**3. Ionic liquid as quasi solid and solid electrolyte**

Liquid electrolytes need additional encapsulation. Thus, SCs applied ILs face difficulty in integration and manufacturing flexible devices. These problems can be addressed by solidifying the ILs because the ILs maintain the conductivity even when solidified, unlike a typical organic electrolyte. Generally, polymers are applied for solid state with various devices, and ILs adopted polymer structure are described in **Figure 10**.

**Figure 10.** Ionic liquid-polymer electrolyte.

#### **3.1. Ionic liquid-blended polymer electrolyte**

ILs blended polymer electrolyte can be referred to ion gel (iongel, ionogel) because most of the ILs blended polymer electrolytes were synthesized in gel polymer type. The initial gel polymer electrolyte consisted of ion conducting salts (ILs), organic solvent, and polymers like gel polymer electrolyte of LIBs. In 1997, Fuller et al. reported iongel that contains ILs, organic solvent, and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) [74]. The iongel which was composed of [EMI][BF4] and [EMI][CF<sup>3</sup> SO<sup>3</sup> − ] and PVdF-HFP achieved the highest ionic conductivity of 5.8 mS cm−1. Various carbon materials were adopted from classical AC to nanostructured carbon such as CNT and graphene [75–77]. Pandey et al. also proposed [EMI] [B(CN)<sup>4</sup> ] and PVdF-HFP-based iongel electrolyte [77]. SCs applied ion gel was tested with multiwall carbon nanotube (MWCNT) electrode. The SCs adopting [EMI][B(CN)<sup>4</sup> ] based iongel shows good thermal stability up to 310°C, a wider electrochemical window of ∼3.8 V, and a high ionic conductivity of ∼9 × 10−3 S cm−1 at room temperature. The SCs also show a specific energy of ∼3.5 W h kg−1 and a specific power of ∼4.2 kW kg−1. Lu et al. also studied ion gel with PVdF-HFP and [EMI][FSI] [78]. They compared different polymer composites. The ion gels were flexible and mechanically strong and show 28 Wh kg−1 of the energy density with 2317 W kg−1 of the power density. The features of iongel and the structure of the SCs using the iongel are shown in **Figure 11**.

specific current of 2.67 A g−1, and energy density and power density of 25.7 Wh kg−1 and 35.2 kW kg−1, respectively. The ion gel stretched up to 150%. Other polymeric matrixes such as poly(acrylonitrile), polyvinylidene fluoride/polyvinyl acetate, poly(ethylene oxide), poly(vinylalcohol), poly(methylmethacrylate), poly(tetrafluoroethylene), and chitosan were

**Figure 11.** (a) Photographs of ILGPEs prepared by different methods: (A) PVdF-HFP/EMIMTf2N by ionic-liquid–

N] by ionic-liquid–polymer membrane method. (b) Photograph of a capacitor before assembly

N]/zeolite ILGPE.

N]/zeolite by ionic-liquid–inorganicpolymer composite method,

Ionic Liquid for High Voltage Supercapacitor http://dx.doi.org/10.5772/intechopen.73053 37

Most of the introduced ILs and iongel operated at 2.0–3.5 V. These operating voltages were not enough to overcome intrinsic limitation of ILs because ILs have low ionic conductivity and high viscosity, such properties cause decrement of specific capacity. To overcome relatively low specific capacity, operating voltage have to be enhanced at 3.5–4.0 V. Additionally, high operating voltage is helpful to manage power supply. SCs are regarded as excellent energy storage devices due to their high power density and permanent life cycles. However, SCs face with critical challenge of low energy density. Therefore, LIBs and SCs are applied together for the complementary purpose. The typical case is a wireless detection system that needs two kinds of power supply units. One is a continuous power supply for operation, and the other is a pulse power to transmit data to the control system. Continuous power systems require

investigated [80–83].

