Unprotected Organic Cations─The Dilemma of Highly Li-Concentrated Ionic Liquid Electrolytes

Highly Li-concentrated electrolytes have been widely studied to harness their uniquely varying bulk and interface properties that arise from their distinctive physicochemical properties and coordination structures. Similar strategies have been applied in the realm of ionic liquid electrolytes to exploit their improved functionalities. Despite these prospects, the impact of organic cation behavior on interfacial processes remains largely underexplored compared to the widely studied anion behavior. The present study demonstrates that the weakened interactions between cations and anions engender "unprotected" organic cations in highly Li-concentrated ionic liquid electrolytes, leading to the decomposition of electrolytes during the initial charge. This decomposition behavior is manifested by the substantial irreversible capacities and inferior initial Coulombic efficiencies observed during the initial charging of graphite negative electrodes, resulting in considerable electrolyte consumption and diminished energy densities in full-cell configurations. The innate cation behavior is ascertained by examining the coordination environment of ionic liquid electrolytes with varied Li concentrations, where intricate ionic interactions between organic cations and anions are unveiled. In addition, anionic species with high Lewis basicity were introduced to reinforce the ionic interactions involving organic cations and improve the initial Coulombic efficiency. This study verifies the role of unprotected organic cations while highlighting the significance of the coordination environment in the performance of ionic liquid electrolytes.

Do non-coordinating polymers function as host materials for solid polymer electrolytes? The case of PVdF-HFP

In the search for novel solid polymer electrolytes (SPEs), primarily targeting battery applications, a range of different polymers is currently being explored. In this context, the non-coordinating poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) polymer is a frequently utilized system. Considering that PVdF-HFP should be a poor solvent for cation salts, it is counterintuitive that this is a functional host material for SPEs. Here, we do an in-depth study of the salt dissolution properties and ionic conductivity of PVdF-HFP-based electrolytes, using two different fabrication methods and also employing a low-molecular-weight solvent analogue. It is seen that PVdF-HFP is remarkably poor as an SPE host, despite its comparatively high dielectric constant, and that the salt dissolution properties instead are controlled by fluorophilic interactions of the anion with the polymer.

High Performance Ternary Solid Polymer Electrolytes Based on High Dielectric Poly(vinylidene fluoride) Copolymers for Solid State Lithium-Ion Batteries

Renewable energy sources require efficient energy storage systems. Lithium-ion batteries stand out among those systems, but safety and cycling stability problems still need to be improved. This can be achieved by the implementation of solid polymer electrolytes (SPE) instead of the typically used separator/electrolyte system. Thus, ternary SPEs have been developed based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene), P(VDF-TrFE-CFE) as host polymers, clinoptilolite (CPT) zeolite added to stabilize the battery cycling performance, and ionic liquids (ILs) (1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN])), 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([PMPyr][TFSI]) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), incorporated to increase the ionic conductivity. The samples were processed by doctor blade with solvent evaporation at 160 °C. The nature of the polymer matrix and fillers affect the morphology and mechanical properties of the samples and play an important role in electrochemical parameters such as ionic conductivity value, electrochemical window stability, and lithium-transference number. The best ionic conductivity (4.2 × 10^-5 S cm^-1) and lithium transference number (0.59) were obtained for the PVDF-HFP-CPT-[PMPyr][TFSI] sample. Charge-discharge battery tests at C/10 showed excellent battery performance with values of 150 mAh g^-1 after 50 cycles, regardless of the polymer matrix and IL used. In the rate performance tests, the best SPE was the one based on the P(VDF-TrFE-CFE) host polymer, with a discharge value at C-rate of 98.7 mAh g^-1, as it promoted ionic dissociation. This study proves for the first time the suitability of P(VDF-TrFE-CFE) as SPE in lithium-ion batteries, showing the relevance of the proper selection of the polymer matrix, IL type, and lithium salt in the formulation of the ternary SPE, in order to optimize solid-state battery performance. In particular, the enhancement of the ionic conductivity provided by the IL and the effect of the high dielectric constant polymer P(VDF-TrFE-CFE) in improving battery cyclability in a wide range of discharge rates must be highlighted.

