Understanding the electrochemistry of “water-in-salt” electrolytes: basal plane highly ordered pyrolytic graphite as a model system

A new approach to expand the accessible voltage window of electrochemical energy storage systems, based on so-called “water-in-salt” electrolytes, has been expounded recently. Although studies of transport in concentrated electrolytes date back over several decades, the recent demonstration that concentrated aqueous electrolyte systems can be used in the lithium ion battery context has rekindled interest in the electrochemical properties of highly concentrated aqueous electrolytes. The original aqueous lithium ion battery conception was based on the use of concentrated solutions of lithium bis(trifluoromethanesulfonyl)imide, although these electrolytes still possess some drawbacks including cost, toxicity, and safety. In this work we describe the electrochemical behavior of a simple 1 : 1 electrolyte based on highly concentrated aqueous solutions of potassium fluoride (KF). Highly ordered pyrolytic graphite (HOPG) is used as well-defined model carbon to study the electrochemical properties of the electrolyte, as well as its basal plane capacitance, from a microscopic perspective: the KF electrolyte exhibits an unusually wide potential window (up to 2.6 V). The faradaic response on HOPG is also reported using K₃Fe(CN)₆ as a model redox probe: the highly concentrated electrolyte provides good electrochemical reversibility and protects the HOPG surface from adsorption of contaminants. Moreover, this electrolyte was applied to symmetrical supercapacitors (using graphene and activated carbon as active materials) in order to quantify its performance in energy storage applications. It is found that the activated carbon and graphene supercapacitors demonstrate high gravimetric capacitance (221 F g⁻¹ for activated carbon, and 56 F g⁻¹ for graphene), a stable working voltage window of 2.0 V, which is significantly higher than the usual range of water-based capacitors, and excellent stability over 10 000 cycles. These results provide fundamental insight into the wider applicability of highly concentrated electrolytes, which should enable their application in future of energy storage technologies.

The stability of water-in-salt electrolyte systems is investigated using highly concentrated solutions of KF(aq) with graphite as a model system.     

Effect of Urea on the Thermodynamics of Hexadecyltrimethylammonium Bromide Micelle Formation in Aqueous Solutions

The effect of urea on the thermodynamics of hexadecyltrimethylammonium bromide (CTAB) micelle formation in aqueous urea solutions was studied by isothermal titration microcalorimetry. The thermodynamic functions of ΔH, ΔG, and ΔS of CTAB micelle formation were calculated. The critical micelle concentrations (CMC) were determined. The addition of urea to the solution decreased the micelle formation entropy. This was attributed to the “lowering” of the structural temperature of the solution, which led to an increased number of hydrogen bonds and structure formation of water.

     

Effect of alkaline and alkaline–earth cations on the supercapacitor performance of MnO2 with various crystallographic structures

The electrochemical performances of the α-, γ-, and δ-MnO₂ with different crystallographic structures were systematically investigated in 0.5 mol/L Li₂SO₄, 0.5 mol/L Na₂SO₄, 1 mol/L Ca(NO₃)₂, and 1 mol/L?Mg(NO₃)₂ electrolytes. The results showed that the electrochemical performances of the manganese dioxides depended strongly on the crystallographic structures of MnO₂ as well as the cation in the electrolytes. Because the δ-MnO₂ consists with layers of structure and the interlayer separation is 7 Å, which is suitable for insertion/extraction of some alkaline and alkaline–earth cations, the δ-MnO₂ electrode showed the higher specific capacitance than that of α-MnO₂ and γ-MnO₂. We also found that the α-, γ-, and δ-MnO₂ electrodes in the Mg(NO₃)₂ electrolyte showed a higher specific capacitance, while all the α-, γ-, and δ-MnO₂ electrodes in the Li₂SO₄ electrolyte exhibited a better cycle life. The reason for the different behavior of Li⁺ and Mg²⁺ during the charge/discharge process can be ascribed to the charge effect of the cations in the electrolytes. The ex situ X-ray diffraction (XRD) and long-time cyclic voltammogram measurements were used to systematically study the energy storage mechanism of MnO₂-based electrodes. A progressive crystallinity loss of the materials is also observed upon potential cycling at the oxidized states. A reasonable charge/discharge mechanism is proposed in this work.     

