Dielectric Properties of Ionic Liquids Proposed to Be Used in Batteries

Ionic liquids (ILs) opened new technological possibilities (Marsh, 2004) at the end of the 1990s for many application fields (Mutelet & Jaubert, 2006) due to their special physical and chemical properties (Welton, 1999). They have several advantages compared to commercial organic solvents (Mutelet & Jaubert, 2006), or electrolyte liquids (Plechkova & Seddon, 2008) since they are liquid over a wide temperature range, easy to recycle, noncombustible (Matsumoto, 2005), non-flammable (Saruwatari, 2010), display wide electrochemical windows (Giroud, 2008), high inherent conductivities (Stracke, 2009), have negligible vapor pressure (Endres, 2008), high thermal (Wasserschied & Welton, 2007) and electrochemical stability (Arnold, 2004), tuneable physicochemical properties (Koel, 2008) and lack of reactivity in various electrochemical or industrial applications (Seddon, 2003). Therefore ILs have been intensively studied recently as novel, much safer electrolyte materials (Sakaebe, 2007) for electrochemical devices (Ohno 2005) and energy storing devices, such as Li batteries for cellular phones (Xu, 2006), batteries for vehicles, fuel cells, supercapacitors (Sato, 2004), solar cells (Stathatos, 2005.), etc. In order to decide whether an IL could be successfully used as electrolyte it is necessary to first gain enough information about the electrochemical properties, hence the dielectric behaviour of the selected liquid. However, several methods have already been published about determination of the dielectric properties of ILs – generally characterized by the dielectric constant (┝’) (Wakai, 2005) and by the dielectric loss factor (┝”) (Bright & Baker, 2006; Wakai, 2006), so far no measuring method was introduced. This can be explained by the fact that the dielectric properties of ionic liquids cannot be measured by classical methods due to their high conductivity (Krossing, 2006). In this chapter we intend to describe a novel method and a self-designed microwave dielectrometric apparatus built to measure the (e’) and (e”) values and the electrical conductivity (G) of several ionic liquids, which might be used as electrolyte in batteries, at 2.45 GHz and at different temperatures. Furthermore the connections between the structure of the investigated ionic liquids and the dielectric characteristics will be determined.


Introduction
Ionic liquids (ILs) opened new technological possibilities (Marsh, 2004) at the end of the 1990s for many application fields (Mutelet & Jaubert, 2006) due to their special physical and chemical properties (Welton, 1999).They have several advantages compared to commercial organic solvents (Mutelet & Jaubert, 2006), or electrolyte liquids (Plechkova & Seddon, 2008) since they are liquid over a wide temperature range, easy to recycle, noncombustible (Matsumoto, 2005), non-flammable (Saruwatari, 2010), display wide electrochemical windows (Giroud, 2008), high inherent conductivities (Stracke, 2009), have negligible vapor pressure (Endres, 2008), high thermal (Wasserschied & Welton, 2007) and electrochemical stability (Arnold, 2004), tuneable physicochemical properties (Koel, 2008) and lack of reactivity in various electrochemical or industrial applications (Seddon, 2003).Therefore ILs have been intensively studied recently as novel, much safer electrolyte materials (Sakaebe, 2007) for electrochemical devices (Ohno 2005) and energy storing devices, such as Li batteries for cellular phones (Xu, 2006), batteries for vehicles, fuel cells, supercapacitors (Sato, 2004), solar cells (Stathatos, 2005.), etc.In order to decide whether an IL could be successfully used as electrolyte it is necessary to first gain enough information about the electrochemical properties, hence the dielectric behaviour of the selected liquid.However, several methods have already been published about determination of the dielectric properties of ILs -generally characterized by the dielectric constant ( ') (Wakai, 2005) and by the dielectric loss factor ( ") (Bright & Baker, 2006;Wakai, 2006), so far no measuring method was introduced.This can be explained by the fact that the dielectric properties of ionic liquids cannot be measured by classical methods due to their high conductivity (Krossing, 2006).In this chapter we intend to describe a novel method and a self-designed microwave dielectrometric apparatus built to measure the (ε') and (ε") values and the electrical conductivity (G) of several ionic liquids, which might be used as electrolyte in batteries, at 2.45 GHz and at different temperatures.Furthermore the connections between the structure of the investigated ionic liquids and the dielectric characteristics will be determined.
that the IL solution were damaged above 450°C hence they could be used even for electric vehicle applications.It was also stated that the IL electrolyte showed better cycling performances than organic electrolyte and that operating potentials for IL electrolyte batteries might be higher than that of conventional lithium ion batteries (Giroud, 2008).Most of the authors having studied the electrochemical properties of ILs and their possible application in batteries as electrolyte concluded that ILs could be successfully used to create new types of rechargeable systems or to even replace the standard lead-acid batteries.

