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

Synergistic Combination of Cross-Linked Polymer and Concentrated Ionic Liquid for Electrolytes with High Stability in Solid-State Lithium Metal Batteries

Poly(ethylene oxide)-(PEO-based solid polymer electrolytes (SPEs) are regarded as excellent candidates for solid-state lithium metal batteries (SSLMBs) due to their inherent safety advantages, processability, low cost, and excellent Li+ ion solvation. However, they suffer from limited oxidation stability (up to 4 V vs Li+/Li). In this study, a crosslinked polymer-in-concentrated ionic liquid (PCIL) SPE consisting of PEO, N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI) ionic liquid (IL), and lithium bis(fluorosulfonyl)imide (LiFSI) salt is developed. The adopted UV-crosslinking strategy synergistically reduces PEO crystallinity while increasing the amount of encompassed lithium salt and IL and improves PEO oxidative stability, therefore leading to enhanced electrochemical performance. The physical and electrochemical properties of both linear and crosslinked SPEs are explored and compared. The designed cross-linked SPEs exhibited a promisingly high oxidative stability of 4.9 V vs Li+/Li and high ambient temperature ionic conductivity of 4 × 10-4 S cm-1. Stable and reversible lithium plating/stripping is demonstrated in symmetrical Li||Li cells over hundreds of hours. High-loading solid-state lithium iron phosphate (LFP)||Li cells show favorable cycling with over 90% capacity retention at 0.1C over 100 cycles at 50 °C. High voltage solid-state lithium manganese oxide (LMO)||Li cells exhibit promising cycling with a 93% capacity retention at a 0.2 C rate over 50 cycles at 50 °C. Thus, the combination of concentrated ionic liquid electrolytes in a crosslinked PEO-based matrix enables a pathway for designing high-performing SPEs for high energy density solid-state LMBs.

Poly(Ionic Liquid) Electrolytes at an Extreme Salt Concentration for Solid-State Batteries

Polymer-in-salt electrolytes were introduced three decades ago as an innovative solution to the challenge of low Li-ion conductivity in solvent-free solid polymer electrolytes. Despite significant progress, the approach still faces considerable challenges, ranging from a fundamental understanding to the development of suitable polymers and salts. A critical issue is maintaining both the stability and high conductivity of molten salts within a polymer matrix, which has constrained their further exploration. This research offers a promising solution by integrating cationic poly(ionic liquids) (polyIL) with a crystallization-resistive salt consisting of asymmetric anions. A stable polymer-in-salt electrolyte with an exceptionally high Li-salt content of up to 90 mol % was achieved, providing a valuable opportunity for the in-depth understanding of these electrolytes at an extremely high salt concentration. This work explicates how increased salt concentration affects coordination structures, glass transitions, ionic conductivity, and the decoupling and coupling of ion transport from structural dynamics in a polymer electrolyte, ultimately enhancing electrolyte performance. These findings provide significant knowledge advancement in the field, guiding the future design of polymer-in-salt electrolytes.

Long-life potassium metal batteries enabled by anion-derived solid electrolyte interphase using concentrated ionic liquid electrolytes

Potassium metal batteries (PMBs) show great potential as next-generation energy storage systems yet face challenges such as the dendritic growth of the potassium anode, leading to issues with cycle life and safety. This study reports a potassium salt-concentrated ionic liquid electrolyte (PCIL) consisting of potassium bis(fluorosulfonyl)imide (KFSI) and 1-methyl-1-propyl pyrrolidinium bis(fluorosulfonyl)imide (Pyr13FSI) to achieve long-life and, safe PMBs. PCIL presents several advantages including outstanding oxidation stability (≈5.2 V), decent ionic conductivity (4.0 mS cm-1 at 25 °C), and negligible flammability. Moreover, PCIL promotes the development of anion-derived solid-electrolyte interphase (SEI) with high inorganic content. This not only hinders the growth of potassium dendrites but also facilitates facile interfacial charge transfer kinetics. Benefiting from these advantages, PMBs (K||KVPO4F) employing PCIL exhibit remarkable cycle performances at both ambient and elevated temperatures (capacity retention after 300 cycles: 74.8% at 25 °C and 82.9% at 45 °C), surpassing the performance of conventional carbonate (1 M KPF6 EC/PC) and dilute potassium ionic liquid electrolyte (PIL). This work demonstrates the tangible capability of PCIL in realizing practical PMBs.

