Understanding Polymerized Ionic Liquids as Solid Polymer Electrolytes for Sodium Batteries.

Solid polymer electrolytes (SPEs) have emerged as promising candidates for sodium-based batteries due to their cost-effectiveness and excellent flexibility. However, achieving high ionic conductivity and desirable mechanical properties in SPEs remains a challenge. In this study, we investigated an AB diblock copolymer, PS-PEA(BuImTFSI), as a potential SPE for sodium batteries. We explored binary and ternary electrolyte systems by combining the polymer with salt and [C3mpyr][FSI] ionic liquid (IL) and analyzed their thermal and electrochemical properties. Differential scanning calorimetry revealed phase separation in the polymer systems. The addition of salt exhibited a plasticizing effect localized to the polyionic liquid (PIL) phase, resulting in an increased ionic conductivity in the binary electrolytes. Introducing the IL further enhanced the plasticizing effect, elevating the ionic conductivity in the ternary system. Spectroscopic analysis, for the first time, revealed that the incorporation of NaFSI and IL influences the conformation of TFSI- and weakens the interaction between TFSI- and the polymer. This establishes correlations between anions and Na+, ultimately enhancing the diffusivity of Na ions. The electrochemical properties of an optimized SPE in Na/Na symmetrical cells were investigated, showcasing stable Na plating/stripping at high current densities up to 0.7 mA cm-2, maintaining its integrity at 70 °C. Furthermore, we evaluated the performance of a Na|NaFePO4 cell cycled at different rates (C/10 and C/5) and temperatures (50 and 70 °C), revealing remarkable high-capacity retention and Coulombic efficiency. This study highlights the potential of solvent-free diblock copolymer electrolytes for high-performance sodium-based energy storage systems, contributing to advanced electrolyte materials.

Zwitterionic materials with disorder and plasticity and their application as non-volatile solid or liquid electrolytes.

Zwitterionic materials can exhibit unique characteristics and are highly tunable by variation to the covalently bound cationic and anionic moieties. Despite the breadth of properties and potential uses reported to date, for electrolyte applications they have thus far primarily been used as additives or for making polymer gels. However, zwitterions offer intriguing promise as electrolyte matrix materials that are non-volatile and charged but non-migrating. Here we report a family of zwitterions that exhibit molecular disorder and plasticity, which allows their use as a solid-state conductive matrix. We have characterized the thermal, morphological and structural properties of these materials using techniques including differential scanning calorimetry, scanning electron microscopy, solid-state NMR and X-ray crystallography. We report the physical and transport properties of zwitterions combined with lithium salts and a lithium-functionalized polymer to form solid or high-salt-content liquid electrolytes. We demonstrate that the zwitterion-based electrolytes can allow high target ion transport and support stable lithium metal cell cycling. The ability to use disordered zwitterionic materials as electrolyte matrices for high target ion conduction, coupled with an extensive scope for varying the chemical and physical properties, has important implications for the future design of non-volatile materials that bridge the choice between traditional molecular and ionic solvent systems.

New organic ionic plastic crystals utilizing the morpholinium cation.

Organic ionic plastic crystals (OIPCs) are emerging candidates as safer, quasi solid-state ion conductors for various applications, especially for next-generation batteries. However, a fundamental understanding of these OIPC materials is required, particularly concerning how the choice of cation and anion can affect the electrolyte properties. Here, we report the synthesis and characterisation of a range of new morpholinium-based OIPCs and demonstrate the benefit of the ether functional group in the cation ring. Specifically, we investigate the 4-ethyl-4-methylmorpholinium [C2mmor]+ and 4-isopropyl-4-methylmorpholinium [C(i3)mmor]+ cations paired with bis(fluorosulfonyl)imide [FSI]- and bis(trifluoromethanesulfonyl)imide [TFSI]- anions. A fundamental study of the thermal behaviour and transport properties was performed using differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and electrochemical impedance spectroscopy (EIS). The free volume within the salts has been investigated by positron annihilation lifetime spectroscopy (PALS) and the ion dynamics using solid-state nuclear magnetic resonance (NMR) analysis. Finally, the electrochemical stability window was studied using cyclic voltammetry (CV). Out of the four morpholinium salts, [C2mmor][FSI] exhibits the widest phase I range from 11 to 129 °C, which is advantageous for their application. [C(i3)mmor][FSI] displayed the highest conductivity of 1 × 10-6 S cm-1 at 30 °C, whereas the largest vacancy volume of 132 Å3 was found for [C2mmor][TFSI]. These insights into the properties of new morpholinium-based OIPCs will be important for developing new electrolytes with optimised thermal and transport properties for a range of clean energy applications.

