Electrolyte Solutions For Lithium Batteries Research Articles

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.

Properties of Ion Complexes and Their Impact on Charge Transport in Organic Solvent-Based Electrolyte Solutions for Lithium Batteries: Insights from a Theoretical Perspective

Electrolyte formulations in standard lithium ion and lithium metal batteries are complex mixtures of various components. In this article, we review molecular key principles of ion complexes in multicomponent electrolyte solutions in regards of their influence on charge transport mechanisms. We outline basic concepts for the description of ion–solvent and ion–ion interactions, which can be used to rationalize recent experimental and numerical findings concerning modern electrolyte formulations. Furthermore, we discuss benefits and drawbacks of empirical concepts in comparison to molecular theories of solution for a more refined understanding of ion behavior in organic solvents. The outcomes of our discussion provide a rational for beneficial properties of ions, solvent, co-solvent and additive molecules, and highlight possible routes for further improvement of novel electrolyte solutions.

On the Correlation between Free Volume, Phase Diagram and Ionic Conductivity of Aqueous and Non-Aqueous Lithium Battery Electrolyte Solutions over a Wide Concentration Range

There is little known about the transport behavior of ions in electrolyte solutions at very high concentrations and there is currently no one widely-accepted theory or equation to describe it over the whole concentration range. In this work, the ionic conductivity (κ) of lithium salts in aqueous and non-aqueous electrolyte solutions have been measured as a function of concentration (C) and have been fitted to known theoretical and empirical equations. A new, isothermal, semi-empirical equation based on free volume theory: where Vo and Vf are the occupied and unoccupied “free” volume, respectively, gives better fit over the whole concentration range than the known equations. Vf and V02, ϕ, the apparent molar volume of the salt, were calculated from density measurements and it is found that free volume decreases with concentration in both the aqueous and non-aqueous solutions over the whole range. We hypothesize that the changes to transport properties in solution with concentration are caused by structural changes that switches the conductivity mechanism from vehicular to a Grotthuss-type or a mixture of both. We, for the first time, correlate the origin of Cmax, the concentration of highest conductivity, to the eutectic composition in the salt-solvent phase diagram.

Correlation Between Free Volume and Ionic Conductivity of Non-Aqueous Lithium Battery Electrolyte Solutions over a Wide Concentration Range

There is little known about the transport behavior of ions in electrolyte solutions at very high concentration and there is currently no satisfactory theory/equation to describe it [1,2]. In this work electrolyte solutions of lithium salts of various anions: [(CF3SO2)2N]°¥, (TFSI), [PF6]°¥ and [BF4]°¥ dissolved in polar solvents and solvent mixtures up to saturation have been prepared. The solvents are propylene carbonate (PC), acetonitrile (ACN), adiponitrile (ADN) and ethylene carbonate:dimethyl carbonate (1:1 v/v) (EC:DMC). The Ionic conductivity and density of the electrolyte solutions have been measured over a wide concentration range (up to 5.4 M). The specific ionic conductivity (κ) vs. C plots show a non-ideal behavior with very strong dependence on concentration (a typical Gaussian-like function) while the molar conductivity (Λ) vs. C plots show an exponential decrease similar to the behavior of weak electrolyte solutions. The conductivity vs. concentration data were fitted to various equations of known models (Debye-Hückel-Onsager’s square-rate law: Λ vs. C^1/2, Cubic-root law: Λ vs. C^3, Casteel-Amis equation: κ/κmax vs. C/Cmax , or the corresponding state-law equation based on the pseudo-lattice theory: κ/κmax vs. C/Cmax .The models show good fit only in the low concentration, with the last two models fail to show universal behavior beyond the concentration of maximum conductivity, Cmax. However, we have derived a new, semi-empirical equation that shows a very good fit over the whole concentration range and can correlate molar ionic conductivity to changes in free volume within the liquid solution: Λ = A' exp [-B C], B =-γ Vo/Vf, and Vf = V-Vo Where Vƒ is the free volume which can be calculated from the difference between the measured volume, V, of the liquid (from density measurements) and the Van der Waal “molecular “volume, Vo, which can be obtained from XRD data at low temperatures or calculated using chemical models. C is concentration, A’ is a pre-exponential factor that can be related to limiting molar conductivity and square root of temerature. γ is a correction factor for overlapping holes with a value between 0.5 and 1. These preliminary results are encouraging in bringing us closer to a free volume-based approach to explain the behavior of ionic conductivity of electrolyte solutions at very high concentrations. However, more work is underway to study more solutions with different chemical and physical properties, including aqueous solutions, to test the validity of this approach. [1] S. I. Smedley, The Interpretation of Ionic Conductivity in Liquids, Springer, 1980 [2] C.A. Angell, J. Phys. Chem., 70, 1966, 3988