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.

Machine learning-guided discovery of ionic polymer electrolytes for lithium metal batteries

As essential components of ionic polymer electrolytes (IPEs), ionic liquids (ILs) with high ionic conductivity and wide electrochemical window are promising candidates to enable safe and high-energy-density lithium metal batteries (LMBs). Here, we describe a machine learning workflow embedded with quantum calculation and graph convolutional neural network to discover potential ILs for IPEs. By selecting subsets of the recommended ILs, combining with a rigid-rod polyelectrolyte and a lithium salt, we develop a series of thin (~50 μm) and robust (>200 MPa) IPE membranes. The Li|IPEs|Li cells exhibit ultrahigh critical-current-density (6 mA cm^-2) at 80 °C. The Li|IPEs|LiFePO₄ (10.3 mg cm^-2) cells deliver outstanding capacity retention in 350 cycles (>96% at 0.5C; >80% at 2C), fast charge/discharge capability (146 mAh g^-1 at 3C) and excellent efficiency (>99.92%). This performance is rarely reported by other single-layer polymer electrolytes without any flammable organics for LMBs.

Locally Concentrated Ionic Liquid Electrolytes for Lithium-Metal Batteries

Non-flammable ionic liquid electrolytes (ILEs) are well-known candidates for safer and long-lifespan lithium metal batteries (LMBs). However, the high viscosity and insufficient Li⁺ transport limit their practical application. Recently, non-solvating and low-viscosity co-solvents diluting ILEs without affecting the local Li⁺ solvation structure are employed to solve these problems. The diluted electrolytes, i.e., locally concentrated ionic liquid electrolytes (LCILEs), exhibiting lower viscosity, faster Li⁺ transport, and enhanced compatibility toward lithium metal anodes, are feasible options for the next-generation high-energy-density LMBs. Herein, the progress of the recently developed LCILEs are summarised, including their physicochemical properties, solution structures, and applications in LMBs with a variety of high-energy cathode materials. Lastly, a perspective on the future research directions of LCILEs to further understanding and achieve improved cell performances is outlined.

Cationic polymer-in-salt electrolytes for fast metal ion conduction and solid-state battery applications

Polymer electrolytes provide a safe solution for future solid-state high-energy-density batteries. Materials that meet the simultaneous requirement of high ionic conductivity and high transference number remain a challenge, in particular for new battery chemistries beyond lithium such as Na, K and Mg. Herein, we demonstrate the versatility of a polymeric ionic liquid (PolyIL) as a polymer solvent to achieve this goal for both Na and K. Using molecular simulations, we predict and elucidate fast alkali metal ion transport in PolyILs through a structural diffusion mechanism in a polymer-in-salt environment, facilitating a high metal ion transference number simultaneously. Experimental validation of these computationally designed Na and K polymer electrolytes shows good ionic conductivities up to 1.0 × 10^-3 S cm^-1 at 80 °C and a Na⁺ transference number of ~0.57. An electrochemical cycling test on a Na∣2:1 NaFSI/PolyIL∣Na symmetric cell also demonstrates an overpotential of 100 mV at a current density of 0.5 mA cm^-2 and stable long-term Na plating/stripping performance of more than 100 hours. PolyIL-based polymer-in-salt strategies for new solid-state electrolytes thus offer an alternative route to design high-performance next-generation sustainable battery chemistries.

Bridging Database and Experimental Analysis to Reveal Super-hydrodynamic Conductivity Scaling Regimes in Ionic Liquids

Ion transport through electrolytes critically impacts the performance of batteries and other devices. Many frameworks used to model ion transport assume hydrodynamic mechanisms and focus on maximizing conductivity by minimizing viscosity. However, solid-state electrolytes illustrate that non-hydrodynamic ion transport can define device performance. Increasingly, selective transport mechanisms, such as hopping, are proposed for concentrated electrolytes. However, viscosity-conductivity scaling relationships in ionic liquids are often analyzed with hydrodynamic models. We report data-centric analyses of hydrodynamic transport models of viscosity-conductivity scaling in ionic liquids by merging three databases to bridge physical properties and computational descriptors. With this expansive database, we constrained scaling analyses using ion sizes defined from simulated volumes, as opposed to estimating sizes from activity coefficients. Remarkably, we find that many ionic liquids exhibit positive deviations from the Nernst-Einstein model, implying ions move faster than hydrodynamics should allow. We verify these findings using microrheology and conductivity experiments. We further show that machine learning tools can improve predictions of conductivity from molecular properties, including predictions from solely computational features. Our findings reveal that many ionic liquids exhibit super-hydrodynamic viscosity-conductivity scaling, suggesting mechanisms of correlated ion motion, which could be harnessed to enhance electrochemical device performance.

