High-Voltage Resistant Ionic Liquids for Lithium-Ion Batteries

Fluorinated Linkers Enable High-Voltage Pyrrolidinium-based Dicationic Ionic Liquid Electrolytes

Novel fluorinated, pyrrolidinium-based dicationic ionic liquids (FDILs) as high-performance electrolytes in energy storage devices have been prepared, displaying unprecedented electrochemical stabilities (up to 7 V); thermal stability (up to 370 °C) and ion transport (up to 1.45 mS cm-1). FDILs were designed with a fluorinated ether linker and paired with TFSI/FSI counterions. To comprehensively assess the impact of the fluorinated spacer on their electrochemical, thermal, and physico-chemical properties, a comparison with their non-fluorinated counterparts was conducted. With a specific focus on their application as electrolytes in next-generation high-voltage lithium-ion batteries, the impact of the Li-salt on the characteristics of dicationic ILs was systematically evaluated. The incorporation of a fluorinated linker demonstrates significantly superior properties compared to their non-fluorinated counterparts, presenting a promising alternative towards next-generation high-voltage energy storage systems.

Dynamics and thermochemistry of the negatively charged clusters in a 2-hydroxyethylhydrazinium nitrate ionic liquid system

The formation and fragmentation of negatively charged 2-hydroxyethylhydrazinium nitrate ([HOCH2CH2NH2NH2]+NO3-, HEHN) ionic liquid clusters were examined using a guided-ion beam tandem mass spectrometer furnished with collision-induced dissociation of selected ions with Xe atoms. Measurements included the compositions of cluster ions formed in the ionization source, and the dissociation products, cross sections, and 0 K threshold energies for individually selected cluster ions. To identify the structures of the main cluster ion series [(HEHN)n(HNO3)0-1NO3]- formed, molecular dynamics simulations were employed to create initial geometry guesses, followed by optimization at the ωB97XD/6-31+G(d,p) level of theory, from which global minimum structures were identified for reaction thermodynamics analyses. A comparison was made between the cluster formation and fragmentation in the negatively charged 2-hydroxyethylhydrazinium nitrate with those in the positive mode (reported by W. Zhou et al., Phys. Chem. Chem. Phys., 2023, 25, 17370). In both modes, the cluster ions were predominantly composed of m/z below 350; loss of a neutral 2-hydroxyethylhydrazinium nitrate ion pair represents the most important cluster fragmentation pathway, followed by intra-ion pair proton transfer-mediated 2-hydroxyethylhydrazine and HNO3 elimination; and all clusters started to dissociate at threshold energies less than 1.5 eV. The overwhelming similarities in the formation and fragmentation chemistry of positively vs. negatively charged 2-hydroxyethylhydrazinium nitrate clusters may be attributed to their inherent ionic nature and high electric conductivities.

Ionic liquid electrolyte selection for high voltage supercapacitors in high-temperature applications

Systematic analyses of electrolyte physicochemical properties are important to screen ionic liquids (ILs) and understand the electrochemical performance of supercapacitor electrolytes. This study harmonizes the evaluation of electrochemical performance and transport properties of eight shortlisted ILs from 22 commercially available hydrophobic ILs toward achieving a ≥ 5 V supercapacitor capable of high-temperature operation (up to 353.15 K). The eight ILs are N-Propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([Pyr 1, 3] [TFSI], N-Pentyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([Pyr 1, 5] [TFSI]), N-Propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([Pyr 1, 3] [FSI]), 1-Methyl-1-(2-methoxyethyl)pyrrolidinium Bis(trifluoromethanesulfonyl)imide ([Pyr 1, 102] [TFSI]), 1-Methyl-1-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([Pip 1, 3] [TFSI]), 1-Methyl-1-propylpiperidinium bis(fluorosulfonyl)imide ([Pip 1, 3] [FSI]), N-Trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide ([N 111, 3] [TFSI]), N-Trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide ([N 111, 6] [TFSI]). The density, viscosity, and ionic conductivity of the eight ILs were measured between 278.15 and 373.15 K to confirm the effects of temperature and ion structure before electrochemical characterization. The [FSI]-based ILs ([Pip 1, 3] [FSI] and [Pyr 1, 3] [FSI]) showed lower densities and viscosities compared to other ILs among the eight based on [TFSI]. Consequently, the highest conductivity was obtained for [Pyr 1, 3] [FSI]. Cyclic voltammetry and impedance spectroscopy was performed on supercapacitors assembled with the eight ILs as electrolytes between 298.15-353.15 K. Conclusion from the two-electrode supercapacitors using multi-walled carbon nanotubes showed the 6 most-applicable ILs towards the targeted ≥ 5 V SC at high temperature are [Pip 1, 3] [TFSI] (5.4 V), [Pip 1, 3] [FSI] (5 V), [N 111, 3] [TFSI] (5.1 V), [N 111, 6] [TFSI] (5.2 V), [Pyr 1, 102] [TFSI] (5.2 V), and [Pyr 1, 5] [TFSI] (5.2 V).

