**2. Ionic liquid as liquid electrolyte for supercapacitors**

commercialization due to the disadvantage of obtaining hydrogen from petrochemistry and the reluctance of establishing a hydrogen station. As an alternative to this situation, SCs can be proposed. Up to now, SCs have been applied to an electric bus using regenerative break and electric train powered by pulse power, since the large power density of SCs is suitable for intermittent power supply [4]. However, SCs are facing with critical challenge of low energy density. Therefore, LIBs and SCs are applied together for the complementary purpose. The operating voltage of LIBs is from 3.5 to 4.0 V and that of SCs is approximately under 2.8 V. 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 [5]. In other words, if SCs archive high energy density without sacrificing power density, SCs can be applied as an

There are two ways to increase the energy density of SCs. One is to enhance capacitance, and the other is to increase the operating voltages, since the energy density is proportional to capacitance and operating voltage. At the initial stage, many researchers focused on widening the electrode area for enhancing electric double layer (EDL) capacitance. Various types of activated carbon (AC) are commercialized as a result of these efforts. Carbon nanomaterials like carbon nanotubes (CNTs) and graphene are also highlighted as large capacitance materials [6]. Additionally, redox active electrode materials like metal oxide and conductive polymer are proposed to enlarge capacitance by pseudocapacitance. However, those materials struggle with process abilities, limiting of electrolyte adoption, charging/discharging properties, minimizing high cost, and checking suitability for commercial production lines. Another approach of redox active materials is electrolyte that contains redox active couples. Representative redox couples are halides, vanadium complexes, copper salts, hydroquinone, methylene blue, indigo carmine, p-phenylenediamine, m-phenylenediamine, lignosulfonates, and sulfonated polyaniline [7]. Most of electrolyte adopting redox couple has a problem of solvent selection because such redox couples are effective in aqueous medium. This means that the operating voltage of aqueous electrolyte containing redox active couple is limited under 1.23 V, which is the electrolysis voltage of water. Such boundaries with electrode materials motivate the development of advanced electrolyte with high operating voltages for high energy densities because the operating voltages for SCs depend on the electrochemical stabil-

At early stages, classical salts and systems are suggested. The solvent is organic solvents such as acetonitrile (AN) and propylene carbonate (PC) [8]. Quaternary ammonium salts and AN are representative electrolyte of SCs, and their operating voltage is about 2.8 V. The electrolytes adopting AN exhibit large specific capacitance due to high ionic conductivity derived from low viscosity of AN. However, SCs applied AN-based electrolyte have to be operated under 80°C due to the boiling point of AN. Otherwise, PC is less toxic and has higher thermal stability than AN. Thus, the electrolyte adopting PC is generally considered a safe electrolyte. Using PC-based electrolytes can achieve slightly higher operating voltages for SCs

by reaction of carbon electrode and PC have been identified as the main causes of performance degradation at high voltages [9]. To overcome these drawbacks, many electrolytes have been proposed over the past several years as an alternative electrolyte to increase the

and CO<sup>2</sup>

caused

than those using AN. However, formation of carbonate and evolution of H<sup>2</sup>

alternative energy storage system replacing LIBs and FCs.

24 Supercapacitors - Theoretical and Practical Solutions

ity window of the electrolyte.

## **2.1. Neat ionic liquid as liquid electrolyte for electric double layer**

In 1807, Humphry Davy pioneered the study for the electrolysis of molten salts which is referred as ILs later and presented the electrochemical theory of the molten electrolyte, despite the research was concentrated on reactive metal preparation [13]. After that, synthetic method of aluminum with electrolysis of aluminum oxide dissolved in cryolite was suggested by Hall [14]. This method is meaningful that eutectic molten salts were formed at low temperature with electrolysis, and this method is still applied in aluminum industrial production.

ILs have been studied for electrochemical devices such as SCs, FCs, rechargeable batteries, photovoltaic cell, and actuator etc. due to the mobility and flexibility of ions [5, 14–20]. ILs are composed of large and asymmetrical organic cation and charge delocalized inorganic/ organic anion by weak interaction [21]. Despite that ILs exist in the cation-anion state, these structures lower the tendency to crystallize, so they provide a fluid phase with reasonable ion conductivity, and they show no decomposition or significant vapor pressure [21, 22]. As mentioned, ILs are composed of organic ions and can be combined to various structures with easy preparation. Thus, various kinds of ILs can be used for the given application with desired properties (**Figure 1**).

Wide tunability of ILs can be combined to satisfy the desired characteristics of SCs such as working voltage, operating temperature range, and internal resistances [23]. For these reasons, ILs have been widely investigated as electrolyte material. As mentioned above, quaternary

**Figure 1.** Synthetic mechanism of imidazolium-based ILs.

ammonium salts which is one of ILs are widely investigated with organic solvents, and some of them were already commercialized (1 M of tetraethylammonium tetrafluoroborate ([Et<sup>4</sup> N] [BF<sup>4</sup> ]) in AN or PC) [24]. Ammonium with different alkyl groups ([Et<sup>4</sup> N], tetrapropylene ammonium ([Pr<sup>4</sup> N]), and tetrabutyl ammonium ([Bu<sup>4</sup> N])) and various anions ([BF<sup>4</sup> − ], hexafluorophosphate ([PF<sup>6</sup> ]), perchlorate ([ClO<sup>4</sup> ]), and triplate ([CF<sup>3</sup> SO<sup>3</sup> − ])) was applied in organic solvents such as PC, AN, γ-butyrolactone (GBL), and N,N-dimethylformamide (DMF). The ionic conductivities generally decrease in the following order: [Et<sup>4</sup> N] > [Pr<sup>4</sup> N] > [Bu<sup>4</sup> N] > [Me<sup>4</sup> N], [BF<sup>4</sup> ] > [PF<sup>6</sup> ] > [ClO<sup>4</sup> ] > [CF<sup>3</sup> SO<sup>3</sup> ]. Similarly, nonaqueous electrolyte with various alkyl imidazolium salts was studied [25]. According to development of organic electrolyte, carbon electrode material which is effective for enhancing EDL capacitance was investigated. These studies were focused about suitability between the ion size of ammonium salts and the pore size of carbon electrode [26].

