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

Pyrrolidium- and Imidazolium-Based Ionic Liquids and Electrolytes with Flexible Oligoether Anions

A new class of fluorine-free ionic liquids (ILs) and electrolytes based on aliphatic flexible oligoether anions, 2-(2-methoxyethoxy)acetate (MEA) and 2-[2-(2-methoxyethoxy)ethoxy]acetate (MEEA), coupled with pyrrolidinium and imidazolium cations is introduced. For the ILs with MEEA anions, Li+ conducting electrolytes are created by doping the ILs with 30 mol % of LiMEEA. The structural flexibility of the oligoether functionality in the anion results in glass transition temperatures (Tg) as low as -60 °C for the neat ILs and the electrolytes. The imidazolium-based ILs and electrolytes reveal better thermal stabilities but higher Tg and lower electrochemical stabilities than the corresponding pyrrolidinium-based analogues. All neat ILs show comparable transport properties for the cations and these decrease by the addition of lithium salt - the pyrrolidinium-based electrolyte being affected the most.

Building Stable Anodes for High-Rate Na-Metal Batteries

Due to low cost and high energy density, sodium metal batteries (SMBs) have attracted growing interest, with great potential to power future electric vehicles (EVs) and mobile electronics, which require rapid charge/discharge capability. However, the development of high-rate SMBs has been impeded by the sluggish Na+ ion kinetics, particularly at the sodium metal anode (SMA). The high-rate operation severely threatens the SMA stability, due to the unstable solid-electrolyte interface (SEI), the Na dendrite growth, and large volume changes during Na plating-stripping cycles, leading to rapid electrochemical performance degradations. This review surveys key challenges faced by high-rate SMAs, and highlights representative stabilization strategies, including the general modification of SMB components (including the host, Na metal surface, electrolyte, separator, and cathode), and emerging solutions with the development of solid-state SMBs and liquid metal anodes; the working principle, performance, and application of these strategies are elaborated, to reduce the Na nucleation energy barriers and promote Na+ ion transfer kinetics for stable high-rate Na metal anodes. This review will inspire further efforts to stabilize SMAs and other metal (e.g., Li, K, Mg, Zn) anodes, promoting high-rate applications of high-energy metal batteries towards a more sustainable society.

Fluoroborate ionic liquids as sodium battery electrolytes

High-voltage sodium batteries are an appealing solution for economical energy storage applications. Currently available electrolyte materials have seen limited success in such applications therefore the identification of high-performing and safer alternatives is urgently required. Herein we synthesise six novel ionic liquids derived from two fluoroborate anions which have shown great promise in recent battery literature. This study reports for the first time the electrochemically applicable room-temperature ionic liquid (RTIL) N-ethyl-N,N,N-tris(2-(2-methoxyethoxy)ethyl)ammonium (tetrakis)hexafluoroisopropoxy borate ([N2(2O2O1)3][B(hfip)4]). The RTIL shows promising physical properties with a very low glass-transition at -73 °C and low viscosity. The RTIL exhibits an electrochemical window of 5.3 V on a glassy carbon substrate which enables high stability electrochemical cycling of sodium in a 3-electrode system. Of particular note is the strong passivation behaviour of [N2(2O2O1)3][B(hfip)4] on aluminium current-collector foil at potentials as high as 7 V (vs. Na+/Na) which is further improved with the addition of 50 mol% Na[FSI]. This study shows [B(hfip)4]- ionic liquids have the desired physical and electrochemical properties for high-voltage sodium electrolytes.

Nuclear magnetic resonance characterisation of ionic liquids and organic ionic plastic crystals: common approaches and recent advances

Ionic liquids, and their solid-state equivalents organic ionic plastic crystals, show many useful and tailorable properties that make them interesting for a wide range of applications including as electrolytes for energy storage devices. Nuclear magnetic resonance spectroscopy and related techniques offer a powerful and versatile toolkit for the characterisation of structure, interactions and dynamics in these materials. This article summarises both commonly used methods and some recent advances in this area, including solution- and solid-state methods, dynamic nuclear polarisation, imaging, diffusion and relaxation measurements, and example applications of some less commonly studied nuclei.

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.

Application of Magnetic Resonance Techniques to the In Situ Characterization of Li-Ion Batteries: A Review

In situ magnetic resonance (MR) techniques, such as nuclear MR and MR imaging, have recently gained significant attention in the battery community because of their ability to provide real-time quantitative information regarding material chemistry, ion distribution, mass transport, and microstructure formation inside an operating electrochemical cell. MR techniques are non-invasive and non-destructive, and they can be applied to both liquid and solid (crystalline, disordered, or amorphous) samples. Additionally, MR equipment is available at most universities and research and development centers, making MR techniques easily accessible for scientists worldwide. In this review, we will discuss recent research results in the field of in situ MR for the characterization of Li-ion batteries with a particular focus on experimental setups, such as pulse sequence programming and cell design, for overcoming the complications associated with the heterogeneous nature of energy storage devices. A comprehensive approach combining proper hardware and software will allow researchers to collect reliable high-quality data meeting industrial standards.

