Ionic electrochemical stability

A key criterion for selection of a solvent for electrochemical studies is the electrochemical stability of the solvent [12]. This is most clearly manifested by the range of voltages over which the solvent is electrochemically inert. This useful electrochemical potential window depends on the oxidative and reductive stability of the solvent. In the case of ionic liquids, the potential window depends primarily on the resistance of the cation to reduction and the resistance of the anion to oxidation. (A notable exception to this is in the acidic chloroaluminate ionic liquids, where the reduction of the heptachloroaluminate species [Al2Cl7] is the limiting cathodic process). In addition, the presence of impurities can play an important role in limiting the potential windows of ionic liquids. 

In the tradition of previous reviews [1-22], this section addresses various aspects of nonaqueous electrolytes, including intrinsic properties, such as local structures caused by ion-ion and ion-solvent interactions and bulk properties, such as ionic conductivity, viscosity, and electrochemical stability (voltage window), and their relationships to intrinsic properties. 

However, even if electrolytes have sufficiently large voltage windows, their components may not be stable (at least ki-netically) with lithium metal for example, acetonitrile shows very large voltage windows with various salts, but is polymerized at deposited lithium if this reaction is not suppressed by additives, such as S02 which forms a protective ionically conductive layer on the lithium surface. Nonetheless, electrochemical stability ranges from CV experiments may be used to choose useful electrolytes. 

It is essential from the point of view of high power-density to ensure the electrochemical stability of the system at possibly high voltages. Broad electrochemical stability windows are typical if ionic liquids, however, the... 

Table 5. Electrochemical stability window of ionic liquids (IL) at the glassy carbon (potentials [V/ expressed versus Ag/Agf 0.01M in DMSO reference) [26 /.

Table 5 shows cathodic and anodic limits of electrochemical stability windows of a number of ionic liquids. The cathodic limit of the stability window of the ILs based on the EMIm+ and BMIm+ cations, investigated at the glassy carbon electrode, is -2.1 V against the Ag/Ag+ (0.01M in DMSO) reference. The BMPy+ cation is reduced at the glassy carbon at considerably more positive potential, at ca. -1.0 V. 

We have also demonstrated that well-behaved quantized charging of gold MPCs is possible in air- and water-stable room-temperature ionic liquids, such as 1-hexyl-3-methylimidazolium tris(penta-fluoroethyl)-trifluorophosphate (HMImEEP), Fig. 30c, d [334, 335]. As ionic liquids have very attractive features, including nearzero vapor pressure, considerable thermal stability, and an electrochemical stability window that often exceeds 4 V, this demonstration is particularly significant from a technological point of view. 

A polymer electrolyte with acceptable conductivity, mechanical properties and electrochemical stability has yet to be developed and commercialized on a large scale. The main issues which are still to be resolved for a completely successful operation of these materials are the reactivity of their interface with the lithium metal electrode and the decay of their conductivity at temperatures below 70 °C. Croce et al. found an effective approach for reaching both of these goals by dispersing low particle size ceramic powders in the polymer electrolyte bulk. They claimed that this new nanocomposite polymer electrolytes had a very stable lithium electrode interface and an enhanced ionic conductivity at low temperature. combined with good mechanical properties. Fan et al. has also developed a new type of composite electrolyte by dispersing fumed silica into low to moderate molecular weight PEO. 

Imidazolium-based ionic liquids (ILs) have been used extensively as media for the formation and stabilization of transition-metal nanoparticles [14—17]. These 1,3-dialkylimidazolium salts (Figure 15.3) possess very interesting properhes they have a very low vapor pressure, they are nonflammable, have high thermal and electrochemical stabilities, and display different solubilities in organic solvents [18-20]. 

Recently, room-temperature ionic liquids (molten salts) have been extensively studied in order to replace volatile organic solvents by them in electrochemical devices such as batteries. Interest in these materials is stimulated by their properties (e.g., high ionic conductivity, good electrochemical stability, and low volatility). Among these properties, the low volatility is the most critical for ensuring the long-term stability of electrochemical devices. Room-temperature ionic liq-... 

The lithium polymer battery (LPB), shown schematically in Fig. 7.21, is an all-solid-state system which in its most common form combines a lithium ion conducting polymer separator with two lithium-reversible electrodes. The key component of these LPBs is the polymer electrolyte and extensive work has been devoted to its development. A polymer electrolyte should have (1) a high ionic conductivity (2) a lithium ion transport number approaching unity (to avoid concentration polarization) (3) negligible electronic conductivity (4) high chemical and electrochemical stability with respect to the electrode materials (5) good mechanical stability (6) low cost and (7) a benign chemical composition. 

