Electrolytes with other solvents

The modification of electrolytes via additives is attractive to industry as an economical approach however, its impact on electrolyte performance is mainly restricted to tuning interfacial-related properties because of their small concentration in the electrolyte, while other challenges for the state-of-the-art electrolytes such as temperature limits, ion conductivity, and Inflammability are still determined by the physical properties of the bulk components. Improvements in these bulk-related properties can only be realized by replacing the bulk components of the electrolytes with new solvents and salts, but such efforts have been met with difficulty, since more often than not the improvement in the individually targeted properties is achieved at the expense of other properties that are also of vital importance to the performance of electrolytes. Such collateral damage undermines the significance of the improvements achieved and, in some cases, even renders the entire effort unworthy. 

Z values cover a range from 94.6 (water) to about 60 kcal/mol (z-octane) and were originally measured for 21 pure solvents and 35 binary solvent mixtures [5, 56], as well as some electrolytes [57] and surfactant solutions [58]. Various authors have since gradually extended this to include 45 pure solvents. Z values for a further 41 pure solvents have been determined by Griffiths and Pugh [172], who also compiled all available Z values and their relationships with other solvent polarity scales. A selection of Z values together with some other spectroscopic solvent polarity parameters is given in Table 7-2. 

Fluorosulfonic acid (i.e., HSO3F) (Fig. 1.19b) was first reported in 1918 [236]. LiS03F-based electfolytes with several aprotic solvents were found to have a similar oxidative stability to electrolytes with other lithium salts such as LiSOsCFs [237]. The conductivity of 1 M LiS03F electrolytes with PC and GBL is 1.4 and... 

The model of the EDL that is currently used was proposed by Bockris et al. [25], who suggested that a water layer is present at the surface of the electrode (within the inner Helmholtz plane) and some other water molecules are displaced by adsorbed ions. This model may be extended to charge transfer reactions occurring in organic electrolytes with polar solvents, for example, acetonitrile (AN), contributing to the potential drop across the electrode-electrolyte plane. 

The chemical properties of solvents that are relevant to their dissolution abilities for electrolytes and the ionic dissociation of the latter include their structuredness or self-association and their donor (electron pair donation, basicity) and acceptor (hydrogen bonding ability, acidity) properties as well as their softness. The mutual solubility with other solvents, in particular water, is also of importance as are the windows for making spectroscopic and electrochemical measurements on solutions of ions in the solvents. 

Anhydrous silver hexafluorophosphate [26042-63-7] AgPF, as well as other silver fluorosalts, is unusual in that it is soluble in ben2ene, toluene, and xylene and forms 1 2 molecular crystalline complexes with these solvents (91). Olefins form complexes with AgPF and this characteristic has been used in the separation of olefins from paraffins (92). AgPF also is used as a catalyst. Lithium hexafluorophosphate [21324-40-3] LiPF, as well as KPF and other PF g salts, is used as electrolytes in lithium anode batteries (qv). 

The metallic salts of trifluoromethanesulfonic acid can be prepared by reaction of the acid with the corresponding hydroxide or carbonate or by reaction of sulfonyl fluoride with the corresponding hydroxide. The salts are hydroscopic but can be dehydrated at 100°C under vacuum. The sodium salt has a melting point of 248°C and decomposes at 425°C. The lithium salt of trifluoromethanesulfonic acid [33454-82-9] CF SO Li, commonly called lithium triflate, is used as a battery electrolyte in primary lithium batteries because solutions of it exhibit high electrical conductivity, and because of the compound s low toxicity and excellent chemical stabiUty. It melts at 423°C and decomposes at 430°C. It is quite soluble in polar organic solvents and water. Table 2 shows the electrical conductivities of lithium triflate in comparison with other lithium electrolytes which are much more toxic (24). 

Whereas the electrochemical decomposition of propylene carbonate (PC) on graphite electrodes at potentials between 1 and 0.8 V vs. Li/Li was already reported in 1970 [140], it took about four years to find out that this reaction is accompanied by a partially reversible electrochemical intercalation of solvated lithium ions, Li (solv)y, into the graphite host [64], In general, the intercalation of Li (and other alkali-metal) ions from electrolytes with organic donor solvents into fairly crystalline graphitic carbons quite often yields solvated (ternary) lithiated graphites, Li r(solv)yC 1 (Fig. 8) [7,24,26,65,66,141-146],... 

Numerous research activities have focused on the improvement of the protective films and the suppression of solvent cointercalation. Beside ethylene carbonate, significant improvements have been achieved with other film-forming electrolyte components such as C02 [156, 169-177], N20 [170, 177], S02 [155, 169, 177-179], S/ [170, 177, 180, 181], ethyl propyl carbonate [182], ethyl methyl carbonate [183, 184], and other asymmetric alkyl methyl carbonates [185], vinylpropylene carbonate [186], ethylene sulfite [187], S,S-dialkyl dithiocarbonates [188], vinylene carbonate [189], and chloroethylene carbonate [190-194] (which evolves C02 during reduction [195]). In many cases the suppression of solvent co-intercalation is due to the fact that the electrolyte components form effective SEI films already at potential which are positive relative to the potentials of solvent co-intercalation. An excess of DMC or DEC in the electrolyte inhibits PC co-intercalation into graphite, too [183]. 

In electrolytes based on solvent mixtures both solvent compounds may react to form films of scarcely soluble materials. PC/THF mixtures yield alkoxides and alkylcarbonates [188] EC/ether blends mainly yield alkylcarbonates, which are thought to be the reason for smaller lithium loss during cycling [188]. PC based electrolytes with LiAsF6and LiC104 form films containing alkylcarbonates which allow the access of other molecules, such... 

In screening electrolyte redox systems for use in PEC the primary factor is redox kinetics, provided the thermodynamics is not prohibitive, while consideration of properties such as toxicity and optical transparency is important. Facile redox kinetics provided by fast one-electron outer-sphere redox systems might be well suited to regenerative applications and this is indeed the case for well-behaved couples that have yielded satisfactory results for a variety of semiconductors, especially with organic solvents (e.g., [21]). On the other hand, many efficient systems reported in the literature entail a more complicated behaviour, e.g., the above-mentioned polychalcogenide and polyiodide redox couples actually represent sluggish redox systems involving specific interactions with the semiconductor... 

In electrochemical reactions, the catalyst surface is in contact not only with the reacting species but also with other species (i.e., the solvent molecules and the electrolyte ions), which in turn influence the properties of the surface and give rise to special reaction features. 

Ordinary water behaves very differently under high temperature and high pressure. Early studies of aqueous solutions under high pressure showed a unique anomaly that was not observed with any other solvent.11 The electrolytic conductance of aqueous solutions increases with an increase in pressure. The effect is more pronounced at lower... 

Electrodialysis is a process for the separation of an electrolyte from the solvent and is used, for example, in desalination. This process occurs in a system with at least three compartments (in practice, a large number is often used). The terminal compartments contain the electrodes and the middle compartment is separated from the terminal compartments by ion-exchanger membranes, of which one membrane (1) is preferentially permeable for the cations and the other one (2) for the anions. Such a situation occurs when the concentration of the electrolyte in the compartments is less than the concentration of bonded ionic groups in the membrane. During current flow in the direction from membrane 1 to membrane 2, cations pass through membrane 1 in the same direction and anions pass through membrane 2 in the opposite direction. In order for the electrolyte to be accumulated in the central compartment, i.e. between membranes 1 and 2 (it is assumed for simplicity that a uni-univalent electrolyte is involved), the relative flux of the cations with respect to the flux of the solvent, /D +, and the relative flux of the anions with respect to... 

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