Tuning Sodium Interfacial Chemistry with Mixed-Anion Ionic Liquid Electrolytes

The interphase layer that forms on either the anode or the cathode is considered to be one of the critical components of a high performing battery. This solid-electrolyte interphase (SEI) layer determines the stability of the electrode in the presence of a given electrolyte as well as the internal resistance of a battery, and hence the overpotential of a cell. In the case of lithium ion batteries where carbonate based electrolytes are used, additives including hexafluorophosphate (PF₆), bis-trifluoromethylsulfonimide (TFSI), (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI), and fluorosulfonimde (FSI) are used to obtain favorable SEI layers. Ionic liquids and salts based on anions containing nitrile groups, including dicyanamide (DCA), offer a less expensive alternative to a fluorinated anion and have also been shown to support stable electrochemistry in lithium and sodium systems. However, longer term cycling leads to the eventual passivation of the electrode, presumed to be due to the instability of the DCA anion. We herein consider the use of a fluorinated anion to control the interfacial electrochemistry and provide a more stable SEI in DCA ILs. We investigate the addition of NaDCA, NaFSI, NaTFSI, and NaFTFSI to the methylpropylpyrrolidinium dicyanamide ([C3mpyr]DCA) ionic liquid. NaFSI was found to generate a more stable SEI layer, as evidenced by extended symmetric cell cycling, while the TFSI and FTFSI salts both lead to thicker, highly passivating surfaces. We use molecular dynamics, infrared spectroscopy and X-ray photoelectron spectroscopy to interrogate and discuss the influence of the anion on the bulk electrolyte, the interfacial electrolyte structure, and the formation of the SEI layer, in order to rationalize the contrasting electrochemical observations.

Electrolyte selection for supercapacitive devices: a critical review

Electrolytes are one of the vital constituents of electrochemical energy storage devices and their physical and chemical properties play an important role in these devices' performance, including capacity, power density, rate performance, cyclability and safety. This article reviews the current state of understanding of the electrode-electrolyte interaction in supercapacitors and battery-supercapacitor hybrid devices. The article discusses factors that affect the overall performance of the devices such as the ionic conductivity, mobility, diffusion coefficient, radius of bare and hydrated spheres, ion solvation, viscosity, dielectric constant, electrochemical stability, thermal stability and dispersion interaction. The requirements needed to design better electrolytes and the challenges that still need to be addressed for building better supercapacitive devices for the competitive energy storage market have also been highlighted.

Electrolytes and Electrolyte/Electrode Interfaces in Sodium-Ion Batteries: From Scientific Research to Practical Application

Sodium-ion batteries (SIBs) have drawn considerable interest as power-storage devices owing to the wide abundance of their constituents and low cost. To realize a high performance-price ratio, the cathode and anode materials must be optimized. As essential components of SIBs, electrolytes should have wide electrochemical windows, high thermal stability, and exceptional ionic conductivity. Therefore, improved electrolytes, based on various materials and compositions, are developed to meet the practical demands of SIBs, including organic electrolytes, ionic liquids, aqueous, solid electrolytes, and hybrid electrolytes. Although mature organic electrolytes are currently used in production, aqueous and solid electrolytes show advantages for future applications, as discussed here in detail. Current efforts in modifying electrolytes to optimize their interfacial compatibility with electrodes, leading to longer battery lifetimes and greater safety, are described. The advanced characterization techniques used to investigate the properties of electrolytes and interfaces are introduced, and the reaction processes and degradation mechanisms of SIBs are revealed. Furthermore, the practical prospects of SIBs promoted by high-quality electrolytes appropriately matched with electrodes are predicted and directions for developing next-generation SIBs are suggested.

High-Safety Nonaqueous Electrolytes and Interphases for Sodium-Ion Batteries

Rapidly developed Na-ion batteries are highly attractive for grid energy storage. Nevertheless, the safety issues of Na-ion batteries are still a bottleneck for large-scale applications. Similar to Li-ion batteries (LIBs), the safety of Na-ion batteries is considered to be tightly associated with the electrolyte and electrode/electrolyte interphase. Although the knowledge obtained from LIBs is helpful, designing safe electrolytes and obtaining stable interphases in Na-ion batteries is still a huge challenge. Therefore, it is of significance to investigate the key factors and develop new strategies for the development of high-safety Na-ion batteries. This comprehensive review introduces the recent efforts from nonaqueous electrolytes and interphase aspects of Na-ion batteries, proposes their design strategies and requirements for improving safety characteristics, and discusses the potential issues for practical applications. The insight to formulate safe electrolytes and design the stable interphase for Na-ion batteries with high safety is intended to be provided herein.