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.

Probing the molecular interactions and physicochemical properties of a cobalt-based redox electrolyte system for thermo-electrochemical cells.

Redox-active materials play a primary role in the high-performance electrochemical device research field. Their bulk ion dynamics and performances can be studied using different electrochemical analysis methods, but their molecular level interactions and dynamics on which these depend are often not well understood. Here, nuclear magnetic resonance (NMR) relaxation and double-stimulated echo pulsed field gradient (PFG) techniques have been used to gain insights into the molecular level interactions, exchange dynamics and self-diffusivity of the various species present in a cobalt-based redox active electrolyte system used for thermo-electrochemical applications, including how these factors depend on the oxidation state and concentration of the redox species. A series of liquid electrolyte samples consisting of a Co2+/3+(bpy)3(NTf2)2/3 redox couple (where bpy = bipyridyl and NTf2 = bis(trifluoromethanesulfonyl)imide) in 3-methoxypropionitrile (MPN) have been investigated using NMR as well as viscosity and conductivity measurements carried out over a temperature range 293 to 353 K. The results provide insights into the mobilities and interactions between the various species present, including the exchange of the NTf2- anions between the solvation shells of the Co(bpy)3 species. Such information will be useful in understanding the behaviour of these electrolytes in devices such as thermo-electrochemical cells.

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.

An Ultrasensitive Silicon Photonic Ion Sensor Enabled by 2D Plasmonic Molybdenum Oxide.

Silicon photonics has demonstrated great potential in ultrasensitive biochemical sensing. However, it is challenging for such sensors to detect small ions which are also of great importance in many biochemical processes. A silicon photonic ion sensor enabled by an ionic dopant-driven plasmonic material is introduced here. The sensor consists of a microring resonator (MRR) coupled with a 2D restacked layer of near-infrared plasmonic molybdenum oxide. When the 2D plasmonic layer interacts with ions from the environment, a strong change in the refractive index results in a shift in the MRR resonance wavelength and simultaneously the alteration of plasmonic absorption leads to the modulation of MRR transmission power, hence generating dual sensing outputs which is unique to other optical ion sensors. Proof-of-concept via a pH sensing model is demonstrated, showing up to 7 orders improvement in sensitivity per unit area across the range from 1 to 13 compared to those of other optical pH sensors. This platform offers the unique potential for ultrasensitive and robust measurement of changes in ionic environment, generating new modalities for on-chip chemical sensors in the micro/nanoscale.

Insight into Three-Coordinate Aluminum Species on Ethanol-to-Olefin Conversion over ZSM-5 Zeolites.

Commercial bioethanol can be readily converted into ethylene by a dehydration process using solid acids, such as Brønsted acidic H-ZSM-5 zeolites, and thus, it is an ideal candidate to replace petroleum and coal for the sustainable production of ethylene. Now, strong Lewis acidic extra-framework three-coordinate Al3+ species were introduced into H-ZSM-5 zeolites to improve their catalytic activity. Remarkably, Al3+ species working with Brønsted acid sites can accelerate ethanol dehydration at a much lower reaction temperature and shorten the unsteady-state period within 1-2 h, compared to >9 h for those without Al3+ species, which can significantly enhance the ethanol dehydration efficiency and reduce the cost. The reaction mechanism, studied by solid-state NMR, shows that strong Lewis acidic EFAl-Al3+ species can collaborate with Brønsted acid sites and promote ethanol dehydration either directly or indirectly via an aromatics-based cycle to produce ethylene.

On the measurement of intermolecular heteronuclear cross relaxation rates in ionic liquids.