Molecular dynamics study of fluorosulfonyl ionic liquids as electrolyte for electrical double layer capacitors

The development of high-performance supercapacitors is an important goal in the field of energy storage. Ionic liquids (ILs) are promising electrolyte materials for efficient energy storage in supercapacitors, because of the high stability, low volatility, and wider electrochemical stability window than traditional electrolytes. However, ILs-based supercapacitors usually show a relatively lower power density owing to the inherent viscosity-induced low electrical conductivity. Fluorosulfonyl ILs have aroused much attention in energy storage devices due to its low toxicity and excellent stability. Here, we propose that structural modification is an effective way to improve the energy storage performance of fluorosulfonyl ILs through the classical molecular dynamics (MD) method. Four fluorosulfonyl ILs with different sizes and symmetries were considered. Series of properties including conductivity, interface structure, and double-layer capacitance curves were systematically investigated. The results show that smaller size and more asymmetric structure can enhance self-diffusion coefficient and conductivity, and improve the electrochemical performance. Appropriate modification of the electrodes can further enhance the capacitive performance. Our work provides an opportunity to further understand and develop the fluorosulfonyl ILs electrolyte in supercapacitors.

Exploring ionic liquid-laden metal-organic framework composite materials as hybrid electrolytes in metal (ion) batteries

This review focuses on the combination of metal-organic frameworks (MOFs) and ionic liquids (ILs) to obtain composite materials to be used as solid electrolytes in metal-ion battery applications. Benefiting from the controllable chemical composition, tunable pore structure and surface functionality, MOFs offer great opportunities for synthesizing high-performance electrolytes. Moreover, the encapsulation of ILs into porous materials can provide environmentally benign solid-state electrolytes for electrochemical devices. Due to the versatility of MOF-based materials, in this review we also explore their use as anodes and cathodes in Li- and Na-ion batteries. Finally, solid IL@MOF electrolytes and their implementation into Li and Na batteries have been analyzed, as well as the design and advanced manufacturing of solid IL@MOF electrolytes embedded on polymeric matrices.

Ionic Liquid-Type Additive for Lithium Metal Batteries Operated in LiPF6 Based-Electrolyte Containing 2500 ppm H2O

The presence of trace amounts of moisture in the electrolyte can cause hydrolysis of LiPF₆ and deteriorate the stability of lithium metal batteries. Herein, we propose a multifunctional ionic liquid-type additive constituting a 1-methyl-1-butyl pyrrolidium cation (Py₁₄⁺) and an acetate anion (CH₃COO^-) (denoted as IL-AC in this study), which can effectively adsorb the trace moisture and thus prevent the hydrolysis of LiPF₆ via intermolecular interactions. The prepared IL-AC can also remove HF to suppress the dissolution of transition metal ions from cathode materials through the reaction CH₃COO^- + HF → CH₃COOH + F^-. Compared with the baseline electrolyte, the contents of HF and transition metal ions are significantly lower in the electrolyte with 0.5% IL-AC. Upon the addition of 0.5% IL-AC additive and 2500 ppm H₂O, the Li||NCM811 battery shows a capacity of 153.7 mAh g^-1 after 300 cycles, while the Li||LNMO battery possesses stable capacity retention of 93.22% after 500 cycles at 1C and a Coulombic efficiency greater than 99%. Thus, this work provides a convenient and effective method to absorb trace amounts of water and remove HF in the electrolyte and provides a new path for the expensive and tedious process of water removal from the electrolyte in industry.

