Enhanced ionic conductivity in halloysite nanotube-poly(vinylidene fluoride) electrolytes for solid-state lithium-ion batteries

Solid composite electrolytes have gained increased attention, thanks to the improved safety, the prolonged service life, and the effective suppression on the lithium dendrites. However, a low ionic conductivity (<10⁻⁵ S cm⁻¹) of solid composite electrolytes at room temperature needs to be greatly enhanced. In this work, we employ natural halloysite nanotubes (HNTs) and poly(vinylidene fluoride) (PVDF) to fabricate composite polymer electrolytes (CPEs). CPE-5 (HNTs 5 wt%) shows an ionic conductivity of ∼3.5 × 10⁻⁴ S cm⁻¹, which is ∼10 times higher than the CPE-0 (without the addition of HNTs) at 30 °C. The greatly increased ionic conductivity is attributed to the negatively-charged outer surface and a high specific surface area of HNTs, which facilitates the migration of Li⁺ in PVDF. To make a further illustration, a solid-state lithium-ion battery with CPE-5 electrolyte, LiMn₂O₄ cathode and Li metal anode was fabricated. An initial discharge capacity of ∼71.9 mA h g⁻¹ at 30 °C in 1C is obtained, and after 250 cycles, the capacity of 73.5 mA h g⁻¹ is still maintained. This study suggests that a composite polymer electrolyte with high conductivity can be realized by introducing natural HNTs, and can be potentially applied in solid-state lithium-ion batteries.

The special structure of HNTs and the further formation of amorphous PVDF contribute to the enhancement of the Li⁺ transfer.     

1-Alkenyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids: novel and low-viscosity ionic liquid electrolytes for dye-sensitized solar cells

Dye-sensitized Solar Cells (DSCs) based on ruthenium complex N719 as sensitizer have received much attention due to their affordability and high efficiency. However, their best performance is only achieved when using volatile organic solvents as electrolyte solutions, which are unstable under prolonged thermal stress. Thus, we developed a new series of 1-alkenyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids used as robust DSC electrolytes. These ionic liquids exhibit low viscosity, high conductivity, and thermal stability. The implementation of 1-but-3-enyl-3-methyl-imidazolium trifluoromethanesulfonate, [ButMIm]OTf, into DSCs gave the best photovoltaic performance. The results are fairly comparable to those reports for other popular ionic liquid electrolytes currently used in DSC field. An insightful discussion on the relationship between the structure of these new ionic liquids and the J–V characterization as well as electrochemical impedance measurement of DSCs will give more interesting information. The results are useful for large-scale outdoor application of DSCs.

A new series of 1-alkenyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids was prepared under microwave irradiation for application in DSC electrolytes.     

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 (PP_1,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 PP_1,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% PP_1,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 PP_1,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 PP_1,CNFSI addition drives the formation of solid electrolyte interphase (SEI) film which suppresses oxidative decomposition of the electrolyte and protects the structure cathode material.

Cells with 5 wt%, 10 wt%, and 15 wt% PP1, CNFSI addition exhibit higher initial discharge capacities than the cell with blank electrolyte. The addition of IL with suitable amount significantly increases the cycle performance..     

High ionic conduction,toughness and self-healing poly(ionic liquid)-based electrolytes enabled by synergy between flexible units and counteranions

Polymer electrolytes offer great potential for emerging wearable electronics. However, the development of a polymer electrolyte that has high ionic conductivity, stretchability and security simultaneously is still a considerable challenge. Herein, we reported an effective approach for fabricating high-performance poly(ionic liquids) (PILs) copolymer (denoted as PIL-BA) electrolytes by the interaction between flexible units (butyl acrylate) and counteranions. The introduction of butyl acrylate units and bis(trifluoromethane-sulfonyl)imide (TFSI⁻) counteranions can significantly enhance the mobility of polymer chains, resulting in the effective improvement of ion transport, toughness and self-healability. As a result, the PIL-BA copolymer-based electrolytes containing TFSI⁻ counterions achieved the highest ionic conductivity of 2.71 ± 0.17 mS cm⁻¹, 1129% of that of a PIL homopolymer electrolyte containing Cl⁻ counterions. Moreover, the PIL-BA copolymer-based electrolytes also exhibit ultrahigh tensile strain of 1762% and good self-healable capability. Such multifunctional polymer electrolytes can potentially be applied for safe and stable wearable electronics.