Permittivity
The permittivity ( ) is a characteristic of a material, which describes how any electric field affects and is affected by a material (a dielectric medium).In nonconducting materials (insulators or dielectrics) charges do not move freely only might be slightly displaced from their equilibrium position (Heaviside, 2007).Permittivity is determined by the ability of a material to polarize in response to the field, and reduce the total electric field inside the material, hence it can be calculated by Equation 1, which gives the electric field of a point charge (Q) at the distance r from Q.
Equation 2 gives the unit of , which is conventionally measured in farads per meter [F/m], or coulombs per volt-meter [C/Vm], as well.

Dielectric constant
For a given system the ratio of the electric flux density (D) to the electric field strength (E) is a constant, known as the absolute permittivity (Equation 3), which can also be defined as the product of the permittivity of free space (or vacuum permittivity) ( 0 ) and the relative permittivity ( r ), or sometimes also called as dielectric constant ( ') as shown in Equation 4 (Housecroft & Sharpe, 2005).
where the value of vacuum permittivity is; 0 = 8.8541878176x10 -12 As/Vm (Fleisch, 12008).The dielectric constant, or relative permittivity, which is the permittivity of a material relative to that of free space and equals to 1 for vacuum by definition.Due to ability of any material to polarize more than vacuum the dielectric constant of a given material is always higher than 1.The ' is dimensionless since it is just a ratio of two similar quantities.
The most common application of dielectric materials is to increase the capacitance and maximum operating voltage of capacitors.The capacitance (ability to store charge) of a parallel-plate capacitor (C) is proportional to the area of the plates (A) and the distance between the plates (d) and is given by Equation 5 (Carrier & Soga, 1999).
The capacitance of a capacitor is increased by placing a dielectric between the capacitor plates and is given by Equation 6 (Benenson, 2002).
The dielectric constant ( ') in reality is not always constant.In general, it can be different depending on the position in the medium, the frequency of the electric field, temperature, humidity, and other parameters.

Dielectric loss factor
When a capacitor of value C is connected to a generator of RMS voltage (U) at angular frequency (ω) will draw a current (I) according Equation 7.
Supposing that the permittivity has an imaginary component and is written by Equation 8'' ' then substituting Equation 6and Equation 8 into Equation 7 the current will be; The first therm in parentheses shows the component of current in phase quadrature with the voltage as it would be in the case of lossless capacitors.The second term is a component in phase with the applied voltage and therefore it reperesents power dissipation (P), which is given by Equation 10.
where ε" is called the dielectric loss factor of a dielectric and it can quantify the power dissipation in a dielectric filled capacitor.The dielectric loss angle ( ) is defined as the angle by which the resultant current is different from the ideal 90° phase angle relative to the voltage and is given by Equation 11 (Meredith, 1998).

Electrical conductivity
The electrical conductivity is a measure of how easy it is for an electrical current to flow through a material or how easily electrical charge may be transported through it.High value corresponds to the materials that carry electrical currant easily and do not readily remain electrically charge.Due to this behaviour these material might be excellent candidates for electrolytes in certain batteries (Heiken, 1991).Electrical conductivity is defined by Ohm's law for direct current (Equation 12), measured in units of Siemens (S) and is given by the inverse of the resistance as shown in Equation 13 (Mohos, 2010). where

Measurement methods for dielectric properties
Several different techniques have been developed for the determination of the dielectric constant and the dielectric loss factor of solid and liquid materials.The most commonly used ones are the following.

