Conductivity Of Electrolyte Solutions Research Articles

Conductivity Improvement of LiBF4 Containing Electrolyte for Enhanced Application in Lithium-Ion Batteries

In view of a possible cost reduction and safety improvement of Lithium-Ion-Batteries (LIBs), the exchangeability of the common electrolyte salt lithium hexafluorophosphate LiPF6 with lithium tetrafluoroborate LiBF4 was investigated.[1,2] Replacement of LiPF6 with LiBF4 was considered due to the salt’s superior thermal stability and moisture stability compared to LiPF6. While LiBF4 was repeatedly studied for application in LIBs, low conductivity compared to analogous electrolyte solutions with LiPF6 is referred to as one of the main drawbacks of LiBF4 electrolytes.[1,3] These differences in electrolyte solution conductivity are usually related to an increased tendency for ion pair formation between the [BF4]– anion and Li+.[1,4] To overcome the strong ion-pairing between Li+ and [BF4]–, the conductivity of a LiBF4 electrolyte solution was improved by introduction of different amounts of 1,2-dimethoxyethane (DME) as bidentate ligand into the commercially available electrolyte solvent L57 (ethyl carbonate : ethylmethyl carbonate, 30 : 70, by wt%). The solvent modification was carried out in consideration of the published superior oxidative stability of lithium associated oligoethers compared to non-associated analogues.[5] All electrolyte solutions were investigated for their electrochemical behavior and their performance in Lithium-Ion-Battery (LIB) cells with commercially available cell components. All results were referred to analogous measurements with LiPF6 and LiBF4 electrolyte solution in the unmodified L57 solvent. Electrochemical characterization was performed by conductivity measurements and cyclic voltammetry. Battery cell experiments were carried out for Lithium-Metal cells with excess of electrolyte solution and Lithium-Ion cells. Lithium nickel cobalt manganese oxide (NCM622) electrodes were implemented as positive electrodes and Lithium-Ion cells were assembled with graphite electrodes as negative electrodes. All battery cells were investigated for their ambient temperature cycle life. Furthermore, ambient temperature C-rate stability of Li-Ion cells was examined. In view of the electrochemical behavior, conductivity of LiBF4 electrolytes significantly increases with the introduction of DME to the chosen electrolyte solvent, which is shown in the figure presenting the temperature dependent conductivities of the investigated electrolyte solutions. This conductivity enhancement is decreasing towards the contribution of more than two equivalents of DME per Li+ ion in the electrolyte solution. Cyclic voltammetry experiments of the electrolyte solutions show negligible effects on reductive stability of the electrolyte solution system, while the respective oxidative stability distinctly decreases with the addition of DME into the electrolyte solvent. A decreased oxidative stability is considered to induce parasitic reactions at the positive electrode, which presumably can be affected by change of positive electrode material. Despite the differences in conductivity, battery cell experiments present LiBF4 as a comparable electrolyte salt to LiPF6, with a performance difference that could possibly be overcome by utilization of the right electrolyte additive(s). The investigated oligoether containing electrolyte solutions show inferior battery performance compared to the unmodified carbonate-based electrolyte solvent. Since the battery cycling experiments were prepared at the oxidative stability limit of these electrolyte solutions, these electrolyte solutions might still create promising battery cell performance with a change of positive electrode material. In order to further investigate the ion pair association of lithium cations and tetrafluoroborate anions, the utilized system was additionally investigated spectroscopically and with quantum chemical calculations on the basis of density functional theory. Spectroscopic characterization was carried out with help of stimulated spin echo experiments in nuclear magnetic spectroscopy and Raman spectroscopy measurements. Both spectroscopies present changes in the associated structure of lithium cation and tetrafluoroborate anions by addition of DME to the commercial electrolyte solvent. For further conclusions, these results were compared with the calculations prepared for the electrolyte solution system. In summary, spectroscopic characterization and quantum chemical calculations indicate a modification of the associated structures in solution by the addition of DME to the chosen electrolyte solvent. By comparison of all collected results, research concerning the application of LiBF4 electrolytes should be continued with adaption of electrolyte solution additives or positive electrode material, respectively.[1] C. Daniel, J. O. Besenhard (Hrsg.) Handbook of battery materials, Wiley-VCH-Verl., Weinheim, 2011.[2] M. Winter, R. J. Brodd, Chem. Rev. 2004, 104, 4245.[3] R. Korthauer (Hrsg.) Handbuch Lithium-Ionen-Batterien, Springer Vieweg, Berlin, Heidelberg, 2013.[4] H. Tsunekawa, A. Narumi, M. Sano, A. Hiwara, M. Fujita, H. Yokoyama, J. Phys. Chem. B 2003, 107, 10962.[5] K. Yoshida, M. Nakamura, Y. Kazue, N. Tachikawa, S. Tsuzuki, S. Seki, K. Dokko, M. Watanabe, J. Am. Chem. Soc. 2011, 133, 13121. Figure 1

Electrophoresis of ions and electrolyte conductivity: From bulk to nanochannels.