Polymer electrolytes offer great potential for emerging wearable electronics.     

Synthesis of a three-dimensional cross-linked Ni–V2O5 nanomaterial in an ionic liquid for lithium-ion batteries

A three-dimensional cross-linked Ni–V₂O₅ nanomaterial with a particle size of 250–300 nm was successfully prepared in a 1-butyl-3-methylimidazole bromide ionic liquid (IL). The formation of this structure may follow the rule of dissolution–recrystallization and the ionic liquid, as both a dissolution and structure-directing agent, plays an important role in the formation of the material. After calcination of the precursor, the active material (Ni–V₂O₅–IL) was used as an anode for lithium-ion batteries. The designed anode exhibited excellent electrochemical performance with 765 mA h g⁻¹ at a current density of 0.3 A g⁻¹ after 300 cycles, which is much higher than that of a NiVO–W material prepared via a hydrothermal method (305 mA h g⁻¹). These results show the remarkable superiority of this novel electrode material synthesized in an ionic liquid.

A three-dimensional cross-linked Ni–V₂O₅ nanomaterial with a particle size of 250–300 nm was successfully prepared in a 1-butyl-3-methylimidazole bromide ionic liquid (IL).     

A low-cost average valence detector for mixed electrolytes in vanadium flow batteries

The decay in capacity has hindered the applications of vanadium redox flow batteries (VFBs), which are promising energy storage devices with many benefits. Mixing positive and negative electrolytes can recover some of the lost capacity but is ineffective towards the increasing average valence of the mixed electrolyte caused by the side reactions. In this study, a low-cost optical average valence detector has been developed to monitor the average valence of the mixed electrolyte in VFBs. We demonstrate experimentally that with the aid of the average valence detector, the capacity of VFBs can be regularly recovered via electrolyte mixing and online electrolysis. This low-cost average valence detector has great potential for recovering the decayed capacities of VFBs in large-scale energy storage stations, which consist of thousands of VFBs.

An optical average valence detector has been developed to enable the capacity recovery of VFBs via electrolyte mixing and online electrolysis.     

Dissolution of metal oxides in task-specific ionic liquid

Due to their typically low reactivity, the activation of metal oxides, as found in ores, earths and minerals, is in general performed by high temperature reactions, which consume much energy. Owing to the prevalence of fossil fuels, this is accompagnied by the generation of large amounts of CO₂. Alternatively, ionic liquids (ILs) can be used as solvents for metal oxide dissolution and downstream chemistry at much lower temperatures. The dissolution ability of the dry ionic liquid betainium bis(trifluoromethylsulfonyl)imide, [Hbet][NTf₂], was investigated for 30 metal oxides at 175 °C and compared to chloride containing IL [Hbet]₂[NTf₂]Cl. A general dissolution-promoting effect of chloride anions was found, regarding reaction time as well as the variety of dissolved metal oxides. Up to now, the dissolution in [Hbet]₂[NTf₂]Cl is limited to basic or amphoteric metal oxides and assumed to be influenced by multiple factors, such as reaction conditions and the lattice energy of the metal oxide as well as its crystal structure. Comprehensive investigations were performed for the dissolution of CuO, which led to the discovery of the water-free complex compound [Cu₂(bet)₄(NTf₂)₂][NTf₂]₂. Proceeding from this compound, a complete exchange of the O-coordination sphere by other ligands was demonstrated, opening up promising possibilities for downstream chemistry.