Ambient temperature liquid salt electrolytes

Alkali metal salts usually have high melting points due to strong electrostatic interactions and solvents are needed to create ambient temperature liquid electrolytes. Here, we report on six phosphate-anion-based alkali metal salts, Li/Na/K, all of which are liquids at room temperature, with glass transition temperatures ranging from -61 to -29 °C, and are thermally stable up to at least 225 °C. While the focus herein is on various physico-chemical properties, these salts also exhibit high anodic stabilities, up to 6 V vs. M/M⁺ (M = Li/Na/K), and deliver some battery performance - at elevated temperatures as there are severe viscosity limitations at room-temperature. While the battery performance arguably is sub-par, solvent-free electrolytes based on alkali metal salts such as these should pave the way for conceptually different Li/Na/K-batteries, either by refined anion design or by using several salts to create eutectic mixtures.

Influence of alkali metals on water dynamics inside imidazolium-based ionic liquid nano-domains

The global need to expand the design of energy-storage devices led to the investigation of alkali metal - Ionic Liquid (IL) mixtures as a possible class of electrolytes. In this study, 1D and 2D Nuclear Magnetic Resonance (NMR) and Electrochemical Impedance Spectroscopy (EIS) as well as Molecular Dynamics (MD) simulations were used to study the intermolecular interactions in imidazolium-based IL - water - alkali halide ternary mixtures. The 1H and 23Na 1D and 1H DOSY NMR spectra revealed that the presence of small quantities of NaCl does not influence the aggregation of water molecules in the IL nano-domains. The order of adding ionic compounds to water, as well as the certain water and NaCl molecular ratios, lead to the formation of isolated water clusters. Two ternary solutions representing different orders of compounds mixing (H2O+ IL + NaCl or H2O+ NaCl + IL) showed a strong dependence of the initial solvation shell of Na+ and the self-clustering of water. Furthermore, the behaviour of water was found to be independent from the conditions applied during the solution preparation, such as temperature and/or duration of stirring and aging. These findings could be confirmed by large differences in the amount of ionic species, observed in the ternary solutions and depending on the order of mixing/solute preparation.

Controlling Li+ transport in ionic liquid electrolytes through salt content and anion asymmetry: a mechanistic understanding gained from molecular dynamics simulations

In this work, we report the results from molecular dynamics simulations of lithium salt-ionic liquid electrolytes (ILEs) based either on the symmetric bis[(trifluoromethyl)sulfonyl]imide (TFSI^-) anion or its asymmetric analogue 2,2,2-(trifluoromethyl)sulfonyl-N-cyanoamide (TFSAM^-). Relating lithium's coordination environment to anion mean residence times and diffusion constants confirms the remarkable transport behaviour of the TFSAM^--based ILEs that has been observed in recent experiments: for increased salt doping, the lithium ions must compete for the more attractive cyano over oxygen coordination and a fragmented landscape of solvation geometries emerges, in which lithium appears to be less strongly bound. We present a novel, yet statistically straightforward methodology to quantify the extent to which lithium and its solvation shell are dynamically coupled. By means of a Lithium Coupling Factor (LCF) we demonstrate that the shell anions do not constitute a stable lithium vehicle, which suggests for this electrolyte material the commonly termed "vehicular" lithium transport mechanism could be more aptly pictured as a correlated, flow-like motion of lithium and its neighbourhood. Our analysis elucidates two separate causes why lithium and shell dynamics progressively decouple with higher salt content: on the one hand, an increased sharing of anions between lithium limits the achievable LCF of individual lithium-anion pairs. On the other hand, weaker binding configurations naturally entail a lower dynamic stability of the lithium-anion complex, which is particularly relevant for the TFSAM^--containing ILEs.