When electrolyte solutions are confined in micro- and nanochannels their conductivity is significantly different from those in a bulk phase. Here we revisit the theory of this phenomenon by focusing attention on the reduction in the ion mobility with the concentration of salt and a consequent impact to the conductivity of a monovalent solution, from bulk to confined in a narrow slit. We first give a systematic treatment of electrophoresis of ions and obtain equations for their zeta potentials and mobilities. The latter are then used to obtain a simple expression for a bulk conductivity, which is valid in a concentration range up to a few molars and more accurate than prior analytic theories. By extending the formalism to the electrolyte solution in the charged channel the equations describing the conductivity in different modes are presented. They can be regarded as a generalization of prior work on the channel conductivity to a more realistic case of a nonzero reduction of the electrophoretic mobility of ions with salt concentration. Our analysis provides a framework for interpreting measurements on the conductivity of electrolyte solutions in the bulk and in narrow channels.

HARDWARE AND SOFTWARE COMPLEX FOR CONDUCTOMETRIC MEASUREMENTS WITH HIGH METROLOGICAL RELIABILITY

The article summarizes and supplements the results of the development of methods and tools for improving the reliability of differential measurement of local changes in electrical conductivity in electrolyte solutions using a pair of interdigitated transducers, which are used in conductometric biosensors. The aim of the work is a deeper suppression of the error due to the influence of background changes in a solution when the parameters of a pair of transducers are not completely identical. The essence of the issue, the causes and mechanism of the resulting error, and possible ways to reduce it are considered. The structure and algorithms of operation of a computer biosensor system with an intelligent module of a differential conductometric channel based on an alternating current bridge, with balancing in absolute value and phase of the disequilibrium signal, are described. Methods have been developed for tuning such a bridge to a special quasi- equilibrium state, in which a change in the background electrical conductivity does not change its output informative signal. The results of experimental studies are presented.

ЗМЕНШЕННЯ ВПЛИВУ ОПОРІВ ПЕРЕНОСУ ЗАРЯДУ У БІОСЕНСОРНИХ КОНДУКТОМЕТРИЧНИХ ВИМІРЮВАЛЬНИХ ПЕРЕТВОРЮВАЧАХ

The article proposes the further development of a method of reducing the error of measuring local changes in the electrical conductivity of electrolyte solutions caused by a change in the background electrical conductivity of the measuring medium. Such an error occurs in differential conductometric biosensors with working and reference transducers, if their electrical parameters differ significantly. In contrast to the previously known, the described method takes into account the influence of near-electrode charge transfer resistances, which becomes especially noticeable when there is a significant difference in the values of these parameters of the converters. The goal is to ensure deep suppression of the influence of background changes in electrical conductivity of solutions under significant differences in the reactive and active component impedances of pairs of sensor transducers, including near-electrode charge transfer resistances. Mathematical expressions characterizing the process of bringing the bridge measuring circuit to a specific state of quasi-balance, in which the influence of changes in the background electrical conductivity of electrolyte solutions is minimal, are considered. Formulas for calculating the voltage parameters on the reference converter have been obtained, which make it possible to establish the indicated mode of the measuring circuit. A technique for determining the values of charge transfer resistances using the results of additional measurements of the impedance parameters of the converters at a frequency higher than the operating frequency is proposed. The results of research on suppressing the effect of changes in the background electrical conductivity of the solution in the differential conductometric channel using a computer model are presented, which showed the possibility of significantly reducing the effect of changes in the background electrical conductivity of the solution compared to the known method. References 9, figures 2, table 1.