The HOESY (Heteronuclear Overhauser Effect SpectroscopY) NMR experiment is commonly used to study interactions and structuring in ionic liquids (ILs) via the measurement of the cross relaxation rate σ between two spins. In the intermolecular case, σ is proportional to r-n, where r is the internuclear distance and n can vary between 1 and 6 depending on the frequency of the nuclei and their dynamics, thus σ can potentially provide detailed information on the liquid phase structure. However, in HOESY studies of ILs only relative values for σ are typically reported, making comparisons between different samples difficult. Herein we discuss the quantitative measurement of intermolecular cross relaxation rates based on the normalisation of HOESY signal intensities to the nuclear Boltzmann polarisation, demonstrated for 7Li-1H spin pairs in a lithium-containing pyrrolidinum-based ionic liquid electrolyte. We also use a simple model based on diffusing hard spheres for interpreting these quantities in terms of a distance of closest approach.

Effect of mixed anions on the physicochemical properties of a sodium containing alkoxyammonium ionic liquid electrolyte.

The physicochemical properties of the ionic liquid N-ethyl-2-(2-methoxyethoxy)-N,N-bis(2-(2-methoxyethoxy)ethyl)ethan-1-ammonium bis(trifluoromethylsulfonyl)imide (N2(2O2O1)3TFSI) as well as its solutions with sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) and sodium bis(fluorosulfonyl)imide (NaFSI) are compared in order to study the effects of the anion. The NaFSI solutions show weaker interactions as suggested by lower glass transition temperatures, lower densities, lower viscosities and higher conductivities as compared to their more strongly coordinating NaTFSI analogues. The transport properties follow Vogel-Tamman-Fulcher behaviour suggesting that the mixtures are fragile glass formers. The addition of a higher concentration of Na salts results in decoupling of the translational motion from the viscosity, as observed from Walden plots, and 23Na NMR data suggests that the sodium speciation is independent of the nature of the Na salt or the temperature but is affected by the salt concentration.

Optimisation of excitation schemes for 14N overtone MAS NMR using numerically exact simulations.

Numerically exact simulations of the 14N overtone (14NOT) MAS NMR experiment are used to investigate the effects of the applied magnetic field strength as well as three types of excitation pulse. The results show that both the resolution and sensitivity of 14NOT MAS NMR increase linearly with the applied static magnetic field strength. Standard RF excitation pulses are compared with frequency-swept WURST pulses as well as several composite pulses. WURST pulses are demonstrated to provide the largest bandwidths, while the direction of the frequency sweep is shown to be important when these pulses are used for the direct observation of 14NOT signals. A composite pulse is shown to provide the most efficient excitation overall, but only when applied on resonance. WURST excitation pulses are therefore the best option when studying a sample with unknown 14N NMR parameters.

Ion Dynamics in a Mixed-Cation Alkoxy-Ammonium Ionic Liquid Electrolyte for Sodium Device Applications.

The ion dynamics in a novel sodium-containing room-temperature ionic liquid (IL) consisting of an ether-functionalised quaternary ammonium cation and bis(trifluoromethylsulfonyl)amide [NTf2 ] anion with various concentrations of Na[NTf2 ] have been characterised using differential scanning calorimetry, impedance spectroscopy, diffusometry and NMR relaxation measurements. The IL studied has been specifically designed to dissolve a relatively large concentration of Na[NTf2 ] salt (over 2 mol kg-1 ) as this has been shown to improve ion transport and conductivity. Consistent with other studies, the measured ionic conductivity and diffusion coefficients show that the overall ionic mobility decreases with decreasing temperature and increasing salt content. NMR relaxation measurements provide evidence for correlated dynamics between the ether-functionalised ammonium and Na cations, possibly with the latter species acting as cross-links between multiple ammonium cations. Finally, preliminary cyclic voltammetry experiments show that this IL can undergo stable electrochemical cycling and could therefore be potentially useful as an electrolyte in a Na-based device.

