Highly Conductive, Flexible, and Nonflammable Double-Network Poly(ionic liquid)-Based Ionogel Electrolyte for Flexible Lithium-Ion Batteries

The solid-state lithium-ion battery is proposed as the ultimate form of battery and has rapidly become an updated attentive research field due to its high safety and extreme temperature tolerance. However, current solid-state electrolytes hardly meet the requirement in practical applications due to its low ionic conductivity, weak mechanical properties, and poor interfacial contact between the electrolyte and the electrode. In this work, we developed a double-network-supported poly(ionic liquid)-based ionogel electrolyte (DN-Ionogel) via a one-step method. Due to its compact cross-linking structure, the leakage-free DN-Ionogel electrolyte exhibits outstanding flexibility and favorable mechanical properties. In this ionogel electrolyte, the double network favors dissociation of lithium bis(trifluoromethanesulfony)imide (LiTFSI), further resulting in remarkable ionic conductivity (1.8 × 10^-3 S/cm, room temperature), wide electrochemical window (up to 5.0 V), and high lithium-ion transference number (0.33). Furthermore, the cell (LiFePO₄||DN-Ionogel||Lithium) delivers a discharge capacity as high as 150.5 mAh/g, stable cyclic performance (over 200 cycles), and superior rate behavior.

A robust, freeze-resistant and highly ion conductive ionogel electrolyte towards lithium metal batteries workable at -30 °C

The wide applications of lithium metal batteries have encountered a severe conductivity issue when operating in cold weather. Here we report a freeze-resistant lithium metal battery, which displays outstanding rate performance, negligible polarization deterioration, and a good capacity retention of 94.25% after 700-cycles of use at -30 °C, the lowest temperature ever reported for gel electrolyte-based lithium metal batteries. Remarkably, the lithium metal batteries are even workable at temperatures down to -60 °C. The key point of the innovative design is the utilization of a newly created anti-freezing ionogel as an electrolyte, which is produced by gelation of an electrochemically inert ionic liquid, 1-butyl-3-methylimidazolium tetrafluoro-borate ([BMIM]BF₄), via dynamic condensation of a specially designed benzaldehyde-terminated polyethylene glycol (PEG-CHOs) with the tetra-hydrazide derivative of p-tert-butyl-calix[4]arene (CTH). The as-prepared ionogel electrolyte demonstrates a high ionic conductivity (0.43 mS cm^-1), a broad stability window (2.4-4.3 V vs. Li⁺/Li), and high flexibility at -30 °C. The outstanding property of the ionogel electrolyte is ascribed to its unique gel network structure as it enables enrichment of Li⁺ and enhances its efficient transportation. Further tests demonstrate that the ionogel electrolyte could be also used for the assembly of flexible lithium metal batteries.

Healable, Highly Conductive, Flexible, and Nonflammable Supramolecular Ionogel Electrolytes for Lithium-Ion Batteries

High-performance solid-state electrolytes with healability to repair mechanical damages are important for the fabrication of Li-ion batteries (LIBs) with enhanced safety and prolonged service life. In this study, we present the fabrication of healable, highly conductive, flexible, and nonflammable ionogel electrolytes for use in LIBs by loading ionic liquids and Li salts within a hydrogen-bonded supramolecular poly(ionic liquid) copolymer network. The ionogel electrolytes exhibit ionic conductivities as high as 10^-3 S/cm, which is comparable to the conventional liquid electrolytes. The Li/LiFePO₄ battery assembled with the ionogel membrane exhibits excellent cycling performance and delivers a steady high discharge capacity of 147.5 mA h g^-1 and Coulombic efficiency of 99.7% after 120 cycles at the charge/discharge rate of 0.2 C. Importantly, the ionogel membranes can heal damages outside or inside a battery because of the reversible nature of the supramolecular interactions between the components. The damaged ionogel membranes after being healed can effectively restore the original performance of the LIBs.