Locally Concentrated Ionic Liquid Electrolytes Enabling Low-Temperature Lithium Metal Batteries

Lithium metal is a promising anode material for next-generation high-energy-density batteries but suffers from low stripping/plating Coulombic efficiency and dendritic growth particularly at sub-zero temperatures. Herein, a poorly-flammable, locally concentrated ionic liquid electrolyte with a wide liquidus range extending well below 0 °C is proposed for low-temperature lithium metal batteries. Its all-anion Li⁺ solvation and phase-nano-segregation solution structure are sustained at low temperatures, which, together with a solid electrolyte interphase rich in inorganic compounds, enable dendrite-free operation of lithium metal anodes at −20 °C and 0.5 mA cm⁻², with a Coulombic efficiency of 98.9 %. As a result, lithium metal batteries coupling thin lithium metal anodes (4 mAh cm⁻²) and high-loading LiNi_0.8Co_0.15Al_0.05O₂ cathodes (10 mg cm⁻²) retain 70 % of the initial capacity after 100 cycles at −20 °C. These results, as a proof of concept, demonstrate the applicability of locally concentrated ionic liquid electrolytes for low-temperature lithium metal batteries.     

Temperature effects on the aggregation of decylpyridinium chloride in aqueous solution

Surface tension and conductivity measurements were used to study micelle formation in an aqueous solution of decylpyridinium chloride at 20, 25, 30, 35, and 40°C. The critical micelle concentration (CMC) was minimum at about 30–35°C. After CMC₁, aggregation with CMC₂ and CMC₃ occurred as the concentration increased.     

Water structure and electron trapping in aqueous ionic solutions

The optical absorption spectra observed by pulse radiolysis of alkaline (NaOH, KOH, RbOH), chloride (LiCl, MgCl₂, CaCl₂, NaCl, KCl) and perchloride (NaClO₄) solutions at temperature 298 K are reported. Some measurements were performed at low temperature with aqueous ionic glasses. With increasing concentration of the above solutes a uniform blue-shift of the maximum of the solvated electron (e¯_sol) absorption band is observed. Near infrared (NIR) spectroscopy was so used to examine the properties of water in several concentrated electrolyte solutions. It is shown that some inorganic electrolytes (e.g. NaOH, NaClO₄) substantially change the water structure whereas some others (e.g. LiCl, CaCl₂) influence water structure insignificantly. The correlation between the ability of excess electron trapping in electrolyte solutions and water structure deduced from NIR spectroscopy is discussed.     

The Micellization and Clouding Phenomena of a Nonionic Surfactant,Poly(ethylene glycol) t-octylphenyl ether (Triton X-100): Effect of (Chloride Salt) Electrolytes

Herein, we report the micellization and the clouding of a nonionic surfactant, poly(ethylene glycol) t-octylphenyl ether (Triton X-100), in aqueous solutions in the absence and presence of (chloride salt) electrolytes. In the absence and presence of electrolytes, the critical micelle concentration (CMC) of Triton X-100 was measured by surface tension measurements. Upon increasing the temperature as well as the concentration of electrolytes, the CMCs decreased. The surface properties and the thermodynamic parameters of the micellar systems were evaluated. From these evaluated thermodynamic parameters, it was found that in the presence of an electrolyte, the stability of the micellar system is high. The cloud points (CPs) of Triton X-100 were also measured in the absence and presence of metallic ions of electrolytes. Upon the addition of metallic ions of chloride salts (electrolytes), the decrease in CP values was observed and the order was found to be: K⁺ > Na⁺ > Li⁺ > NH⁺₄.     

Diffusion coefficients of lithium chloride and potassium chloride in hydrogel membranes derived from acrylamide

Diffusion of non-associated electrolytes (potassium chloride and lithium chloride) in concentrated aqueous solutions (0.1-1.0 mol dm⁻³) has been studied in hydrogels derived from acrylamide and methyl methacrylate to study the mechanism of electrolyte transport. The preparation of two gels with different monomer ratio compositions resulted in obtaining membranes of substantially different hydrophilic character with polymer fractions of 0.3 and 0.5.Cukier hydrodynamic model was applied to explain the dependence of the diffusion coefficients of KCl and LiCl on the electrolyte concentration in hydrogel obtained experimentally. It was shown that the increase of the diffusion coefficients is accompanied with a decrease of the mean distance of approach of the ions. This can be explained by the formation of ion-pairs, resulting in a further contribution to diffusion once there is a decrease in the hydrodynamic resistance of the medium to the diffusing particles. Parameters, which characterise such a behaviour quantitatively, are different for different electrolytes and depend on water content in the gel.     