Impact of Low Temperatures on the Lithiation and Delithiation Properties of Si-Based Electrodes in Ionic Liquid Electrolytes

Lithium-ion batteries are used in various extreme environments, such as cold regions and outer space; thus, improvements in energy density, safety, and cycle life in these environments are urgently required. We investigated changes in the charge and discharge properties of Si-based electrodes in ionic liquid electrolytes with decreasing temperature and the cycle life at low temperature. The reversible capacity at low temperature was determined by the properties of the surface film on the electrodes and/or the ionic conductivity of the electrolytes. The electrode coated with a surface film formed at a low temperature exhibited insufficient capacity. In contrast, a Si-only electrode precoated with the surface film at room temperature exhibited a cycle life at low temperatures in ionic liquid electrolytes longer than that in conventional organic liquid electrolytes. Doping phosphorus into Si led to improved cycling performance, and its impact was more noticeable at lower temperatures.

Challenges for Safe Electrolytes Applied in Lithium-Ion Cells-A Review

The aspect of safety in electronic devices has turned out to be a huge challenge for the world of science. Thus far, satisfactory power and energy densities, efficiency, and cell capacities have been achieved. Unfortunately, the explosiveness and thermal runaway of the cells prevents them from being used in demanding applications such as electric cars at higher temperatures. The main aim of this review is to highlight different electrolytes used in lithium-ion cells as well as the flammability aspect. In the paper, the authors present liquid inorganic electrolytes, composite polymer-ceramic electrolytes, ionic liquids (IL), polymeric ionic liquids, polymer electrolytes (solvent-free polymer electrolytes (SPEs), gel polymer electrolytes (GPEs), and composite polymer electrolytes (CPEs)), and different flame retardants used to prevent the thermal runaway and combustion of lithium-ion batteries (LIBs). Additionally, various flame tests used for electrolytes in LIBs have been adopted. Aside from a detailed description of the electrolytes consumed in LIBs. Last section in this work discusses hydrogen as a source of fuel cell operation and its practical application as a global trend that supports green chemistry.

Ionic Liquid Electrolyte-Based Switchable Mirror with Fast Response and Improved Durability

Electrochemically tunable devices based on reversible metal electrodeposition have attracted extensive attention for energy-saving smart windows, information displays, digital signage, and variable reflectance mirrors, owing to their excellent optical modulation characteristics, low operation voltage, and superb electrochemical stability. Here, we study the effects of ionic liquid (IL)-based electrolytes on electrodeposition of the reversible electrochemical mirrors (REMs) by changing the organic cations of the ILs to obtain devices with the desired spectroelectrochemical and electrodeposited properties. Spectroelectrochemical measurements and scanning electron microscopy images show that organic cations drastically affect the switching speed and cycling durability, which we proposed on the basis of the difference in the absorption energies between cations and Ag(111) surfaces. Higher adsorption energy indicates strong adhesion between organic cations and Ag(111) surfaces, and this strong adsorption would prevent aggregation and agglomeration during the nucleation of Ag nanoparticles (AgNPs), leading to a denser and more compact electrodeposited Ag film and faster switching speeds (3.3 s for coloring and 14.3 s for bleaching). These findings allow us to fabricate dynamic devices that exhibit reversibly switchable light modulation at fast switching speeds and excellent cycling stability over thousands of cycles without attenuation. The combination of rapid switching and durable cycling stability enables tunable windows, which are based on reversible electrodeposition of metal Ag and IL-based electrolytes, make REM devices a competitive and promising alternative to traditional intelligent response materials.