P = *<sup>V</sup>*<sup>2</sup> \_\_\_\_\_\_

**Figure 2.** Basic types of ionic liquids: Aprotic, protic, and zwitterionic types [21].

decomposition voltage (**Figure 3**).

E = \_\_<sup>1</sup>

respectively [34].

electrolyte (5.2 Wh kg−1).

This problem appears to be more severe at room and low temperatures, as evidenced by comparative studies between organic electrolytes and IL electrolytes [22, 28, 31]. These issues are more serious below room temperature. In addition, the EDL capacitance values of ILs adopting SCs can be decreased especially at high scan rates or high charging/discharging rates [32]. Despite these problems, ILs are still attractive materials for electrolytes. ILs show wide stability windows because the ILs are composed of individual ions, which do not participate in any considerable electrochemical reaction over a wide range of potential. Additionally, properties of ILs derived from ionic structure, such as high viscosity, increase the electrochemical

In terms of the energy density, increment of the operating voltage is advantageous rather than increase the capacitance. The energy density of the SCs is proportional to the square of the voltage as shown in Eq. 2, and E and C are energy density and specific capacitance of SCs,

**Figure 4** shows the energy density deference of typical electrolytes of SCs such as KOH aqueous solution, battery electrolyte, and ILs [35]. The hierarchical carbon nanostructure with mesoporous carbon CMK-5 intercalated between reduced graphene oxide (RGO) sheets was proposed as electrode in this research. According to cyclo-voltammograms, capacitive behaviors of applied electrolyte are very similar; however, the differences of stable windows result

increased to 60.7 Wh kg−1, and this value was 11 times higher than energy density of KOH

The electrochemical properties of SCs, especially their capacitances, are highly dependent on the suitability of the electrode material and the electrolyte. ILs are also heavily influenced by the electrode materials. In carbon material including porous activated carbon (AC), the graphitic edges are twisted, which can lead to uneven charge distribution [36]. This effect is

in significant differences of energy density. When adopting [EMI][BF<sup>4</sup>

<sup>4</sup> <sup>×</sup> *ESR* (1)

Ionic Liquid for High Voltage Supercapacitor http://dx.doi.org/10.5772/intechopen.73053 27

<sup>2</sup> *C V*<sup>2</sup> (2)

], energy density was

ILs can be classified as aprotic, protic, and zwitterionic types. Aprotic type ILs have been used for SCs [21]. In the published literature so far, most of ILs used in SCs are based on imidazolium, pyrrolidinium, ammonium, sulfonium, and phosphonium cations. Anions of ILs are [BF<sup>4</sup> ], [PF<sup>6</sup> ], bis(trifluoromethanesulfonyl)imide ([TFSI]), bis(fluorosulfonyl)imide ([FSI]), and dicyanamide ([DCA]). Among them, iomidazolium and pyrrolidinium based ILs were widely investigated because of relatively lower viscosity and reasonable ionic conductivity [27]. Generally, imidazolium salts were used for high ionic conductivity, and pyrrolidinium salts were applied for wide electrolchemical stability window [28, 29] (**Figure 2**). **Table 1** shows representative ILs for electrolyte of SCs.

Most ILs in **Table 1** have several problems, such as high viscosity, low ionic conductivity, and high cost comparison to typical SCs electrolytes. These issues prevent actual application in SCs. (Numerous ILs maintain solid phase at room temperature.) 1-ethyl-3-methyl imidazolium ([EMI]) [BF4] shows high ionic conductivity among ILs; however, the ionic conductivity value of [EMI][BF<sup>4</sup> ] is 23% level in comparison to 1 M [Et<sup>4</sup> N][BF<sup>4</sup> ] in AN. The viscosity of [EMI][BF<sup>4</sup> ] is also 130 times higher than common organic electrolyte of SCs [29]. Generally, low ionic conductivity and high viscosity of electrolyte occur increment of internal resistance (equivalent series resistance, ESR) and limit both the energy density and the power density. The energy density is decreased due to Ohmic drop and the power density is also reduced because the power density was oppositely proportional to ESR described as Eq. 1.P is power density, and V is working voltage [30].

**Figure 2.** Basic types of ionic liquids: Aprotic, protic, and zwitterionic types [21].

ammonium salts which is one of ILs are widely investigated with organic solvents, and some of them were already commercialized (1 M of tetraethylammonium tetrafluoroborate ([Et<sup>4</sup>

solvents such as PC, AN, γ-butyrolactone (GBL), and N,N-dimethylformamide (DMF). The

imidazolium salts was studied [25]. According to development of organic electrolyte, carbon electrode material which is effective for enhancing EDL capacitance was investigated. These studies were focused about suitability between the ion size of ammonium salts and the pore