On the estimation of equivalent conductivity of electrolyte solutions: The effect of relative static permittivity and viscosity

The effect of a concentration-dependent relative static permittivity (RSP) and viscosity on the predictions of the equivalent conductivity models is studied. The full version and simplified version of the mean spherical approximation models along with four Debye-Hückel-Onsager (DHO) based equivalent conductivity models are used to predict the equivalent conductivity of nine 1:1 aqueous electrolyte solutions at 25°C. Three of the DHO based models have been developed in this study by combining the electrophoretic and relaxation effects that have been previously presented in the literature. To investigate the effect of RSP on the equivalent conductivity predictions, we have used three empirical RSP models along with the solvent's RSP. It has been shown that using a concentration-dependent RSP reduces the deviation of equivalent conductivity predictions from the experimental data. We have also investigated the effect of a concentration-dependent viscosity on the equivalent conductivity predictions. Our observations show that using a concentration-dependent viscosity sometimes improves and sometimes worsens the predictions.

OPTIMIZATION OF BALANCING IN A BRIDGE MEASURING CIRCUIT WITH A DIFFERENTIAL CONDUCTOMETRIC SENSOR

The article is devoted to solving the problem of the occurrence of an additive error in determining local changes in the electrical conductivity of electrolyte solutions under conditions of changes in the background electrical conductivity of the measurement medium, which often occurs in biosensor and other systems with a differential pair of conductometric transducers, if their electrical parameters are not identical. The goal is to provide a deep suppression of the influence of background changes with significant differences in both reactance and active resistance in the transducers of a pair of sensor. The essence of the issue, the causes and mechanism of this type of error, as well as the methods and means of its reduction, developed earlier, are briefly considered. A diagram and description of the structure of a differential conductometric channel of a biosensor system based on an AC bridge, an algorithm for its balancing operations by controlling the module and phase of the test voltage, as well as a vector diagram of currents and voltages in the bridge circuit during this process. The balancing of the bridge has been was modeled analytically, bringing it to a quasi-equilibrium state, in which changes in the background electrical conductivity do not change its output signal. Additional operations for balancing the bridge are determined to achieve such a state with significant differences in both capacitances and active resistances in the impedances of a pair of conductometric transducers of a differential sensor. The results of experimental studies of the suppression of the influence of changes in the background electrical conductivity of a solution in a differential conductometric channel with using its computer model and experimental sample of a conductometric instrument with an electrical equivalent of a differential sensor are presented. A comparison of the results obtained and the corresponding data for balancing bridge circuits by previously developed methods is given. References 16, figures 3, tables 3.

Fast evaluation technique for the shear viscosity and ionic conductivity of electrolyte solutions

With the growing need to obtain ideal materials for various applications, there is an increasing interest in computational methods to rapidly and accurately search for materials. Molecular dynamics simulation is one of the successful methods used to investigate liquid electrolytes with high transport properties applied in lithium-ion batteries. However, further reduction in computational cost is required to find a novel material with the desired properties from a large number of combinations. In this study, we demonstrate an effective fast evaluation technique for shear viscosity and ionic conductivity by molecular dynamics simulation for an exhaustive search of electrolyte materials with high transport properties. The proposed model was combined with a short-time correlation function of the stress tensor and empirical relationships to address the issues of inefficient and uncertain evaluation by conventional molecular dynamics methods. Because we focus on liquid electrolytes consisting of organic solvents and lithium salts, our model requires dissociation ratio and effective diffusion size of lithium salts. Our method is applied to search for the compositional combinations of electrolytes with superior transport properties even at low temperatures. These results correlate well with experimental results.

Comparison of Models for the Prediction of the Electrical Conductivity of Electrolyte Solutions

Comparison of Models for the Prediction of the Electrical Conductivity of Electrolyte Solutions

A new photoelectrochemical reactor designed for solar hydrogen fuel production: Experimental study

A novel photoanode configuration of meshy type dome design is examined to exploit the maximum accessible light during available sun time at any solar rays angle due to its unique structure. This photoanode shape aims to receive the maximum light quantity by utilizing the possible illumination area. Also, the meshy design lets the solar ray spread into the body of the dome photoanode at any solar rays angle. Titanium dioxide is prepared to coat the stainless steel photoanode using the sol–gel dip-coating method. Two concentrations of potassium dioxide solution are used to study the electrolyte solution conductivity effects on the hydrogen production rate and the associated energy and exergy efficiencies. The vertical and tilted light boundaries are studied to evaluate the enhancement of the hydrogen production rate and the corresponding system efficiencies due to the new dome mesh photoanode design. The present experimental results indicate that increasing the solution concentration significantly affects the hydrogen production rates and associated system efficiencies. The greatest hydrogen mass production rates produced at 0.25 M KOH under artificial tilted (45°) and vertical light boundaries are 31.55 µg/s and 32.22 µg/s, respectively. The overall energy and exergy system efficiencies can be reached to be 5.98% and 4.08% under vertical light, while it is 5.89% and 4.02% at tilted light boundaries. The development of the hydrogen production rate obtained at artificial tilted light boundaries changes between 74% and 80% compared to the enhancement which occurred at vertical artificial light boundaries.