Li+ transference number and dynamic ion correlations in glyme-Li salt solvate ionic liquids diluted with molecular solvents

Highly concentrated electrolytes (HCEs) have attracted significant interest as promising liquid electrolytes for next-generation Li secondary batteries, owing to various beneficial properties both in the bulk and at the electrode/electrolyte interface. One particular class of HCEs consists of binary mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and oligoethers that behave like ionic liquids. [Li(G4)][TFSA], which comprises an equimolar mixture of LiTFSA and tetraglyme (G4), is an example. In our previous works, the addition of low-polarity molecular solvents to [Li(G4)][TFSA] was found to effectively enhance the conductivity while retaining the unique Li-ion solvation structure. However, it remains unclear how the diluents affect another key electrolyte parameter-the Li⁺ transference number-despite its critical importance for achieving the fast charging/discharging of Li secondary batteries. Thus, in this study, the effects of diluents on the extremely low Li⁺ transference number under anion-blocking conditions in [Li(G4)][TFSA] were elucidated, with a special focus on the polarity of the additional solvents. The concentration dependence of the dynamic ion correlations was further studied in the framework of the concentrated electrolyte theory. The results revealed that a non-coordinating diluent is not involved in the modification of the ion transport mechanism, and therefore the low Li⁺ transference number is inherited by the diluted electrolytes. In contrast, a coordinating diluent effectively reduces the anti-correlated ion motions of [Li(G4)][TFSA], thereby improving the Li⁺ transference number. This is the first time that the significant effects of the coordination properties of the diluting solvents on the dynamic ion correlations and Li⁺ transference numbers have been reported for diluted solvate ionic liquids.

Synthesis, Physical Properties and Electrochemical Applications of Two Ionic Liquids Containing the Asymmetric (Fluoromethylsulfonyl)(Trifluoromethylsulfonyl)imide Anion

Novel ionic liquid (IL) electrolytes based on the asymmetric (fluoromethylsulfonyl)(trifluoromethylsulfonyl)imide (FTFSI)− anion, combined with the N-trimethyl-N-butyl-ammonium (N1114)+ and N,N-diethyl-N-methyl-N(2-methoxyethyl)-ammonium (N122(2O1))+ cations, were successfully synthesized and investigated in terms of thermal, vibrational and electrochemical properties. Thermogravimetric measurements revealed that the ionic liquids are stable up to 300 °C (2% mass loss). Differential scanning calorimetry measurements evidenced no phase transition down to −90 °C, suggesting a transition towards a glass state at lower temperatures. Infrared spectroscopy measurements, for the first time performed on ILs containing FTFSI, could not detect any crystallization down to −140 °C. The frequency of the main absorption bands of the ILs are in good agreement with DFT calculations. The FTFSI ionic liquid electrolytes, containing 20% mol of LiTFSI, show no solid-liquid phase transition due to the asymmetry of the FTFSI− anion, increasing the −10 °C conductivity up to 10−4 S cm−1. These interesting ion transport properties remarkably extend the operative temperature range down to low temperatures. The FTFSI electrolytes exhibit remarkable electrochemical stability up to 4.8 V, this making them appealing for realizing safer and highly reliable lithium battery systems operating at high voltages.

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.