Lithium insertion into manganese dioxide polymorphs in aqueous electrolytes

The electrochemical behaviour of the spinel-like LiMn₂O₄ was studied in non-aqueous and aqueous saturated alkali nitrate electrolytes in comparison with the layered manganese dioxide δ-MnO₂. The results obtained by galvanostatic and cyclic voltammetry techniques showed that the insertion of Li⁺/e^– or H⁺/e^– depends on both the host lattice and the type of electrolyte. The spinel-like LiMn₂O₄ preferably allowed the insertion of Li⁺/e^– in non-aqueous and aqueous saturated LiNO₃ electrolytes, as observed from the similarity of the electrochemical behaviour in these electrolytes and the stability of the structure. This was explained by the presence of a three-dimensional network of vacant tetrahedral and half-filled octahedral sites in LiMn₂O₄, which guarantee high mobility of Li⁺ ions. The layered manganese dioxide could be inserted by Li⁺/e^– only in non-aqueous electrolytes. The work described herein was carried out at the Institut für Anorganische und Analytische Chemie, Technische Universit?t Berlin, Germany     

Temperature structural transformations of reverse micelles of oxyethylated surfactants

The dependence of the hydrodynamic radius of reverse micelles of Tergitol NP-4 on temperature (20–50°C), solubilization capacity (0–7 vol %), electrolyte composition (NH₄NO₃, KNO₃, HNO₃, NH₄OH, and KOH), and electrolyte concentration (0–8 mol/l) in an aqueous pseudophase was studied by photon-correlation spectroscopy. The hydrodynamic radius of micelles was shown to decrease with increasing temperature regardless of the concentration and type of electrolyte, and spherical micelles were formed in the process. It was ascertained that temperature aligns the features of interaction between solubilisate and micelles due to dehydration of the surfactant molecules: the radius of spherical micelles at 50°C depended only on solubilization capacity.     

Ionic conductivity in crystalline–amorphous polymer electrolytes – P(EO)6:LiX phases

Recent reports have indicated higher ionic conductivities in crystalline polymer electrolytes consisting of isostructural P(EO)₆:LiX (X=PF₆, AsF₆, SbF₆) phases relative to the analogous amorphous materials. These reports challenge the conventional wisdom in polymer electrolyte research that amorphous electrolytes are much more conductive than crystalline ones. The higher conductivity in the crystalline materials was attributed to the structures in which Li⁺ cations are located within PEO cylinders uncoordinated by the anions. The conductivity and crystallinity of P(EO)_n–LiClO₄ (EO/Li=6 and 10) electrolytes have been examined here. In contrast to the recent reports, much lower conductivities are found for the isostructural P(EO)₆:LiClO₄ crystalline electrolyte relative to the same fully amorphous electrolyte.     

SINGLET-TRIPLET SPIN TRANSITION IN AN IRON(II) COMPLEX OF 1,10-PHENANTHROLINE-2-CARBOTHIOAMIDE

Abstract

The ⁵⁷Fe Mössbauer effect for the solid complex FeLCl₂·H₂O (L=10-phenanthroline-2-carbothioamide) has been studied between 4.2°K and 298°K. Two overlapping doublets (I and II) are observed, their relative intensity being strongly temperature dependent. The doublets are characterized by δE_Q(I)=0.53 mm sec^?1, δ^IS(I)=+ 0.22 mm sec^?1 and δE_Q(II)=1.33 mm sec^?1, δ^IS(II)=+0.23 mm sec^?1 at 4.2°K. In conjunction with magnetic data, the Mössbauer spectra are interpreted in terms of a singlet-triplet spin transition of the central iron(II) ion. In an applied magnetic field at 4.2°K, H_int=0, V_ZZ(I)>0, and V_ZZ(II)>0. The results are consistent with optical electronic and infrared vibrational spectra.     