On the molecular correlations that result in field-dependent conductivities in electrolyte solutions.

Employing recent advances in response theory and nonequilibrium ensemble reweighting, we study the dynamic and static correlations that give rise to an electric field-dependent ionic conductivity in electrolyte solutions. We consider solutions modeled with both implicit and explicit solvents, with different dielectric properties, and at multiple concentrations. Implicit solvent models at low concentrations and small dielectric constants exhibit strongly field-dependent conductivities. We compare these results to Onsager-Wilson theory of the Wien effect, which provides a qualitatively consistent prediction at low concentrations and high static dielectric constants but is inconsistent away from these regimes. The origin of the discrepancy is found to be increased ion correlations under these conditions. Explicit solvent effects act to suppress nonlinear responses, yielding a weakly field-dependent conductivity over the range of physically realizable field strengths. By decomposing the relevant time correlation functions, we find that the insensitivity of the conductivity to the field results from the persistent frictional forces on the ions from the solvent. Our findings illustrate the utility of nonequilibrium response theory in rationalizing nonlinear transport behavior.

Concentration and temperature empirical relationships of the electrical conductivity of electrolyte solutions

We have considered the dependences of the specific (κ) and molar (Λ) electrical conductivity (EC) of aqueous electrolyte solutions on the molar concentration and temperature for sulfates of divalent metals (Mn, Co, Ni, Cu, Zn, Cd) in a wide concentration range at 5 – 35°C. To describe such systems we propose a modified cubic equation (MCE): κ = C∙c3k + Q∙c2k + L∙ck, where C, Q, L, k are empirical parameters, fixed parameter k = 0.5 has been considered as well. From the correlation between the calculated parameters we assume that two of them are sufficient. The maximum of specific EC (κm) and the corresponding concentration (cm) have been calculated. We also assume that the systems under study are isomorphic in the normalized coordinates (κ/κm via c/cm). For the dependences like κ = A∙cx + B∙cy it is shown that x = 1 is a good approximation over the generalized sample. Empirical dependences with y = 5/4 and y = 4/3 are also considered. It is shown that they give comparable results to MCE. The proposed approach is tested on EC data of aqueous solutions of some salts. Similar two-parameter κ(κm, cm; c) equations of other authors have been considered. In order to describe the dependence of the specific EC on temperature and concentration we propose an equation κ = (A25 + a∙θ)∙c – (B25 + b∙θ)∙c5/4, where θ is the reduced temperature and A25, a, B25 and b are empirical parameters. Also a generalized equation for the molar EC of concentrated electrolyte solutions is proposed: Λ(Λ*, Λm, cm; c), where Λ* is the effective limiting molar EC, and Λm is the molar EC at c = cm. It was found that Λ* and Λm depend linearly on temperature. The average value of the exponent is close to 1/3, which brings the generalized molar EC equation closer to the equation derived from the quasi-lattice model of electrolyte solutions.