Investigation on the dissolution of 30 metal oxides in the water-free ionic liquid [Hbet][NTf₂] and the catalytic effect of chloride for the application in green ore processing.     

Fundamental properties of TEMPO-based catholytes for aqueous redox flow batteries: effects of substituent groups and electrolytes on electrochemical properties,solubilities and battery performance

Water-soluble 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivatives have been frequently utilized as catholytes for aqueous redox flow batteries to achieve cost-effective renewable energy storage. However, fundamental knowledge of TEMPO derivatives is still largely underdeveloped. Herein, a comprehensive study on the properties of TEMPO derivatives has been conducted in aqueous electrolytes. The results confirm that the redox potential, diffusion coefficient, electron transfer rate constant and solubility are clearly influenced by functional groups of TEMPO derivatives and supporting electrolytes. The charge–discharge cycling performance is evaluated using a symmetric redox flow battery configuration. The capacity decay for TEMPO-based catholytes is mainly derived from the crossover of the oxidized state. The presented study not only advances an in-depth understanding of TEMPO-based RFB applications, but also highlights the challenge of crossover of redox-active TEMPO derivative molecules applied in aqueous RFBs.

The effects of substituent groups of TEMPO-based catholytes and supporting electrolytes on electrochemical properties, solubility and battery performance were examined systematically for aqueous redox flow batteries.     

Physicochemical characterizations of novel dicyanamide-based ionic liquids applied as electrolytes for supercapacitors

Novel ionic liquids (ILs), containing a dicyanamide anion (DCA⁻), are synthesized and applied as suitable electrolytes for electrochemical double layer capacitors (EDLCs). The prepared ILs are either composed of triethyl-propargylammonium (N_222pr⁺) or triethyl-butylammonium (N₂₂₂₄⁺) cations paired with the DCA⁻ anion. The structure of the cation influences its electrostatic interaction with the DCA⁻ anion and highly impacts the physical and electrochemical properties of the as-prepared ILs. The geometry and the length of the alkyl chain of the propargyl group in N_222pr⁺ enhance the ionic conductivity of N_222pr–DCA (11.68 mS cm⁻¹) when compared to N₂₂₂₄–DCA (5.26 mS cm⁻¹) at 298 K. It is demonstrated that the Vogel–Tammann–Fulcher model governs the variations of the transport properties investigated over the temperature range of 298–353 K. A maximum potential window of 3.29 V is obtained when N_222pr–DCA is used as electrolyte in a graphene based symmetric EDLC system. Cyclic voltammetry and galvanostatic measurements confirm that both electrolytes exhibit an ideal capacitive behavior. The highest specific energy of 55 W h kg⁻¹ is exhibited in the presence of N₂₂₂₄–DCA at a current density of 2.5 A g⁻¹.

Novel ionic liquids (ILs), containing a dicyanamide anion (DCA⁻), are synthesized and applied as suitable electrolytes for electrochemical double layer capacitors (EDLCs).     

One-pot synthesis of poly(ionic liquid)s with 1,2,3-triazolium-based backbones via clickable ionic liquid monomers

Clickable α-azide-ω-alkyne ionic liquid monomers were developed and subsequently applied to the one-pot synthesis of ionically conducting poly(ionic liquid)s with 1,2,3-triazolium-based backbones through a click chemistry strategy. This approach does not require the use of solvents, polymerisation mediators, or catalysts. The obtained poly(ionic liquid)s were characterized by NMR, differential scanning calorimetry, thermogravimetric analysis, and impedance spectroscopy analysis. Moreover, these poly(ionic liquid)s were cross-linked via N-alkylation with a dianion quarternizing agent to achieve enhanced ionic conductivity and mechanical strength. The resulting free-standing films showed a Young''s modulus up to 4.8 MPa and ionic conductivities up to 4.60 × 10⁻⁸ S cm⁻¹ at 30 °C. This facile synthetic strategy has the potential to expand the availability of poly(ionic liquid)s and promote the development of functional materials.