Superionicity in Ionic-Liquid-Based Electrolytes Induced by Positive Ion-Ion Correlations

In ionic-liquid (IL)-based electrolytes, relevant for current energy storage applications, ion transport is limited by strong ion-ion correlations, generally yielding inverse Haven ratios (ionicities) of below 1. In particular, Li is transported in anionic clusters into the wrong direction of the electric field, requiring compensation by diffusive anion fluxes. Here, we present a concept to exploit ion-ion correlations in concentrated IL electrolytes beneficially by designing organic cations with a Li-coordinating chain. ¹H NMR and Raman spectra show that IL cations with seven or more ether oxygens in the side chain induce Li coordination to organic cations. An unusual behavior of an inverse Haven ratio of >1 is found, suggesting an ionicity larger than that of an ideal electrolyte with uncorrelated ion motion. This superionic behavior is consistently demonstrated in both NMR transport/conductivity measurements and molecular dynamics (MD) simulations. Key to this achievement is the formation of long-lived Li-IL cation complexes, which invert the Li drift direction, yielding positive Li⁺ ion mobilities for the first time in a single IL-solvent-based electrolyte. Onsager correlation coefficients are derived from MD simulations and demonstrate that the main contributions to the inverse Haven ratio, which induce superionicity, arise from enhanced Li-IL cation correlations and a sign inversion of Li-anion correlation coefficients. Thus, the novel concept of coordinating cations not only corrects the unfortunate anionic drift direction of Li in ILs but even exploits strong ion correlations in the concentrated electrolyte toward superionic transport.

Extremely Fast Interfacial Li Ion Dynamics in Crystalline LiTFSI Combined with EMIM-TFSI

Materials providing fast transport pathways for ionic charge carriers are at the heart of future all-solid state batteries that completely rely on sustainable, nonflammable solid electrolytes. The mobile ions in fast ion conductors may take benefit from structural disorder, cation and anion substitution, or dimensionality effects. While these effects concern the bulk regions of a given material, one may also manipulate the surface or interfacial regions of a polycrystalline poorly conducting electrolyte to enhance its transport properties. Here, we used ⁷Li NMR to characterize interfacial effects in crystalline lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) to which a small amount of ionic liquid EMIM-TFSI (EMIM: 1-ethyl-3-methylimidazolium cation, C₆H₁₁N₂ ⁺) was added. We recorded longitudinal spin-lattice relaxation (SLR) curves M _z (t _d) that directly mirror the ⁷Li spin-fluctuations controlled by motional processes in such ionic-liquids-in-salt composites. Already at room temperature we observe strongly bimodal buildup curves M _z (t _d) leading to two distinct SLR rates differing by a factor of 100. While the slower rate does exactly reflect the temperature behavior expected for poorly conducting LiTFSI, the faster rate mirrors rapid motional processes that are governed by an activation energy as low as 73 meV. We attribute these fast processes to highly mobile Li⁺ ions in or near the contact area of crystalline LiTFSI and EMIM-TFSI. By using a method that characterizes motional processes from the atomic-scale point of view, we emphasize the importance of interfacial regions as reservoirs for fast Li⁺ ions in such solid composite electrolytes.

Application of Ionic Liquids for Batteries and Supercapacitors

Nowadays, the rapid development and demand of high-performance, lightweight, low cost, portable/wearable electronic devices in electrical vehicles, aerospace, medical systems, etc., strongly motivates researchers towards advanced electrochemical energy storage (EES) devices and technologies. The electrolyte is also one of the most significant components of EES devices, such as batteries and supercapacitors. In addition to rapid ion transport and the stable electrochemical performance of electrolytes, great efforts are required to overcome safety issues due to flammability, leakage and thermal instability. A lot of research has already been completed on solid polymer electrolytes, but they are still lagging for practical application. Over the past few decades, ionic liquids (ILs) as electrolytes have been of considerable interest in Li-ion batteries and supercapacitor applications and could be an important way to make breakthroughs for the next-generation EES systems. The high ionic conductivity, low melting point (lower than 100 °C), wide electrochemical potential window (up to 5-6 V vs. Li+/Li), good thermal stability, non-flammability, low volatility due to cation-anion combinations and the promising self-healing ability of ILs make them superior as "green" solvents for industrial EES applications. In this short review, we try to provide an overview of the recent research on ILs electrolytes, their advantages and challenges for next-generation Li-ion battery and supercapacitor applications.