and (C) PTFE/[EMIM][Tf<sup>2</sup>

polymer gelation method, (B) PVdF-HFP/[EMIM][Tf<sup>2</sup>

from two carbon electrodes and a PVdF-HFP/[EMIM][Tf<sup>2</sup>

Each carbon electrode was attached on a polypropylene plate on the aluminum foil side. ([EMIM][Tf<sup>2</sup> N]: [EMI][FSI] and PTFE: Poly(tetrafluoroethylene)) [78]. Other type of polymers and ILs has been tried as ion gel electrolyte. Tamilarasan et al. reported ion gel as stretchable electrolyte. The synthesized [BMI][TFSI] incorporated stretchable poly(methyl methacrylate)(PMMA) electrolyte [79]. The device has specific capacitance of 83 F g−1,

**Figure 11.** (a) Photographs of ILGPEs prepared by different methods: (A) PVdF-HFP/EMIMTf2N by ionic-liquid– polymer gelation method, (B) PVdF-HFP/[EMIM][Tf<sup>2</sup> N]/zeolite by ionic-liquid–inorganicpolymer composite method, and (C) PTFE/[EMIM][Tf<sup>2</sup> N] by ionic-liquid–polymer membrane method. (b) Photograph of a capacitor before assembly from two carbon electrodes and a PVdF-HFP/[EMIM][Tf<sup>2</sup> N]/zeolite ILGPE.

**3.1. Ionic liquid-blended polymer electrolyte**

**Figure 10.** Ionic liquid-polymer electrolyte.

36 Supercapacitors - Theoretical and Practical Solutions

which was composed of [EMI][BF4] and [EMI][CF<sup>3</sup>

[B(CN)<sup>4</sup>

([EMIM][Tf<sup>2</sup>

iongel are shown in **Figure 11**.

ILs blended polymer electrolyte can be referred to ion gel (iongel, ionogel) because most of the ILs blended polymer electrolytes were synthesized in gel polymer type. The initial gel polymer electrolyte consisted of ion conducting salts (ILs), organic solvent, and polymers like gel polymer electrolyte of LIBs. In 1997, Fuller et al. reported iongel that contains ILs, organic solvent, and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) [74]. The iongel

ionic conductivity of 5.8 mS cm−1. Various carbon materials were adopted from classical AC to nanostructured carbon such as CNT and graphene [75–77]. Pandey et al. also proposed [EMI]

gel shows good thermal stability up to 310°C, a wider electrochemical window of ∼3.8 V, and a high ionic conductivity of ∼9 × 10−3 S cm−1 at room temperature. The SCs also show a specific energy of ∼3.5 W h kg−1 and a specific power of ∼4.2 kW kg−1. Lu et al. also studied ion gel with PVdF-HFP and [EMI][FSI] [78]. They compared different polymer composites. The ion gels were flexible and mechanically strong and show 28 Wh kg−1 of the energy density with 2317 W kg−1 of the power density. The features of iongel and the structure of the SCs using the

Each carbon electrode was attached on a polypropylene plate on the aluminum foil side.

mers and ILs has been tried as ion gel electrolyte. Tamilarasan et al. reported ion gel as stretchable electrolyte. The synthesized [BMI][TFSI] incorporated stretchable poly(methyl methacrylate)(PMMA) electrolyte [79]. The device has specific capacitance of 83 F g−1,

N]: [EMI][FSI] and PTFE: Poly(tetrafluoroethylene)) [78]. Other type of poly-

multiwall carbon nanotube (MWCNT) electrode. The SCs adopting [EMI][B(CN)<sup>4</sup>

SO<sup>3</sup> −

] and PVdF-HFP-based iongel electrolyte [77]. SCs applied ion gel was tested with

] and PVdF-HFP achieved the highest

] based ion-

specific current of 2.67 A g−1, and energy density and power density of 25.7 Wh kg−1 and 35.2 kW kg−1, respectively. The ion gel stretched up to 150%. Other polymeric matrixes such as poly(acrylonitrile), polyvinylidene fluoride/polyvinyl acetate, poly(ethylene oxide), poly(vinylalcohol), poly(methylmethacrylate), poly(tetrafluoroethylene), and chitosan were investigated [80–83].