Spontaneous Proton Chemistry Enables Ultralow-temperature and Long-life Aqueous Copper Metal Batteries

Aqueous copper metal batteries with acidic electrolytes are regarded as promising candidates for low-temperature energy storage, benefiting from fast kinetics of protons and acid resistance of copper. Here, a Cu(BF₄)₂ electrolyte that spontaneously generates protons is developed for ultralow-temperature copper metal batteries. Systematic studies demonstrate that the hydrolysis of BF₄⁻ generates more protons, rendering the Cu(BF₄)₂ among the most effective aqueous electrolyte capable of breaking hydrogen bonds in water molecules. This electrolyte endows a polyaniline/Cu battery to deliver a short charging time of 21 s and a charge/discharge capability of up to 10 A g⁻¹ at −30 °C, along with a high discharge specific capacity of 70 mAh g⁻¹ and a supercapacitor-comparable power density of 3000 W kg⁻¹. Furthermore, it can exhibit a long and stable cycling lifespan over 10 000 cycles at −50 °C and works well at −70 °C. This work provides an opportunity for intrinsically acidic electrolytes.     

Micellar Formation Study of Crown Ether Surfactants

Fluorescence probe and nuclear magnetic resonance (NMR) methods were employed to investigate the micellation of prepared crown ether surfactants, e.g. decyl 15‐crown‐5 and decyl 18‐crown‐6. Pyrene was employed as the fluorescence probe to evaluate the critical micellar concentration (CMC) of these surfactants in aqueous solutions while spin lattice relaxation times (T₁) and chemical shifts of H‐1 NMR were applied in non‐aqueous solutions. Decyl 15‐crown‐5 with lower CMC forms micelles much easier than decyl 18‐crown‐6 with higher CMC in aqueous solutions, whereas decyl 18‐crown‐6 forms micelles easier than decyl 15‐crown‐5 in nonaqueous solutions. Comparison of the CMC of crown ether surfactants and other polyoxyethylene surfactants such as decylhexaethylene glycol was made. Effects of salts and solvents on the micellar formation were also investigated. In general, additions of both alkali metal salts and polar organic solvents into the aqueous surfactant solutions increased in the CMC of these surfactants. The formation of micelles in organic solvents such as methanol and acetonitrile was successfully observed by the NMR method while it was difficult to study these surfactants in organic solutions by the pyrene fluorescence probe method. The NMR study revealed that the formation of micelles resulted in the decrease in all H‐1 spin lattice relaxation times (T₁) of hydrophobic groups, e.g. CH₃ and CH₂, and hydrophilic group OCH₂ of these surfactants. However, upon the micellar formation, the H‐1 chemical shifts (δ) of these surfactant hydrophobic groups were found to shift to downfield (increased δ) while the chemical shift of the hydrophilic group OCH₂ moved to up‐field. Comparison of the spin lattice relaxation time and H‐1 chemical shift methods was also made and discussed.     

Anionic and cationic nanocomposite hydrogels reinforced with cellulose and chitin nanowhiskers: effect of electrolyte concentration on mechanical properties and swelling behaviors

Various nanocomposite gels were prepared using cellulose nanowhiskers (CNWs) or chitin nanowhiskers (ChNWs) as reinforcing fillers and hydroxypropyl cellulose (HPC), carboxymethyl cellulose (CMC), or chitosan as network polymers. The use of CNWs with low surface charge induced significant CNWs aggregations, which were well explained by depletion effect. Young's modulus E and swelling ratio Q of CNWs/HPC · CMC gels were highest at zero electrolyte concentration and decreased above 0.01 M electrolyte, whereas stress at break σ of the gels showed its minimum at zero electrolytes and increased with an addition of electrolytes. In the case of ChNWs/chitosan gels, maximum of E and Q was located at 0.01 M electrolyte concentration, and σ did not indicate clear tendency with electrolyte concentration. Although all gels indicated an increase in E and a decrease in Q with an increase in whisker content, the most remarkable changes were observed under an absence of electrolytes, whereas the changes under the presence of electrolytes were somewhat negligible. Copyright © 2014 John Wiley & Sons, Ltd.     

