Ionic liquids in electrochemical devices and processes : managing interfacial electrochemistry

Many ionic liquids offer a range of properties that make them attractive to the field of electrochemistry; indeed it was electrochemical research and applications that ushered in the modern era of interest in ionic liquids. In parallel with this, a variety of electrochemical devices including solar cells, high energy density batteries, fuel cells, and supercapacitors have become of intense interest as part of various proposed solutions to improve sustainability of energy supply in our societies. Much of our work over the last ten years has been motivated by such applications. Here we summarize the role of ionic liquids in these devices and the insights that the research provides for the broader field of interest of these fascinating liquids.

Lithium-functionalised silica nanoparticles for enhanced ionic conductivity in an organic ionic plastic crystal

Addition of silica nanoparticles functionalised with lithium propane sulfonate to the organic ionic plastic crystal N-ethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C₂mpyr][NTf₂]) results in a significant increase in ionic conductivity. Analysis of these nanocomposites by impedance spectroscopy, NMR, positron annihilation lifetime spectroscopy (PALS) and Raman spectroscopy suggests that this is the result of higher matrix mobility due to an increase in defect size and concentration. The effect of these functionalised nanoparticles is compared to that previously observed for unfunctionalised nanoparticles in the lithium-doped and pure plastic crystal.

Anisotropic MRI contrast reveals enhanced ionic transport in plastic crystals

Organic ionic plastic crystals (OIPCs) are attractive as solid-state electrolytes for electrochemical devices such as lithium-ion batteries and solar and fuel cells. OIPCs offer high ionic conductivity, nonflammability, and versatility of molecular design. Nevertheless, intrinsic ion transport behavior of OIPCs is not fully understood, and their measured properties depend heavily on thermal history. Solid-state magnetic resonance imaging experiments reveal a striking image contrast anisotropy sensitive to the orientation of grain boundaries in polycrystalline OIPCs. Probing triethyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1222FSI) samples with different thermal history demonstrates vast variations in microcrystallite alignment. Upon slow cooling from the melt, microcrystallites exhibit a preferred orientation throughout the entire sample, leading to an order of magnitude increase in conductivity as probed using impedance spectroscopy. This investigation describes both a new conceptual window and a new characterization method for understanding polycrystalline domain structure and transport in plastic crystals and other solid-state conductors.

Recycling textiles : The use of ionic liquids in the separation of cotton polyester blends

Textiles are commonly made from intimate blends of polyester and cotton, which makes recycling very difficult. We report for the first time the use of ionic liquid in the separation of polyester cotton blends. By selective dissolution of the cotton component, the polyester component can be separated and recovered in high yield. This finding presents an environmentally benign approach to recycling textile waste. © 2014 The Royal Society of Chemistry.

Comparison of various NMR methods for the indirect detection of nitrogen-14 nuclei via protons in solids

We present an experimental comparison of several through-space Hetero-nuclear Multiple-Quantum Correlation experiments, which allow the indirect observation of homo-nuclear single- (SQ) or double-quantum (DQ) ¹⁴N coherences via spy ¹H nuclei. These ¹H-{¹⁴N} D-HMQC sequences differ not only by the order of ¹⁴N coherences evolving during the indirect evolution, t<inf>1inf>, but also by the radio-frequency (rf) scheme used to excite and reconvert these coherences under Magic-Angle Spinning (MAS). Here, the SQ coherences are created by the application of center-band frequency-selective pulses, i.e. long and low-power rectangular pulses at the ¹⁴N Larmor frequency, ν<inf>0inf>(¹⁴N), whereas the DQ coherences are excited and reconverted using rf irradiation either at ν<inf>0inf>(¹⁴N) or at the ¹⁴N overtone frequency, 2ν<inf>0inf>(¹⁴N). The overtone excitation is achieved either by constant frequency rectangular pulses or by frequency-swept pulses, specifically Wide-band, Uniform-Rate, and Smooth-Truncation (WURST) pulse shapes. The present article compares the performances of four different ¹H-{¹⁴N} D-HMQC sequences, including those with ¹⁴N rectangular pulses at ν<inf>0inf>(¹⁴N) for the indirect detection of homo-nuclear (i) ¹⁴N SQ or (ii) DQ coherences, as well as their overtone variants using (iii) rectangular or (iv) WURST pulses. The compared properties include: (i) the sensitivity, (ii) the spectral resolution in the ¹⁴N dimension, (iii) the rf requirements (power and pulse length), as well as the robustness to (iv) rf offset and (v) MAS frequency instabilities. Such experimental comparisons are carried out for γ-glycine and l-histidine.HCl monohydrate, which contain ¹⁴N sites subject to moderate quadrupole interactions. We demonstrate that the optimum choice of the ¹H-{¹⁴N} D-HMQC method depends on the experimental goal. When the sensitivity and/or the robustness to offset are the major concerns, the D-HMQC sequence allowing the indirect detection of ¹⁴N SQ coherences should be employed. Conversely, when the highest resolution and/or adjusted indirect spectral width are needed, overtone experiments are the method of choice. The overtone scheme using WURST pulses results in broader excitation bandwidths than that using rectangular pulses, at the expense of reduced sensitivity. Numerically exact simulations also show that the sensitivity of the overtone ¹H-{¹⁴N} D-HMQC experiment increases for larger quadrupole interactions.