Most of the introduced ILs and iongel operated at 2.0–3.5 V. These operating voltages were not enough to overcome intrinsic limitation of ILs because ILs have low ionic conductivity and high viscosity, such properties cause decrement of specific capacity. To overcome relatively low specific capacity, operating voltage have to be enhanced at 3.5–4.0 V. Additionally, high operating voltage is helpful to manage power supply. SCs are regarded as excellent energy storage devices due to their high power density and permanent life cycles. However, SCs face with critical challenge of low energy density. Therefore, LIBs and SCs are applied together for the complementary purpose. The typical case is a wireless detection system that needs two kinds of power supply units. One is a continuous power supply for operation, and the other is a pulse power to transmit data to the control system. Continuous power systems require high-energy densities, while pulse power systems need high-power densities. Consequently, LIBs are responsible for device operation, while SCs are used for data transmission. A critical issue in this system is voltage leveling between the LIBs and SCs. The operating voltage of LIBs is from 3.5 to 4.0 V. Thus two or more SCs in series are needed for a wireless detection system. If SCs achieve the 3.5–4.0 V rated voltage with high energy densities or LIBs achieve high power densities, then the power supply unit can be minimized and integrated. LIBs have intrinsic limitations in power densities due to their energy storage mechanisms. Hence, SCs that show high energy densities and wide operating voltages have been researched extensively. Thus, new approaches were reported to enhance working voltage of SCs. Pandey et al. adopted zeolite as additives for enhancing operating voltage, and the zeolite added SCs was stable up to 4.1 V [78]. Cross-linked polymer matrix was suggested as solid electrolyte for high-voltage SCs. Choi et al. suggested all printable SCs using UV curable materials. [BMI] [BF<sup>4</sup> ] and UV-cured ethoxylated trimethylolpropane triacrylate (ETPTA)-based gel polymer electrolytes are incorporated to produce the solid-state SCs. They showed various shapes of inkjet-printed SCs described in **Figure 12**. Interestingly, proposed UV-cured ion gel was water proof despite of adopting [BMI][BF<sup>4</sup> ] [84].

Our team proposed a quasi-solid polymer electrolyte based on a composite cross-linked poly-4-vninyphenol (c-P<sup>4</sup> VPh) embedded [EMI][TFSI] [5]. [EMI][TFSI] was selected for its high ionic conductivity, and c-P<sup>4</sup> VPh was used for its ability to enhance electrochemical stability by hydrogen bonding between [EMI][TFSI] and c-P<sup>4</sup> VPh in addition to maintaining a quasi-solid state. Also, cross-linked polymers can keep a larger amount of ILs than other polymer by swelling. The composite electrolytes are highly ionic conductive solid state due to the rigid framework of c-P4 VPh and high ionic conductivity from large contents of [EMI][TFSI] over 60 wt%. The IL-CPs are thermally stable over 300°C and electrochemically stable over 7.0 V, since there are hydrogen bonds between c-P<sup>4</sup> VPh and [EMI][TFSI]. We also introduced all-solid state SCs that operate at 4.0 V and have high energy density without sacrificing power density. SCs showed the best electrochemical performances and had capacitance of 172.5 F g-1 and energy density of 72.3 Wh kg−1. Their structure and electrochemical characteristics are displayed in **Figure 13**.

#### **3.2. Poly ionic liquid as solid electrolyte**

A poly IL (PIL) or polymeric ionic liquid is a polymer electrolyte containing a polymer backbone and an IL species in the monomer repeat unit. Certain characteristics of IL such as negligible vapor pressure, thermal stability, nonflammability, relatively high ionic conductivity, and broad electrochemical stability window are transferred to the polymer chain (from oligomers to high molecular weight polymers) [85]. Various PIL structures and properties are required for various applications of polymer electrolytes, electrochemical devices, smart materials, catalyst supports, porous polymer structures, and antibacterial PILs. The standard synthetic pathway for PILs relies on a basic strategy: (1) direct chain growth polymerization of IL with or without nonionic monomers; (2) step-growth polymerization of IL monomers; and (3) post-modification of polymer chains with IL monomers.