ILs can be classified as aprotic, protic, and zwitterionic types. Aprotic type ILs have been used for SCs [21]. In the published literature so far, most of ILs used in SCs are based on imidazolium, pyrrolidinium, ammonium, sulfonium, and phosphonium cations. Anions of ILs are

dicyanamide ([DCA]). Among them, iomidazolium and pyrrolidinium based ILs were widely investigated because of relatively lower viscosity and reasonable ionic conductivity [27]. Generally, imidazolium salts were used for high ionic conductivity, and pyrrolidinium salts were applied for wide electrolchemical stability window [28, 29] (**Figure 2**). **Table 1** shows

Most ILs in **Table 1** have several problems, such as high viscosity, low ionic conductivity, and high cost comparison to typical SCs electrolytes. These issues prevent actual application in SCs. (Numerous ILs maintain solid phase at room temperature.) 1-ethyl-3-methyl imidazolium ([EMI]) [BF4] shows high ionic conductivity among ILs; however, the ionic conductiv-

low ionic conductivity and high viscosity of electrolyte occur increment of internal resistance (equivalent series resistance, ESR) and limit both the energy density and the power density. The energy density is decreased due to Ohmic drop and the power density is also reduced because the power density was oppositely proportional to ESR described as Eq. 1.P is power

] is 23% level in comparison to 1 M [Et<sup>4</sup>

], bis(trifluoromethanesulfonyl)imide ([TFSI]), bis(fluorosulfonyl)imide ([FSI]), and

] is also 130 times higher than common organic electrolyte of SCs [29]. Generally,

]), and triplate ([CF<sup>3</sup>

]) in AN or PC) [24]. Ammonium with different alkyl groups ([Et<sup>4</sup>

N]), and tetrabutyl ammonium ([Bu<sup>4</sup>

ionic conductivities generally decrease in the following order: [Et<sup>4</sup>

SO<sup>3</sup>

]), perchlorate ([ClO<sup>4</sup>

] > [CF<sup>3</sup>

[BF<sup>4</sup>

[Me<sup>4</sup>

[BF<sup>4</sup>

], [PF<sup>6</sup>

ity value of [EMI][BF<sup>4</sup>

[EMI][BF<sup>4</sup>

ammonium ([Pr<sup>4</sup>

N], [BF<sup>4</sup>

fluorophosphate ([PF<sup>6</sup>

] > [PF<sup>6</sup>

size of carbon electrode [26].

representative ILs for electrolyte of SCs.

density, and V is working voltage [30].

] > [ClO<sup>4</sup>

**Figure 1.** Synthetic mechanism of imidazolium-based ILs.

26 Supercapacitors - Theoretical and Practical Solutions

N]

N] >

N], tetrapropylene

N] > [Bu<sup>4</sup>

])) was applied in organic

] in AN. The viscosity of

− ], hexa-

N])) and various anions ([BF<sup>4</sup>

N] > [Pr<sup>4</sup>

SO<sup>3</sup> −

]. Similarly, nonaqueous electrolyte with various alkyl

N][BF<sup>4</sup>

$$\mathbf{P} = \frac{V^2}{4 \times ESR} \tag{1}$$

This problem appears to be more severe at room and low temperatures, as evidenced by comparative studies between organic electrolytes and IL electrolytes [22, 28, 31]. These issues are more serious below room temperature. In addition, the EDL capacitance values of ILs adopting SCs can be decreased especially at high scan rates or high charging/discharging rates [32]. Despite these problems, ILs are still attractive materials for electrolytes. ILs show wide stability windows because the ILs are composed of individual ions, which do not participate in any considerable electrochemical reaction over a wide range of potential. Additionally, properties of ILs derived from ionic structure, such as high viscosity, increase the electrochemical decomposition voltage (**Figure 3**).

In terms of the energy density, increment of the operating voltage is advantageous rather than increase the capacitance. The energy density of the SCs is proportional to the square of the voltage as shown in Eq. 2, and E and C are energy density and specific capacitance of SCs, respectively [34].

$$\mathbf{E} = \frac{1}{2}\mathbf{C}V^2\tag{2}$$

**Figure 4** shows the energy density deference of typical electrolytes of SCs such as KOH aqueous solution, battery electrolyte, and ILs [35]. The hierarchical carbon nanostructure with mesoporous carbon CMK-5 intercalated between reduced graphene oxide (RGO) sheets was proposed as electrode in this research. According to cyclo-voltammograms, capacitive behaviors of applied electrolyte are very similar; however, the differences of stable windows result in significant differences of energy density. When adopting [EMI][BF<sup>4</sup> ], energy density was increased to 60.7 Wh kg−1, and this value was 11 times higher than energy density of KOH electrolyte (5.2 Wh kg−1).

The electrochemical properties of SCs, especially their capacitances, are highly dependent on the suitability of the electrode material and the electrolyte. ILs are also heavily influenced by the electrode materials. In carbon material including porous activated carbon (AC), the graphitic edges are twisted, which can lead to uneven charge distribution [36]. This effect is

suggest that functional groups, especially hydroxyl groups, prevent ILs ions from interacting directly with the electrode surface [40]. These phenomena can be occurred polarity and hydrophilic/hydrophobic physical property of functional groups. In case of partially negative functional groups of carbonaceous materials like chlorine treated carbon prevent anion adsorption, or hydrophilic ILs indirectly bonded with the functionalities on surface of porous carbon electrode by hydrogen bonding and ions of ILs cannot be penetrated into narrow micropores. These ions block pores, and consequently, the specific capacitance was reduced especially at the high scan rate [41, 42]. Interestingly, these functional groups of carbonaceous materials are helpful to increase the specific capacitance of SCs in conventional electrolytes especially the aqueous electrolytes due to pseudoreaction. Thus, functional groups of carbonaceous materials have disadvantageous for ILs electrolytes system, while they are not effective with the conventional electrolyte adopted SCs. Pinker et al. controlled the specific

working and counter electrode: Platinum, reference electrode: Ag/AgCl electrode [33]. (DEME: N,N-diethyl-N-methyl-

**Figure 3.** Cyclic voltammogram of the ionic liquids based on the DEME cation and EMI-BF<sup>4</sup>

: [EMI][BF<sup>4</sup>

]).