Physical and Electrolytic Properties of Dialkoxyethanes

1,2-Dimethoxyethane (DME, ethylene glycol dimethyl ether) and 1,2-diethoxyethane (DEE, ethylene glycol diethyl ether) are dialkyl ethers of ethylene glycol. DME shows a high boiling point as compared to diethyl ether and tetrahydrofuran. DME is also used as a reagent in organometallic chemistry and as a solvent in some electrolyte solutions for lithium batteries. The solubility of lithium salts in DEE is much less than that in DME. Partial fluorination of DEE can improve the solubility of lithium salts. In contrast, the polyfluorinated and perfluorinated organic solvents show very low polarity. These solvents are the so-called fluorous media and are often immiscible with organic solvents as well as water. We report the physical and electrochemical properties of fluorinated DEEs: monofluorinated, trifluorinated, and tetrafluorinated DEEs (EFEE, ETFEE, and FETFEE, respectively).The relative permittivity reflects the ease of progress of dielectric polarization. The relative permittivities of FETFEE, EFEE, and EFTEE decreased linearly with an increase in temperature. The relative permittivities of FETFEE, EFEE, and EFTEE decreased linearly with an increase in temperature. The relative permittivity of FETFEE was higher than those of ETFEE and EFEE: FETFEE > ETFEE ≈ EFEE > DEE. The viscosity is the property which opposes the relative motion of the adjacent layers of the liquid. It is regarded as the internal friction and affects electrolytic conductivity. The viscosity (η) of FETFEE was also higher than those of EFEE and ETFEE: FETFEE > EFEE > ETFEE > DEE. The kinematic viscositsiy (v) of ETFEE was as low as that of DEE above 323 K. The plots of log10(η/mPas) vs. T −1 and the plots of log10(v / 10−2 m2 s−1) vs. T −1 gave straight lines.The conductivity of an electrolyte solution is a key factor determining the internal resistance and rate performance of lithium batteries. The conductivities (κ) of electrolyte solutions in ETFEE and FETFEE increased with an increase in temperature (T). The viscosities of the fluorinated DEEs decreased rapidly with an increase in temperature and approached that of DEE at high temperatures. Interestingly, the conductivity of LiPF6 solution in FETFEE was higher than that in ETFEE: EFEE > FETFEE > ETFEE. The partial fluorination can increase the ability of the solvent molecules to form hydrogen bonds. The number of hydrogen atoms that are bound to the same terminal carbon atom as fluorine atoms are as given below: FETFEE (2), EFEE (2), ETFEE (0), and DEE (0). The attraction between FETFEE molecules can be based on nonconventional weak intermolecular hydrogen bonding (CF−H···O or C−H···F−C). The weak hydrogen-bonding system does not exchange its proton and therefore it is no more a genuine hydrogen bond; it is an electrostatic attraction between positive charge on the hydrogen and negative charge on the organic fluorine or the organic oxygen.The hydrogen bonding leads to the high acceptability of an electron-pair of a donor atom from a solute. This effect may result in the increased solvation of PF6 − ions and, consequently, in the higher degree of ionic dissociation. The fluoromethyl group (CH2F-) may serve as an anion-attracting group. In contrast, the polyfluorination can decrease the electron-pair donability of an oxygen atom of the C-O-C moiety. The trifluoromethyl group (CF3-) is a strong electron-withdrawing substituent. The anodic stability of FETFEE was higher than that of EFEE: FETFEE > ETFEE > EFEE. The finding suggests that the electron-pair donability decreases in the inverse order: FETFEE < ETFEE < EFEE. Fluorine is the most electronegative of all the elements. Most conventional measures for the electronic effect of substituents are provided as Taft constants (σ*) for substituents attached to aliphatic chains. The Taft constants of a trifluorometheyl group (CF3-: Taft σ* = 2.61, σ(calcd.) = 2.60) and a fluorometheyl group (CH2F-: Taft σ* = 1.10, σ(calcd.) = 1.17) are substantially larger than that of a metheyl group (CH3-: Taft σ* = 0, σ(calcd.) = −0.07). The value of 1.10 is comparable with a formyl group (-CHO: Taft σ* = 1.1, σ(calcd.) = 1.09).The product of conductivity and viscosity (κη) observed for FETFEE, ETFEE, and EFEE was slightly decreasing at high temperatures. The plots of log10(η/mPas) vs. T −1 gave straight lines. In contrast, the plots of log10(κ/mScm−1) vs. T −1 displayed upward curvature. The decrease in the κη and the convex curve show that the conductivities of LiPF6 solutions in FETFEE, ETFEE, and EFEE are not inversely proportional to the viscosities at elevated temperatures.