Clickable ionic liquid monomers realize the one-pot synthesis of ionically conducting poly(ionic liquid)s with 1,2,3-triazolium-based backbones via click chemistry.     

Understanding the ionic activity and conductivity value differences between random copolymer electrolytes and block copolymer electrolytes of the same chemistry

Herein, a systematic study where the macromolecular architectures of poly(styrene-block-2-vinyl pyridine) block copolymer electrolytes (BCE) are varied and their activity coefficients and ionic conductivities are compared and rationalized versus a random copolymer electrolyte (RCE) of the same repeat unit chemistry. By performing quartz crystal microbalance, ion-sorption, and ionic conductivity measurements of the thin film copolymer electrolytes, it is found that the RCE has higher ionic activity coefficients. This observation is ascribed to the fact that the ionic groups in the RCE are more spaced out, reducing the overall chain charge density. However, the ionic conductivity of the BCE is 50% higher and 17% higher after the conductivity is normalized by their ion exchange capacity values on a volumetric basis. This is attributed to the presence of percolated pathways in the BCE. To complement the experimental findings, molecular dynamics (MD) simulations showed that the BCE has larger water cluster sizes, rotational dynamics, and diffusion coefficients, which are contributing factors to the higher ionic conductivity of the BCE variant. The findings herein motivate the design of new polymer electrolyte chemistries that exploit the advantages of both RCEs and BCEs.

Random copolymer electrolytes have better permselectivity but lower ionic conductivity than block copolymer electrolytes of the same repeat unit chemistry.     

Enhancing the solubility of 1,4-diaminoanthraquinones in electrolytes for organic redox flow batteries through molecular modification

1,4-Diaminoanthraquinones (DAAQs) are a promising class of redox-active molecules for use in nonaqueous redox flow batteries (RFBs) because they can have up to five electrochemically accessible and reversible oxidation states. However, most of the commercially available DAAQs have a low solubility in the polar organic solvents that are typically used in RFBs, in particular when supporting electrolyte salts are present. This significantly limits the energy densities that can be achieved. We have functionalized the amino groups in the DAAQ structure with three types of chains, namely alkyl chains, cationic alkyl chains and oligoethylene glycol ether chains, and measured the solubility of these derivatives in various organic solvents by quantitative UV-Vis absorption spectroscopy. The DAAQ derivatives with higher polarity exhibit a significantly higher solubility in commonly used organic electrolytes in comparison to apolar derivatives. Cyclic voltammetry was used to assess the viability of the DAAQs as redox-active species for RFBs. Although the cationic DAAQ derivatives have an enhanced solubility in the electrolytes, the cathodic redox reactions have a poor reversibility, most likely due to an internal decomposition reaction of their reduced forms. The oligoethylene-glycol-ether-functionalized DAAQs are the most promising compounds for use in organic RFB electrolytes because they have the optimal combination of high solubility and a high reversibility of the redox couples.

The redox-active 1,4-diaminoanthraquinone structure was modified with several side chains in order to increase the solubility in organic electrolytes for redox flow batteries.     

Protic ionic liquids with primary alkylamine-derived cations: the dominance of hydrogen bonding on observed physicochemical properties

Novel protic ionic liquids (PILs) were synthesized by neutralization of primary alkylamines with bis(trifluoromethanesulfonyl)amide acid. An extensive hydrogen bonding network in these PILs was observed via lower thermal stability, temperature dependent inversion from non-Newtonian to Newtonian fluidic behavior, and lower ionicity compared to their secondary and tertiary analogues.

The dominance of hydrogen bonds (networking) over the physicochemical features of primary alkylamine-PILs based on an amide acid.     