N-(2-methoxyethyl)ammonium, EMI-BF<sup>4</sup>

capacitance by surface treatment of ordered mesoporous carbon [41]. [EMI][BF<sup>4</sup>

as the electrolyte. The surface treatment of pristine-ordered mesoporous carbide-derived carbon (OM-CDC) in chlorine gas and oxygen was evaluated. The chlorine gas protects the surface from reoxidation and lowers the surface polarity, thus enhancing the rate capability. The surface treatment of OM-CDC in air introduces oxygen functionalities, which result in a significant decrease of the rate capability. Silicon nanowires coated have attracted attention as a promising candidate for electrolytes for ILs electrolyte-based SCs due to their excellent cycle performance and thermal stability [43, 44]. Since the silicon electrode is not suitable for aqueous electrolytes, the importance of ionic liquid electrolytes has been further emphasized. Along with the compatibility with the electrodes, the behavior of the ionic liquid at the electrode/electrolyte interface is also important. Without destructive chemical interactions of the IL ion, it is electrochemically stable, but structural changes during cycling are still an important

] was used

at 25°C. Scan rate: 1 mV s−1,

Ionic Liquid for High Voltage Supercapacitor http://dx.doi.org/10.5772/intechopen.73053 29

**Table 1.** Representative cations, anions of ILs for SCs.

more important in ILs because the entire medium is made up of charged ions, so ions interact directly with the localized charge on the electrode surface [37]. Aligned mesoporous carbon provides a good opportunity for diffusion of electroactive species through well-ordered structures, but the chemical composition and degree of graphitization play a decisive role in electrochemical behavior and EDL formation [38]. This effect is more prevalent in ILs, in which the ionic medium is fully interacting. The nature of IL has a profound effect on the capacitive behavior of well-defined mesoporous carbon [39]. Graphene and similar carbonaceous materials are generally covered with various functional groups. Molecular dynamics simulations

**Figure 3.** Cyclic voltammogram of the ionic liquids based on the DEME cation and EMI-BF<sup>4</sup> at 25°C. Scan rate: 1 mV s−1, working and counter electrode: Platinum, reference electrode: Ag/AgCl electrode [33]. (DEME: N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium, EMI-BF<sup>4</sup> : [EMI][BF<sup>4</sup> ]).

suggest that functional groups, especially hydroxyl groups, prevent ILs ions from interacting directly with the electrode surface [40]. These phenomena can be occurred polarity and hydrophilic/hydrophobic physical property of functional groups. In case of partially negative functional groups of carbonaceous materials like chlorine treated carbon prevent anion adsorption, or hydrophilic ILs indirectly bonded with the functionalities on surface of porous carbon electrode by hydrogen bonding and ions of ILs cannot be penetrated into narrow micropores. These ions block pores, and consequently, the specific capacitance was reduced especially at the high scan rate [41, 42]. Interestingly, these functional groups of carbonaceous materials are helpful to increase the specific capacitance of SCs in conventional electrolytes especially the aqueous electrolytes due to pseudoreaction. Thus, functional groups of carbonaceous materials have disadvantageous for ILs electrolytes system, while they are not effective with the conventional electrolyte adopted SCs. Pinker et al. controlled the specific capacitance by surface treatment of ordered mesoporous carbon [41]. [EMI][BF<sup>4</sup> ] was used as the electrolyte. The surface treatment of pristine-ordered mesoporous carbide-derived carbon (OM-CDC) in chlorine gas and oxygen was evaluated. The chlorine gas protects the surface from reoxidation and lowers the surface polarity, thus enhancing the rate capability. The surface treatment of OM-CDC in air introduces oxygen functionalities, which result in a significant decrease of the rate capability. Silicon nanowires coated have attracted attention as a promising candidate for electrolytes for ILs electrolyte-based SCs due to their excellent cycle performance and thermal stability [43, 44]. Since the silicon electrode is not suitable for aqueous electrolytes, the importance of ionic liquid electrolytes has been further emphasized.

Along with the compatibility with the electrodes, the behavior of the ionic liquid at the electrode/electrolyte interface is also important. Without destructive chemical interactions of the IL ion, it is electrochemically stable, but structural changes during cycling are still an important

more important in ILs because the entire medium is made up of charged ions, so ions interact directly with the localized charge on the electrode surface [37]. Aligned mesoporous carbon provides a good opportunity for diffusion of electroactive species through well-ordered structures, but the chemical composition and degree of graphitization play a decisive role in electrochemical behavior and EDL formation [38]. This effect is more prevalent in ILs, in which the ionic medium is fully interacting. The nature of IL has a profound effect on the capacitive behavior of well-defined mesoporous carbon [39]. Graphene and similar carbonaceous materials are generally covered with various functional groups. Molecular dynamics simulations

Tetrafluoroborate

Bis(fluorosulfonyl)imide

Dicyanamide

**Cation Anion**

28 Supercapacitors - Theoretical and Practical Solutions

Pyrolidinium Hexafluorophosphate

Sulfonium Bis(trifluoromethylsulfonyl)imide

Imidazolium

Ammonium

Phosphonium

**Table 1.** Representative cations, anions of ILs for SCs.

[EMI][BF<sup>4</sup>

(EMImBF<sup>4</sup>

EMImN(SO<sup>2</sup>

viscosity, and 1

: [EMI][BF<sup>4</sup>

F)2

], EMImB(CN)<sup>4</sup>

: [EMI][B(CN)<sup>4</sup>

of 2 V. SCs adopting [EMI][BF<sup>4</sup>

ized double-walled carbon nanotubes (DWCNT).