Enhanced Li+ Transport in Ionic Liquid-Based Electrolytes Aided by Fluorinated Ethers for Highly Efficient Lithium Metal Batteries with Improved Rate Capability

FSI^- -based ionic liquids (ILs) are promising electrolyte candidates for long-life and safe lithium metal batteries (LMBs). However, their practical application is hindered by sluggish Li⁺ transport at room temperature. Herein, it is shown that additions of bis(2,2,2-trifluoroethyl) ether (BTFE) to LiFSI-Pyr₁₄ FSI ILs can effectively mitigate this shortcoming, while maintaining ILs' high compatibility with lithium metal. Raman spectroscopy and small-angle X-ray scattering indicate that the promoted Li⁺ transport in the optimized electrolyte, [LiFSI]₃ [Pyr₁₄ FSI]₄ [BTFE]₄ (Li₃ Py₄ BT₄ ), originates from the reduced solution viscosity and increased formation of Li⁺ -FSI^- complexes, which are associated with the low viscosity and non-coordinating character of BTFE. As a result, Li/LiFePO₄ (LFP) cells using Li₃ Py₄ BT₄ electrolyte reach 150 mAh g^-1 at 1 C rate (1 mA cm^-2 ) and a capacity retention of 94.6% after 400 cycles, revealing better characteristics with respect to the cells employing the LiFSI-Pyr₁₄ FSI (operate only a few cycles) and commercial carbonate (80% retention after only 218 cycles) electrolytes. A wide operating temperature (from -10 to 40 °C) of the Li/Li₃ Py₄ BT₄ /LFP cells and a good compatibility of Li₃ Py₄ BT₄ with LiNi_0.5 Mn_0.3 Co_0.2 O₂ (NMC532) are demonstrated also. The insight into the enhanced Li⁺ transport and solid electrolyte interphase characteristics suggests valuable information to develop IL-based electrolytes for LMBs.

Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications

Ionic liquids are attractive and safe electrolytes for diverse electrochemical applications such as advanced rechargeable batteries with high energy densities. Their properties that are beneficial for energy storage and conversion include negligible vapor-pressure, intrinsic conductivity as well as high stability. To explore the suitability of a series of ionic liquids with small ammonium cations for potential battery applications, we investigated their thermal and transport properties. We studied the influence of the symmetrical imide-type anions bis(trifluoromethanesulfonyl)imide ([TFSI]−) and bis(fluorosulfonyl)imide ([FSI]−), side chain length and functionalization, as well as lithium salt content on the properties of the electrolytes. Many of the samples are liquid at ambient temperature, but their solidification temperatures show disparate behavior. The transport properties showed clear trends: the dynamics are accelerated for samples with the [FSI]− anion, shorter side chains, ether functionalization and lower amounts of lithium salts. Detailed insight was obtained from the diffusion coefficients of the different ions in the electrolytes, which revealed the formation of aggregates of lithium cations coordinated by anions. The ionic liquid electrolytes exhibit sufficient stability in NMC/Li half-cells at elevated temperatures with small current rates without the need of additional liquid electrolytes, although Li-plating was observed. Electrolytes containing [TFSI]− anions showed superior stability compared to those with [FSI]− anions in battery tests.

Acidic Ionic Liquids Enabling Intermediate Temperature Operation Fuel Cells

Herein we show that protic ionic liquids (PILs) are promising electrolytes for fuel cells operating in the temperature range 100-120 °C. N,N-Diethyl-N-methyl-3-sulfopropan-1-ammonium hydrogen sulfate ([DEMSPA][HSA]), N,N-diethyl-N-methyl-3-sulfopropan-1-ammonium triflate ([DEMSPA][TfO]), N,N-diethyl-3-sulfopropan-1-ammonium hydrogen sulfate ([DESPA][HSA]), and N,N-diethyl-3-sulfopropan-1-ammonium triflate ([DESPA][TfO]) are investigated in this study with regard to their specific conductivity, thermal stability, viscosity, and electrochemical properties. The [DEMSPA][TfO] and [DESPA][TfO] electrolytes offer high limiting current densities for the oxygen reduction reaction (ORR) on platinum electrodes, that is, about 1 order of magnitude larger than 98% H₃PO₄. This is explained by the minor poisoning of the Pt catalyst and the significantly larger product of the oxygen self-diffusion coefficient and concentration in these two PILs.