Perturbation technique
The perturbation technique is normally used in the case of homogeneous materials due to its simplicity, high temperature capability and easy data reduction (Tong, & Lentz, 1993).One of its greatest advantages is that it can be highly accurate in the determination of small loss factors.It is based on the measurement of either reflection coefficients or resonant frequencies (Sheen, 2009).In this later case the investigated materials are characterized to load a resonant cavity and the sample permittivity is evaluated from the shift of the resonant frequency value, compared to that of the empty cavity (Murthy & Raman, 1989).
For the evaluation of dielectric properties of homogeneous dielectric materials a new, even more accurate perturbation technique has been developed where a special cavity has been designed with very small slot at the center of the broader side of the wave-guide in order to insert the sample (Kumar & Sharma, 2007).

Waveguide and coaxial transmission line method
The determination of dielectric constant and loss factor is based on the transmission line theory (Ebara, 2006).According to this theory the values of ' and " can be determined by measuring the phase and amplitude of reflected microwave signals both from a solid or liquid sample placed against the end of a short circuit transmission -waveguide or coaxialline (Kozhevnikov, 2009).Charreyre and co-workers were among the first researchers who have described an apparatus designed to measure dielectric parameters by this technique (Charreyre, 1984).

Open-ended probe technique
In this case a coaxial line with a tip, which can sense the signal reflected from the investigated material, must be applied (Sheen & Woodhead, 1999).The tip is either in contact with the sample by touching the probe to a flat face of the solid material (Gardner & Anantaraman, 1995) or it is immersed into the liquid sample during the measurements (Guo, 2011).The dielectric properties are calculated from the phase and amplitude of the reflected signal at the end of the open-ended coaxial line.This technique is valid for materials with loss factor higher than 1, between the frequencies of 915 and 2450 MHz.

Time domain spectroscopy method
Although, this method is rather expensive it is an excellent tool to investigate the interaction of the electromagnetic energy and materials over a wide frequency range (Afsar, 1986).The reflection characteristics of the investigated material are used to compute the dielectric properties.The measurement is very rapid and the accuracy is very high, within a few percent error range (Lee, 2003).

Free-space transmission technique
It is considered to be a non-destructive and contact-less measuring method, which is particularly suitable for determining the dielectric properties of materials at high temperature and for inhomogeneous dielectrics.It can also be easily implemented in industrial applications for continuous monitoring and control (Kraszewski, 1980).The accuracy of measurement depends mainly on the performance of the measuring system and the validity of the equations used for the calculation (Håkansson, 2007).
The above described conventional techniques to measure the dielectric properties of ionic liquids or electrolyte solutions mostly tend to fail because the samples are largely short circuited by the high electrical conductance (Wakai, 2005).Hence it is quite difficult to measure the dielectric constant and loss factor of conducting materials.The main challenge is to develop an accurate method for determining the dielectric properties of such materials while the method is not only effected by the frequency of interest, the required accuracy, the geometry of the sample holder (Roumeliotis & Kokkorakis, 1994) and the adequate modeling of the circuit, but also by the physical and electrical characteristics of the studied liquid (Venkatesh & Raghavan, 2005).Therefore we intended and managed to design and build a so-called microwave dielectrometric measurement device in which -based on the compensation of phase change due to the microwave energy absorption of the liquid sample -the dielectric properties of ionic liquids can be successfully measured in a continuous and automatic way.

Materials
All ILs used in this work were synthesized at the Research Institute of Chemical and Process Engineering, University of Pannonia, Hungary.Their names, structural formula and water content can be seen in Table 1.The water content of the ILs was measured by Karl-Fischer titration and the values ranged between 0.08% and 5.11% (w/w).After a 24 h long vacuum treatment all ILs contained less than 0.01% (w/w) water.

Experimental set-up
The schematic representation of the self-designed experimental set-up applied for automatic and online measurement of dielectric properties of well conducting liquids in a definite temperature range is shown in Figure 1.It is composed of the following devices and instruments; cylindrical sample holder unit, thermostat, peristaltic pump, waveguide, temperature sensor, displaceable piston, stepper motor, magnetron, detectors, control unit, and a PC.Furthermore it controls the position of the short circuit displaceable piston on the basis of algorithm software elaborated for this purpose.