], and [EMI][FSI] showed excellent cycling stability with the same potential window

maximum energy density of 67 Wh kg−1 at a current density of 1 Ag−1 [48] (**Figure 5**).

Other types of imidazolium-based ILs were also investigated. Bettini et al. studied film SCs by adopting ILs electrolytes. The electrolyte was varied out through cation based on [FSI] anion. The applied cations are EMI, 1-butyl-3-methylimidazolium ([BMI]), 1-dodecyl-3-methylimidazolium ([C12MI]), and 1-butyl-1-methylpyrrolidinium ([PYR14]), and electrode was nanostructured carbon (ns-C) [49]. The ionic conductivities were decreased depending on cation size in the order of [EMI] > [BMI] > [PYR14] > [C12MI]. [BMI][FSI] adopted SCs exhibited the highest specific capacitance of 75 F g−1 despite of secondly high ionic conductivity. This means that [BMI][FSI] have chemical affinity with ns-C. Borges et al. use 1-butyl-2,3-dimethylimidazolium [EBDMI] cation and [TFSI] anion as electrolyte of temperature stable SCs [50]. The proposed electrolyte achieved 4.4 V of electrochemical stability window with functional-

The pyrrolidinium cations were extensively studied for the wide operating voltage. Most of cases, [PYR14] cation was selected. Largeot et al. introduced high-temperature carbon-carbon SCs working at 100°C with [PYR14][TFSI] electrolyte and microporous carbide-derived carbon electrode [51]. High operating temperature means that ILs are thermally stable, and the viscosity of ILs is not enough to obtain reasonable specific capacitance (Most of pyrrolidinium based ILs exist in quasi solid state at room temperature). The specific capacitance of suggested SCs reached a maximum at 130 F g−1. Mastragostino group also reported several studies using pyrrolidinium cation, [PYR14], [TFIS] was used as anion [52–54]. They tested the ILs electrolytes using hybrid SCs (asymmetric SCs). The AC and the conductive polymer (poly(3-methylthiophene)) were adopted as electrodes. These SCs achieved 3.6 V of the working voltage and 16,000 cycle of the cycle-ability at 60°C. The hybrid supercapacitor delivered 24 Wh kg−1 and 14 kW kg−1 as maximum values. However, low ionic conductivity remains as problems.

**Figure 5.** (a) Cyclic voltammograms measured at cell voltage scan rate 1 mV s−1 for SC test cells filled with RTILs.

(C2 F5 )3

H NMR chemical shift of the proton in the 2-position of the imidazole ring of the ILs studied [48].

: [EMI][FSI], and EMImSCN: [EMI][SCN]) [47], (b) the relationship between the electrical conductivity,

: [EMI][PF<sup>3</sup>

(C2 F5 )3

], EMImN(SO<sup>2</sup>

CF3 )2

: [EMI][TFSI],

], EMImPF3

] had the widest potential window of 4 V, which displayed a

Ionic Liquid for High Voltage Supercapacitor http://dx.doi.org/10.5772/intechopen.73053 31

**Figure 4.** CV behaviors (a), Ragone plots (b), and change of energy density with current density (c) of the RGO– CMK-5 composite electrode measured in different electrolytes. (d) Cycle performance of the RGO–CMK-5 electrode measured in EMIMBF4 electrolyte, with the arrows showing the beginning of a new cycle [35] (CV: cyclic voltammetry, EMIMBF<sup>4</sup> :[EMI][BF<sup>4</sup> ]).

issue and should be carefully considered. To understand the aging or failure mechanism of ILs, electrochemical decomposition of ILs was investigated using in situ techniques (infrared and electrochemical spectroscopy methods and XPS) [45, 46]. These studies were performed to understand the EDL structure of ILs, enhancing electrochemical properties by modifying positive or negative ions or adopting pseudoelectrode material.

As previously mentioned, imidazolium-based ILs are one of the candidates for SCs electrolyte. The electrochemical characteristics of various ILs electrolytes with [EMI] cations and different anions ([BF<sup>4</sup> ], tetracyanoborate([B(CN)<sup>4</sup> ]), tris(pentafluoroethyl)trifluorophosphate ([PF<sup>3</sup> (C2 F5 )3 ]), [TFSI], [FSI], and thiocyanate ([SCN])) was reported using carbon cloth electrodes [47]. Working voltage was changed according to anions, and [EMI][BF<sup>4</sup> ], [EMI][TFSI], and [EMI][B(CN)<sup>4</sup> ] were stable up to 3.2 V. The highest energy values at 3.2 V for [EMI] [B(CN)<sup>4</sup> ] were 49 Wh kg−1. Graphene sheet electrode was also evaluated [EMI] cation with various anions, which were categorized representative chemical species. The five anions were [BF<sup>4</sup> ] as inorganic fluoride, [FSI] as organic fluoride, [DCA] as cyano functionality, ethylsulphate ([EtSO<sup>4</sup> ]) as ester functionality, and acetate ([OAc]) as acid. The hydrogen-bond-accepting ability of anions was closely related to the viscosity of the ILs. Moreover, [EMI][DCA], [EMI][BF<sup>4</sup> ], and [EMI][FSI] showed excellent cycling stability with the same potential window of 2 V. SCs adopting [EMI][BF<sup>4</sup> ] had the widest potential window of 4 V, which displayed a maximum energy density of 67 Wh kg−1 at a current density of 1 Ag−1 [48] (**Figure 5**).