Competitive Salt Precipitation/Dissolution During Free-Water Reduction in Water-in-Salt Electrolyte

Water-in-salt electrolytes based on highly concentrated bis(trifluoromethyl)sulfonimide (TFSI) promise aqueous electrolytes with stabilities nearing 3 V. However, especially with an electrode approaching the cathodic (reductive) stability, cycling stability is insufficient. While stability critically relies on a solid electrolyte interphase (SEI), the mechanism behind the cathodic stability limit remains unclear. Now, two distinct reduction potentials are revealed for the chemical environments of free and bound water and that both contribute to SEI formation. Free water is reduced about 1 V above bound water in a hydrogen evolution reaction (HER) and is responsible for SEI formation via reactive intermediates of the HER; concurrent LiTFSI precipitation/dissolution establishes a dynamic interface. The free-water population emerges, therefore, as the handle to extend the cathodic limit of aqueous electrolytes and the battery cycling stability.

Phase-Inversion Polymer Composite Separators Based on Hexagonal Boron Nitride Nanosheets for High-Temperature Lithium-Ion Batteries

By preventing electrical contact between anode and cathode electrodes while promoting ionic transport, separators are critical components in the safe operation of rechargeable battery technologies. However, traditional polymer-based separators have limited thermal stability, which has contributed to catastrophic thermal runaway failure modes that have conspicuously plagued lithium-ion batteries. Here, we describe the development of phase-inversion composite separators based on carbon-coated hexagonal boron nitride (hBN) nanosheets and poly(vinylidene fluoride) (PVDF) polymers that possess high porosity, electrolyte wettability, and thermal stability. The carbon-coated hBN nanosheets are obtained through a scalable liquid-phase shear exfoliation method using ethyl cellulose as a polymer stabilizer and source of the carbon coating following thermal pyrolysis. When incorporated within the PVDF matrix, the carbon-coated hBN nanosheets promote favorable interfacial interactions during the phase-inversion process, resulting in porous, flexible, free-standing composite separators. The unique chemical composition of these carbon-coated hBN separators implies high wettability for a wide range of liquid electrolytes. This combination of high porosity and electrolyte wettability enables enhanced ionic conductivity and lithium-ion battery electrochemical performance that exceeds incumbent polyolefin separators over a wide range of operating conditions. The hBN nanosheets also impart high thermal stability, providing safe lithium-ion battery operation up to 120 °C.

Computational Investigation of Mixed Anion Effect on Lithium Coordination and Transport in Salt Concentrated Ionic Liquid Electrolytes

The use of high concentrations of alkali metal ion salts in ionic liquids (ILs) has been demonstrated to significantly improve electrolyte performance, increase alkali metal ion transference numbers, and promote the formation of favorable SEI structures enabling long-term stable cycling. One challenge in using this material is the overall low ionic conductivity, which is a common effect of increased salt concentration. This simulation work first investigated the strategy of using mixed anions to tune the ionic conductivity in a concentrated IL (or "ionic liquid-in-salt") system having 50 mol % lithium salt. The effects of binding strength, size, and mobility of selected anions on coordination and dynamics of lithium ions were discussed. The results confirm its feasibility and provide general guidance for the selection of anions to improve the ionic conductivity of salt-concentrated electrolyte systems based on ionic liquids and other solvent systems.