Measurement method
The method is based on the compensation of phase change due to the microwave energy absorption of the liquid sample.The short circuit piston situated behind the sample must be actuated for compensation.The energy conditions created by the wave front in the waveguide are measured by four diode-detectors.Based on the diode detector signals the wave-front pictures are developed in the transmission-line as it is represented on Figure 2. As it shown, essentially three arrangements can be formed.Figure 2 a) shows the case when the sample holder is empty and the piston is at the position x 0, then a field maximum is detected by diode D 3 .The short circuit piston must be moved to a distance 3/2λ from the diode detector D 1 , and the signal of D 1 -being at a distance λ/4 from the detector D 3 -exhibits a minimum.As a result of suitable geometric and electric arrangements, the signals of the detector D 2 and D 4 -which are placed with λ/4 spacing -are identical.The incorrect position of the short circuit piston is indicated by the difference between D 2 and D 4 .The differences between the signals determine the distance from the required position of the piston, while its sign refers to the direction where the short circuit piston must be moved.These latter detectors are applied to measure the equilibrium state, while D 1 and D 3 serve for the detection of the standing wave ratio.Figure 2 c) represents the state when the sample is still in the sample holder, but the equilibrium is restored by moving the short circuit piston.At the correct position of the piston the wave-front is restored to the state demonstrated in Figure 2 a).The position of the short circuit piston will be at a new x 1 position after the detector signals turn into the initial positions on the standing wave.The difference between position x 1 and x 0 (Δx) is used to calculate the dielectric constant ' (Equation 14).The standing wave ratio is equivalent to the quotient of the maximum and minimum values of the electromagnetic waves developed in the transmission line, which are measured by the detectors marked with odd numbers.
When the sample holder is empty the standing wave ratio is equal to 1 at equivalent state (D 2 = D 4 ).When the sample is inserted into the sample holder the standing wave ratio increases and the calculation of the dielectric loss factor " is based on the extent of this increase of the standing wave ratio (Equation 15).
where β = 2π/ λ T and Δx = x 1 -x 0 where 1/r = √ (U 1 /U 3 ), a is the large waveguide dimension, d is the inner diameter of the sample holder λ and λ T are wavelength in vacuum and in the waveguide, respectively, U 1 /U 3 is the so called standing wave ratio, arising from D 1 and D 3 detector signals and Δx is the displacement of short circuit piston (Charreyre, 1984).

Determination of the dielectric constant, dielectric loss factor and the electrical conductivity of ionic liquids
Dielectric constants, dielectric loss factors and the temperature dependence of the dielectric properties of ionic liquids intended to be used in batteries were determined by the above described self-designed microwave dielectrometric apparatus (Figure 3) at the frequency of 2.45 GHz and at different temperatures (30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C and 120°C).The speed of the change in temperature depends on the electrical field strength in the material (E), the absorbed microwave power, (P v ) density (ρ), the specific heat capacity (C p ) and the dielectric loss factor " and can be given by Equation 16 (Göllei, 2009).
The electrical conductivity values (G) were calculated by Equation 12 and Equation 13.Temperature (°C)  It is worth noticing that the ILs containing the same cation and different anions have quite different E 1 values, and that their E 2 values become more distinct with the increase in temperature between the temperature range of 80°C and 120°C.
It can be concluded that there might be a connection between the dielectric constant, dielectric loss factor and electrical conductivity of the investigated ILs and the structure of the ILs but the connection is not yet correctly defineable and further experiments should be carried out in this field.

Effect of cation on the dielectric properties
The dielectric constant (E 1 ), dielectric loss factor (E 2 ] a sudden break can be noticed in the constant values between 50°C and 70°C.This might be associated with some sudden changes in the structure of the ILs or in the physicochemical interactions between the ions.Similarly to the dielecetric constant, the highest dielectric loss of 4. ] anion it can be stated that the cation has an effect on the dielectric constant and the dielectric loss factor of the investigated liquids.Furtehrmore it can be concluded that the [DiEtMeIm][PF 6 ] and [DiEtetIm][PF 6 ] ILs seem to be the most adequate for battery applications, however further investigation must be carried out.Taking into account the dielectric constant, dielectric loss and electrical conductivity results of all the investigated ILs it can be concluded that the alkyl chain length of the cation and the structure of the anion strongly influence the dielectric properties of the ILs and that the highest G value is exhibited by the [DiEtMeIm][BF 4 ] IL at 30°C temperature, hence it is the most suitable candidate for battery applications.Furtermore it can be stated that implementing the knowledge about the connection between the IL structure and the dielectric properties another ILs should be studied in order to select the ones most adequate for electrolyte application.