Enhanced ionic mobility in organic ionic plastic crystal - dendrimer solid electrolytes

We report the first study of the characterisation of the organic ionic plastic crystal (OIPC) N-ethyl-N-methylpyrrolidinium tetrafluoroborate (C2mpyrBF4) upon mixing with a dendrimer additive. Whereas previous reports of OIPC composite formation (i.e. with ceramics and polymers) have typically reported a decrease in the conductivity when lithium salt had been added, the addition of dendrimer is shown to lead to a substantial enhancement in the lithium containing system, approaching 3 orders of magnitude at 30°C. Mechanical analysis indicates that dendrimer addition leads to a softer more ductile material while microscopy shows that the dendrimer is uniformly distributed and that the crystal microstructure is substantially disrupted, ultimately adopting a dendritic microstructure at 1mol% dendrimer content. Thermal analysis indicates a new phase in the lithium OIPC system, the crystallisation of which is suppressed in the presence of dendrimer. Instead, a decrease in the phase transition enthalpies indicates a large increase in the amorphous component of the Lithium OIPC, particularly for the most conductive system -C2mpyrBF4 +10mol% LiBF4 +0.1mol% dendrimer. Variable temperature powder X-ray diffraction confirms the presence of a new distinct phase and its absence in the presence of dendrimer. A change in the progression of the thermal phase behaviour of the OIPC in the presence of dendrimer is also shown, exhibiting the phase I (high temperature) structure at temperatures below the phase II-I transition.

NMR studies of modified nasicon-like, lithium conducting solid electrolytes*1

²⁷Al, ³¹P and ⁷Li NMR measurements have been performed on lithium conducting ceramics based on the LiTi₂(PO₄)₃ structure with Al, V and Nb metal ions substituted for either Ti or P within the framework NASICON structure. The ²⁷Al magic angle spinning NMR measurements have revealed that, although Al is intended to substitute for octahedral Ti sites, additional substitution into tetrahedral environments (presumably phosphorous sites) occurs with increasing amount of Al addition. This tetrahedral substitution appears to occur more readily in the presence of vanadium, in Li_1+xAl_xTi_2&minus;x(PO₄)_2.9(VO₄)_0.1, whereas similar niobium additions (in place of vanadium) appear to stifle tetrahedral substitution. ⁷Li static NMR spectra reveal quadrupolar structure with C_q approximately 42 kHz, largely independent of substitution. Measurement of the ⁷Li central transition linewidth at room temperature reveals a relatively mobile lithium species (300–900 Hz) with linewidth tending to decrease with Al substitution and increase with increasing V or Nb. This new structural information is discussed in the context of ionic conduction in these ceramics.

New insights into ordering and dynamics in organic ionic plastic crystal electrolytes

Solid state phases of organic salts, whose chemistry is closely related to that of ionic liquids, often show interesting phase behavior and dynamics resulting in solid-state conductors that have potential application in electrochemical devices such as solid state batteries. The mechanism of conduction in these solid-state plastic crystal phases is still not entirely understood. We have recently shown using molecular dynamics (MD) simulations that the introduction of defects, such as vacancies, leads to heterogeneous dynamics in the OIPC arising from amorphous and mobile domains in these materials. Advanced magnetic resonance imaging (MRI) analysis indicates that these domains can exhibit distinct orientations, leading to anisotropic ionic conductivity with enhanced values in a particular direction. This paper will review this new understanding, drawing links between the molecular and macroscopic-level information provided by these two techniques.

Microscopic interactions in nanocomposite electrolytes

Nanocomposite electrolytes of a fully amorphous trifunctional polyether (3PEG) and poly- (methylene ethylene oxide) (PMEO) have been complexed with two lithium salts and nanoparticulate (~20 nm) fillers of TiO2 and Al2O3. Addition of the fillers to the polymer salt complexes shows a significant change in the conformational modes of both polymers, especially the D-LAM region between 200 and 400 cm-1, indicating a reduced segmental flexibility of the chain. These changes are more pronounced with the use of TiO2 than Al2O3. Incorporation of the nanoparticulate fillers to the electrolytes fails to influence the degree of ion association, suggesting that the number of charge carriers available for conduction in both polymers using both LiClO4 and LiCF3SO3 is not the source of any conductivity increase. Addition of the fillers, which was seen to increase the conductivity in PEO-based systems, generally lowers the conductivity in the present PMEO systems, while the addition of TiO2 has little or no effect except in the cases of 3PEG 1.5 and 1.25 mol/kg LiClO4. In this case, 10 wt % TiO2 provides a conductivity increase of half an order of magnitude at approximately 60 °C. We also report for the first time a Raman spectroscopy investigation into the PEO-based nanocomposite electrolytes. The present results are discussed in terms of the electrostatic interactions involving dielectric properties of the fillers, of special interest being the interactions between the polymer and the fillers and between the ionic species and the fillers, when the effect of crystallization can be ignored.

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^&minus;5 to 10^&minus;6 S cm^&minus;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^&minus;5 S cm^&minus;1 at r.t., which is over 60°C above T_g, 10^&minus;8 S cm^&minus;1 at 25°C above T_g, and conductivity was not measurable at T_g.