Catalytic synthesis of chloroacetates with thermoregulated ionic liquids based on vanadium-substituted polyoxometalate

A series of polyoxometalate-based ionic liquid (POM-IL) catalysts with functional sulfonic acid groups, [TEAPS]_3+nPW_12−nV_nO₄₀ (n = 1, 2, 3) were synthesized and characterized by nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectrophotometry (FT-IR), UV-Vis spectrophotometry (UV), potentiometric titration and thermogravimetry-differential scanning calorimetry (TG-DSC). The catalytic ability and reusability of the POM-IL catalysts were evaluated on esterification of chloroacetic acid and n-amyl alcohol. The optimum reaction conditions, 0.2 g of the catalyst amount, 10 mL of water carrier, 140 °C of reaction temperature, and 1.2/1 of the molar ratio of alcohol/acid, were obtained by an orthogonal test. [TEAPS]₅PW₁₀V₂O₄₀ was found to be the best active catalyst with an esterification rate of 98.75% and could be reused five times without significant decrease in activity. The ionic liquid acted as a temperature-responsive catalyst, forming a homogeneous mixture with the reactants at reaction temperature, and could be precipitated and separated from products when the reaction ends at ambient temperature. Therefore, an environmentally friendly and highly efficient approach for the synthesis of chloroacetates is provided.

The ionic liquid catalyst [TEAPS]₅PW₁₀V₂O₄₀ exhibited good activity and reusability in the esterification process owing to its acidity and thermoregulated properties.

Thermoresponsive Polyoxometalate/Ionic Liquid Supramolecular Gel Electrolytes for Supercapacitors: Fabrication, Structure, and Heteropolyanion Structure Effect

We report the fabrication, structure, and heteropolyanion structure effect of polyoxometalate (POM)/ionic liquid (IL) supramolecular gels. These supramolecular gels exhibit ordered structures, as a result of their excellent reversible self-assembly, and they show various physicochemical properties, determined by the heteropolyanion structure effect of POM anions. Specifically, the formation of POM/IL supramolecular gels results in a highly ordered layer-shape structure, which has been calculated using X-ray powder diffraction patterns and proven by transmission electron microscopy images for the first time. When these POM/IL supramolecular gels are heated, they become viscous liquid sols, with melting isotropic drops and even flowerlike structures on microscopic scales, while it undergoes a reversible gel-sol phase transformation from gel to sol. The heteropolyanion structure effect in these two IL gels, [TBTP]₅PW₁₀V₂O₄₀ and [TBTP]₈P₂W₁₆V₂O₆₂, on their physicochemical properties is demonstrated. The POM structures have a strong structure effect on the physicochemical properties. As the size of heteropolyanions increases, there is a significant improvement in the conductivity, thermal performance, and oxidizability, with a lower phase inversion temperature, which means that the Dawson-type compound, [TBTP]₈P₂W₁₆V₂O₆₂, has a higher conductivity, lower melting point, stronger oxidizability, and better thermal performance than the Keggin-type compound, [TBTP]₅PW₁₀V₂O₄₀, under the same conditions.

Synthesis and conductive performance of indium-substituted ternary heteropoly acids with Keggin structures

Keggin-type ternary polyoxometalates H₄[In(H₂O)PW₁₁O₃₉]·11H₂O and H₅[In(H₂O)SiW₁₁O₃₉]·8H₂O are solid high-proton conductors with a conductivity of 2.60 × 10⁻⁴ S cm⁻¹ and 5.25 × 10⁻⁴ S cm⁻¹, respectively, at 18 °C and 80% relative humidity.

Substitution effect in reversible gel-liquid phase transformation polyoxometalate ionic liquid compounds

The substitution effect on a series of POM-type ionic liquids, has been investigated. The physicochemical properties of the ILs are highly related to their structures. When protons are substituted for ammonium, high conductivity and better thermostability are observed.

The synthesis and electrical properties of hybrid gel electrolytes derived from Keggin-type heteropoly acids and 3-(pyridin-1-ium-1-yl)propane-1-sulfonate (PyPs)

Cyclic voltammetry of PyPs–H₃PWMo at room temperature between −4 and +4 V vs. Ag/AgCl at a 10 mV s⁻¹ scan rate and comparison of electrical conductivity of PyPs–H₃PWMo with other known promising proton conductors.

Keggin-type polyoxometalate-based ionic liquid gels

A series of reversible phase transformation ammonium- and phosphonium-based polyoxometalate ionic liquid (POM-IL) gels were synthesized and studied with a focus on the correlation between their physicochemical properties and their chemical structure. The products were successfully characterized by IR, UV, XRD and TG-DTA, and their ionic conductivities were measured. The Keggin-type heteropolyanion clusters decorated with long alkyl chains demonstrated a tendency to exhibit a gel state at room temperature, while all the gels transformed into liquids after heating and then recovered after cooling. With a decrease in the alkyl chain length, a significant improvement in the thermal stability and conductivity of the ammonium-based POM-IL gels can be achieved. Moreover, compared with the corresponding ammonium compound, phosphonium-based POM-IL gel was found to be more stable at high temperature and exhibited better conductivity.