Conclusion
Due to the special characteristics of ILs, such as wide electrochemical windows, high inherent conductivities, high thermal and electrochemical stability, tuneable physicochemical properties, etc., they might be excellent candidates for environmentally sound, green electrolites in batteries.In order to decide whether they could be successfully applied it is essential to gain information about their dielectric properties.
However, several techniques have already been published about determination of the dielectric properties of ILs, so far no measuring methods were introduced due to the fact that such measurements cannot be realized by classical methods because of the high conductivity of ILs.
In this chapter first a self-designed microwave dielectrometric apparatus and a novel method for the measurement of the dielectric properties of ILs were described, then the dielectric constant, dielectric loss factor and electrical conductivity of several ILs were measured.Furthermore the connections between the structure of the investigated ionic liquids and the dielectric characteristics were determined.It was concluded that there was a certain connection between the structure of the investigated ILs and that this knowledge was worth implementing in studying further ILs that might be applied as novel, environmentally sound electrolytes.

Acknowledgement
The research work was supported by the research program "Livable environment and healthier people -Bioinnovation and Green Technology Research at the University of Pannonia TÁMOP-4.2.2-08/1/2008-0018"

Fig. 1 .
Fig. 1.Scheme of the experimental set-upThe IL sample, which is placed in the thermostat to keep it at the desired constant temperature, is flown across the waveguide having a length of about 3λ through the sample holder tube with the help of a peristaltic pump.The electric energy is transformed into microwave energy by the magnetron.The stepper motor is controlled by a microprocessor control unit, which contains an Intel 8-bit microcontroller, 12 bit A/D converters for receiving the four diode-detector signals, a stepper motor driver and a RS-232 serial interface to connect it to the PC.The control unit collects the detector signals and the temperature data determined by the temperature sensor and sends them to the PC.Furthermore it controls the position of the short circuit displaceable piston on the basis of algorithm software elaborated for this purpose.

Fig. 2
Fig. 2. a-c.Illustration of wave-fronts developed in the transmission line Figure 2 b) shows the situation when the sample is introduced into the sample holder such a way, that the position of the piston remains unchanged.However, the reflection on the short circuit takes place in the same way as previously, the signals of the detectors are different from the previous state (demonstrated in Figure 2 a) when the shape of the wave-front formed in the transmission line is modified by the sample of dielectric properties.Due to the equilibrium shift between the D 2 and D 4 detectors, their signals become different and the quotient of the signals of D 1 and D 3 decreases because of the change of the traveling/reflected wave ratio, where the latter depends on the dielectric constant.Figure2 c) represents the state when the sample is still in the sample holder, but the equilibrium is restored by moving the short circuit piston.At the correct position of the piston the wave-front is restored to the state demonstrated in Figure2 a).The position of the short circuit piston will be at a new x 1 position after the detector signals turn into the initial positions on the standing wave.The difference between position x 1 and x 0 (Δx) is used to calculate the dielectric constant ' (Equation14).The standing wave ratio is equivalent to the

Fig. 5 .
Fig. 5. Dielectric loss factor of ILs containing common [DiEtEtIm] cation and different anions The dielectric loss values are almost the same and quite low for all six ILs at the temperature of 30°C.Except for the E 2 of [DiEtEtIm][Br], which reaches only the value of 2 at 120°C, the dielectric loss of the other five investigated ILs increase constantly with higher temperature.It is interesting to note that the highest E 2 value belongs to the [DiEtEtIm][N(CN) 2 ] IL, which has the lowest E 1 value.

Table 1 .
Name, structural formula and water content of the studied ionic liquids www.intechopen.com 3 belongs to [DiEtMeIm][PF 6 ], while the other ILs have more than four times lower E 2 values.With increasing temperature between the range of 30°C and 120°C the dielectric loss value of [DiEtMeIm][PF 6 ] shows a sharp increase, the values for [DiEtEtIm][PF 6 ] and [DiEtBuIm][PF 6 ] moderately increase and the increase of [DiEtPrIm][PF 6 ] E 2 value exhibits such a sudden change in the values between 100°C and 110°C as it is shown in the case of E 1 .