Strong and Flexible Composite Solid Polymer Electrolyte Membranes for Li-Ion Batteries

A composite solid polymer electrolyte (CSPE) is studied in this work to alleviate the concerns associated with poor mechanical strength of a solid polymer electrolyte (SPE) system composed of poly(ethyleneglycol)diacrylate, an electrolyte lithium bis(trifluoromethane)sulfonamide, and a plasticizer succinonitrile. CSPE is fabricated by incorporating the ingredients of SPE in the macroporous membranes of syndiotactic polystyrene to render flexibility and mechanical robustness with a 6-fold increase in tensile strength over SPE. The data from differential scanning calorimetry and wide-angle X-ray diffraction confirm the amorphous nature of the polymeric domains of SPE that produce high room-temperature ionic conductivity of ∼0.43 mS/cm. The flexible CSPE membranes are used as the electrolyte in Li-ion battery (LIB) half cells in conjunction with lithium iron phosphate as the counter electrode. The use of CSPE helps expand the electrochemical window of the cell to 5 V, indicating strong potential in the fabrication of flexible rechargeable LIBs.

Concentrated Ionic-Liquid-Based Electrolytes for High-Voltage Lithium Batteries with Improved Performance at Room Temperature

Ionic liquids (ILs) have been widely explored as alternative electrolytes to combat the safety issues associated with conventional organic electrolytes. However, hindered by their relatively high viscosity, the electrochemical performances of IL-based cells are generally assessed at medium-to-high temperature and limited cycling rate. A suitable combination of alkoxy-functionalized cations with asymmetric imide anions can effectively lower the lattice energy and improve the fluidity of the IL material. The Li/Li_1.2 Ni_0.2 Mn_0.6 O₂ cell employing N-N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium (fluorosulfonyl)(trifluoromethanesulfonyl)imide (DEMEFTFSI)-based electrolyte delivered an initial capacity of 153 mAh g^-1 within the voltage range of 2.5-4.6 V, with a capacity retention of 65.5 % after 500 cycles and stable coulombic efficiencies exceeding 99.5 %. Moreover, preliminary battery tests demonstrated that the drawbacks in terms of rate capability could be improved by using Li-concentrated IL-based electrolytes. The improved room-temperature rate performance of these electrolytes was likely owing to the formation of Li⁺ -containing aggregate species, changing the concentration-dependent Li-ion transport mechanism.

Glassy Dynamics and Translational-Rotational Coupling of an Ionically Conducting Pharmaceutical Salt-Sodium Ibuprofen

In this work, we study the structural dipolar relaxation and ionic conductivity relaxation in an ionized derived from a nonionized glass former. The latter is the salt form of a well-studied active pharmaceutical ingredient, sodium ibuprofen, and the former is ibuprofen. Quantum mechanical calculations were employed to study the variation in its molecular electrostatic potentials, and its spatial extent on its salt formation with Na⁺ ions. Measurements have been made using differential scanning calorimetry and broadband dielectric spectroscopy, and the characterization is assisted by density functional theory. The dielectric data contain information on both ionic and dipolar molecular mobility of NaIb and were extracted by representation in terms of the electric modulus and permittivity. A secondary β-conductivity relaxation coexists with the primary α-conductivity relaxation. By use of the coupling model, we show that the β-conductivity relaxation is connected to the α-conductivity relaxation and is the analogue of the relation of the Johari-Goldstein β-relaxation to the structural α-relaxation, shown valid also in ibuprofen. This remarkable result has an impact on the fundamental understanding of the dynamics of ionic conductivity. By representing the data as permittivity, a dipolar β-relaxation was found to have practically the same relaxation times as the β-conductivity relaxation in the glassy state and translational-rotational coupling is valid at a more local secondary relaxation level. However, the α-conductivity relaxation decouples from structural α-relaxation because the structural glass transition temperature is lower than the conductivity counterpart by 29 K. These are novel findings. The study elucidates the effects on the dynamics by the change in the nature of bonding and in size on introducing sodium ions to ibuprofen in the glassy and supercooled liquid states.

