Lithium electrolyte solutions

So far, very few attempts at improving ion conductivity have been realized via the salt approach, because the choice of anions suitable for lithium electrolyte solute is limited. Instead, solvent composition tailoring has been the main tool for manipulating electrolyte ion conductivity due to the availability of a vast number of candidate solvents. Considerable knowledge has been accumulated on the correlation between solvent properties and ion conductivity, and the most important are the two bulk properties of the solvents, dielectric constant e and viscosity rj, which determine the charge carrier number n and ion mobility (w ), respectively. 

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). 

Capillary tube isotachophoresis using a potential gradient detector is another technique that has been applied to the analysis of alcohol sulfates, such as sodium and lithium alcohol sulfates [303]. The leading electrolyte solution is a mixture of methyl cyanate and aqueous histidine buffer containing calcium chloride. The terminating electrolyte solution is an aqueous solution of sodium octanoate. 

The corrosion resistance of lithium electrodes in contact with aprotic organic solvents is due to a particular protective film forming on the electrode surface when it first comes in contact witfi tfie solvent, preventing further interaction of the metal with the solvent. This film thus leads to a certain passivation of lithium, which, however, has the special feature of being efiective only while no current passes through the external circuit. The passive film does not prevent any of the current flow associated with the basic current-generating electrode reaction. The film contains insoluble lithium compounds (oxide, chloride) and products of solvent degradation. Its detailed chemical composition and physicochemical properties depend on the composition of the electrolyte solution and on the various impurity levels in this solution. 

Tadic et al. studied the polymer poly-vynilidene fluoride/hexa-fluoropropylene ( PVdF/HFP ) containing lithium salt solution in Ethylene carbonate/diethylene carbonate ( EC/DEC )- In order to understand better the effect of anion size in the electrolyte, two Li salts were compared, namely LiN(CF3S02)2 (termed Liimide by the authors) and LiN(C2F5S02)2 (termed Libeti ). 

The ion solvating polymers have found application mainly in power sources (all-solid lithium batteries, see Fig. 2.19), where polymer electrolytes offer various advantages over liquid electrolyte solutions. 

There is no question that the development and commercialization of lithium ion batteries in recent years is one of the most important successes of modem electrochemistiy. Recent commercial systems for power sources show high energy density, improved rate capabilities and extended cycle life. The major components in most of the commercial Li-ion batteries are graphite electrodes, LiCo02 cathodes and electrolyte solutions based on mixtures of alkyl carbonate solvents, and LiPF6 as the salt.1 The electrodes for these batteries always have a composite structure that includes a metallic current collector (usually copper or aluminum foil/grid for the anode and cathode, respectively), the active mass comprises micrometric size particles and a polymeric binder. 

Endo E., Tanaka K. and Seika K. Initial Reaction in the Reduction Decomposition of Electrolyte Solutions for Lithium Batteries, J. Electrochem. Soc., 147, 4029-4033 (2000). 

Electrolytes are ubiquitous and indispensable in all electrochemical devices, and their basic function is independent of the much diversified chemistries and applications of these devices. In this sense, the role of electrolytes in electrolytic cells, capacitors, fuel cells, or batteries would remain the same to serve as the medium for the transfer of charges, which are in the form of ions, between a pair of electrodes. The vast majority of the electrolytes are electrolytic solution-types that consist of salts (also called electrolyte solutes ) dissolved in solvents, either water (aqueous) or organic molecules (nonaqueous), and are in a liquid state in the service-temperature range. [Although nonaqueous has been used overwhelmingly in the literature, aprotic would be a more precise term. Either anhydrous ammonia or ethanol qualifies as a nonaqueous solvent but is unstable with lithium because of the active protons. Nevertheless, this review will conform to the convention and use nonaqueous in place of aprotic .]... 

Most compositions of lithium electrolytes are based on solutions of one or more lithium salts in mixtures of two or more solvents, and single-solvent formulations are very rare, if there are any. The rationale behind this mixed solvent formulation is that the diverse and often contradicting requirements of bat-... 

An ideal electrolyte solute for ambient rechargeable lithium batteries should meet the following minimal requirements (1) It should be able to completely dissolve and dissociate in the nonaqueous media, and the solvated ions (especially lithium cation) should be able to move in the media with high mobility. (2) The anion should be stable against oxidative decomposition at the cathode. (3) The anion should be inert to electrolyte solvents. (4) Both the anion and the cation should remain inert toward the other cell components such as separator, electrode substrate. 

Electrolytes for Lithium-Based Rechargeable Batteries Table 3. Lithium Salts as Electrolyte Solutes... 

LiPFe was proposed as an electrolyte solute for lithium-based batteries in the late 1960s, and soon its chemical and thermal instabilities were known. Even at room temperature, an equilibrium exists ... 

The above merits made LiPFe the salt of choice when lithium ion technology leaped from concept into product. In 1990, it was used by Sony in the first generation lithium ion cell, and since then, its position in the lithium ion industry has remained unchallenged. Like EC as an indispensable solvent component, LiPFe has become the indispensable electrolyte solute for almost all lithium ion devices manufactured in the past decade. 

In contrast to that of solvents, the effect of the electrolyte solute, LiPFe, on the thermal decomposition of the cathode, LiCo02, was found to be suppression instead of catalyzation. The SHR of a partially delithiated cathode was measured in a series of electrolytes with various salt concentrations, and a strong suppression of the self-heating behavior was found as the concentration of LiPEe increased above 0.50 M. The mechanistic rationale behind this salt effect is still not well understood, but the authors speculated that the salt decomposition coated the cathode with a protective layer that acted as a combustion retardant. On the basis of these results, the authors recommended a higher salt concentration (>1.50 M) for LiCo02-based lithium ion cells is preferred in terms of thermal safety. 

Pointing out that the acidic nature of LiPFe assists the dissolution of Mn + into electrolyte solution, the authors speculated that HF in the electrolyte solution efficiently cleans up the spinel surface of MnO therefore, the bulk electrolyte solvents can be continuously exposed to the fresh surface of Mn204 and be oxidized. As a result, solvent oxidation would proceed more rapidly as compared with the case of an electrolyte that is less acidic. Therefore, for a spinel manganese-based lithium ion cell, a higher thermal safety would be obtained with lower salt concentration, and the authors suggested 0.5 M as the optimum concentration at which the ion... 

Figure 48. Anodic stability as measured on a spinel LL-Mn204 cathode surface for EMS-based electrolytes (a) Lilm (b) LiC104 (c) LiTf. In all cases, 1.0 m lithium salt solutions were used, and slow scan voltammetry was conducted at 0.1 mV s with lithium as counter and reference electrodes and spinel LiJV[n204 as working electrode. (Reproduced with permission from ref 75 (Figure 3). Copyright 1998 The Electrochemical Society.)...

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