Ionic Liquid MixturesVariations in Physical Properties and Their Origins in Molecular Structure

In order to explore the various possible property trends in ionic liquid mixtures, five different ionic liquids were mixed with <i>N</i>-methyl-<i>N</i>-propylpyrrolidinium bis­(trifluoromethylsulfonyl)­amide ([C₃mpyr]­[NTf₂]), and the viscosities, excess molar volumes, ionic conductivities, and phase diagrams of the mixtures were determined over a range of temperatures. In a number of the mixtures the crystallization of both components was completely suppressed and no melting point was observable. Such mixtures of similar ionic liquids thus have potential for use in low-temperature applications by extending the liquid range to <i>T</i>_g. The molar conductivities and viscosities are described as approximating predictable or “simple” mixing behaviors, while excess molar volumes were found to show a variety of mixing and nonideal mixing effects. Mixture equations for viscosity and conductivity are discussed and analyzed. An immiscibility window was observed in the trihexyl­(tetradecyl)­phosphonium bis­(trifluoromethylsulfonyl)­amide ([P_6,6,6,14]­[NTf₂]) in the [C₃mpyr]­[NTf₂] system in the [C₃mpyr]­[NTf₂]-rich region. Unusual physical properties are exhibited by miscible compositions near the demixing line. These compositions are described as [P_6,6,6,14]­[NTf₂]-like, even up to 0.5 mol fraction of [C₃mpyr]­[NTf₂]

Dynamic Heterogeneity and Ionic Conduction in an Organic Ionic Plastic Crystal and the Role of Vacancies

Dynamic heterogeneity was investigated for the first time in a conductive organic ionic plastic crystal by molecular dynamics simulation. A minority fraction of ions that possess above-average dynamics were identified in the plastic crystal phase. The signature of this unusual motional behavior is found in the significant increase in the non-Gaussian parameter α­(<i>t</i>). A study by incorporation of vacancies into the crystal structure shows explicit evidence of coexistence of mobile species with an otherwise rigid matrix, which particularly supports the previous explanation on heterogeneous motional narrowing in nuclear magnetic resonance. It is also found that the origin of dynamic heterogeneity here is inseparable from the inherent structural characteristics of organic ions. This work reveals the profound effect brought by heterogeneous dynamics on the conduction mechanism of this material, as well as the important role of defects on ions dynamics

Dynamic Heterogeneity and Ionic Conduction in an Organic Ionic Plastic Crystal and the Role of Vacancies

Dynamic heterogeneity was investigated for the first time in a conductive organic ionic plastic crystal by molecular dynamics simulation. A minority fraction of ions that possess above-average dynamics were identified in the plastic crystal phase. The signature of this unusual motional behavior is found in the significant increase in the non-Gaussian parameter α­(<i>t</i>). A study by incorporation of vacancies into the crystal structure shows explicit evidence of coexistence of mobile species with an otherwise rigid matrix, which particularly supports the previous explanation on heterogeneous motional narrowing in nuclear magnetic resonance. It is also found that the origin of dynamic heterogeneity here is inseparable from the inherent structural characteristics of organic ions. This work reveals the profound effect brought by heterogeneous dynamics on the conduction mechanism of this material, as well as the important role of defects on ions dynamics

In Situ, Real-Time Visualization of Electrochemistry Using Magnetic Resonance Imaging

The drive to develop better electrochemical energy storage devices requires the development of not only new materials, but also better understanding of the underpinning chemical and dynamical processes within such devices during operation, for which new analytical techniques are required. Currently, there are few techniques that can probe local composition and transport in the electrolyte during battery operation. In this paper, we report a novel application of magnetic resonance imaging (MRI) for probing electrochemical processes in a model electrochemical cell. Using MRI, the transport and zinc and oxygen electrochemistry in an alkaline electrolyte, typical of that found in zinc-air batteries, are investigated. Magnetic resonance relaxation maps of the electrolyte are used to visualize the chemical composition and electrochemical processes occurring during discharge in this model metal-air battery. Such experiments will be useful in the development of new energy storage/conversion devices, as well as other electrochemical technologies

Redox Chemistry of the Superoxide Ion in a Phosphonium-Based Ionic Liquid in the Presence of Water

Stable electrogenerated superoxide ion has been observed for the first time in a phosphonium-based ionic liquid in the presence of water, leading to a chemically reversible O₂/O₂^•– redox couple instead of the disproportionation reaction that is usually observed. It appears that the cation solvates the superoxide anion, stabilizing it against the disproportionation reaction. The electrogeneration is studied at various levels of water or other diluents including toluene to explore the limits of stability of the superoxide ion under these conditions

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 ([P_1,2,2,4]­[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 [P_1,2,2,4]­[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

Solid State Li Metal/LMO Batteries based on Ternary Triblock Copolymers and Ionic Binders

Triblock copolymers containing an ionophilic polymerized ionic liquid block, sandwiched between two ionophobic polystyrene blocks, were investigated as solid polymer electrolytes (SPE) to simultaneously provide mechanically robust, free-standing membranes with high lithium conductivity and an optimized electrolyte composition. The conductivity reached 8 × 10–5 S cm–1 and 6.5 × 10–4 S cm–1 at 30 and 80 °C, respectively, with an anodic stability above 4.5 V. Highly stable Li metal symmetric cycling was demonstrated, with an overpotential of 130 mV for over 300 h at 50 °C at a current density of 0.5 mA cm–2/0.5 mAh cm–2. Attempts were also made to incorporate the SPE as the binder in an LMO cathode formulation. The best cell performance, however, was obtained when substituting the SPE in the LMO cathode formulation with a PMA solid-state gel electrolyte, resulting in a high-performance solid-state Li|polymer eletrolyte|LMO device with stable cycling at C/5, and an impressive capacity retention (i.e., 105 mAh g–1 after 150 cycles at 0.1 mA cm–2) with a Coulombic efficiency around 99.4%

Novel Na⁺ Ion Diffusion Mechanism in Mixed Organic–Inorganic Ionic Liquid Electrolyte Leading to High Na⁺ Transference Number and Stable, High Rate Electrochemical Cycling of Sodium Cells.