Solubility of sulfur dioxide in tetraglyme-NH4SCN ionic liquid: high absorption efficiency

An easily prepared ionic liquid was synthesized by a one-step method and applied in SO₂ absorption efficiently. The cation of the ionic liquid is a supramolecular structure consisting of NH⁺₄ and tetraglyme, similar to the structure of NH⁺₄ and crown ether, and the anion is selected as SCN⁻. The ionic liquid has good thermal stability. Under the conditions of 293 K and 1 bar, one mol ionic liquid can absorb 2.73 mol SO₂, which is about 30% higher than tetraglyme. The absorption mechanism was characterized using IR and NMR. And the results confirmed that the interaction mechanism between SO₂ and the ionic liquid is a physical interaction rather than a chemical interaction.

Simple prepared tetraglyme-NH₄SCN ionic liquid is used to absorb sulfur dioxide efficiently and regenerate.     

Development of advanced electrolytes in Na-ion batteries: application of the Red Moon method for molecular structure design of the SEI layer

This review aims to overview state-of-the-art progress in the collaborative work between theoretical and experimental scientists to develop advanced electrolytes for Na-ion batteries (NIBs). Recent investigations were summarized on NaPF₆ salt and fluoroethylene carbonate (FEC) additives in propylene carbonate (PC)-based electrolyte solution, as one of the best electrolytes to effectively passivate the hard-carbon electrode with higher cycling performance for next-generation NIBs. The FEC additive showed high efficiency to significantly enhance the capacity and cyclability of NIBs, with an optimal performance that is sensitive at low concentration. Computationally, both microscopic effects, positive and negative, were revealed at low and high concentrations of FEC, respectively. In addition to the role of FEC decomposition to form a NaF-rich solid electrolyte interphase (SEI) film, intact FECs play a role in suppressing the dissolution to form a compact and stable SEI film. However, the increase in FEC concentration suppressed the organic dimer formation by reducing the collision frequency between the monomer products during the SEI film formation processes. In addition, this review introduces the Red Moon (RM) methodology, recent computational battery technology, which has shown a high efficiency to bridge the gap between the conventional theoretical results and experimental ones through a number of successful applications in NIBs.

This review aims to overview state-of-the-art progress in the collaborative work between theoretical and experimental scientists to develop advanced electrolytes for Na-ion batteries (NIBs).     

Realizing outstanding electrochemical performance with Na3V2(PO4)2F3 modified with an ionic liquid for sodium-ion batteries

Na₃V₂(PO₄)₂F₃ is a typical NASICON structure with a high voltage plateau and capacity. Nevertheless, its applications are limited due to its low conductivity and poor rate performance. In this study, nitrogen–boron co-doped carbon-coated Na₃V₂(PO₄)₂F₃ (NVPF-CNB) was prepared by a simple sol–gel method using an ionic liquid (1-vinyl-3-methyl imidazole tetrafluoroborate) as a source of nitrogen and boron for the first time. The morphology and electrochemical properties of NVPF-CNB composites were investigated. The results show that a nitrogen–boron co-doped carbon layer could increase the electron and ion diffusion rate, reduce internal resistance, and help alleviate particle agglomeration. NVPF-CNB-30 exhibited better rate performance under 5C and 10C charge/discharge with initial reversible capacities of 99 and 90 mA h g⁻¹, respectively. Furthermore, NVPF-CNB-30 illustrates excellent cyclic performance with the capacity retention rate reaching 91.9% after 500 cycles at 5C, as well as a capacity retention rate of about 95.5% after 730 cycles at 10C. The evolution of the material''s structure during charge/discharge processes studied by in situ X-ray diffraction confirms the stable structure of nitrogen–boron co-doped carbon-coated Na₃V₂(PO₄)₂F₃. Co-doping of nitrogen and boron also provides more active sites on the surface of Na₃V₂(PO₄)₂F₃, revealing a new strategy for the modification of sodium-ion batteries.

An ionic liquid is used as a new nitrogen and boron source to synthesize nitrogen–boron co-doped carbon-coated Na₃V₂(PO₄)₂F₃ (NVPF-CNB).     