VDAC Gating Thermodynamics, but Not Gating Kinetics, Are Virtually Temperature Independent

The voltage-dependent anion channel (VDAC) is the most abundant protein in the mitochondrial outer membrane and an archetypical β-barrel channel. Here, we study the effects of temperature on VDAC channels reconstituted in planar lipid membranes at the single- and multichannel levels within the 20°C to 40°C range. The temperature dependence of conductance measured on a single channel in 1 M KCl shows an increase characterized by a 10°C temperature coefficient Q10 = 1.22 ± 0.02, which exceeds that of the bathing electrolyte solution conductivity, Q10 = 1.17 ± 0.01. The rates of voltage-induced channel transition between the open and closed states measured on multichannel membranes also show statistically significant increases, with temperatures that are consistent with activation energy barriers of ∼10 ± 3 kcal/mol. At the same time, the gating thermodynamics, as characterized by the gating charge and voltage of equipartitioning, does not display any measurable temperature dependence. The two parameters stay within 3.2 ± 0.2 elementary charges and 30 ± 2 mV, respectively. Thus, whereas the channel kinetics, specifically its conductance and rates of gating response to voltage steps, demonstrates a clear increase with temperature, the conformational voltage-dependent equilibria are virtually insensitive to temperature. These results, which may be a general feature of β-barrel channel gating, suggest either an entropy-driven gating mechanism or a role for enthalpy-entropy compensation.

Experimental and Modeling of Conductivity for Electrolyte Solution Systems.

Studying the concentration and temperature dependence of the conductivity of electrolyte solution is of great significance for the evaluation and improvement of the performance of the electrochemical system. In this paper, based on the influence of the number of free ions and ion mobility on the conductivity, a semiempirical conductivity model with five parameters was proposed to correlate the conductivity, concentration and temperature data of electrolyte solutions at medium and high concentrations. The conductivities of NaCl and CaCl2 in propylene carbonate–H2O binary solvents were measured at temperatures varying from 283.15 to 333.15 K. The validity of the model was verified by the experimental data of this paper and the conductivity, concentration, and temperature data of 28 electrolyte solution systems in the literature. The electrolyte solutions investigated in this paper included binary organic solvent systems, pure organic solvent systems, and aqueous solution systems. The results showed that the proposed model can fit the experimental data well for both pure solvent and mixed solvents systems, which is of great value to practical engineering applications.

Constructing a simple electrolyte conductivity meter for chemistry learning

This research is based on the idea of the need for an innovative experimental learning media in the study of physical chemistry on electrolyte solutions conductivity concept. Through Research and Development method with the stages of analysis, design and development, a simple electrolyte conductivity measurement device has been produced. Based on the results of the research, the product was obtained in the form of an electrolyte solution conductivity measuring device that equipped with a user guidebook and an experimental worksheet of the electrolyte solution conductivity measurement. The electrolyte solution conductivity device is designed to measure the conductivity of an electrolyte solution. This measurement device is able to compare the conductivity value of fruit juices, vegetable juices and isotonic drinks quantitatively based on Kohlrausch’s law with simple and easy devices and materials. The validation result shows that the product is valid. The product was tested to chemistry education students using a feasibility test instrument. The result shows that the average of the value of the feasibility test is 93.85%. These results indicate that the constructed electrolyte conductivity meter is feasible to be used.

Investigation of Influence of Area of Electrodes on the Characteristics of Electrode Polarization-Based Flow Sensor

The electrode polarization-based flow meter is a simple and cost-effective flow meter, which utilizes the effect of flow of fluid on the polarization impedance that exists between the two electrodes for the measurement of flow rate of conductive fluid. It consists of two pairs of electrodes which are placed diametrically opposite to each other. Among these four electrodes, two diagonally placed electrodes are driven electrodes and other two diagonally placed electrodes serve as an output pickup. The output of polarization impedance-based flow metergets influence of excitation frequency, current density, ions constituents, conductivity of electrolyte solution, and temperature and area of electrodes. In this paper, we have analyzed and investigated the influence of electrodes area on the characteristics of electrode polarization-based flow meter. The experiment has been performed after the theoretical investigation and its results are reported in this paper. The performance of the flow meter is investigated by finite element method (FEM) usingComsol 5.4 Multiphysics simulation software. The simulation result shows that the variation in output of flow sensor with respect to the variation of diameter. The experiment results match quite well with simulations. It has been also observed that the result derived from the experiment exactly follows the theoretically derived equations.