Anion effects on Li ion transference number and dynamic ion correlations in glyme-Li salt equimolar mixtures

To achieve single-ion conducting liquid electrolytes for the rapid charge and discharge of Li secondary batteries, improvement in the Li+ transference number of the electrolytes is integral. Few studies have established a feasible design for achieving Li+ transference numbers approaching unity in liquid electrolytes consisting of low-molecular-weight salts and solvents. Previously, we studied the effects of Li+-solvent interactions on the Li+ transference number in glyme- and sulfolane-based molten Li salt solvates and clarified the relationship between this transference number and correlated ion motions. In this study, to deepen our insight into the design principles of single-ion conducting liquid electrolytes, we focused on the effects of Li+-anion interactions on Li ion transport in glyme-Li salt equimolar mixtures with different counter anions. Interestingly, the equimolar triglyme (G3)-lithium trifluoroacetate (Li[TFA]) mixture ([Li(G3)][TFA]) demonstrated a high Li+ transference number, estimated via the potentiostatic polarization method (tPPLi = 0.90). Dynamic ion correlation studies suggested that the high tPPLi could be mainly ascribed to the strongly coupled Li+-anion motions in the electrolytes. Furthermore, high-energy X-ray total scattering measurements combined with all-atom molecular dynamics simulations showed that Li+ ions and [TFA] anions aggregated into ionic clusters with a relatively long-range ion-ordered structure. Therefore, the collective motions of the Li ions and anions in the form of highly aggregated ion clusters, which likely diminish rather than enhance ionic conductivity, play a significant role in achieving high tPPLi in liquid electrolytes. Based on the dynamic ion correlations, a potential design approach is discussed to accomplish single-ion conducting liquid electrolytes with high ionic conductivity.

Pyrrolidinium Containing Ionic Liquid Electrolytes for Li-Based Batteries

Ionic liquids are potential alternative electrolytes to the more conventional solid-state options under investigation for future energy storage solutions. This review addresses the utilization of IL electrolytes in energy storage devices, particularly pyrrolidinium-based ILs. These ILs offer favorable properties, such as high ionic conductivity and the potential for high power drain, low volatility and wide electrochemical stability windows (ESW). The cation/anion combination utilized significantly influences their physical and electrochemical properties, therefore a thorough discussion of different combinations is outlined. Compatibility with a wide array of cathode and anode materials such as LFP, V₂O₅, Ge and Sn is exhibited, whereby thin-films and nanostructured materials are investigated for micro energy applications. Polymer gel electrolytes suitable for layer-by-layer fabrication are discussed for the various pyrrolidinium cations, and their compatibility with electrode materials assessed. Recent advancements regarding the modification of typical cations such a 1-butyl-1-methylpyrrolidinium, to produce ether-functionalized or symmetrical cations is discussed.

UV-Cured Poly(Siloxane-Urethane)-Based Polymer Composite Materials for Lithium Ion Batteries-The Effect of Modification with Ionic Liquids

The results of studies on the synthesis and characterization of conductive polymer composite materials designed as potential separators for lithium ion batteries are presented. The conductive polymer composites were prepared from UV-cured poly(siloxane-urethanes)s (PSURs) containing poly(ethylene oxide) (PEO) segments and modified with lithium salts and ionic liquids (ILs). The most encouraging results in terms of specific conductivity and mechanical properties of the composite were obtained when part of UV-curable PSUR prepolymer was replaced with a reactive UV-curable IL. Morphology of the composites modified with ILs or containing a standard ethylene carbonate/dimethyl carbonate mixture (EC/DMC) as solvent was compared. It was found that the composites showed a two-phase structure that did not change when non-reactive ILs were applied instead of EC/DMC but was much affected when reactive UV-curable ILs were used. The selected IL-modified UV-cured PSUR composite that did not contain flammable EC/DMC solvent was preliminarily tested as gel polymer electrolyte and separator for lithium ion batteries.

Structural and Ion Dynamics in Fluorine-Free Oligoether Carboxylate Ionic Liquid-Based Electrolytes

Here, we investigate the physicochemical and electrochemical properties of fluorine-free ionic liquid (IL)-based electrolytes with two different cations, tetrabutylphosphonium, (P4,4,4,4)+, and tetrabutylammonium, (N4,4,4,4)+, coupled to a new anion, 2-[2-(2-methoxyethoxy)ethoxy]acetate anion (MEEA)-, for both neat and (P4,4,4,4)(MEEA) also doped with 10-40 mol % of Li(MEEA). We find relatively weaker cation-anion interactions in (P4,4,4,4)(MEEA) than in (N4,4,4,4)(MEEA), and for both ILs, the structural flexibility of the oligoether functionality in the anion results in low glass transition temperatures, also for the electrolytes made. The pulsed field gradient nuclear magnetic resonance (PFG NMR) data suggest faster diffusion of the (MEEA)- anion than (P4,4,4,4)+ cation in the neat IL, but the addition of a Li salt results in slightly lower mobility of the former than the latter and lower ionic conductivity. This agrees with the combined 7Li NMR and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy data, which unambiguously reveal preferential interactions between the lithium cations and the carboxylate groups of the IL anions, which also increased as a function of the lithium salt concentration. In total, these systems provide a stepping stone for further design of fluorine-free and low glass transition temperature IL-based electrolytes and also stress how crucial it is to control the strength of ion-ion interactions.