14 N Solid-State NMR Spectroscopy of Amino Acids.

14 N ultra-wideline solid-state NMR (SSNMR) spectra were obtained for 16 naturally occurring amino acids and four related derivatives by using the WURST-CPMG (wideband, uniform rate, and smooth truncation Carr-Purcell-Meiboom-Gill) pulse sequence and frequency-stepped techniques. The 14 N quadrupolar parameters were measured for the sp3 nitrogen moieties (quadrupolar coupling constant, CQ , values ranged from 0.8 to 1.5 MHz). With the aid of plane-wave DFT calculations of the 14 N electric-field gradient tensor parameters and orientations, the moieties were grouped into three categories according to the values of the quadrupolar asymmetry parameter, ηQ : low (≤0.3), intermediate (0.31-0.7), and high (≥0.71). For RNH3+ moieties, greater variation in N-H bond lengths was observed for systems with intermediate ηQ values than for those with low ηQ values (this variation arose from different intermolecular hydrogen-bonding arrangements). Strategies for increasing the efficiency of 14 N SSNMR spectroscopy experiments were discussed, including the use of sample deuteration, high-power 1 H decoupling, processing strategies, high magnetic fields, and broadband cross-polarization (BRAIN-CP). The temperature-dependent rotations of the NH3 groups and their influence on 14 N transverse relaxation rates were examined. Finally, 14 N SSNMR spectroscopy was used to differentiate two polymorphs of l-histidine through their quadrupolar parameters and transverse relaxation time constants. The strategies outlined herein permitted the rapid acquisition of directly detected 14 N SSNMR spectra that to date was not matched by other proposed methods.

Unexpected effect of tetraglyme plasticizer on lithium ion dynamics in PAMPS based ionomers.

Li(+) cation conducting ionomers based on poly(2-acrylamido-2-methyl-1-propane sulphonic acid) (PAMPS) incorporating a low molecular weight plasticizer have been characterized. Previously we have observed an apparent decoupling of ionic conductivity and lithium ion dynamics from the Tg of this ionomer along with an increase in ionic conductivity obtained by incorporating a quaternary ammonium co-cation. The incorporation of tetraglyme as a coordinating plasticizer was investigated in order to further improve the ion dissociation and dynamics. Solid-state NMR, thermal analysis, impedance spectroscopy and infrared spectroscopy were used to characterize these systems. As expected, the glass transition temperature Tg decreased upon the addition of the plasticizer. However, in contrast to the previously reported Na-conducting systems, the ionic conductivity was also decreased by several orders of magnitude, indicating that the tetraglyme recouples the conductivity back to the polymer dynamics. Temperature dependent (7)Li NMR line width and T1 measurements were used to probe the Li(+) dynamics, which were found to be dependent on the Li(+) concentration, the nature of the co-cation and the presence or absence of tetraglyme.

New insights into the thermal behaviour of organic ionic plastic crystals: magnetic resonance imaging of polycrystalline morphology alterations induced by solid-solid phase transitions.

Organic ionic plastic crystals (OIPCs) show strong potential as solid-state electrolytes for lithium battery applications, demonstrating promising electrochemical performance and eliminating the need for a volatile and flammable liquid electrolyte. The ionic conductivity (σ) in these systems has recently been shown to depend strongly on polycrystalline morphology, which is largely determined by the sample's thermal history. [K. Romanenko et al., J. Am. Chem. Soc., 2014, 136, 15638]. Tailoring this morphology could lead to conductivities sufficiently high for battery applications, so a more complete understanding of how phenomena such as solid-solid phase transitions can affect the sample morphology is of significant interest. Anisotropic relaxation of nuclear spin magnetisation provides a new MRI based approach for studies of polycrystalline materials at both a macroscopic and molecular level. In this contribution, morphology alterations induced by solid-solid phase transitions in triisobutyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1444FSI) and diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate (P1224PF6) are examined using magnetic resonance imaging (MRI), alongside nuclear magnetic resonance (NMR) spectroscopy, diffusion measurements and conductivity data. These observations are linked to molecular dynamics and structural behaviour crucial for the conductive properties of OIPCs. A distinct correlation is established between the conductivity at a given temperature, σ(T), and the intensity of the narrow NMR signal that is attributed to a mobile fraction, fm(T), of ions in the OIPC. To explain these findings we propose an analogy with the well-studied relationship between permeability (k) and void fraction (θ) in porous media, with k(θ) commonly quantified by a power-law dependence that can also be employed to describe σ(fm).

Solid-state NMR as a probe of anion binding: molecular dynamics and associations in a [5]polynorbornane bisurea host complexed with terephthalate.

