattice relaxation measurements with apparent activation energies E_a of 20 to 40 kJ/mol, long-range macroscopic transport was monitored by Pulsed-Field Gradient (PFG) NMR, providing an E_a of 61 kJ/mol. The latter is in good agreement with the values determined from bulk conductivity data, indicating the major contribution of ion transport was only detected by PFG NMR. However, the μm-diffusion is rather slow, emphasizing the strong lithium coordination to the carbonyl oxygens, which hampers sufficient ion conductivities and suggests exploring ‘softer’ solvating moieties in future electrolytes.rn

Effect of height of bridge above water on salt deposition levels

Maintenance of bridge structures is a major issue for the Queensland Department of Main Roads. In the previous phase of this CRC project an initial approach was made towards the development of a program for lifetime prediction of metallic bridge components. This involved the analysis of five representative bridge structures with respect to salt deposition (a major contributor to metallic corrosion) to determine common elements to be used as “cases” - those defined for buildings are not applicable. The five bridges analysed included the Gladstone Port Access Road Overpass, Stewart Road Overpass, South Johnstone River Bridge, Johnson Creek Bridge and the Ward River Bridge.

Report : effect of natural cleaning on salt deposition on Queensland bridges

Maintenance of bridge structures is a major issue for the Queensland Department of Main Roads. In the previous phase of this CRC project an initial approach was made towards the development of a program for lifetime prediction of metallic bridge components. This involved the analysis of five representative bridge structures with respect to salt deposition (a major contributor to metallic corrosion) to determine common elements to be used as “cases” - those defined for buildings are not applicable. The five bridges analysed included the Gladstone Port Access Road Overpass, Stewart Road Overpass, South Johnstone River Bridge, Johnson Creek Bridge and the Ward River Bridge.