**Figure 12.** Esthetic versatility and IoT applications of the inkjet-printed SCs as object-tailored/monolithically integrated power sources. (a) Photograph of the inkjet-printed Korea map, wherein the inkjet-printed SCs (marked by red boxes) were seamlessly connected to LED lamps (marked by blue boxes) via the inkjet-printed electric circuits. (b) SEM image of the LED lamp connected to the inkjet-printed electric circuits. (c) CV profile (scan rate = 1.0 mV s−1) of the inkjet-printed SC in the map. (d) Photograph of the inkjet-printed, letter ("OR")-shaped SCs that were seamlessly connected to the letter ("HOT" and "COLD")-shaped electric circuits onto A4 paper. (e) Galvanostatic charge/discharge profile (current density = 1.0 mA cm−2) of the inkjet-printed, letter ("OR")-shaped SCs that were composed of 4 cells connected in series. (f) Photograph depicting the operation of the blue LED lamp in the smart cup (for cold water (∼10°C)), wherein the inset is a photograph of a temperature sensor assembled with an Arduino board. (g) Photograph depicting the operation of the red LED lamp in the smart cup (for hot water

Ionic Liquid for High Voltage Supercapacitor http://dx.doi.org/10.5772/intechopen.73053 39

(∼80°C)) [84].

Properties required as solid electrolytes include ionic conductivity, thermal stability, and electrochemical stability. The ionic conductivity of PILs is an important property when applied as

high-energy densities, while pulse power systems need high-power densities. Consequently, LIBs are responsible for device operation, while SCs are used for data transmission. A critical issue in this system is voltage leveling between the LIBs and SCs. The operating voltage of LIBs is from 3.5 to 4.0 V. Thus two or more SCs in series are needed for a wireless detection system. If SCs achieve the 3.5–4.0 V rated voltage with high energy densities or LIBs achieve high power densities, then the power supply unit can be minimized and integrated. LIBs have intrinsic limitations in power densities due to their energy storage mechanisms. Hence, SCs that show high energy densities and wide operating voltages have been researched extensively. Thus, new approaches were reported to enhance working voltage of SCs. Pandey et al. adopted zeolite as additives for enhancing operating voltage, and the zeolite added SCs was stable up to 4.1 V [78]. Cross-linked polymer matrix was suggested as solid electrolyte for high-voltage SCs. Choi et al. suggested all printable SCs using UV curable materials. [BMI]

] and UV-cured ethoxylated trimethylolpropane triacrylate (ETPTA)-based gel polymer electrolytes are incorporated to produce the solid-state SCs. They showed various shapes of inkjet-printed SCs described in **Figure 12**. Interestingly, proposed UV-cured ion gel was water

Our team proposed a quasi-solid polymer electrolyte based on a composite cross-linked poly-

Also, cross-linked polymers can keep a larger amount of ILs than other polymer by swelling. The composite electrolytes are highly ionic conductive solid state due to the rigid framework

operate at 4.0 V and have high energy density without sacrificing power density. SCs showed the best electrochemical performances and had capacitance of 172.5 F g-1 and energy density of 72.3 Wh kg−1. Their structure and electrochemical characteristics are displayed in **Figure 13**.

A poly IL (PIL) or polymeric ionic liquid is a polymer electrolyte containing a polymer backbone and an IL species in the monomer repeat unit. Certain characteristics of IL such as negligible vapor pressure, thermal stability, nonflammability, relatively high ionic conductivity, and broad electrochemical stability window are transferred to the polymer chain (from oligomers to high molecular weight polymers) [85]. Various PIL structures and properties are required for various applications of polymer electrolytes, electrochemical devices, smart materials, catalyst supports, porous polymer structures, and antibacterial PILs. The standard synthetic pathway for PILs relies on a basic strategy: (1) direct chain growth polymerization of IL with or without nonionic monomers; (2) step-growth polymerization of IL monomers;

Properties required as solid electrolytes include ionic conductivity, thermal stability, and electrochemical stability. The ionic conductivity of PILs is an important property when applied as

VPh and high ionic conductivity from large contents of [EMI][TFSI] over 60 wt%. The IL-CPs are thermally stable over 300°C and electrochemically stable over 7.0 V, since there are

VPh) embedded [EMI][TFSI] [5]. [EMI][TFSI] was selected for its high ionic

VPh was used for its ability to enhance electrochemical stability by hydro-

VPh and [EMI][TFSI]. We also introduced all-solid state SCs that

VPh in addition to maintaining a quasi-solid state.

] [84].