A range of solid-state NMR techniques is used to characterise a molecular host:guest complex consisting of a [5]polynorbornane bisurea host binding a terephthalate dianion guest. Detailed information is obtained on the molecular dynamics and associations from the point of view of both the host and guest molecules. The formation of the complex in the solid state is confirmed using (1)H 2D exchange NMR, and the 180° flipping of the (2)H-labelled terephthalate guest and its eventual expulsion from the complex at elevated temperatures are quantified using variable-temperature (2)H spin-echo experiments. Two-dimensional (1)H-(13)C HETCOR spectra obtained under fast magic angle spinning conditions (60 kHz) show a high resolution despite the poor crystallinity of the solid complex, and clearly reveal changes in the rigidity of the host molecule when complexed. Short-range intra- and intermolecular (1)H-(1)H proximities are also detected using 2D SQ-DQ correlation methods, providing insight into the molecular packing in the solid phase.

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.

Insight into local structure and molecular dynamics in organic solid-state ionic conductors.

Elucidating the rate and geometry of molecular dynamics is particularly important for unravelling ion-conduction mechanisms in electrochemical materials. The local molecular motions in the plastic crystal 1-ethyl-1-methylpyrrolidinium tetrafluoroborate ([C2 mpyr][BF4 ]) are studied by a combination of quantum chemical calculations and advanced solid-state nuclear magnetic resonance spectroscopy. For the first time, a restricted puckering motion with a small fluctuation angle of 25° in the pyrrolidinium ring has been observed, even in the low-temperature phase (-45 °C). This local molecular motion is deemed to be particularly important for the material to maintain its plasticity, and hence, its ion mobility at low temperatures.

Dynamic nuclear polarisation enhanced (14)N overtone MAS NMR spectroscopy.

Dynamic nuclear polarisation (DNP) has been used to obtain magic angle spinning (14)N(OT) (nitrogen-14 overtone) solid-state NMR spectra from several model amino acids, with both direct and indirect observation of the (14)N(OT) signal. The crystalline solids were impregnated with biradical solutions of organic liquids that do not dissolve the crystalline phase. The bulk phase was then polarized via(1)H spin diffusion from the highly-polarized surface (1)H nuclei, resulting in (1)H DNP signal enhancements of around two orders of magnitude. Cross polarisation from (1)H nuclei directly to the (14)N overtone transition is demonstrated under magic angle spinning, using a standard pulse sequence with a relatively short contact time (on the order of 100 µs). This method can be used to acquire (14)N overtone MAS powder patterns that match closely with simulated line shapes, allowing isotropic chemical shifts and quadrupolar parameters to be measured. DNP enhancement also allows the rapid acquisition of 2D (14)N(OT) heteronuclear correlation spectra from natural abundance powder samples. (1)H-(14)N(OT) HETCOR and (13)C-(14)N(OT) HMQC pulse sequences were used to observe all single-bond H-N and C-N correlations in histidine hydrochloride monohydrate, with the spectra obtained in a matter of hours. Due to the high natural abundance of the (14)N isotope (99.6%) and the advantages of observing the overtone transition, these methods provide an attractive route to the observation of C-N correlations from samples at natural isotopic abundance and enable the high resolution measurement of (14)N chemical shifts and quadrupolar interaction parameters.

Insights into the Carbon Dioxide Separation Performance of Bis(trifluoromethylsulfonyl)imide-based Plastic Crystal Composite Membranes with Fluorinated Polar Polymers.

Membrane-based gas separation technologies are one solution towards mitigating global emissions of CO2. New membrane materials with improved separation performance are still highly sought after. Composite membranes based on organic ionic plastic crystals (OIPCs) have shown preferential interaction for CO2 over N2, leading in some cases to competitive CO2/N2 selectivities. However, these ionic materials have been scarcely studied in the field of gas separation. Here, OIPCs based on the bis(trifluoromethylsulfonyl)imide ([TFSI]-) anion were investigated for use as gas separation membranes for the first time. The effect of the polymer type was also investigated, through the comparison of poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP) OIPC membranes. A strong temperature dependence of the gas separation performance was found, particularly in the N-methyl-N-ethylpyrrolidinium-based composites where the material undergoes a solid-solid phase transition within the testing temperature range. The polymer type was noted to induce a strong effect on the structure of the composites, as well as affecting the gas and ionic transport. Thus, this research provides insights on the influence of the [TFSI]- anion on the structure and separation properties of OIPC-based composites, and new information towards the development of novel OIPC-based membranes with enhanced gas separation performance.