Improving electrochemical properties of room temperature ionic liquid (RTIL) based electrolyte for Li-ion batteries

40 Years of Low-Temperature Electrolytes for Rechargeable Lithium Batteries

Rechargeable lithium batteries are one of the most appropriate energy storage systems in our electrified society, as virtually all portable electronic devices and electric vehicles today rely on the chemical energy stored in them. However, sub-zero Celsius operation, especially below -20 °C, remains a huge challenge for lithium batteries and greatly limits their application in extreme environments. Slow Li+ diffusion and charge transfer kinetics have been identified as two main origins of the poor performance of RLBs under low-temperature conditions, both strongly associated with the liquid electrolyte that governs bulk and interfacial ion transport. In this review, we first analyze the low-temperature kinetic behavior and failure mechanism of lithium batteries from an electrolyte standpoint. We next trace the history of low-temperature electrolytes in the past 40 years (1983-2022), followed by a comprehensive summary of the research progress as well as introducing the state-of-the-art characterization and computational methods for revealing their underlying mechanisms. Finally, we provide some perspectives on future research of low-temperature electrolytes with particular emphasis on mechanism analysis and practical application.

A Review of Nonaqueous Electrolytes, Binders, and Separators for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are the most important electrochemical energy storage devices due to their high energy density, long cycle life, and low cost. During the past decades, many review papers outlining the advantages of state-of-the-art LIBs have been published, and extensive efforts have been devoted to improving their specific energy density and cycle life performance. These papers are primarily focused on the design and development of various advanced cathode and anode electrode materials, with less attention given to the other important components of the battery. The “nonelectroconductive” components are of equal importance to electrode active materials and can significantly affect the performance of LIBs. They could directly impact the capacity, safety, charging time, and cycle life of batteries and thus affect their commercial application. This review summarizes the recent progress in the development of nonaqueous electrolytes, binders, and separators for LIBs and discusses their impact on the battery performance. In addition, the challenges and perspectives for future development of LIBs are discussed, and new avenues for state-of-the-art LIBs to reach their full potential for a wide range of practical applications are outlined.

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The Impact of Polymer Electrolyte Properties on Lithium-Ion Batteries

In recent decades, the enhancement of the properties of electrolytes and electrodes resulted in the development of efficient electrochemical energy storage devices. We herein reported the impact of the different polymer electrolytes in terms of physicochemical, thermal, electrical, and mechanical properties of lithium-ion batteries (LIBs). Since LIBs use many groups of electrolytes, such as liquid electrolytes, quasi-solid electrolytes, and solid electrolytes, the efficiency of the full device relies on the type of electrolyte used. A good electrolyte is the one that, when used in Li-ion batteries, exhibits high Li+ diffusion between electrodes, the lowest resistance during cycling at the interfaces, a high capacity of retention, a very good cycle-life, high thermal stability, high specific capacitance, and high energy density. The impact of various polymer electrolytes and their components has been reported in this work, which helps to understand their effect on battery performance. Although, single-electrolyte material cannot be sufficient to fulfill the requirements of a good LIB. This review is aimed to lead toward an appropriate choice of polymer electrolyte for LIBs.

Ionic Liquid Electrolytes for Electrochemical Energy Storage Devices

For decades, improvements in electrolytes and electrodes have driven the development of electrochemical energy storage devices. Generally, electrodes and electrolytes should not be developed separately due to the importance of the interaction at their interface. The energy storage ability and safety of energy storage devices are in fact determined by the arrangement of ions and electrons between the electrode and the electrolyte. In this paper, the physicochemical and electrochemical properties of lithium-ion batteries and supercapacitors using ionic liquids (ILs) as an electrolyte are reviewed. Additionally, the energy storage device ILs developed over the last decade are introduced.

Deposition of Ion-Conductive Membranes from Ionic Liquids via Initiated Chemical Vapor Deposition

In this study, liquid droplets of 1-allyl-3-methylimidazolium dicyanamide have been processed by initiated chemical vapor deposition (iCVD) with a cross-linked polymer film consisting of (hydroxyethyl)methacrylate and ethylene glycol dimethacrylate to develop free-standing, ion-conductive membranes. We found that the obtained films are solids and have a conductivity of up to 18 ± 6 mS/cm, associated with the negatively charged counterion, indicating no loss of conductivity, compared to the ionic liquid in the liquid state. The membranes were conductive within a large process window and in air, thanks to the fact that the iCVD process does not affect the mobility of the anion in the ionic liquid. Furthermore, we demonstrate that varying the deposition conditions can influence the homogeneity and conductivity of the resulting membranes. The promising results of this study represent an important stepping stone on the way to novel ion-conductive membranes.