The Effect of Support Salt on Penetration Depth of Reaction of Electrochemical Reduction of Sulfur into Volume of Positive Electrode of Lithium-Sulfur Batteries

Lithium-sulfur batteries are batteries with a liquid cathode. Electrochemical transformations of sulfur and the products of its reduction (lithium polysulfides and lithium sulfide) occur on the surface of carbon particles, forming a porous carbon conductive matrix (porous carbon electrode).The depth and rate of electrochemical transformations of sulfur and lithium polysulfides are determined by both the properties of the carbon matrix (porosity, specific surface area, electrical conductivity, sorption capacity towards sulfur and lithium polysulfides) and the properties of electrolyte systems. One of the most important parameters of a porous electrode is the characteristic length - the penetration depth of the electrochemical process into the electrode volume, the value of which is determined by the ratio of the conductivity of electrolyte and charge transfer resistance (equation).We studied the effect of the conductivity of electrolyte solutions and the surface capacity of sulfur electrodes on the depth of the electrochemical reduction of sulfur (figure). The sulfur electrodes (d=2.85 cm, A=6.38 cm2) used in this study had the following composition: S (99.5%, Acros) - 70% wt., Ketjenblack® EC-600JD (Akzo Nobel) - 10% wt., Polyethylene oxide (ММ=4x106, Aldrich) - 20% wt. The negative electrodes (d=2.55 cm, A=5.10 cm2) were made of lithium foil (99.9%, Russia) with a thickness of 120 micrometers. 1M solution of LiSO3CF3 in sulfolane (specific conductivity is 0.87 mS/cm at 30 0C) and 1M solution of LiClO4 in sulfolane (specific conductivity is 2.23 mS/cm at 30 0C) were used as electrolytes. The content of electrolytes into lithium-sulfur cells was 4 microliters per 1 mAh, which is equivalent to 6.7 microliters per 1 mg of sulfur. The electrolyte was introduced into lithium-sulfur cells using a “MICROLITERTM Syringe” (Hamilton Company, USA) with a volume of 50 microliters. The accuracy of electrolyte dosing into the cells was ±5 %.It was established that the penetration depth of the reactions of electrochemical reduction of sulfur and lithium polysulfides into volume of the sulfur electrode is higher in 1M solution of LiClO4 in sulfolane than 1M solution of LiSO3CF3 in sulfolane. The observed patterns are explained from the standpoint of the theory of a porous electrode.This work was performed as part Government Order of the Ministry of Science and Higher Education of the Russian Federation (theme No. АААА-А17-117011910031-7) and was also financially supported by Russian Science Foundation (Project No 17-73-20115) and by Russian Foundation for Basic Research (Project No 16-29-06190). Figure 1

A theoretical study on resistance of electrolytic solution: Measurement of electrolytic conductivity

We propose a theoretical approach to determine the conductivity of electrolytic solution using sine waveform voltage as a triggering signal. The approach is based on the acquisition of the complex impedance in two-electrode conductance cell by analyzing the equivalent circuit. We deduce the formula of resistance in electrolytic solution by doing a series of calculations in transformation, simplification, and replacement for the complex impedance and solving the quadratic equation with unknown solution resistance. Even though there exists electrode polarization or the distributed capacitance which may affect the measurement, we can still obtain the electrolytic conductivity. In this paper, we focus on deducing the expression for the value of resistance in electrolytic solution. To our knowledge, this is the first report on directly deducing the resistance’s value. In particular, the extent of the electrode polarization is also analyzed and mathematically formulated. The experimental results show that the method proposed in this paper can achieve high precision.

ĐỘ DẪN ĐIỆN RIÊNG CỦA DUNG DỊCH ĐIỆN PHÂN THEO NỒNG ĐỘ

ĐỘ DẪN ĐIỆN RIÊNG CỦA DUNG DỊCH ĐIỆN PHÂN THEO NỒNG ĐỘ

Drude-jellium model for the microwave conductivity of electrolyte solutions

The microwave conductivity characteristics of electrolyte solutions have attracted much interest of researchers because a good understanding of their properties plays a key role to study fundamental processes in biology and chemistry. In this work, we consider the solution of sodium chloride as a plasma consisting of ions with water background. Its plasmon frequency is calculated by the jellium theory. The linear dependence of the microwave conductivity on the ion concentration of the electrolyte solutions is explained by a microscopic approach and described by a combination of this plasmon relationship and the simplified Drude formula for dielectric constant. Furthermore, the dependence of the microwave conductivity on the frequency of the salt solution is also examined. We suggest that it obeys the logistic distribution. We found a good agreement between theoretical calculations and experimental data. The values of the damping coefficient γ for the conductive solutions at low frequencies and the cutting frequency are estimated. The linear dependence of the diffusion coefficient on the temperature of the salt solution is also shown, in similarity with the result in the other model. The application of the Drude-jellium model could be done for the other electrolyte solutions in order to study theirs electro-dynamic properties.