[BF<sup>4</sup>

of c-P4

proof despite of adopting [BMI][BF<sup>4</sup>

38 Supercapacitors - Theoretical and Practical Solutions

gen bonding between [EMI][TFSI] and c-P<sup>4</sup>

**3.2. Poly ionic liquid as solid electrolyte**

and (3) post-modification of polymer chains with IL monomers.

4-vninyphenol (c-P<sup>4</sup>

conductivity, and c-P<sup>4</sup>

hydrogen bonds between c-P<sup>4</sup>

**Figure 12.** Esthetic versatility and IoT applications of the inkjet-printed SCs as object-tailored/monolithically integrated power sources. (a) Photograph of the inkjet-printed Korea map, wherein the inkjet-printed SCs (marked by red boxes) were seamlessly connected to LED lamps (marked by blue boxes) via the inkjet-printed electric circuits. (b) SEM image of the LED lamp connected to the inkjet-printed electric circuits. (c) CV profile (scan rate = 1.0 mV s−1) of the inkjet-printed SC in the map. (d) Photograph of the inkjet-printed, letter ("OR")-shaped SCs that were seamlessly connected to the letter ("HOT" and "COLD")-shaped electric circuits onto A4 paper. (e) Galvanostatic charge/discharge profile (current density = 1.0 mA cm−2) of the inkjet-printed, letter ("OR")-shaped SCs that were composed of 4 cells connected in series. (f) Photograph depicting the operation of the blue LED lamp in the smart cup (for cold water (∼10°C)), wherein the inset is a photograph of a temperature sensor assembled with an Arduino board. (g) Photograph depicting the operation of the red LED lamp in the smart cup (for hot water (∼80°C)) [84].

(110°C). Flexible backbones yield lower T<sup>g</sup>

resulting PIL exhibits a low T<sup>g</sup>

30°C and ~10−3 S cm−1 at 90°C.

on PIL thermal stability [89]. [CF<sup>3</sup>

[C16H34PO<sup>4</sup>

− ]. values. Similarly, Hu et al. directly grafted TFSI

Ionic Liquid for High Voltage Supercapacitor http://dx.doi.org/10.5772/intechopen.73053

value (up to −14°C) and an average conductivity of 10−5 S cm−1 at

H4 SO<sup>3</sup> − ] > [PF<sup>6</sup>

] > bromide >

41

anions onto the polyethylene oxide backbone via anionic ring-opening polymerization [88]. The

The thermal stability of the PILs is directly related to the lifetime and stability of the capacitor and chemical and electrochemical stability. In thermogravimetric analysis (TGA) experiments, the decomposition starting temperature (Tonset) is usually controlled through the nature of the PIL backbone chemical structure. Imidazolium-based PILs have improved thermal stability due to conjugate structure and steric hindrance. They have higher Tonset values than pyrrolidinium-based PILs. The thermal stability of PILs increases with the length of the substituent of the cation. The chemical structure of anions also affects the Tonset value of PILs. Poly (1-vinyl-3-ethyl-imazazolium) X-PIL was studied by comparing the effects of counter anions

] > [TFSI] > [C12H25C6

PIL-based polymer electrolytes have high ionic conductivity (up to 10−3 S cm−1 at 25°C), wide electrochemical windows (up to 5 V), and high thermal stability (up to 350°C) [86]. Pyrrolidinium-based PILs have been reported to have better electrochemical stability than imidazolium-based PILs. The pyrrolidinium cations also exhibit a much larger cathodic electrolysis potential than the quaternary ammonium cations of the noncyclic and unsaturated rings [90]. Because of excellent electrochemical stability and reasonable ionic conductivity in solid phase, PIL electrolytes have been considered as an ideal electrolyte for supercapacitors. However, only a few studies have been reported in this area by Marcilla group, and (Diallyldimethylammonium) bis (trifluoromethanesulfonyl) imide dissolved in [PYR14] was prepared and applied as a solid state electrolyte [91, 92]. Impregnating the electrode with the electrolyte before assembling the SCs is an important process for improving the carbonelectrolyte contact. However, these manufactured SCs high ESR values due to poor electrode/ electrolyte interface performance. Further studies using pyrrolidium-based PIL electrolytes with high ionic conductivity have greatly improved relative dielectric constant. **Figure 14**