Recent Progress in Electrolytes for Zn-Air Batteries

Zn-air battery is considered as one of the most promising candidates for next-generation batteries for energy storage due to safety, high energy density, and low cost. There are many challenges in electrolytes for developing high-performance rechargeable Zn-air cells as well as electrocatalysts. An electrolyte is the crucial part of the rechargeable Zn-air batteries that determine their capacity, cycling stability, and lifetime. This paper reviews the most recent progress in designing and fabricating electrolytes in aqueous and flexible Zn-air batteries. The discussion on the surface reaction relationships was covered between air-catalyst-electrolyte and electrolyte-zinc reaction mechanism. We highlight the recent developments of three different electrolytes in zinc-air battery: aqueous electrolyte, room temperature ionic liquid, and quasi-solid flexible electrolyte. Furthermore, the general perspective is proposed for designing and fabricating electrolytes to improve the performance and prolong the lifetime of Zn-air batteries.

Toward adequate control of internal interfaces utilizing nitrile-based electrolytes

Methods to control internal interfaces in lithium ion batteries often require sophisticated procedures to deposit coating layers or introduce interphases, which are typically difficult to apply. This particularly holds for protection from parasitic reactions at the current collector, which reflects an internal interface for the electrode composite material and the electrolyte. In this work, electrolyte formulations based on aliphatic cyclic nitriles, cyclopentane-1-carbonitrile and cyclohexane-1-carbonitrile, are introduced that allow for successful suppression of aluminum dissolution and control of internal interfaces under application-relevant conditions. Such nitrile-based electrolytes show higher intrinsic oxidative and thermal stabilities as well as similar capacity retentions in lithium nickel-manganese-cobalt oxide LiNi_3/5Mn_1/5Co_1/5O₂ (NMC622)||graphite based full cells compared to the state-of-the-art organic carbonate-based electrolytes, even when bis(trifluoro-methane)sulfonimide lithium salt is utilized. Moreover, the importance of relative permittivity, degree of ion dissociation, and viscosity of the applied electrolyte formulations for the protection of current collector interfaces is emphasized.

Ionic Liquid-Based Electrolytes for Energy Storage Devices: A Brief Review on Their Limits and Applications

Since the ability of ionic liquid (IL) was demonstrated to act as a solvent or an electrolyte, IL-based electrolytes have been widely used as a potential candidate for renewable energy storage devices, like lithium ion batteries (LIBs) and supercapacitors (SCs). In this review, we aimed to present the state-of-the-art of IL-based electrolytes electrochemical, cycling, and physicochemical properties, which are crucial for LIBs and SCs. ILs can also be regarded as designer solvents to replace the more flammable organic carbonates and improve the green credentials and performance of energy storage devices, especially LIBs and SCs. This review affords an outline of the progress of ILs in energy-related applications and provides essential ideas on the emerging challenges and openings that may motivate the scientific communities to move towards IL-based energy devices. Finally, the challenges in design of the new type of ILs structures for energy and environmental applications are also highlighted.

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.

Computational study of halogen-free Boron based dicationic ionic liquids of [bis-Mim][BMB]2 and [bis-Mim][BScB]2

In this paper, the structures and energetics of 1,3-bis[3-methylimidazolium-yl] pentane ([bis-Mim]²⁺) dication and bis(mandelato) borate [BMB]^- and bis(salicylato) borate [BScB]^-anions in isolated forms, ion pairs (IPs) and dicationic ionic liquids (DILs) were studied by Density Functional Theory (DFT) at the M06-2X/6-31G(d,p) level of theory. According to the IUPAC criteria, the hydrogen bonds between anion and cation were characterized and classified in the optimized geometries of the isolated ions, IPs and DILs. Inspection of the optimized structures revealed that the interionic hydrogen binding has important effect on the ions structures. The interaction energies between a dication and anions have been described in terms of NBO charge distribution, the stabilization energy E⁽²⁾ values, changes of vibrational frequencies and the reduced density gradient (RDG) analysis. Also, the reactivity and interactions between chemical species were interpreted in terms of global electronic properties. Electrostatic potential surfaces (ESP) have been applied for visualizing the charge related properties and characterization of the most energetic sites of isolated ions and ion complexes.