Other types of imidazolium-based ILs were also investigated. Bettini et al. studied film SCs by adopting ILs electrolytes. The electrolyte was varied out through cation based on [FSI] anion. The applied cations are EMI, 1-butyl-3-methylimidazolium ([BMI]), 1-dodecyl-3-methylimidazolium ([C12MI]), and 1-butyl-1-methylpyrrolidinium ([PYR14]), and electrode was nanostructured carbon (ns-C) [49]. The ionic conductivities were decreased depending on cation size in the order of [EMI] > [BMI] > [PYR14] > [C12MI]. [BMI][FSI] adopted SCs exhibited the highest specific capacitance of 75 F g−1 despite of secondly high ionic conductivity. This means that [BMI][FSI] have chemical affinity with ns-C. Borges et al. use 1-butyl-2,3-dimethylimidazolium [EBDMI] cation and [TFSI] anion as electrolyte of temperature stable SCs [50]. The proposed electrolyte achieved 4.4 V of electrochemical stability window with functionalized double-walled carbon nanotubes (DWCNT).

The pyrrolidinium cations were extensively studied for the wide operating voltage. Most of cases, [PYR14] cation was selected. Largeot et al. introduced high-temperature carbon-carbon SCs working at 100°C with [PYR14][TFSI] electrolyte and microporous carbide-derived carbon electrode [51]. High operating temperature means that ILs are thermally stable, and the viscosity of ILs is not enough to obtain reasonable specific capacitance (Most of pyrrolidinium based ILs exist in quasi solid state at room temperature). The specific capacitance of suggested SCs reached a maximum at 130 F g−1. Mastragostino group also reported several studies using pyrrolidinium cation, [PYR14], [TFIS] was used as anion [52–54]. They tested the ILs electrolytes using hybrid SCs (asymmetric SCs). The AC and the conductive polymer (poly(3-methylthiophene)) were adopted as electrodes. These SCs achieved 3.6 V of the working voltage and 16,000 cycle of the cycle-ability at 60°C. The hybrid supercapacitor delivered 24 Wh kg−1 and 14 kW kg−1 as maximum values. However, low ionic conductivity remains as problems.

issue and should be carefully considered. To understand the aging or failure mechanism of ILs, electrochemical decomposition of ILs was investigated using in situ techniques (infrared and electrochemical spectroscopy methods and XPS) [45, 46]. These studies were performed to understand the EDL structure of ILs, enhancing electrochemical properties by modifying

**Figure 4.** CV behaviors (a), Ragone plots (b), and change of energy density with current density (c) of the RGO– CMK-5 composite electrode measured in different electrolytes. (d) Cycle performance of the RGO–CMK-5 electrode measured in EMIMBF4 electrolyte, with the arrows showing the beginning of a new cycle [35] (CV: cyclic voltammetry,

As previously mentioned, imidazolium-based ILs are one of the candidates for SCs electrolyte. The electrochemical characteristics of various ILs electrolytes with [EMI] cations and

]), [TFSI], [FSI], and thiocyanate ([SCN])) was reported using carbon cloth elec-

] were 49 Wh kg−1. Graphene sheet electrode was also evaluated [EMI] cation with

]) as ester functionality, and acetate ([OAc]) as acid. The hydrogen-bond-accept-

various anions, which were categorized representative chemical species. The five anions were

ing ability of anions was closely related to the viscosity of the ILs. Moreover, [EMI][DCA],

] as inorganic fluoride, [FSI] as organic fluoride, [DCA] as cyano functionality, ethylsul-

] were stable up to 3.2 V. The highest energy values at 3.2 V for [EMI]

]), tris(pentafluoroethyl)trifluorophosphate

], [EMI][TFSI],

positive or negative ions or adopting pseudoelectrode material.

], tetracyanoborate([B(CN)<sup>4</sup>

trodes [47]. Working voltage was changed according to anions, and [EMI][BF<sup>4</sup>

different anions ([BF<sup>4</sup>

:[EMI][BF<sup>4</sup>

]).

30 Supercapacitors - Theoretical and Practical Solutions

and [EMI][B(CN)<sup>4</sup>

([PF<sup>3</sup> (C2 F5 )3

EMIMBF<sup>4</sup>

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

phate ([EtSO<sup>4</sup>

[BF<sup>4</sup>

**Figure 5.** (a) Cyclic voltammograms measured at cell voltage scan rate 1 mV s−1 for SC test cells filled with RTILs. (EMImBF<sup>4</sup> : [EMI][BF<sup>4</sup> ], EMImB(CN)<sup>4</sup> : [EMI][B(CN)<sup>4</sup> ], EMImPF3 (C2 F5 )3 : [EMI][PF<sup>3</sup> (C2 F5 )3 ], EMImN(SO<sup>2</sup> CF3 )2 : [EMI][TFSI], EMImN(SO<sup>2</sup> F)2 : [EMI][FSI], and EMImSCN: [EMI][SCN]) [47], (b) the relationship between the electrical conductivity, viscosity, and 1 H NMR chemical shift of the proton in the 2-position of the imidazole ring of the ILs studied [48].