Aqueous films on silica in the presence of cationic surfactants

Summary The existence of thick, stable water films on silica as described byDerjaguin and Kussakov (1939) has been confirmed in several laboratories. There is much evidence showing that their thicknesses can be substantially accounted for by electrical double layer theory. Such wetting films can be influenced by a third component. For example, they are thinned in the presence of inorganic electrolytes or rendered unstable (or metastable) by prior methylation of the silica surface.Here, we consider the influence of alkyltrimethyl ammonium bromides on the stability of aqueous films on polished fused silica. In particular, the thickness and stability of aqueous films of C₅, C₈, C₁₀ and C₁₆ trimethyl ammonium bromides were studied over a range of surfactant concentrations. Contact angles of the TAB solutions on silica were also measured.The concentration of each cationic surfactant proved important in determining film stability. In dilute solution (well below the bulk CMC) uniform films could be formed only below a critical concentration which varied with the chainlength of the surfactant. The longer the chainlength, the lower was the critical concentration. In this regime, the film thickness decreased significantly with increasing surfactant concentration. This decrease was greater than that found for a simple 1:1 electrolyte such as KCl at the same concentration. As the surfactant concentration increased towards the critical value, the uniform films became increasingly more prone to rupture.Once the critical concentration of each surfactant was exceeded, uniform wetting films on silica could not be formed. The films ruptured immediately regardless of their initial diameter. In this regime, the contact angles, measured through the aqueous phase, increased with TAB concentration up to a maximum value which occurred near the CMC.For C₈, C₁₀ and C₁₆ trimethyl ammonium bromides, wetting films were again formed at concentrations above the CMC. At these concentrations, films were studied in detail only for the C_l6 surfactant. In this case, the wetting films were found to be somewhat thicker than free aqueous films of anionic surfactants formed at similar concentrations and hydrostatic pressures.

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Zusammenfassung Die Existenz dicker stabiler Wasserfrlme auf SiO₂ (Derjaguin undKussakov (1939)) ist in verschiedenen Laboratorien bestätigt worden. Es gibt zahlreiche Hinweise, nach denen die Dicke durch die elektrische Doppelschichttheorie gedeutet werden kann. Solche benetzenden Filme können durch eine dritte Komponente beeinflußt werden; in Gegenwart anorganischer Elektrolyte werden sie dünner, durch vorhergehende Methylierung der SiO₂-Oberfläche instabil oder metastabil.In der Arbeit wird der Einfluß von Alkyltrimethylammoniumbromiden (C₅, C₈, C₁₀ und C₁₆ auf die Stabilität von Wasserfilmen auf polierten Quarzglasoberflächen untersucht. Die Konzentration der kationischen Tenside auf die Filmstabilität erwies sich als wichtig. In verdünnten Lösungen bilden sich einheitliche Filme nur unter einer kritischen Konzentration, die von der Kettenlänge des Tensids abhängt. Über der kritischen Konzentration bilden sich keine einheitlich benetzenden Filme; sie reißen sofort unabhängig von ihrem ursprünglichen Durchmesser. Es wird versucht, die Ergebnisse auf der Basis einer Doppelschichttheorie zu deuten.

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With 7 figures and 2 tables     

Dilute solutions of polyelectrolytes—III. Unperturbed dimensions of poly(methylene) N,N-dimethylpiperidinium chloride) in aqueous and methanolic solutions in presence of tetraethylammonium bromide

The behaviour of poly(methylene N,N-dimethylpiperidinium chloride) in aqueous and methanolic solutions in the presence of tetraethylammonium bromide has been investigated by light scattering; the parameters characterizing the macromolecular chain have been calculated. Peculiar behaviour of the macromolecules in aqueous solutions was attributed to the influence of the simple electrolyte co-ion due to hydrophobic interactions.     