Water-in-Bisalt Electrolyte with Record Salt Concentration and Widened Electrochemical Stability Window

Water-in-salt and water-in-bisalt electrolytes have recently attracted much attention due to their expanded electrochemical stability windows. The concentration limit of such electrolytes is constrained by the solubility of the lithium salts employed, ca. 21 m (mol kg^-1) for LiTFSI (lithium bis(trifluoromethanesulfonyl)imide). By adding a second lithium salt, the total salt concentration can be increased, but the hydrogen evolution keeps limiting the application of such systems in batteries with low potential anodes. Herein we report a water-in-bisalt electrolyte with a record salt concentration (31.4 m LiTFSI + 7.9 m Li[N(CH₃)₂((CH₂)₃SO₃)((CH₂)₄SO₃)]) in which the bulky anion completely prevents the crystallization, even at such low water contents. Although the hydrogen evolution reaction is not completely suppressed, the expanded electrochemical stability window allows for low potential reactions such as aluminum-lithium alloying. The high salt concentration favors the formation of a suitable passivation layer that can be further engineered by modifying the anion structure.

Application of Operando X-ray Diffractometry in Various Aspects of the Investigations of Lithium/Sodium-Ion Batteries

The main challenges facing rechargeable batteries today are: (1) increasing the electrode capacity; (2) prolonging the cycle life; (3) enhancing the rate performance and (4) insuring their safety. Significant efforts have been devoted to improve the present electrode materials as well as to develop and design new high performance electrodes. All of the efforts are based on the understanding of the materials, their working mechanisms, the impact of the structure and reaction mechanism on electrochemical performance. Various operando/in-situ methods are applied in studying rechargeable batteries to gain a better understanding of the crystal structure of the electrode materials and their behaviors during charge-discharge under various conditions. In the present review, we focus on applying operando X-ray techniques to investigate electrode materials, including the working mechanisms of different structured materials, the effect of size, cycling rate and temperature on the reaction mechanisms, the thermal stability of the electrodes, the degradation mechanism and the optimization of material synthesis. We demonstrate the importance of using operando/in-situ XRD and its combination with other techniques in examining the microstructural changes of the electrodes under various operating conditions, in both macro and atomic-scales. These results reveal the working and the degradation mechanisms of the electrodes and the possible side reactions involved, which are essential for improving the present materials and developing new materials for high performance and long cycle life batteries.

Cation effect on small phosphonium based ionic liquid electrolytes with high concentrations of lithium salt

Ionic liquid electrolytes with high alkali salt concentrations have displayed some excellent electrochemical properties, thus opening up the field for further improvements to liquid electrolytes for lithium or sodium batteries. Fundamental computational investigations into these high concentration systems are required in order to gain a better understanding of these systems, yet they remain lacking. Small phosphonium-based ionic liquids with high concentrations of alkali metal ions have recently shown many promising results in experimental studies, thereby prompting us to conduct further theoretical exploration of these materials. Here, we conducted a molecular dynamics simulation on four small phosphonium-based ionic liquids with 50 mol. % LiFSI salt, focusing on the effect of cation structure on local structuring and ion diffusional and rotational dynamics-which are closely related to the electrochemical properties of these materials.

Connection between Lithium Coordination and Lithium Diffusion in [Pyr12O1 ][FTFSI] Ionic Liquid Electrolytes

The use of highly concentrated ionic liquid-based electrolytes results in improved rate capability and capacity retention at 20 °C compared to Li⁺ -dilute systems in Li-metal and Li-ion cells. This work explores the connection between the bulk electrolyte properties and the molecular organization to provide insight into the concentration dependence of the Li⁺ transport mechanisms. Below 30 mol %, the Li⁺ -containing species are primarily smaller complexes (one Li⁺ cation) and the Li⁺ ion transport is mostly derived from the vehicular transport. Above 30 mol %, where the viscosity is substantially higher and the conductivity lower, the Li⁺ -containing species are a mix of small and large complexes (one and more than one Li⁺ cation, respectively). The overall conduction mechanism likely changes to favor structural diffusion through the exchange of anions in the first Li⁺ solvation shell. The good rate performance is likely directly influenced by the presence of larger Li⁺ complexes, which promote Li⁺ -ion transport (as opposed to Li⁺ -complex transport) and increase the Li⁺ availability at the electrode.