In conclusion, ILs have tremendous potential in electrochemical systems with combination of cations and anion. However, their applicability has been limited by the general perception that they are ionic electrolytes for replacing conventional electrolytes. However, due to its high viscosity, low ionic conductivity, and high price, it is difficult to replace conventional electrolytes directly. In other words, ILs should not be chosen as an alternative to organic solvents. Therefore, it is considered that the ILs-based liquid electrolyte are used to enhance the energy density by increasing the high operating voltage of the SCs. They are also helpful to secure the stability and safety of SCs. In addition, since IL is stable at high temperatures, it can be applied to SCs that must be used under extreme conditions. In the same way, IL is more attractive as a solid electrolyte instead of a conventional electrolyte. In particular, IL-based solid polymer electrolytes appear to be one of the most promising choices for flexible SCs. However, current research on ILs as electrolytes has focused on several ILs, including imidazolium and pyrrolidinium cation. As mentioned, ILs have design flexibility due to the

SO<sup>3</sup> −

shows the PILs structure and electrochemical characteristics.

**Figure 13.** (a) Chemical structure of IL-CPs, (b) LSV for EMITFSI and IL-CPs in the SUS/SUS cell (CR 2032), (c) flexibility of IL-CPs and bending performance of SCs with IL-CPs for 1000 cycles, and (d) photograph of a green light-emittingdiode (LED) powered by a single SC with IL-CP3.5. (IL-CPs: [EMI][TFSI]/c-P<sup>4</sup> VPh composite, and IL-CPx: x = weight ratio of [EMI][TFSI]) [5].

a solid electrolyte. Unlike ILs and ion gels in which both anions and cations can move, PILs are typically single-ion conductors. In this case, anions or cations are structurally constrained as part of the polymer skeleton [86]. Therefore, the ionic conductivity of PIL is generally lower than that of monomeric ILs ionic conductivity. This phenomenon is due to the significant increment of glass transition temperature (Tg ) and depletion of mobile ions after covalent or ionic bonding [85]. The ionic conductivity of PIL is affected by polymer architecture, molecular weight, and chemical nature of polymer chains.

Particularly, the ionic conductivity of PIL is related to the glass transition temperature, and the ionic conductivity is usually increased when the T<sup>g</sup> is low. PIL has weak binding ions and can exhibit low T<sup>g</sup> despite having very high charge densities due to weak electrostatic ion pair interactions. Counter ions affect the T<sup>g</sup> of these polymers. Therefore, a method of lowering the Tg using a different kind of anion has been proposed. Tang et al. found that anion tendency with poly 1-(p-Vinylbenzyl)-3-methyl-imidazolium cation, the T<sup>g</sup> also changes in order [FSI] (3°C) < o-benzoicsulphimide (40°C) < [BF<sup>4</sup> ] (78°C) < [PF<sup>6</sup> ] (85°C) [87]. For PILs with the same anion [BF<sup>4</sup> ], the Tg varied according to the backbone and Poly[(1-butylimidazolium-3)methylethylene oxide (33°C) < 1-[2-(Methacryloyloxy)ethyl]-3-butyl-imidazolium (54°C) < Poly[1- (p-Vinylbenzyl)-3-butyl-imidazolium (78°C) < poly 1-(p-Vinylbenzyl)-3-methyl-imidazolium (110°C). Flexible backbones yield lower T<sup>g</sup> values. Similarly, Hu et al. directly grafted TFSI anions onto the polyethylene oxide backbone via anionic ring-opening polymerization [88]. The resulting PIL exhibits a low T<sup>g</sup> value (up to −14°C) and an average conductivity of 10−5 S cm−1 at 30°C and ~10−3 S cm−1 at 90°C.