Volumetric and Viscometric Studies on Sodium Nitrate and Potassium Nitrate in Aqueous and H<Subscript>2</Subscript>O-urea Solutions

Summary.  Density and viscosity of NaNO₃ and KNO₃ in aqueous and in H₂O-urea solutions were determined as a function of electrolyte concentrations at 308, 313, 318, 323, and 328 K, respectively. The apparent molal volume (φ _v ) of the electrolytes were found to be linear functions of the square root of the solute molality (b). The φ _v and data were fitted to the Masson equation [1] by the least square method to obtain the apparent molar volume at infinite dilution (φ _v ^{^}), which is practically equal to the partial molar volume . The viscosity coefficients A and B were calculated on the basis of the viscosity of the solutions and the solvent concerned using the Jones–Dole [2] equation. The activation parameters for viscous flow (ΔG ^≠, ΔS ^≠, and ΔH ^≠) were calculated according to Eyring [3]. The values of for the two systems were also calculated from B-coefficient data. The results were found to be of opposite nature in the two electrolyte systems. Where sodium nitrate showed structure making behaviour both in aqueous and in H₂O-urea solutions, KNO₃ showed structure breaking behaviour in aqueous solutions and structure making behaviour in 5 molal H₂O-urea solutions in the studied temperature range. The behaviour of these two electrolytes in aqueous binary and in aqueous-urea ternary systems are discussed in terms of charge, size, and hydrogen bonding effects. Corresponding author. E-mail: chemistry_ru@yahoo.com Received January 24, 2002; accepted (revised) April 5, 2002     

Conductance of salts at moderate and high concentrations in propylene carbonate-dimethoxyethane mixtures at temperatures from −45°C to 25°C

The specific conductances of LiClO₄, KSCN, KPF₆, and Et₄NPF₆ in propylene carbonate-dimethoxyethane mixtures are reported for moderate to high concentrations at temperatures between –45°C to 25°C. The data analysis was established on the base of an empirical conductance equation permitting equally well adapted fits for all electrolyte solutions irrespective of temperature, electrolyte compound and solvent composition. The effects on the conductance resulting from ion-ion, ion-solvent interactions and solvent viscosity are rationalized in terms of the thermodynamic and hydrodynamic parameters characterizing the behaviour of electrolytes in dilute solutions. Some basic aspects are discussed for optimizing the conductance of battery electrolytes, especially those of high energy density batteries.     

Dependence of solar cell performance on the nature of alkaline counterion in gel polymer electrolytes containing binary iodides

Performance of dye-sensitized nano-crystalline TiO₂ thin film-based photo-electrochemical solar cells (PECSCs) containing gel polymer electrolytes is largely governed by the nature of the cation in the electrolyte. Dependence of the photovoltaic performance in these quasi-solid state PECSCs on the alkaline cation size has already been investigated for single cation iodide salt-based electrolytes. The present study reports the ionic conductivity dependence on the nature of alkaline cations (counterion) in a gel polymer electrolyte based on binary iodides. Polyacrylonitrile-based gel polymer electrolyte series containing binary iodide salts is prepared using one of the alkaline iodides (LiI, NaI, KI, RbI, and CsI) and tetrapropylammonium iodide (Pr₄NI). All the electrolytes based on binary salts have shown conductivity enhancement compared to their single cation counterparts. When combined with Pr₄NI, each of the Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ cation containing iodide salts incorporated in the gel electrolytes has shown a room temperature conductivity enhancement of 85.59, 12.03, 12.71, 20.77, and 15.36%, respectively. The conductivities of gel electrolytes containing binary iodide systems with Pr₄NI and KI/RbI/CsI are higher and have shown values of 3.28, 3.43, and 3.23 mS cm⁻¹, respectively at room temperature. The influence of the nature of counterions on the performance of quasi-solid state dye-sensitized solar cells is investigated by assembling two series of cells. All the binary cationic solar cells have shown more or less enhancements of open circuit voltage, short circuit current density, fill factor, and efficiency compared to their single cation counterparts. This work highlights the importance of employing binary cations (a large and a small) in electrolytes intended for quasi-solid state solar cells. The percentage of energy conversion efficiency enhancement shown for the PECSCs made with electrolytes containing Pr₄NI along with Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ iodides is 260.27, 133.65, 65.27, 25.32, and 8.36%, respectively. The highest efficiency of 4.93% is shown by the solar cell containing KI and Pr₄NI. However, the highest enhancements of ionic conductivity as well as the energy conversion efficiency were exhibited by the PECSC made with Li⁺-containing binary cationic electrolyte.