Room-temperature ionic liquids electrolyte applications

Industrial exploitation of ionic liquids is in its early days (Freemantle, 2000). The nse of ionic liquids in industry is seen as being highly speculative, but progress is being made. Room temperature ionic liquids have been stndied actively since their discovery in the 1970s, predominantly for their possible application as battery electrolytes. Work on the exploitation of ionic liquids as reaction media started in the early 1980s, but only recently has it attracted industrial interest. 

Room-temperature ionic liquids (denoted RTILs) have been studied as novel electrolytes for a half-century since the discovery of the chloroaluminate systems. Recently another system consisting of fluoroanions such as BF4 and PFg , which have good stability in air, has also been extensively investigated. In both systems the nonvolatile, noncombustible, and heat resistance nature of RTILs, which cannot be obtained with conventional solvents, is observed for possible applications in lithium batteries, capacitors, solar cells, and fuel cells. The nonvolatility should contribute to the long-term durability of these devices. The noncombustibility of a safe electrolyte is especially desired for the lithium battery [1]. RTILs have been also studied as an electrodeposition bath [2]. 

Room temperature ionic liquids (RTILs) are molten salts whose melting points are below room temperature. RTILs are formed when the constituent ions are sterically mismatched, thereby hindering crystal formation [17]. As polar solvents, RTILs have unique applications as tunable and environmentally benign solvents with very low volatility, high fire resistance, excellent chemical and thermal stability and wide liquid temperature range and electrochemical windows [17-19]. Solvent applications of RTILs include, for example, organic synthesis [17,20, 21], separations [22, 23], storage and transportation of hazardous chemicals [24], polymeric electrolytes [25, 26], dissolution of natural products [27] and synthesis of hollow metal oxide microspheres [28]. 

Nakagawa, H. Fujino, Y Kozono, S. Katayama, Y Nukuda, X. Sakaebe, H. Matsumoto, H. Xatsumi, K., Application of nonflammable electrolyte with room temperature ionic liquids (RXILs) for lithium-ion cells, J. Power Sources, 2007, 174, 1021-1026. 

The first room temperature ionic liquid (RTIL), [EtNHs][N03] (mp 12°C), was reported in 1914 (1), and since then a great deal of research effort has been exerted to exploit possible applications of these compounds. Initially, RTILs were used mainly as electrolytes in batteries or for metal electrodeposition however, nowadays they are finding an ever increasing range of applications. 

The establishment of such interfacial potentials is readily envisaged for cases where the net transport of an electrolyte is prevented because one of its constituents cannot partition. What is perhaps less obvious is that such potentials arise continually within solution phases, even where there is no physical separation into distinct phases. These so-called liquid junction potentials or diffusion potentials play an important role in electrochemical experiments, but because there is no well-defined phase boundary, they are intrinsically more difficult to measure. This chapter discusses how these potentials arise, how they may be calculated, what quantities are associated with them, and how they may be minimised. Finally, interfaces between electrolytes (i.e. those interfaces between immiscible electrolyte solutions (ITIES)) and the application of some of the concepts developed earlier in the chapter to non-standard electrolyte systems, such as polymer electrolytes and room-temperature ionic liquids, will be discussed. 

No information is given on the mechanical properties. Angell and coworkers have pointed out that it is not necessary to have a room temperature ionic liquid [26]. Indeed solid electrolytes have many well-known attractive properties for lithium-ion batteries. Several lithium salts have given conductivities comparable with PEO - lithium salt mixtures, which are heavily weighted with polymer. Since these values are not really adequate for most lithium ion battery applications, however, they are not covered further here. 

The early history of ionic liquid research was dominated by their application as electrochemical solvents. One of the first recognized uses of ionic liquids was as a solvent system for the room-temperature electrodeposition of aluminium [1]. In addition, much of the initial development of ionic liquids was focused on their use as electrolytes for battery and capacitor applications. Electrochemical studies in the ionic liquids have until recently been dominated by work in the room-temperature haloaluminate molten salts. This work has been extensively reviewed [2-9]. Development of non-haloaluminate ionic liquids over the past ten years has resulted in an explosion of research in these systems. However, recent reviews have provided only a cursory look at the application of these new ionic liquids as electrochemical solvents [10, 11]. 

Wilkes launched the field of air- and moisture-stable ionic liquids by introducing five new materials, each containing the Tethyl-3-methylimidazolium cation [EMIMJ+ with one of five anions nitrate [NC>3], nitrite [NO2]-, sulfate [SC>4]2, methyl carbonate [CH3CO2]- and tetrafluoroborate [BF [47]. Only the last two materials had melting points lower than room temperature, and the reactive nature of the methyl carbonate would make it unsuitable for many applications. This led to the early adoption of [EMIM][BF4] as a favored ionic liquid, which has since been the subject of over 350 scientific publications. One of the first appeared in 1997 [50], reporting the investigation of [EMIM][BF4] as the electrolyte system for a number of processes, including the electrodeposition of lithium (intended for use in lithium ion batteries). 

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. 

The exploration of novel types of ILs has involved important and intensive work. For the battery use, higher concentration of Li+ in the electrolyte is required and for this purpose the lithium salt itself hopefully could be melted at room temperature. Fujinami et al. have designed a novel bulky anion and demonstrated that certain kinds of Li salts can become viscous fluids at ambient temperature with a conductivity of between 10 and 10 S cm [31]. This result is still far from practical application and the electrochemical window is still unknown, but the work has shown unique progress for ionic liquids. 

Ionic liquids (ILs) are being considered more and more as alternatives for conventional electrolyte materials [5-7]. ILs offer the unique features of nonvolatility and nonflammability even in a liquid state. Systems that show ionic conductivity of over 10 S cm at room temperature have been reported close to the level required for fuel cell applications [8-10]. However, this value is based on the IL itself, and they do not include target ions such as the proton. This is a critical subject of research on making the present system viable. 

Molten salts at room temperature, so-called ionic liquids [1, 2], attracting the attention of many researchers because of their excellent properties, such as high ion content, liquid-state over a wide temperature range, low viscosity, nonvolatility, nonflammability, and high ionic conductivity. The current literature on these unique salts can be divided into two areas of research neoteric solvents as environmentally benign reaction media [3-7], and electrolyte solutions for electrochemical applications, for example, in the lithium-ion battery [8-12], fuel cell [13-15], solar cell [16-18], and capacitor [19-21],... 

Because ionic liquids (ILs) consist only of ions, they offer two brilliant features very high concentration of ions [1] and high mobiUty of component ions at room temperature. Because many ILs show the ionic conductivity of over 10 S cm at room temperature [2, 3], there are plenty of possible applications as electrolyte materials, among these, for rechargeable lithium-ion batteries [4—8], fuel cells [9-12], solar cells [13-17], and capacitors [18-23],... 

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