The thermal stability of the PILs is directly related to the lifetime and stability of the capacitor and chemical and electrochemical stability. In thermogravimetric analysis (TGA) experiments, the decomposition starting temperature (Tonset) is usually controlled through the nature of the PIL backbone chemical structure. Imidazolium-based PILs have improved thermal stability due to conjugate structure and steric hindrance. They have higher Tonset values than pyrrolidinium-based PILs. The thermal stability of PILs increases with the length of the substituent of the cation. The chemical structure of anions also affects the Tonset value of PILs. Poly (1-vinyl-3-ethyl-imazazolium) X-PIL was studied by comparing the effects of counter anions on PIL thermal stability [89]. [CF<sup>3</sup> SO<sup>3</sup> − ] > [TFSI] > [C12H25C6 H4 SO<sup>3</sup> − ] > [PF<sup>6</sup> ] > bromide > [C16H34PO<sup>4</sup> − ].

PIL-based polymer electrolytes have high ionic conductivity (up to 10−3 S cm−1 at 25°C), wide electrochemical windows (up to 5 V), and high thermal stability (up to 350°C) [86]. Pyrrolidinium-based PILs have been reported to have better electrochemical stability than imidazolium-based PILs. The pyrrolidinium cations also exhibit a much larger cathodic electrolysis potential than the quaternary ammonium cations of the noncyclic and unsaturated rings [90]. Because of excellent electrochemical stability and reasonable ionic conductivity in solid phase, PIL electrolytes have been considered as an ideal electrolyte for supercapacitors. However, only a few studies have been reported in this area by Marcilla group, and (Diallyldimethylammonium) bis (trifluoromethanesulfonyl) imide dissolved in [PYR14] was prepared and applied as a solid state electrolyte [91, 92]. Impregnating the electrode with the electrolyte before assembling the SCs is an important process for improving the carbonelectrolyte contact. However, these manufactured SCs high ESR values due to poor electrode/ electrolyte interface performance. Further studies using pyrrolidium-based PIL electrolytes with high ionic conductivity have greatly improved relative dielectric constant. **Figure 14** shows the PILs structure and electrochemical characteristics.

a solid electrolyte. Unlike ILs and ion gels in which both anions and cations can move, PILs are typically single-ion conductors. In this case, anions or cations are structurally constrained as part of the polymer skeleton [86]. Therefore, the ionic conductivity of PIL is generally lower than that of monomeric ILs ionic conductivity. This phenomenon is due to the significant

**Figure 13.** (a) Chemical structure of IL-CPs, (b) LSV for EMITFSI and IL-CPs in the SUS/SUS cell (CR 2032), (c) flexibility of IL-CPs and bending performance of SCs with IL-CPs for 1000 cycles, and (d) photograph of a green light-emitting-

ionic bonding [85]. The ionic conductivity of PIL is affected by polymer architecture, molecu-

Particularly, the ionic conductivity of PIL is related to the glass transition temperature, and

using a different kind of anion has been proposed. Tang et al. found that anion tendency

] (78°C) < [PF<sup>6</sup>

ethylene oxide (33°C) < 1-[2-(Methacryloyloxy)ethyl]-3-butyl-imidazolium (54°C) < Poly[1- (p-Vinylbenzyl)-3-butyl-imidazolium (78°C) < poly 1-(p-Vinylbenzyl)-3-methyl-imidazolium

despite having very high charge densities due to weak electrostatic ion pair

varied according to the backbone and Poly[(1-butylimidazolium-3)methyl-

) and depletion of mobile ions after covalent or

of these polymers. Therefore, a method of lowering the

is low. PIL has weak binding ions and

VPh composite, and IL-CPx: x = weight

] (85°C) [87]. For PILs with the same

also changes in order [FSI]

increment of glass transition temperature (Tg

40 Supercapacitors - Theoretical and Practical Solutions

interactions. Counter ions affect the T<sup>g</sup>

], the Tg

(3°C) < o-benzoicsulphimide (40°C) < [BF<sup>4</sup>

can exhibit low T<sup>g</sup>

ratio of [EMI][TFSI]) [5].