Spectroscopic and Computational Study of Boronium Ionic Liquids and Electrolytes

Boronium cation-based ionic liquids (ILs) have demonstrated high thermal stability and a >5.8 V electrochemical stability window. Additionally, IL-based electrolytes containing the salt LiTFSI have shown stable cycling against the Li metal anode, the "Holy grail" of rechargeable lithium batteries. However, the basic spectroscopic characterisation needed for further development and effective application is missing for these promising ILs and electrolytes. In this work, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and density functional theory (DFT) calculations are used in combination to characterise four ILs and electrolytes based on the [NNBH₂ ]⁺ and [(TMEDA)BH₂ ]⁺ boronium cations and the [FSI]^- and [TFSI]^- anions. By using this combined experimental and computational approach, proper understanding of the role of different ion-ion interactions for the Li cation coordination environment in the electrolytes was achieved. Furthermore, the calculated vibrational frequencies assisted in the proper mode assignments for the ILs and in providing insights into the spectroscopic features expected at the interface created when they are adsorbed on a Li(001) surface. A reproducible synthesis procedure for [(TMEDA)BH₂ ]⁺ is also reported. The fundamental findings presented in this work are beneficial for any future studies that utilise IL based electrolytes in next generation Li metal batteries.

Triethylammonium Hydrogen Sulfate [Et3NH][HSO4]-Catalyzed Rapid and Efficient Multicomponent Synthesis of Pyrido[2,3-d]pyrimidine and Pyrazolo[3,4-b]pyridine Hybrids

An operationally simple, one-pot multicomponent reaction has been developed for the assembly of pyrido[2,3-d]pyrimidine and pyrazolo[3,4-b]pyridine derivatives (4a-4am) in excellent yields (92-94%) with high purity. The reactions were easy to perform simply by mixing of electron-rich amino heterocycles (including aminouracils and aminopyrazoles), aldehyde, and acyl acetonitrile in the presence of [Et3NH][HSO4] under solvent-free conditions. The remarkable feature of the present approach is that the ionic liquid possesses dual solvent-catalytic engineering capability. Results of this study revealed that 1 mmol of the ionic liquid catalyst under solvent-free conditions at 60 °C is the best reaction parameter for the construction of fused pyridine and pyrimidine derivatives in excellent yields. The present methodology showed good results under gram-scale conditions, thereby indicating its applicability in industrial as well as academic settings in the near future.

Novel piperidinium-based ionic liquid as electrolyte additive for high voltage lithium-ion batteries

Conventional carbonate-based electrolyte is prone to oxidative decomposition at high voltage (over 4.5 V vs. Li/Li+), which leads to the bad oxidation stability and inferior cycling performance of lithium ion batteries (LIBs). To solve these problems, a novel ionic liquid (IL) N-butyronitrile-N-methylpiperidinium bis(fluorosulfonyl)imide (PP1,CNFSI) was synthesized and explored as the additive to the LiPF6-ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte. For the cell performance, the addition of PP1,CNFSI not only inhibits overcharge phenomenon, but also improves discharge capacity, thus enhancing capacity retention capability. Compared to the cell with blank electrolyte, the capacity retentions of adding 15 wt% PP1,CNFSI into the electrolyte were improved to 96.8% and 97% from 82.8% and 78.7% at 0.2 C and 5 C, respectively. The effects of PP1,CNFSI on the LNMO cathode surface were further investigated by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It reveals that PP1,CNFSI addition drives the formation of solid electrolyte interphase (SEI) film which suppresses oxidative decomposition of the electrolyte and protects the structure cathode material.