Ambient temperature sodium batteries hold the promise of a new generation of high energy density, low-cost energy storage technologies. Particularly challenging in sodium electrochemistry is achieving high stability at high charge/discharge rates. We report here mixtures of inorganic/organic cation fluorosulfonamide (FSI) ionic liquids that exhibit unexpectedly high Na⁺ transference numbers due to a structural diffusion mechanism not previously observed in this type of electrolyte. The electrolyte can therefore support high current density cycling of sodium. We investigate the effect of NaFSI salt concentration in methylpropylpyrrolidinium (C₃mpyr) FSI ionic liquid (IL) on the reversible plating and dissolution of sodium metal, both on a copper electrode and in a symmetric Na/Na metal cell. NaFSI is highly soluble in the IL allowing the preparation of mixtures that contain very high Na contents, greater than 3.2 mol/kg (50 mol %) at room temperature. Despite the fact that overall ion diffusivity decreases substantially with increasing alkali salt concentration, we have found that these high Na⁺ content electrolytes can support higher current densities (1 mA/cm²) and greater stability upon continued cycling. EIS measurements indicate that the interfacial impedance is decreased in the high concentration systems, which provides for a particularly low-resistance solid-electrolyte interphase (SEI), resulting in faster charge transfer at the interface. Na⁺ transference numbers determined by the Bruce–Vincent method increased substantially with increasing NaFSI content, approaching >0.3 at the saturation concentration limit which may explain the improved performance. NMR spectroscopy, PFG diffusion measurements, and molecular dynamics simulations reveal a changeover to a facile structural diffusion mechanism for sodium ion transport at high concentrations in these electrolytes

Biobased Acrylic Latexes/Sodium Carboxymethyl Cellulose Aqueous Binders for Lithium-Ion NMC 811 Cathodes

The increasing demands for sustainable energy storage technologies have prompted extensive research in the development of eco-friendly materials for lithium-ion batteries (LIBs). This research article presents the design of biobased latexes, which are fluorine-free and rely on renewable resources, based on isobornyl methacrylate (IBOMA) and 2-octyl acrylate (2OA) to be used as binders in batteries. Three different compositions of latexes were investigated, varying the ratio of IBOMA and 2OA: (1) Poly2OA homopolymer, (2) Poly(2OA0,6-co-IBOMA0,4) random copolymer, and (3) PolyIBOMA homopolymer. The combination of the two monomers provided a balance between rigidity from the hard monomer (IBOMA) and flexibility from the soft one (2OA). The study evaluated the performance of the biobased latexes using sodium carboxymethyl cellulose (CMC) as a thickener and cobinder by fabricating LiNi0.8Mn0.1Co0.1O2 (NMC 811) cathodes. Also, to compare with the state of the art, organic processed PVDF electrodes were prepared. Among aqueous slurries, rheological analysis showed that the CMC + Poly(2OA0,6-co-IBOMA0,4) binder system resulted in the most stable and well-dispersed slurries. Also, the electrodes prepared with this latex demonstrated enhanced adhesion (210 ± 9 N m–1) and reduced cracks compared to other aqueous compositions. Electrochemical characterization revealed that the aqueous processed cathodes using the CMC + Poly(2OA0,6-co-IBOMA0,4) biobased latex displayed higher specific capacities than the control with no latex at high C-rates (100.3 ± 2.1 vs 64.5 ± 0.8 mAh g–1 at 5C) and increased capacity retention after 90 cycles at 0.5C (84% vs 81% for CMC with no latex). Overall, the findings of this study suggest that biobased latexes, specifically the CMC + Poly(2OA0,6-co-IBOMA0,4) composition, are promising as environmentally friendly binders for NMC 811 cathodes, contributing to the broader goal of achieving sustainable energy storage systems

Formulation and Characterization of PS-Poly(ionic liquid) Triblock Electrolytes for Sodium Batteries

Solvent-free solid polymer electrolytes (SPE) are gaining more attention to develop postlithium battery technologies due to the safety and performance benefits of solid-state batteries. In this work, we present a new SPE for a sodium metal battery based on high salt concentration polymer electrolyte membranes comprising mixed anions, polymerized ionic liquid (PIL), block copolymer (BCP) polystyrene-b-poly­(diallydimethylammonium)­bis­(trifluoromethanesulfonyl)­imide-b-polystyrene (PS-b-PDADMATFSI-b-PS) and NaFSI salt. The maximum salt concentration incorporated was up to 1:2 mol ratio (PIL block: NaFSI). The ionic conductivity was 10–3 S cm–1 at 70 °C for 1:2 composition, and the anion diffusion as measured by 19F NMR decreased. FTIR measurement indicates that the ion coordination in the polymer–salt mixtures changes with composition. The storage modulus as measured by dynamic mechanical analysis (DMA) was observed in the range 300 MPa at −40 °C to 35.8 MPa at 70 °C. The optimized electrolyte (1:2 mol ratio) membrane was investigated for its long-term stability against Na metal cycling with Na/Na symmetrical cells demonstrating stable Na plating/stripping behavior at 0.2 mA cm–2 at 70 °C. Finally, an Na|NaFePO4 cell cycled with a specific capacity of 118 mAh g–1 at C-rate C/20 at 70 °C and a good Coulombic efficiency (98%), showing the promising potential of these solvent-free triblock copolymer electrolytes in Na metal batteries