Investigating Intermolecular Interactions in a DME-Based Hybrid Ionic Liquid Electrolyte by HOESY NMR

The intermolecular interactions in a hybrid electrolyte based on various compositions of the ionic liquid N-methyl-N-propyl pyrrolidinium bis-fluorosulfonylimide (C₃mpyrFSI), LiFSI salt and an ether-based additive, 1,2-dimethoxy ethane (DME), have been investigated using the HOESY (Heteronuclear Overhauser Effect SpectroscopY) NMR experiment. This NMR technique allows a quantification of the intermolecular interactions in ionic liquids (ILs) by measuring the cross-relaxation rate (σ) between different pairs of nuclei. Thereby, we compare the cross-relaxation rates between the cations, anions and DME in these hybrid electrolyte systems using ¹H-⁷Li and ¹H-¹⁹F HOESY experiments, and interpret the measured parameters in terms of ionic and molecular associations. The results give insights into the local coordination environment of the Li⁺ cations and their solvation by the FSI anions and DME.

Correlating Intermolecular Cross-Relaxation Rates with Distances and Coordination Numbers in Ionic Liquids

The HOESY NMR experiment is commonly used to probe ion associations in ionic liquids and their mixtures. The parameter measured in this experiment is the heteronuclear cross-relaxation rate σ, which has dimensions of s^-1. For intramolecular NOEs this scales as r^-6 where r is the internuclear distance, but in the intermolecular case (as typically probed in studies of ionic liquids), theory predicts a more complex behavior including a distance dependence that is affected by the relative frequencies of the nuclei involved. Specifically, for nuclei with similar resonance frequencies such as ¹H and ¹⁹F, it has been predicted that intermolecular NOEs will be sensitive to longer range distances than for nuclei with very different frequencies such as ¹H and ⁷Li. In this contribution, we test this theory using a combination of quantitative HOESY analysis and molecular dynamics simulations carried out on two different ionic liquid electrolyte systems. In agreement with theoretical predictions, we find excellent correlations between the experimentally measured ¹H-⁷Li NOEs and carbon-lithium distances below 4 Å, while longer distances (>6 Å) must be considered in order to obtain good correlations between ¹H-¹⁹F NOEs and carbon-fluorine coordination numbers. This demonstrates the utility of HOESY NMR in understanding structure and interactions in ionic liquids while also illustrating that care must be taken in interpreting the measured cross-relaxation rates.

Structural, electrical properties and dielectric relaxations in Na+-ion-conducting solid polymer electrolyte

In this paper, we have studied the structural, microstructural, electrical, dielectric properties and ion dynamics of a sodium-ion-conducting solid polymer electrolyte film comprising PEO₈-NaPF₆+  x wt. % succinonitrile. The structural and surface morphology properties have been investigated, respectively using x-ray diffraction and field emission scanning electron microscopy. The complex formation was examined using Fourier transform infrared spectroscopy, and the fraction of free anions/ion pairs obtained via deconvolution. The complex dielectric permittivity and loss tangent has been analyzed across the whole frequency window, and enables us to estimate the DC conductivity, dielectric strength, double layer capacitance and relaxation time. The presence of relaxing dipoles was determined by the addition of succinonitrile (wt./wt.) and the peak shift towards high frequency indicates the decrease of relaxation time. Further, relations among various relaxation times ([Formula: see text]) have been elucidated. The complex conductivity has been examined across the whole frequency window; it obeys the Universal Power Law, and displays strong dependency on succinonitrile content. The sigma representation ([Formula: see text]) was introduced in order to explore the ion dynamics by highlighting the dispersion region in the Cole-Cole plot ([Formula: see text]) in the lower frequency window; increase in the semicircle radius indicates a decrease of relaxation time. This observation is accompanied by enhancement in ionic conductivity and faster ion transport. A convincing, logical scheme to justify the experimental data has been proposed.

Thermophysical and Electrochemical Properties of Ethereal Functionalised Cyclic Alkylammonium-based Ionic Liquids as Potential Electrolytes for Electrochemical Applications

A series of hydrophobic room temperature ionic liquids (ILs) based on ethereal functionalised pyrrolidinium, piperidinium and azepanium cations bearing the bis[(trifluoromethyl)sulfonyl]imide, [TFSI]- , anion were synthesized and characterized. Their physicochemical properties such as density, viscosity and electrolytic conductivity, and thermal properties including phase transition behaviour and decomposition temperature have been measured. All of the ILs showed low melting point, low viscosity and good conductivity and the latter properties have been discussed in terms of the IL fragility, an important electrolyte feature of the transport properties of glass-forming ILs. Furthermore, the studied [TFSI]- -based ILs generally exhibit good electrochemical stabilities and, by coupling electrochemical experiments and DFT calculations, the effect of ether functionalisation at the IL cation on the electrochemical stability of the IL is discussed. Preliminary investigations into the Li-redox chemistry at a Cu working electrode are also reported as a function of ether-functionality within the pyrrolidinium-based IL family. Overall, the results show that these ionic liquids are suitable for electrochemical devices such as battery systems, fuel cells or supercapacitors.

Ion dynamics in halogen-free phosphonium bis(salicylato)borate ionic liquid electrolytes for lithium-ion batteries

Halogen-free and hydrolytically stable phosphonium bis(salicylato)borate ionic liquid electrolytes for enhanced safety and performance of lithium-ion batteries.

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

We study the beneficial effects of multiple anionic species on the ion dynamics in a sodium-containing ionic liquid electrolyte.