Basic ionic liquid as catalyst and surfactant: green synthesis of quinazolinone in aqueous media

Basic imidazolium-based ionic liquids not only possess the extraordinary physicochemical properties of ionic liquids, but also have excellent basicity and surfactivity. 1-Propyl-3-alkylimidazole hydroxide ionic liquids ([PRIm][OH]) were synthesized and their catalytic and surfactant behavior were studied in this work. [PRIm][OH] owned excellent surfactivity, and their alkyl chains and ion pairs benefit hydrophobicity and hydrophilicity respectively. The surfactivity of [PRIm][OH] increased with increasing alkyl chain length. [PRIm][OH] showed better catalytic performance than NaOH in the condensation of 2-aminobenzonitrile with cyclohexanone in aqueous medium, and the catalytic performance was well coincident with their surfactant behavior. [PRIm][OH] could decrease the interfacial tension of solvent effectively and form micelles in water. The formed micelles could solubilise more reactants into water and effectively increase the chance of contact between reactants and catalytic active sites. The catalyst dosage obviously affected catalytic performance. The catalytic system is a promising recyclable system.

[PRIm][OH] showed excellent catalytic properties in synthesis of quinazolinone in aqueous medium, owing to its excellent surfactivity and basicity.     

Mechanical and self-recovery properties of supramolecular ionic liquid elastomers based on host–guest interactions and correlation with ionic liquid content

Supramolecular materials have received considerable attention due to their higher fracture energy and self-recovery capability compared to conventional chemically cross-linked materials. Herein, we focus on the mechanical properties and self-recovery behaviours of supramolecular polymeric elastomers swollen with ionic liquid. We also gained insight into the correlation between ionic liquid content and mechanical properties. These supramolecular polymers with ionic liquid can be easily prepared from bulk copolymerization of the host–guest complex (peracetylated cyclodextrin and adamantane derivatives) and alkyl acrylates and subsequent immersion in ionic liquid such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The supramolecular polymeric elastomers showed a self-recovery ability, which the conventional chemically cross-linked elastomers with ionic liquid cannot achieve.

Supramolecular ionic liquid elastomers showed higher fracture energy than chemically cross-linked ionic liquid elastomers and also self-recovery ability.     

Atomic-scale investigation of the heterogeneous structure and ionic distribution in an ionic liquid using scanning transmission electron microscopy

Ionic liquids show characteristic properties derived from them being composed of only molecular ions, and have recently been used as solvents for chemical reactions and as electrolytes for electrochemical devices. The liquid structures, i.e., ionic distributions, form when solutes are dissolved in ionic liquids and fundamentally affect the reactions and transfer efficiency in such solutions. In this study, we directly observe the liquid structure in a solution of the long-chain ionic liquid 1-octyl-3-methylimidazolium bromide (C₈mim Br) and barium stearate (Ba(C₁₇H₃₅COO)₂) using the annular dark-field method of scanning transmission electron microscopy (ADF-STEM). The ADF image shows a 10 nm-scale heterogeneity in the image intensity, which reflects the heterogeneous ionic distribution in the solution. The number density distributions of all the component ions (C₈mim⁺, Br⁻, Ba²⁺, and C₁₇H₃₅COO⁻) were estimated from the ADF image intensity and then visualized. These ionic distribution maps depicted the spatial relationships between the ions at the sub-nanometer scale and revealed that the heterogeneity is largely derived from the large differences in size, charge distributions, and van der Waals interactions.

Ionic liquids show characteristic properties derived from being composed of only molecular ions. The numbers of all constituent molecules and ions were quantitatively estimated from the image intensity and the origin of the heterogeneity in the ionic liquid was discussed.     

The effects of alcohol on blood pressure and electrolytes

The interaction between alcohol abuse, changes in blood pressure, and electrolyte abnormalities is complex. Some effects of alcohol are seen only with acute ingestion, some during withdrawal, and some only in chronic drinkers. Careful attention to the interactions between the metabolism of various electrolytes can prevent unnecessary morbidity and mortality in alcoholic patients.