A major breakthrough was achieved in 1951 with the report of Hurley and Wier. They noticed that a mixture of N-ethylpyridinium bromide (EtPyBr) and AICI3 with a eutectic composition of 1 2 X(AlCh) = 0.66 h of EtPyBr to AICI3 became liquid at unusually low temperatures [2], They investigated these melts with regard to their potential use in the electrodeposition of aluminum at ambient temperature [3]. Several studies were carried out on this system, however, its use was very limited since it is only liquid at a mole fraction of X(A1C13) = 0.66 and the ease of oxidation of the bromide ion limits the electrochemical stability. In the following years the main interest in ionic liquids was focused on electrochemical applications [4—6]. 

Other cation combinations with [BF4] and [PFelectrochemical systems. Although N-butylpyridinium tetrafluorobo-rate [bpyr][BF4] is known to be a RTIL [53], the lower electrochemical stability of pyridinium-based cations relative to imidazolium limits their electrochemical applicability. On the other hand, pyrrolidinium-based cations are known to be more electrochemically stable than imidazolium salts, N-alkyl-N-methylpyrrolidinium salts of [BF4] and [PFf,] are made less attractive to researchers by the fact that they are solids at room temperature [54, 55]. Therefore, most of the electrochemical investigations of ionic liquids containing [BF4] and [PF6] have focused on [BMIM][PF6], [BMIM][BF4] and, to a lesser extent, [EMIM][BF4]. 

Recently, Merck KGaA developed a convenient method for the synthesis of [FAP] -based ionic liquids as replacement for [PF6]--based ionic liquids [72]. Like their [PFs]- analogs, [FAP]--based ionic liquids form biphasic aqueous mixtures and can be separated and recovered easily from aqueous reaction mixtures. They can be easily obtained with very low water and chloride content by washing with water followed by heating under reduced pressure. The hydrolytic stability (Figure 2.3) and electrochemical stability (Table 2.2) of [FAP]- and its ionic liquids are superior to [PF6] and [BF4] and comparable with [NTF]-. 

Since some ILs have excellent electrochemical stability, as shown in Table 3.14, they are favorable for application as electrolyte materials. Recently, ionic liquids have been investigated as conductive and redox media for lithium ions. Stable electrochemical deposition and dissolution of Li metal (Li/Li+) was observed for the lithium salt solution of [Nni3][TFSI], [N 122,201][TFSI], and [PPi4][TFSI] [101-103]. In order to observe the redox couple of lithium metal, Ni should be used as working electrode because it does not form alloys with lithium metal. In addition to this, the atmosphere must be pure Ar, because Li metal reacts rapidly with N2 to form conductive LiN. 

Further to their role as supporting electrolytes, the conductivity and electrochemical stability of ionic liquids clearly also allows them to be used as solvents for the electrochemical synthesis of conducting polymers, thereby impacting on the properties and performance of the polymers from the outset. Parameters such as the ionic liquid viscosity and conductivity, the high ionic concentration compared to conventional solvent/electrolyte systems, as well as the nature of the cation and... 

Cyclic voltammetry is also an ideal analytical tool for assessing the electrochemical stability of the polymer films. This is a fundamental requirement for any conducting polymer to be considered for long-term use in electrochemical devices. The use of ionic liquids for the electrochemical cycling of poly(aniline) has been reported to enhance lifetimes to over a million cycles [12], and significant improvements in the cycling stability of poly(pyrrole) have also been reported [32]. 

The potential improvements that ionic liquids may impart to conducting polymers have been widely discussed - increased doping levels, smoother films, increased conductivity, decreased over-oxidation and improved electrochemical stability and so on. However, the research to date in this area has only just begun to investigate these hypotheses and demonstrate any material advantages in the use of ionic liquids future directions in this area must focus on some of these issues in addition to simply demonstrating the use of new ionic liquids for conducting polymer synthesis. 

Ionic liquids — A class of preferably organic salts that are liquid at room temperature and may be simultaneously used as both solvents and supporting electrolytes for electrochemical reactions. Their unconventional properties include a negligible vapor pressure, a high thermal and electrochemical stability, and exceptional dissolution properties for both organic and inorganic systems. 

An ionic liquid (IL) , or classically a room-temperature molten salt , is an interesting series of materials being investigated in a drive to find a novel electrolyte system for electrochemical devices. ELs contain anions and cations, and they show a liquid nature at room temperature without the use of any solvents. The combination of anionic and cationic species in ILs gives them a lot of variations in properties, such as viscosity, conductivity, and electrochemical stability. These properties, along with the nonvolatile and flame-resistant nature of ILs, makes this material especially desirable for lithium-ion batteries, whose thermal instability has not yet been resolved despite investigations for a long time. In this chapter we discuss the efforts made for battery application of ILs. 

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