Solvent Activity in Electrolyte Solutions Controls Electrochemical Reactions in Li-Ion and Li-Sulfur Batteries

Solvent–ion and ion–ion interactions have significant effects on the physicochemical properties of electrolyte solutions for lithium batteries. The solvation structure of Li+ and formation of ion pairs in electrolyte solutions composed of triglyme (G3) and a hydrofluoroether (HFE) containing 1 mol dm–3 Li[TFSA] (TFSA: bis(trifluoromethanesulfonyl)amide) were analyzed using pulsed-field gradient spin–echo (PGSE) NMR and Raman spectroscopy. It was found that Li+ is preferentially solvated by G3 and forms a [Li(G3)]+ complex cation in the electrolytes. The HFE scarcely participates in the solvation because of low donor ability and relatively low permittivity. The dissociativity of Li[TFSA] decreased as the molar ratio of G3/Li[TFSA] in the solution decreased. The activity of G3 in the electrolyte diminishes negligibly as the molar ratio approaches unity because G3 is involved in 1:1 complexation with Li+ ions. The negligible activity of G3 in the electrolyte solutions has significant effects on the electroch...

Structural Interactions within Lithium Salt Solvates: Cyclic Carbonates and Esters

Only limited information is available regarding the manner in which cyclic carbonate and ester solvents coordinate Li+ cations in electrolyte solutions for lithium batteries. One approach to gleaning significant insight into these interactions is to examine crystalline solvate structures. To this end, eight new solvate structures are reported with ethylene carbonate, γ-butyrolactone, and γ-valerolactone: (EC)3:LiClO4, (EC)2:LiClO4, (EC)2:LiBF4, (GBL)4:LiPF6, (GBL)1:LiClO4, (GVL)1:LiClO4, (GBL)1:LiBF4, and (GBL)1:LiCF3SO3. The crystal structure of (EC)1:LiCF3SO3 is also re-reported for comparison. These structures enable the factors that govern the manner in which the ions are coordinated and the ion/solvent packing—in the solid-state—to be scrutinized in detail.

Vibrational spectroscopic and conductimetric studies of lithium battery electrolyte solutions

Extremely high concentrated solutions of lithium perchlorate in formamide and acetonitrile equimolar mixture were investigated by spectroscopic and conductimetric techniques. The spectral changes observed in the characteristic bands of the solvents, the appearance of the band at 939 cm −1 and the slight increase of conductivity in all solutions were interpreted in terms of a solvent separated ion pairs model. In fact, the mixture of solvents with considerable differences in the donor–acceptor characters allows to prepare electrolytic solutions where the contact ion pairs formation seems uncertain.

Viscosity Properties of Electrolytes in Propylene Carbonate Based Lithium Battery Electrolyte Solutions

Abstract Viscosities of LiClO4 and LiBr have been measured in solvent mixtures of propylene carbonate (PC) with dimethylformamide (DMF), tetrahydrofuran (THF), acetonitrile (AN) and methyl formate (MF) at 298.15K. The dependence of viscosity on the composition of the mixed solvents was fitted with an equation without adjustable parameter. Viscosity B-coefficients for lithium salts and the corresponding activation free energies (Δμ0,≠) for viscous flow have been evaluated. At the same time, viscosity B-coefficients were predicted by the dielectric friction theory. The unsuccessful prediction of the composition dependence of the B-coefficients indicates that improvements will be necessary on the theory with taking account of the short-range interaction and molecular nature of the solvents. Furthermore, solute–solvent interactions in these mixed solvents are discussed in terms of the B-coefficients and activation parameters.

A partial-molar volume study of electrolytes in propylene carbonate-based lithium battery electrolyte solutions at 298.15 K

Apparent molar volumes (V2, ϕ ) and standard partial-molar volumes (V20, ϕ ) of LiClO4 and LiBr at 298.15 K have been determined from precise density measurements in solvent mixtures of propylene carbonate (PC) with dimethylformamide (DMF), tetrahydrofuran (THF), acetonitrile (AN), and methyl formate (MF). The scaled particle theory is used to calculate the contributions of the cavity formation and the electrolyte-solvent interactions to the standard partial-molar volumes. It is shown that V20, ϕ is strongly dependent on the nature of the solvents, and the trends in V20, ϕ with composition of the solvent mixtures are determined by the interaction volumes of electrolytes with solvents. The results are discussed in terms of ionic preferential solvation, packing effect of solvents in the solvation shell, and electrostriction of solvents by ion.Key words: partial-molar volume, scaled particle theory, lithium salts, propylene carbonate, solvent mixtures, lithium battery electrolytes.