A highly concentrated phosphate-based electrolyte for high-safety rechargeable lithium batteries

We prepare a totally nonflammable phosphate-based electrolyte composed of 5 mol L-1 (M) Li bis(fluorosulfonyl) imide (LiFSI) in a trimethyl phosphate (TMP) solvent. The concentrated 5 M LiFSI/TMP electrolyte shows good compatibility with graphite and no Al corrosion. More attractively, such a concentrated electrolyte can effectively suppress the growth of Li dendrites in Li metal batteries because of a stable LiF-rich SEI layer. Therefore, this highly concentrated electrolyte is promising for safe Li batteries.

High Ion-Conducting Solid-State Composite Electrolytes with Carbon Quantum Dot Nanofillers

Solid-state polymer electrolytes (SPEs) with high ionic conductivity are desirable for next generation lithium- and sodium-ion batteries with enhanced safety and energy density. Nanoscale fillers such as alumina, silica, and titania nanoparticles are known to improve the ionic conduction of SPEs and the conductivity enhancement is more favorable for nanofillers with a smaller size. However, aggregation of nanoscale fillers in SPEs limits particle size reduction and, in turn, hinders ionic conductivity improvement. Here, a novel poly(ethylene oxide) (PEO)-based nanocomposite polymer electrolyte (NPE) is exploited with carbon quantum dots (CQDs) that are enriched with oxygen-containing functional groups. Well-dispersed, 2.0-3.0 nm diameter CQDs offer numerous Lewis acid sites that effectively increase the dissociation degree of lithium and sodium salts, adsorption of anions, and the amorphicity of the PEO matrix. Thus, the PEO/CQDs-Li electrolyte exhibits an exceptionally high ionic conductivity of 1.39 × 10-4 S cm-1 and a high lithium transference number of 0.48. In addition, the PEO/CQDs-Na electrolyte has ionic conductivity and sodium ion transference number values of 7.17 × 10-5 S cm-1 and 0.42, respectively. It is further showed that all solid-state lithium/sodium rechargeable batteries assembled with PEO/CQDs NPEs display excellent rate performance and cycling stability.

Formation of different shell structures in lithium-rich layered oxides and their influence on electrochemical properties

A lithium-rich layered oxide with different shell structures was synthesized by a simple wet-chemical surface deposition method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and other techniques were applied to characterize the crystal structure, morphology, and micro-structure of the samples. The surface of the lithium-rich layered oxide can successively produce island-like spinel, ultra-thin spinel, and thick two-phase (spinel and amorphous manganese oxides) separation shell layers with an increase in the coating amount. The formation process of the different shell structures and the effect of the shell structure on the lattice parameters were discussed. The different shell structures play an important role in the electrochemical performance of the lithium-rich oxide. In particular, when the coating amount is 1 wt%, the lithium-rich material with a uniform Li₄Mn₅O₁₂ spinel shell layer exhibits superior electrochemical performance, and can maintain a discharge capacity of 209.9 mA h g⁻¹ and 166.8 mA h g⁻¹ at rates of 2C and 5C.

An ultra-thin spinel Li₄Mn₅O₁₂ shell layer on the surface of lithium-rich oxide particles was investigated.

Cycling stability of Li metal in a mixed carbonate–ionic liquid electrolyte for lithium secondary batteries

Polymeric ionic liquids (PILs) containing a poly(ethylene glycol) methacrylate (POEM) coating layer significantly suppresses the reduction of the ionic liquid and of solvent molecules on Li metal anode in the Pyr14TFSI/carbonate electrolyte.

Physicochemical and electrochemical characterisation of imidazolium based IL + GBL mixtures as electrolytes for lithium-ion batteries

The effect of C(2) methylation of imidazolium ionic liquids on electrochemical stability in electrolytes with GBL on TiO₂ anatase electrodes.