Engineering high-energy-density sodium battery anodes for improved cycling with superconcentrated ionic-liquid electrolytes

Non-uniform metal deposition and dendrite formation in high-density energy storage devices reduces the efficiency, safety and life of batteries with metal anodes. Superconcentrated ionic-liquid electrolytes (for example 1:1 ionic liquid:alkali ion) coupled with anode preconditioning at more negative potentials can completely mitigate these issues, and therefore revolutionize high-density energy storage devices. However, the mechanisms by which very high salt concentration and preconditioning potential enable uniform metal deposition and prevent dendrite formation at the metal anode during cycling are poorly understood, and therefore not optimized. Here, we use atomic force microscopy and molecular dynamics simulations to unravel the influence of these factors on the interface chemistry in a sodium electrolyte, demonstrating how a molten-salt-like structure at the electrode surface results in dendrite-free metal cycling at higher rates. Such a structure will support the formation of a more favourable solid electrolyte interphase, accepted as being a critical factor in stable battery cycling. This new understanding will enable engineering of efficient anode electrodes by tuning the interfacial nanostructure via salt concentration and high-voltage preconditioning.

Solvent effects on Li ion transference number and dynamic ion correlations in glyme- and sulfolane-based molten Li salt solvates

The Li⁺ transference number of electrolytes is one of the key factors contributing to the enhancement in the charge-discharge performance of Li secondary batteries. However, a design principle to achieve a high Li⁺ transference number has not been established for liquid electrolytes. To understand the factors governing the Li⁺ transference number t_Li, we investigated the influence of the ion-solvent interactions, Li ion coordination, and correlations of ion motions on the Li⁺ transference number in glyme (Gn, n = 1-4)- and sulfolane (SL)-based molten Li salt solvate electrolytes with lithium bis(trifluoromethansulfonyl)amide (LiTFSA). For the 1 : 1 tetraglyme-LiTFSA molten complex, [Li(G4)][TFSA], the Li⁺ transference number estimated using the potentiostatic polarisation method (t = 0.028) was considerably lower than that estimated using the self-diffusion coefficient data with pulsed filed gradient (PFG)-NMR (t = 0.52). The dynamic ion correlations (i.e., cation-cation, anion-anion, and cation-anion cross-correlations) were determined from the experimental data on the basis of Roling and Bedrov's concentrated solution theory, and the results suggest that the strongly negative cross-correlations of the ion motions (especially for cation-cation motions) are responsible for the extremely low t of [Li(G4)][TFSA]. In contrast, t is larger than t in the SL-based electrolytes. The high t of the SL-based electrolytes was ascribed to the substantially weaker anti-correlations of cation-cation and cation-anion motions. Whereas the translational motions of the long-lived [Li(glyme)]⁺ and [TFSA]^- dominate the ionic conduction for [Li(G4)][TFSA], Li ion hopping/exchange conduction was reported to be prevalent in the SL-based electrolytes. The unique Li ion conduction mechanism is considered to contribute to the less correlated cation-cation and cation-anion motions in SL-based electrolytes.

High-Voltage Resistant Ionic Liquids for Lithium-Ion Batteries

With the growing demand for high energy and high power density rechargeable lithium-ion batteries, increasing research is focused on improving the output voltage of these batteries. Herein, a series of pyrrolidinium and piperidinium cations with various N-substituents (including cyanomethyl, benzyl, butyl, hexyl, and octyl groups) were synthesized and investigated with respect to their electrochemical stability under high voltages. The influence of substitutions at the N-position of pyrrolidinium and piperidinium cations on their high-voltage resistance was studied by both theoretical and experimental approaches. The voltage resistance was enhanced as the electron-donating ability of the substitutes increased. Furthermore, 1-hexyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([C₆Py][TFSI]) exhibited the highest decomposition voltage at approximately 5.12 V and showed promising potential in a lithium-ion battery.

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.