Molecular dynamics and the translational-rotational coupling of an ionically conducting glass-former: amlodipine besylate

We studied the conductivity relaxation originating from a glass-former composed of cations and anions, and the relation to the structural α-relaxation at temperatures above and below the glass transition temperature. The material chosen was amorphous amlodipine besylate (AMB), which is also a pharmaceutical with a complex chemical structure. Measurements were made using differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS) and X-ray diffraction, and the characterization was assisted using density functional theory (DFT). The X-ray diffraction pattern confirms the amorphous nature of vitrified AMB. Both the ionic and dipolar aspects of the dynamics of AMB were examined using these measurements and were used to probe the nature of the secondary conductivity and dipolar relaxations and their relation to the conductivity α-relaxation and the structural α-relaxation. The coupling model predictions and quantum mechanical simulations were used side by side to reveal the properties and nature of the secondary conductivity relaxation and the secondary dipolar relaxation. Remarkably, the two secondary relaxations have the same relaxation times, and are one and the same process performing dual roles in conductivity and dipolar relaxations. This is caused by the translation-rotation coupling of the AMB molecule. Thus, AMB has both conductivity α- and β-relaxations, and application of the coupling model shows that these two relaxations are related in the same way as the structural α-relaxation and the Johari-Goldstein β-relaxation are. This important result has an impact on the fundamental understanding of the dynamics of ionic conductivity.

Relevance of ion clusters for Li transport at elevated salt concentrations in [Pyr12O1][FTFSI] ionic liquid-based electrolytes

In binary ionic liquid/Li salt mixtures with the novel asymmetric anion FTFSI, electrophoretic mobility μi values of all ion species were determined using electrophoretic NMR. Li was determined to migrate in negatively charged Li-anion clusters towards the anode. This vehicular transport mechanism was shown to have decreasing relevance at elevated salt concentrations.

"Solvent-in-salt" systems for design of new materials in chemistry, biology and energy research

Inorganic and organic "solvent-in-salt" (SIS) systems have been known for decades but have attracted significant attention only recently. Molten salt hydrates/solvates have been successfully employed as non-flammable, benign electrolytes in rechargeable lithium-ion batteries leading to a revolution in battery development and design. SIS with organic components (for example, ionic liquids containing small amounts of water) demonstrate remarkable thermal stability and tunability, and present a class of admittedly safer electrolytes, in comparison with traditional organic solvents. Water molecules tend to form nano- and microstructures (droplets and channel networks) in ionic media impacting their heterogeneity. Such microscale domains can be employed as microreactors for chemical and enzymatic synthesis. In this review, we address known SIS systems and discuss their composition, structure, properties and dynamics. Special attention is paid to the current and potential applications of inorganic and organic SIS systems in energy research, chemistry and biochemistry. A separate section of this review is dedicated to experimental methods of SIS investigation, which is crucial for the development of this field.

Solid-State Lithium Conductors for Lithium Metal Batteries Based on Electrospun Nanofiber/Plastic Crystal Composites

Organic ionic plastic crystals (OIPCs) are a class of solid-state electrolytes with good thermal stability, non-flammability, non-volatility, and good electrochemical stability. When prepared in a composite with electrospun polyvinylidene fluoride (PVdF) nanofibers, a 1:1 mixture of the OIPC N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([C₂ mpyr][FSI]) and lithium bis(fluorosulfonyl)imide (LiFSI) produced a free-standing, robust solid-state electrolyte. These high-concentration Li-containing electrolyte membranes had a transference number of 0.37(±0.02) and supported stable lithium symmetric-cell cycling at a current density of 0.13 mA cm^-2 . The effect of incorporating PVdF in the Li-containing plastic crystal was investigated for different ratios of PVdF and [Li][FSI]/[C₂ mpyr][FSI]. In addition, Li|LiNi_1/3 Co_1/3 Mn_1/3 O₂ cells were prepared and cycled at ambient temperature and displayed a good rate performance and stability.