Unusual Li-Ion Transfer Mechanism in Liquid Electrolytes: A First-Principles Study

Liquid electrolytes play an important role in commercial lithium-ion (Li-ion) batteries as a conduit for Li-ion transfer between anodes and cathodes. It is generally believed that the Li-ions move along with the salt ions; thus, Li-ion diffusion is only affected by the viscosity and salt concentration in the liquid electrolytes based on the Stokes-Einstein equation. In this study, a novel and faster Li-ion diffusion mechanism in electrolytes containing a cyanogen group is identified from first-principles molecular dynamics (FPMD) simulations. In this mechanism, the Li-ions are first detached from the Li-salt and then diffuse along with the solvent molecules, and the Li-ion diffusion does not obey the traditional Stokes-Einstein equation. The ionic conductivity of the electrolyte systems with this "solvent-assisted Li-ion diffusion" mechanism is further enhanced through Li-ion hopping. This novel Li-ion diffusion process explains recent findings of high Li-ion conductivity in electrolytes with cyanogen groups and furnishes a new paradigm for the design of fast-charging liquid electrolyte for Li-ion batteries.

Alcohols as molecular probes in ionic liquids: evidence for nanostructuration

A comprehensive study of the solution and solvation of linear alcohols (propan-1-ol, butan-1-ol and pentan-1-ol) in ionic liquids (ILs) is presented.

Ring-chain synergy in ionic liquid electrolytes for lithium batteries

Lithium-ion batteries have been attracting much attention which enables the revolution of wireless global communication. Ionic liquids are regarded as promising candidates for lithium-ion battery electrolytes because they can overcome the limitations of high operating temperatures and flammability concerns of traditional electrolytes. However, at low temperatures they suffer from low ionic conductivity and phase transition. In this paper mixed electrolyte systems are described based on N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide (Pyr_1,2O1TFSI) and lithium difluoro(oxalate)borate (LiODFB) lithium salt, with ethylene sulphite (ES) or dimethyl sulphite (DMS) as a cosolvent. The mixed electrolyte system exhibits good ion transport properties (a conductivity of 8.163 mS cm^-1), a wide electrochemical window (5.2 V), non-flammability, the ability to form films to protect the anode and a large operating temperature range (-40 °C to 60 °C). We compare the performance and function of the new mixed electrolyte system with a variety of ionic liquid/cosolvent electrolyte systems developed in previous studies. The ring-chain synergy takes advantage of the availability of both high permittivities based on the ring-like components and low viscosities based on the chain-like components in the mixed electrolyte system and causes the electrolyte to exhibit a good overall performance in safety, ion transport and compatibility with electrodes.

Interaction of High Flash Point Electrolytes and PE-Based Separators for Li-Ion Batteries

In this study, promising electrolytes for use in Li-ion batteries are studied in terms of interacting and wetting polyethylene (PE) and particle-coated PE separators. The electrolytes are characterized according to their physicochemical properties, where the flow characteristics and the surface tension are of particular interest for electrolyte–separator interactions. The viscosity of the electrolytes is determined to be in a range of η = 4–400 mPa∙s and surface tension is finely graduated in a range of γ_L = 23.3–38.1 mN∙m⁻¹. It is verified that the technique of drop shape analysis can only be used in a limited matter to prove the interaction, uptake and penetration of electrolytes by separators. Cell testing of Li|NMC half cells reveals that those cell results cannot be inevitably deduced from physicochemical electrolyte properties as well as contact angle analysis. On the other hand, techniques are more suitable which detect liquid penetration into the interior of the separator. It is expected that the results can help fundamental researchers as well as users of novel electrolytes in current-day Li-ion battery technologies for developing and using novel material combinations.

Ionic Liquid Hybrid Electrolytes for Lithium-Ion Batteries: A Key Role of the Separator-Electrolyte Interface in Battery Electrochemistry

Batteries are multicomponent systems where the theoretical voltage and stoichiometric electron transfer are defined by the electrochemically active anode and cathode materials. While the electrolyte may not be considered in stoichiometric electron-transfer calculations, it can be a critical factor determining the deliverable energy content of a battery, depending also on the use conditions. The development of ionic liquid (IL)-based electrolytes has been a research area of recent reports by other researchers, due, in part, to opportunities for an expanded high-voltage operating window and improved safety through the reduction of flammable solvent content. The study reported here encompasses a systematic investigation of the physical properties of IL-based hybrid electrolytes including quantitative characterization of the electrolyte-separator interface via contact-angle measurements. An inverse trend in the conductivity and wetting properties was observed for a series of IL-based electrolyte candidates. Test-cell measurements were undertaken to evaluate the electrolyte performance in the presence of functioning anode and cathode materials, where several promising IL-based hybrid electrolytes with performance comparable to that of conventional carbonate electrolytes were identified. The study revealed that the contact angle influenced the performance more significantly than the conductivity because the cells containing IL-tetrafluoroborate-based electrolytes with higher conductivity but poorer wetting showed significantly decreased performance relative to the cells containing IL-bis(trifluoromethanesulfonyl)imide electrolytes with lower conductivity but improved wetting properties. This work contributes to the development of new IL battery-based electrolyte systems with the potential to improve the deliverable energy content as well as safety of lithium-ion battery systems.