Tg

anion [BF<sup>4</sup>

lar weight, and chemical nature of polymer chains.

the ionic conductivity is usually increased when the T<sup>g</sup>

diode (LED) powered by a single SC with IL-CP3.5. (IL-CPs: [EMI][TFSI]/c-P<sup>4</sup>

with poly 1-(p-Vinylbenzyl)-3-methyl-imidazolium cation, the T<sup>g</sup>

In conclusion, ILs have tremendous potential in electrochemical systems with combination of cations and anion. However, their applicability has been limited by the general perception that they are ionic electrolytes for replacing conventional electrolytes. However, due to its high viscosity, low ionic conductivity, and high price, it is difficult to replace conventional electrolytes directly. In other words, ILs should not be chosen as an alternative to organic solvents. Therefore, it is considered that the ILs-based liquid electrolyte are used to enhance the energy density by increasing the high operating voltage of the SCs. They are also helpful to secure the stability and safety of SCs. In addition, since IL is stable at high temperatures, it can be applied to SCs that must be used under extreme conditions. In the same way, IL is more attractive as a solid electrolyte instead of a conventional electrolyte. In particular, IL-based solid polymer electrolytes appear to be one of the most promising choices for flexible SCs. However, current research on ILs as electrolytes has focused on several ILs, including imidazolium and pyrrolidinium cation. As mentioned, ILs have design flexibility due to the

numerous possible combinations of anions and cations. We believe this flexibility should be achieved by designing a new type of supercapacitor by thinking outside the box beyond what

Ionic Liquid for High Voltage Supercapacitor http://dx.doi.org/10.5772/intechopen.73053 43

Program in Nano Science and Technology, Graduate School of Convergence Science and

[1] Zhang F, Zhang T, Yang X, Zhang L, Leng K, Huang Y, Chen Y. A high-performance supercapacitor-battery hybrid energy storage device based on graphene-enhanced electrode materials with ultrahigh energy density. Energy & Environmental Science. 2013;

[2] Mahlia TMI, Saktisahdan TJ, Jannifar A, Hasan MH, Matseelar HSC. A review of available methods and development on energy storage; technology update. Renewable and

[3] Hannan MA, Hoque MM, Mohamed A, Ayob A. Review of energy storage systems for electric vehicle applications: Issues and challenges. Renewable and Sustainable Energy

[4] Zhang Z, Zhang X, Chen W, Rasim Y, Salman W, Pan H, Yuan Y, Wang C. A high-efficiency energy regenerative shock absorber using supercapacitors for renewable energy applications in range extended electric vehicle. Applied Energy. 2016;**178**:177-188. DOI:

[5] Ahn Y, Kim B, Ko J, You D, Yin Z, Kim H, Shin D, Cho S, Yoo J, Kim YS. All solid state flexible supercapacitors operating at 4 V with a cross-linked polymer–ionic liquid electrolyte. Journal of Materials Chemistry A. 2016;**4**:4386-4391. DOI: 10.1039/C6TA

[6] Béguin F, Presser V, Balducci A, Frackowiak E. Carbons and electrolytes for advanced supercapacitors. Advanced Materials. 2014;**26**:2219-2251. DOI: 10.1002/adma.201304137

[7] Senthilkumar ST, Selvan RK, Melo JS. Redox additive/active electrolytes: A novel approach to enhance the performance of supercapacitors. Journal of Materials Chemistry

Sustainable Energy Reviews. 2014;**33**:532-545. DOI: 10.1016/j.rser.2014.01.068

is available.

**Author details**

Address all correspondence to: jyoo78@snu.ac.kr

**6**:1623-1632. DOI: 10.1039/C3EE40509E

10.1016/j.apenergy.2016.06.054

00643D

Technology, Seoul National University, Seoul, Republic of Korea

Reviews. 2017;**69**:771-789. DOI: 10.1016/j.rser.2016.11.171

A. 2013;**1**:12386-12394. DOI: 10.1039/C3TA11959A

Jeeyoung Yoo

**References**

**Figure 14.** Charge–discharge characterization of all-solid state supercapacitors with different impregnation ratios. (a) Charge–discharge profiles from 0 to 3.5 V at 1 mAcm−2, (b) equivalent series resistance (ESR), (c) specific capacitance (Cam), (d) specific real energy (Ereal) from charge–discharge profiles at different current densities and (e) schematic representation of all-solid state supercapacitors. (IL-b-PE: Ionic liquid based polymer electrolyte).

numerous possible combinations of anions and cations. We believe this flexibility should be achieved by designing a new type of supercapacitor by thinking outside the box beyond what is available.