Lithiation/Delithiation Properties of Lithium Silicide Electrodes in Ionic-Liquid Electrolytes

We investigated the impact of electrolyte difference on lithiation and delithiation properties of a Li_1.00Si electrode to improve the Coulombic efficiency (CE) of Si-based electrodes. The results of X-ray diffraction, Raman spectroscopy, and soft X-ray emission spectroscopy demonstrated that a portion of the Li in Li_1.00Si desorbed by simply immersing the electrode in an ionic-liquid electrolyte, that is, the phase transition of Li_1.00Si to Si occurred. In contrast, this phenomenon was not confirmed in an organic-liquid electrolyte. Instead, the desorbed Li was consumed for the formation of a surface film; thus, the Li in Li_1.00Si did not elute into the electrolyte. The addition of vinylene carbonate (VC) to the ionic-liquid electrolyte suppressed the phase transition of Li_1.00Si to Si. Although the Li_1.00Si electrode showed a low initial CE and poor cycling performance in a VC-free electrolyte, the electrode exhibited a high CE and a remarkable cycle life in the VC-added electrolyte. It was considered that no desorption of the mechanically added Li in Li_1.00Si contributed to the superior cycle life; thus, the characteristic ductility, malleability, and high electrical conductivity of lithium silicide should improve the electrochemical performance.

Reaction Behavior of a Silicide Electrode with Lithium in an Ionic-Liquid Electrolyte

Silicides are attractive novel active materials for use in the negative-electrodes of next-generation lithium-ion batteries that use certain ionic-liquid electrolytes; however, the reaction mechanism of the above combination is yet to be clarified. Possible reactions at the silicide electrode are as follows: deposition and dissolution of Li metal on the electrode, lithiation and delithiation of Si, which would result from the phase separation of the silicide, and alloying and dealloying of the silicide with Li. Herein, we examined these possibilities using various analysis methods. The results revealed that the lithiation and delithiation of silicide occurred.

Self-Powered and Green Ionic-Type Thermoelectric Paper Chips for Early Fire Alarming

Early fire alarming is of vital importance to lower the damages led by forest fires. Thus far, methods to monitor the forest fires at their early stage are mainly focused on artificial ground patrol, unmanned aerial vehicle cruise monitoring, observation by watchtower, or satellite inspection, whereas these methods are practically encountered with the problems of untimely feedback before the forest fires are out of control. This work proposes a particular kind of self-powered, low-cost, and green thermoelectric paper chips based on the principle of self-assembly and disassembly of ionic liquids on the surface of gold electrodes. By adjustment of the species of ionic liquids, both "n- and p-type" thermoelectric behaviors have been exploited that correspond to the opposite open-circuit voltages. Owing to the fluidic nature of ionic liquids, those "n- and p-type" thermoelectric units can be readily connected in series on one paper chip, leading to remarkable voltage signals in the presence of the temperature difference of 35 K. Followed by signal acquisition and transmission, such a thermoelectric paper chip successfully affords immediate electrical alarming at the early stage of an afire circumstance.

Electrolyte Therapy for Improving the Performance of LiNi0.5Mn1.5O4 Cathodes Assembled Lithium-Ion Batteries

High voltage spinel manganese oxide LiNi₀.₅Mn_1.5O₄ (LNMO) cathodes are promising for practical applications owing to several strengths including high working voltages, excellent operating safety, low costs, and so on. However, LNMO-based lithium-ion batteries (LIBs) fade rapidly mainly owing to unqualified electrolytes, hence becoming a big obstacle toward practical applications. To tackle this roadblock, substantial progress has been made thus far, and yet challenges still remain, while rare reviews have systematically discussed the status quo and future development of electrolyte optimization coupling with LNMO cathodes. Here, we discuss cycling degradation mechanisms at the cathode/electrolyte interface and ideal requirements of electrolytes for LNMO cathode-equipped LIBs, as well as review the recent advance of electrolyte optimization for LNMO cathode-equipped LIBs in detail. And then, the perspectives regarding the future research opportunities in developing state-of-the-art electrolytes are also presented. The authors hope to shed light on the rational optimization of advanced organic electrolytes in order to boost the large-scale practical applications of high voltage LNMO cathode-based LIBs.