Investigations on some electrochemical aspects of lithium-ion ionic liquid/gel polymer battery systems, страница 2

where R is the gas constant and T_m is melting point, in degrees Kelvin, of these liquids; strictly speaking, the ordinary connotation of melting point applies to molten salts and metals, since most gases, organic liquids and ionic liquids are not solids at room temperature; in other words T_m is the temperature of phase change, in degrees Kelvin, that gives rise to the liquid state. Theoretically, Eq. (1) can be interpreted by deducing the heat of activation for transport in liquids in terms of work of hole formation in the liquid, as done by Bockris and coworkers [7–9], following original formulation of Fürth [10]. Thus, one must seek for a battery a liquid (ionic or otherwise) with the lowest melting point, in order to attain the highest conductivity.

Most popular ILs consist of quaternary ammonium cations such as imidazolium, pyridinium, pyrrolidinium, sulfonium, ammonium and phosphonium with anions

−    −                  − having low Lewis basicities; such as BF₄, PF₆, CF₃SO₃,

−

and (CF₃SO₂)₂N . The quaternary ammonium ions produce a low melting point, compared to the inorganic salts of the same anions, which approaches the room temperature. These RTILs are known as green solvents due to their advantages of non-flammability, high electrochemical stability, low vapor pressure and high conductivity [11, 12]. From the point of view of their use in lithium-ion batteries, their most outstanding features are no vapor pressure, thence, enhanced safety, good electrochemical stability, excellent solvent characteristics and a large voltage window (>5 V vs. Li) for many ILs. Other aspect of these ILs is their high thermal stability, many of them showing decomposition temperatures above 300 °C: this allows one to operate batteries at high temperatures and extends the safety range of the battery, particularly when used in an electric vehicle. Owing to these advantages of these electrolytes, they find much interest in batteries and capacitors [13–15]. Since these ILs have more stable cations, which make the electrochemical stability window wider and allow a larger variety of cathodes to be used, including 5.0 V cathodes. Most of the work on the cathode materials has been explored on LiCoO₂ with different ILs [16–20].

Other ILs, different from aromatic rings and hetero cycles, and based on aliphatic quaternary ammonium salts with methoxyethyl groups on the nitrogen atom combined

−     −                  − with anions like (BF₄ ) or [TFSI :CF₃SO₂)₂N ] giving DME-BF₄ and DEME-TFSI, have been also extensively studied [21–25]. Such ILs also have other advantages compared to imidazolium, pyridinium, pyrrolidinium, sulfonium, e.g., large potential window (6.0 V) which makes them very attractive for batteries and particularly capacitors [26–28]. On the anode side the problem of IL is more important due the to the SEI layer formation on some materials such as carbon and anode alloys materials with high capacity. Many researchers study the electrochemical intercalation in graphite anodes in EMI based IL, because of its low viscosity and high conductivity. However, some decomposition of the cations occurs and prevents the intercalation process. Some organic solvents can suppress the reduction of EMI; 5 wt.% VC was added to the EMI-TFSI by the Novak group [29, 30] and they obtained high reversible capacity of the anode with 350 mAh/g. Sato et al. demonstrated the cyclability of

Table 1 Viscosity and ionic conductivity of the used electrolyte

Electrolyte         No salt addition                With adding 0.7 M LiFSI