Page 565 - Handbook of Battery Materials

17.2 Components of the Liquid Electrolyte 539

whereas solvents such as PC and all ILs deviate strongly from this behavior and
follow a Vogel-Fulcher-Tamman temperature dependence. They are very weak
liquids, reaching even the limiting value for fragility parameters estimated by
Viglis [523]. For details, see Ref. [524] and the there cited literature.
For ILs, the conductivity decreases with increasing cation size while the anion
size shows no clear influence. Compared to LiPF 6 in EC/DEC, ILs based on
1-ethyl-3-methylimidazolium (EMIm) cations show similar conductivity of about
10 mS·cm −1 at room temperature [121–123, 127]. Unfortunately, electrochemi-
cally more stable quaternary ammonium-, pyrrolidinium-, or piperidinium-based
ILs show only low conductivities up to 2 mS·cm −1 at room temperature [121,
123]. In common ILs with cations having aliphatic alkyl chains addition of a
lithium salt decreases the conductivity with increasing amounts of the salt. For
example, in the case of lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) in
N-butyl-N-ethylpyrrolidinium TFSI an almost linear decrease is recorded [128].

17.2.3.1.3 Diffusion Coefficient Diffusion coefficients are usually in the rela-
2
tively low range of 10 −10 to 10 −11 m ·s −1 because of the high viscosities. They are
determined by Nuclear Magnetic Resonance (NMR) or electrochemical measure-
ments [122].

17.2.3.1.4 Electrochemical Stability RTILs based on inert anions and cations
usually exhibit wide potential windows. The difference between anodic and cathodic
potential limit is in the range of 4–6 V [121, 123]. Comparison of reported
electrochemical windows is difficult because different electrode materials and
reference electrodes (REFs) are used [122] (see also Section 17.4.1). Moreover,
impurities in the ILs strongly narrow the width of the electrochemical window
[123], while the addition of lithium salts often widens it. The cathodic stability of
ILs also has an influence on the rate capability of the battery [125].

17.2.3.1.5 Thermal Stability Usually, RTILs are thermally stable up to above
◦
200 C, DSC experiments show no endothermic peaks, while TGA experiments
show no weight loss before decomposition. Addition of lithium salts to an RTIL
can result in a slightly higher thermal stability [128, 129].

17.2.3.2 Crystallization and Melting Points
The present research group recently presented [130, 131] an automatic
computer-controlled equipment which is able to record simultaneously
temperature–time and conductivity–time functions of up to 30 samples at very
−1
low heating and cooling rates down to 1.5K h . This equipment is also a valuable
tool to determine reliable crystallization and melting points of IL [132] materials
with a strong tendency to supercooling.

17.2.3.3 Applications of ILs in Lithium-Ion Batteries
Exchange of common organic solvents by RTILs can enhance the safety of
lithium-ion batteries. Since low viscosity is a major requirement for electrolytes,