To overcome the viscosity issue, factionalized electrodes were proposed. Nitrogen-doped reduced graphene oxide aerogel (N-rGO aerogel) was one of them [55]. [PYR14][DCA] was matched with N-rGO aerogel. Reported SCs show excellent electrochemical characteristics of wide working potential of 4.0 V, high specific capacitance of 764.53 F g-1 at 1 A g-1, and a capacity retention of 86% over 3000 charge discharge cycles, and this system provided maximum specific power of 6525.56 W kg−1 and energy of 245.00 Wh kg−1. In addition, ILs with various cations have been studied as electrolytes. The sulfonium and the piperidinium were also investigated to achieve high electrochemical performances [31, 56]. However, these ILs have relatively high meting point about 40–60°C; thus, they applied as electrolytes for hightemperature SCs or as electrolytes with solvents.

performance [60]. This research provided the capacitive behavior of a planar carbon in two kinds of ILs and their mixture both experimentally and computationally. They selected single

ture effect, which makes more counterions pack on and more co-ions leave from the electrode surface, leads to an increase of the counterion density within the EDL and thus a larger capacitance. **Figure 7** compares the distribution of ions at the electrode surface in pure ILs

Another approach for ILs is redox active electrolyte. Until recently, SCs using redox reactions have been studied with electrodes. As a result, various electrode materials such as metal oxides and conductive polymers have been proposed [61–63]. These pseudomaterials have been mostly studied using aqueous electrolytes because of the redox mechanism using proton. However, application of ILs has been studied as alternatives due to the limitation of the operating voltage of aqueous electrolyte and the inadequacy of surface functionality of electrode materials. Protic ILs which contain proton were investigated as the electrolyte for pseudo-type electrode. Protic ILs are advantageous over aprotic ILs; however, the high viscosity and slow proton transfer in the ILs electrolyte could limit the charging rate [64, 65]. And their cycle-ability was not enough to actual application. Aprotic ILs were tried as electrolyte for pseudoelectrode in some cases [66]. The pseudomaterial was manganese dioxide (MnO<sup>2</sup>

and the IL was [BMI][DCA]. The redox mechanisms between ILs and MnO<sup>2</sup>

Eq. (3). Within a potential range of 3 V, a specific capacitance of 70 F g-1 was obtained.

**Figure 7.** (a) Schematic representation of a ILs mixture near the electrode surface, (b) distributions of cations (red line for

positively charged surface = 1.5 V: (a) x = 0, (b) x = 0.25, and (c) x = 1. The inserts are schematics of the EDL structures. (x

] and blue line for [TFSI]) in EDL of pure and mixed ILs near a positive surface with

] and [TFSI]. This study explained that the mix-

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

),

33

were expressed in

cation [EMI] and two different anions of [BF<sup>4</sup>

electrolyte and their mixture.

[EMI]) and anions (green line for [BF<sup>4</sup>

is the weight fraction of [EMI][TFSI]) [61].

#### **2.2. Other approaches of ionic liquid as liquid electrolyte**

Approach for reducing the viscosity of ILs is anion eutectic ILs mixture. Several studies were reported to reduce melting transitions and an enhance liquids range, pyrrolidinium- and piperidinium-based ILs which have relatively high viscosity as previously mentioned were selected [57–59]. SC with mixed ILs gave the assurance that the eutectic mixture of ILs could dramatically extend the temperature range of the electrical energy storage device. This is a sufficient result to dispel the recognition that the ILs cannot be used as a low-temperature electrolyte due to the high viscosity of them. SC with mixed ILs showed electrical double layer capacitors able to operate from −50 to 100°C over a wide voltage window (up to 3.7 V) and at very high charge discharge rates of up to 20 V s−1 [59]. **Figure 6** shows the change of specific capacitance versus the temperature according to various electrolytes.

Recently, Lian et al. introduced mixed anion ILs in order to have the same cations but having two types of anions with different sizes and geometries for enhancing the capacitive

**Figure 6.** Normalized capacitance (C/C20 °C) for the onion like carbon and vertical aligned-CNT electrodes (new ionic liquid mixture electrolyte: ([PIP13][FSI])0.5([PYR14][FSI])0.5, conventional EDLC electrolyte: 1 M of [Et<sup>4</sup> N][BF<sup>4</sup> ] in PC, and [PIP13]: N-methyl-N-propylpiperidinium). Capacitances were calculated at 100 mV s−1, except for the −50°C (1 mV s−1) and −40°C (5 mV s−1) experiments. This plot shows that the use of the IL mixture extends the temperature range for supercapacitors into the −50 to 100°C range, while conventional electrolytes using PC as solvent are limited to the −30 to 80°C range. C20°C was 80 and 4 mF, respectively, for OLC and VA-CNT cells. All cells were cycled from 0 up to 2.8 V [59].

performance [60]. This research provided the capacitive behavior of a planar carbon in two kinds of ILs and their mixture both experimentally and computationally. They selected single cation [EMI] and two different anions of [BF<sup>4</sup> ] and [TFSI]. This study explained that the mixture effect, which makes more counterions pack on and more co-ions leave from the electrode surface, leads to an increase of the counterion density within the EDL and thus a larger capacitance. **Figure 7** compares the distribution of ions at the electrode surface in pure ILs electrolyte and their mixture.