Thermophysical properties of two ammonium-based protic ionic liquids

Experimental data for density, viscosity, refractive index and surface tension are reported, for the first time, in the temperature range between 288.15 K and 353.15 K and at atmospheric pressure for two protic ionic liquids, namely 2-(dimethylamino)-N,N-dimethylethan-1-ammonium acetate, [N_11{2(N11)}H][CH₃CO₂], and N-ethyl-N,N-dimethylammonium phenylacetate, [N_112H][C₇H₇CO₂]. The effect of the anion aromaticity and the cation's aliphatic tails on the studied properties is discussed. From the measured properties temperature dependency the derived properties, such as the isobaric thermal expansion coefficient, the surface entropy and enthalpy, and the critical temperature, were estimated.

Novel mixtures of ether-functionalized ionic liquids and non-flammable methylperfluorobutylether as safe electrolytes for lithium metal batteries

Li/LiFePO₄cells using safe electrolytes composed of ether-functionalized ILs and non-flammable methylperfluorobutylether showed good rate performance.

Branched isomeric 1,2,3-triazolium-based ionic liquids: new insight into structure–property relationships

Series of branched isomeric 1,2,3-triazolium-based ionic liquids (ILs) were synthesized and characterized. The effect of branching on thermal and physical properties is investigated.

Thermophysical properties of sulfonium- and ammonium-based ionic liquids

Experimental data for the density, viscosity, refractive index and surface tension of four sulfonium- and ammonium-based Ionic Liquids (ILs) with the common bis(trifluoromethylsulfonyl)imide anion were measured in the temperature range between 288.15 and 353.15 K and at atmospheric pressure. The ILs considered include butyltrimethylammonium bis(trifluoromethylsulfonyl)imide, [N4111][NTf2], tributylmethylammonium bis(trifluoromethylsulfonyl)imide, [N4441][NTf2], diethylmethylsulfonium bis(trifluoromethylsulfonyl)imide, [S221][NTf2], and triethylsulfonium bis(trifluoromethylsulfonyl)imide, [S222][NTf2]. Based on the gathered results and on data taken from literature, the impact of the cation isomerism and of the size of the aliphatic tails, as well as the effect resulting from the substitution of a nitrogen by a sulfur atom as the cation central atom, on the thermophysical properties of sulfonium- and ammonium-based ILs is here discussed. Remarkably, more symmetric cations present a lower viscosity for the same, and sometimes even for higher, alkyl chain lengths at the cation. Additional derivative properties, such as the isobaric thermal expansion coefficient, the surface thermodynamic properties and the critical temperature for the investigated ILs were also estimated and are presented and discussed.

The effect of the cation alkyl chain length on density and diffusion in dialkylpyrrolidinium bis(mandelato)borate ionic liquids

Diffusion behaviour and non-linear dependence of density of [C_nC₁Pyrr][BMB] ionic liquids on the number of CH₂ groups in the long alkyl chains of the cations were described using an additive model, in which ‘ionic’ and ‘aliphatic’ regions make additive contributions.

N-Allyl-N-Methyl Piperidinium Bis(Trifluoromethanesulfonyl) Imide as a Co-Solvent in Li-Ion Batteries

Mixtures of an ionic liquid (IL) with organic solvents and a lithium salt have been studied in order to develop new electrolytes for lithium-ion cells with enhanced safety profiles. In this work, N-allyl-N-methylpiperidinium bis (trifluoromethanesulfonyl) imide (PP1ATFSI) was synthesized and characterized to exhibit high decomposition temperature and wide electrochemical stability window. The evaluation of the coin cell LiFePO4/Li with the mixed electrolyte based on PP1ATFSI with 0.35mol/kg LiTFSI, and 30 wt% VC/DMC (1:1) shows a nice reversibility and cycle performances. All above prove that PP1ATFSI is one of the most promising safety electrolytes of Li-ion battery.