Initial Reaction in the Reduction Decomposition of Electrolyte Solutions for Lithium Batteries

The rotating ring‐disk electrode system and ab initio molecular orbital calculations were used to investigate the initial reaction in the reductive decomposition of electrolyte solutions for lithium batteries. It was confirmed that electron transfer is induced by the cathodic polarization of the electrode, forming soluble reduced species which can be reoxidized. This electron transfer is an exothermic reaction with a reaction energy of −3 eV, from the electrode to the solvent molecules coordinated with lithium ions. This is the initial reaction in the reductive decomposition of electrolyte solutions for lithium batteries, which must be controlled to improve the performance of lithium‐ion and lithium secondary batteries. © 2000 The Electrochemical Society. All rights reserved.

Kinetic Characterization of the Electrochemical Intercalation of Lithium Ions into Graphite Electrodes

Lithium intercalated graphites have taken the place of metallic lithium as anodes for secondary lithium batteries. Controlling the anode‐electrolyte interface has been a major technical challenge in the development of lithium‐ion battery technologies. The interfacial characteristics can be greatly affected by the kinetics of intercalation. However, the kinetics of the electrochemical intercalation of lithium into graphites has not been well analyzed yet. Very few kinetic and interfacial parameters have been reported. In this work, the electrochemical impedance spectroscopy, constant charge step, and galvanostatic pulse polarization techniques were applied to study the kinetics of the intercalation and deintercalation processes of graphite electrodes in a few important lithium battery electrolyte solutions. Based on the proposed equivalent circuit model, we determined the kinetic and interfacial parameters of the intercalation and deintercalation processes. The measured intercalation charge‐transfer resistance, exchange current densities, and intercalation capacitance range between 11 and , 1.0 and , and 1.0 and , respectively, depending on the electrolyte solution compositions. The dependence of these kinetic and interfacial parameters on solvent composition, electrolyte concentration, storage time, and intercalated state is discussed. In addition, the transfer coefficients have been determined. The results suggest that the intercalation/deintercalation process is electrochemically reversible. © 2000 The Electrochemical Society. All rights reserved.

Interactions Between Tetra(trifluoromethylsulfonyl)-1,4,8,11-tetraazocyclotetradecane and Perchlorate Anion in Propylene Carbonate, Nitromethane, Acetonitrile, and Tetrahydrofuran

Electrical conductance measurements are reported for lithium perchlorate andthe anion receptor tetra(trifluoromethylsulfonyl)-1,4,8,11-tetraazocyclotetradecane(TTCD) in different aprotic solvents (propylene carbonate, nitromethane,acetonitrile, and tetrahydrofuran). The data have been analyzed by a suitablemethod based on the Lee-Wheaton theory on mixed electrolytes in order to obtainthe true thermodynamic formation constants of macrocyclic-anion complexes andthe ion pairs of both the uncomplexed (ClO4−)and complexed (TTCD-ClO4−)anions. The results show that the anion-ligand formation constants increase withdecreasing dielectric constant and that the presence of the ligand increases theionization of lithium perchlorate and enhances the transference number of lithiumion. These findings are of particular interest in view of the technologicalapplication of anion receptors in electrolyte solutions for lithium batteries.

Ion solvation in carbonate-based lithium battery electrolyte solutions

Partial molar volumes at infinite dilution V MX ∞ of three electrolytes (LiClO 4, LiCF 3SO 3 and LiPF 6) have been determined by vibrating-tube densimetry in pure propylene carbonate (PC), ethylene carbonate (EC) and dimethylcarbonate (DMC) and, for LiClO 4, in some mixtures of EC with DMC and PC. V mx ∞ are strongly dependent of the nature of the pure or mixed solvents. Absolute ionic molar volumes of the Li ion and associated anions have been deduced from the partial molar volume of tetra-alkylammonium salts in the same solvents. For this purpose the concept of van der Waals volume and packing density proposed by King has been employed. In all solvents under investigation, the volume of the Li ion has similar values: V Li + ∞ ≈ 5 cm 3/mol. This result is different from what is obtained using the tetraphenylarsonium tetraphenylborate assumption. The differences among polyatomic anions volumes are greater than expected and strongly dependent on the solvents' nature. These effects are discussed in terms of ion-solvent interactions: ion solvation, packing effect in the solvation shell and electrostriction. Implication of these findings for Li batteries is briefly discussed.