Rapid Thermal Annealing of Cathode-Garnet Interface toward High-Temperature Solid State Batteries

High-temperature batteries require the battery components to be thermally stable and function properly at high temperatures. Conventional batteries have high-temperature safety issues such as thermal runaway, which are mainly attributed to the properties of liquid organic electrolytes such as low boiling points and high flammability. In this work, we demonstrate a truly all-solid-state high-temperature battery using a thermally stable garnet solid-state electrolyte, a lithium metal anode, and a V₂O₅ cathode, which can operate well at 100 °C. To address the high interfacial resistance between the solid electrolyte and cathode, a rapid thermal annealing method was developed to melt the cathode and form a continuous contact. The resulting interfacial resistance of the solid electrolyte and V₂O₅ cathode was significantly decreased from 2.5 × 10⁴ to 71 Ω·cm² at room temperature and from 170 to 31 Ω·cm² at 100 °C. Additionally, the diffusion resistance in the V₂O₅ cathode significantly decreased as well. The demonstrated high-temperature solid-state full cell has an interfacial resistance of 45 Ω·cm² and 97% Coulombic efficiency cycling at 100 °C. This work provides a strategy to develop high-temperature all-solid-state batteries using garnet solid electrolytes and successfully addresses the high contact resistance between the V₂O₅ cathode and garnet solid electrolyte without compromising battery safety or performance.

Nanostructured Ceramics: Ionic Transport and Electrochemical Activity

Ceramics with nm-sized dimensions are widely used in various applications such as batteries, fuel cells or sensors. Their oftentimes superior electrochemical properties as well as their capabilities to easily conduct ions are, however, not completely understood. Depending on the method chosen to prepare the materials, nanostructured ceramics may be equipped with a large area fraction of interfacial regions that exhibit structural disorder. Elucidating the relationship between microscopic disorder and ion dynamics as well as electrochemical performance is necessary to develop new functionalized materials. Here, we highlight some of the very recent studies on ion transport and electrochemical properties of nanostructured ceramics. Emphasis is put on TiO

N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide-electrospun polyvinylidene fluoride composite electrolytes: characterization and lithium cell studies

LiFSI doped [C₂mpyr][FSI]–PVdF composites were developed as solid-state, self-standing electrolyte membranes.

Elucidation of transport mechanism and enhanced alkali ion transference numbers in mixed alkali metal–organic ionic molten salts

This work reveals how structure facilitates diffusion of the Li/Na ion in ionic liquids with the high Li/Na concentration.

Sulfonic Acid- and Lithium Sulfonate-Grafted Poly(Vinylidene Fluoride) Electrospun Mats As Ionic Liquid Host for Electrochromic Device and Lithium-Ion Battery

Electrospun polymer nanofibrous mats loaded with ionic liquids (ILs) are promising nonvolatile electrolytes with high ionic conductivity. The large cations of ILs are, however, difficult to diffuse into solid electrodes, making them unappealing for application in some electrochemical devices. To address this issue, a new strategy is used to introduce proton conduction into an IL-based electrolyte. Poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) copolymer is functionalized with sulfonic acid through covalent attachment of taurine. The sulfonic acid-grafted P(VDF-HFP) electrospun mats consist of interconnected nanofibers, leading to remarkable improvement in dimensional stability of the mats. IL-based polymer electrolytes are prepared by immersing the modified mats in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM(+)BF4(-)). It is found that the SO3(-) groups can have Lewis acid-base interactions with the cations (BMIM(+)) of IL to promote the dissociation of ILs, and provide additional proton conduction, resulting in significantly improved ionic conductivity. Using this novel electrolyte, polyaniline-based electrochromic devices show higher transmittance contrast and faster switching behavior. Furthermore, the sulfonic acid-grafted P(VDF-HFP) electrospun mats can also be lithiated, giving additional lithium ion conduction for the IL-based electrolyte, with which Li/LiCoO2 batteries display enhanced C-rate performance.

Ionic liquid based lithium battery electrolytes: fundamental benefits of utilising both TFSI and FSI anions?

Synergetic effects of FSI–TFSI mixed electrolytes?