A New Class of Electrochemically and Thermally Stable Lithium Salts for Lithium Battery Electrolytes: IV. Investigations of the Electrochemical Oxidation of Lithium Organoborates

Synthesis, analysis, and purification of lithium bis[2,2′‐biphenyldiolato(2‐)‐O,O′]borate, lithium bis[salicylato(2‐)]‐borate, lithium phenolate, and dilithium‐2,2′‐biphenyldiolate are described. Electrochemical studies show that lithium organoborates are subject to anodic decomposition mechanisms, which depend on their chemical structure. The anodic decomposition of lithium benzenediolatoborates results in the formation of soluble products including chinones, whereas the anodic oxidation of lithium bis[2,2'‐biphenyldiolato(2‐)‐O,O′]borate, lithium bis[salicylato(2‐)]borate, and lithium phenolates entails the formation of thin electronically insulating lithium ion‐conducting polymer films, preventing further anion decomposition as well as anodic decomposition of solvents which are sensitive to oxidation. These film‐forming materials are useful to enlarge the electrochemical window of electrolyte solutions for lithium batteries and electrochemical capacitors.

The use of in situ Fourier-transform infrared spectroscopy for the study of surface phenomena on electrodes in selected lithium battery electrolyte solutions

This paper presents some examples of surface studies of noble metals and Li electrodes in Li battery electrolyte solutions using in situ FT-IR spectroscopic techniques. These examples include the study of a mixture of solvents, the role of the reduction of salt in the build-up of surface films on the electrodes and the impact of contaminants such as traces of oxgen and water. The techniques included multiple and single internal reflectance modes and external reflectance (SNIFTIRS-type) mode. The following conclusions were drawn from this study: (i) salts containing the -SO 2CF 3 group are much more reactive on Li than LiAsF 6. Their reduction dominates the surface chemistry developed on Li in ethereal solutions; (ii) water reduction on Li in wet 1,3-dioxolane solution may not form stable LiOH films due to the further reaction of the hydroxy group with the solvent; (iii) in spite of its low solubility, oxygen dissolved in propylene carbonate and tetrahydrofuran solutions has some impact on the surface chemistry developed on Li in these solutions (probably due to Li 2O formation).

Safety study of electrolyte solutions for lithium batteries by accelerating-rate calorimetry

Safety study of electrolyte solutions for lithium batteries by accelerating-rate calorimetry

Ion solvation in lithium battery electrolyte solutions. 1. Apparent molar volumes

Apparent molar volumes, Vφ{symbol}(MX), of seven electrolytes (NaClO4, NaCF3SO3, NaBPh4, LiClO4, LiAsF6, Ph4AsCF3SO3 and KCF3SO3) have been determined by vibrating-tube densimetry in nonaqueous solvent mixtures of propylene carbonate (PC) with acetonitrile (AN), dimethoxyethane (DME) and tetrahydrofuran (THF). Vφ{symbol}(MX) was measured at an electrolyte concentration of 0.05M over the entire solvent composition range wherever possible. Ionic apparent molar volumes of transfer, ΔtV φ(ion), were obtained via the tetraphenylarsonium tetraphenylborate (TATB) assumption. ΔtV φ(ion) from PC to the mixed solvents are generally strongly negative for both cations and anions consistent with the greater compressibilities and lower dielectric constants of the cosolvents. In PC/AN mixtures cations and anions have similar values of ΔtV φ(ion) but in PC/DME and PC/THF mixtures they differ considerably. Cationic volumes show the expected dependence on ion-size but the differences among the anion volumes are much greater than expected at high cosolvent compositions. These effects are discussed in terms of preferential solvation and other solvent interactions. The implications of these findings for lithium batteries are briefly discussed.

Ion solvation in lithium battery electrolyte solutions Part II. Solvent effects on lithium-ion intercalation in niobium triselenide

Solvent effects on the intercalation of lithium ions in niobium triselenide electrodes have been investigated by cathodic polarization measurements in propylene carbonate (PC), acetonitrile (AN), 1,2-dimethoxyethane (DME), tetrahydrofuran (THF) and their 50% vol./vol. mixtures with PC, using 1 M LiClO 4 as the supporting electrolyte. The polarization follows the general order PC≈PC/AN≈PC/THF>THF>PC/DME>DME>AN which reflects the probable size of the solvated lithium ion. This suggests that intercalation of Li + into the host lattice occurs without desolvation. Lattice diffusion coefficients of Li + in NbSe 3 in the various solvents have been determined by an intermittent galvanostatic technique and found to be in the order AN>DME≈PC/DME>PC/AN>PC. This has also been explained in terms of solvation effects and the composition of the resultant Li + solvation shell.

Properties and structures of electrolyte solutions for lithium batteries

Rules which have been employed to explain and predict solvent properties of lithium battery electrolytes are described and results reviewed. The equilibrium behavior of moderate to high concentration electrolyte solutions is also reviewed. Recent theoretical approaches to explain the behavior are discussed, and a new theory incorporating contact ion pair concepts into an advanced statistical theory for free ions is proposed.