To overcome the viscosity issue, factionalized electrodes were proposed. Nitrogen-doped reduced graphene oxide aerogel (N-rGO aerogel) was one of them [55]. [PYR14][DCA] was matched with N-rGO aerogel. Reported SCs show excellent electrochemical characteristics of wide working potential of 4.0 V, high specific capacitance of 764.53 F g-1 at 1 A g-1, and a capacity retention of 86% over 3000 charge discharge cycles, and this system provided maximum specific power of 6525.56 W kg−1 and energy of 245.00 Wh kg−1. In addition, ILs with various cations have been studied as electrolytes. The sulfonium and the piperidinium were also investigated to achieve high electrochemical performances [31, 56]. However, these ILs have relatively high meting point about 40–60°C; thus, they applied as electrolytes for high-

Approach for reducing the viscosity of ILs is anion eutectic ILs mixture. Several studies were reported to reduce melting transitions and an enhance liquids range, pyrrolidinium- and piperidinium-based ILs which have relatively high viscosity as previously mentioned were selected [57–59]. SC with mixed ILs gave the assurance that the eutectic mixture of ILs could dramatically extend the temperature range of the electrical energy storage device. This is a sufficient result to dispel the recognition that the ILs cannot be used as a low-temperature electrolyte due to the high viscosity of them. SC with mixed ILs showed electrical double layer capacitors able to operate from −50 to 100°C over a wide voltage window (up to 3.7 V) and at very high charge discharge rates of up to 20 V s−1 [59]. **Figure 6** shows the change of

Recently, Lian et al. introduced mixed anion ILs in order to have the same cations but having two types of anions with different sizes and geometries for enhancing the capacitive

**Figure 6.** Normalized capacitance (C/C20 °C) for the onion like carbon and vertical aligned-CNT electrodes (new ionic

[PIP13]: N-methyl-N-propylpiperidinium). Capacitances were calculated at 100 mV s−1, except for the −50°C (1 mV s−1) and −40°C (5 mV s−1) experiments. This plot shows that the use of the IL mixture extends the temperature range for supercapacitors into the −50 to 100°C range, while conventional electrolytes using PC as solvent are limited to the −30 to 80°C range. C20°C was 80 and 4 mF, respectively, for OLC and VA-CNT cells. All cells were cycled from 0 up to 2.8 V [59].

N][BF<sup>4</sup>

] in PC, and

liquid mixture electrolyte: ([PIP13][FSI])0.5([PYR14][FSI])0.5, conventional EDLC electrolyte: 1 M of [Et<sup>4</sup>

specific capacitance versus the temperature according to various electrolytes.

temperature SCs or as electrolytes with solvents.

32 Supercapacitors - Theoretical and Practical Solutions

**2.2. Other approaches of ionic liquid as liquid electrolyte**

Another approach for ILs is redox active electrolyte. Until recently, SCs using redox reactions have been studied with electrodes. As a result, various electrode materials such as metal oxides and conductive polymers have been proposed [61–63]. These pseudomaterials have been mostly studied using aqueous electrolytes because of the redox mechanism using proton. However, application of ILs has been studied as alternatives due to the limitation of the operating voltage of aqueous electrolyte and the inadequacy of surface functionality of electrode materials. Protic ILs which contain proton were investigated as the electrolyte for pseudo-type electrode. Protic ILs are advantageous over aprotic ILs; however, the high viscosity and slow proton transfer in the ILs electrolyte could limit the charging rate [64, 65]. And their cycle-ability was not enough to actual application. Aprotic ILs were tried as electrolyte for pseudoelectrode in some cases [66]. The pseudomaterial was manganese dioxide (MnO<sup>2</sup> ), and the IL was [BMI][DCA]. The redox mechanisms between ILs and MnO<sup>2</sup> were expressed in Eq. (3). Within a potential range of 3 V, a specific capacitance of 70 F g-1 was obtained.

**Figure 7.** (a) Schematic representation of a ILs mixture near the electrode surface, (b) distributions of cations (red line for [EMI]) and anions (green line for [BF<sup>4</sup> ] and blue line for [TFSI]) in EDL of pure and mixed ILs near a positive surface with positively charged surface = 1.5 V: (a) x = 0, (b) x = 0.25, and (c) x = 1. The inserts are schematics of the EDL structures. (x is the weight fraction of [EMI][TFSI]) [61].

$$\mathrm{MnO}\_{2\times}\mathrm{[DCA]}\_{2\times} + 2\,\mathrm{x}^- \leftrightarrow \mathrm{MnO}\_{2} + 2\,\mathrm{x}\,\mathrm{[DCA]}^- \quad \text{(x}\,\leq\,0.5)}\tag{3}$$

redox active couple. Taniki et al. reported that the N-ethyl-N-methylpyrrolidinium ([EMPyr)) fluorohydrogenate ([(FH)2.3F]) could significantly contribute extra specific capacitance to the SCs through the redox reaction of the electrolyte [72]. In the case of charging up to 2.5 V, the EDL capacitance of the positive electrode is 140 F g−1, and the redox capacitance is 150 F g−1, both of which contribute to the total capacitance. Correspondingly, the negative capacitance is the sum of the double-layer capacitance of 130 F g−1 and the redox capacitance of 116 F g−1. In 2014, Tooming et al. reported redox active electrolyte using 5 wt% [EMI] iodide into [EMI]

**Figure 9.** Cyclo-voltammogram of [EMI][TFSI] and [EMI][I and Br]/[EMI][TFSI] mixture at 1 mV s−1.

], and they archive a nearly 50% increase in specific capacitance in comparison to bare

sity (∼30% at P ∼ 1 kW kg−1/ 1 kW dm−3) as well as in specific energy and energy density (∼60% at P ∼ 1 Wh kg−1 / 1 Wh dm−3) has been achieved by 5 wt% addition of [EMI][I] into [EMI][BF<sup>4</sup>

However, EDL capacitance is proportional to number of the ions (charges). Thus, weight ratio

reason, our group suggested redox active electrolyte, which have same number of ions. We choose bromide and iodide as redox mediator. The specific capacitances were increased by amount of [EMI] halide (**Figure 9**). The SCs adopting 0.12 mole fraction of [EMI][I] show the highest specific capacity of 176.1 F g−1 with 3.5 V of working voltage. The energy density is

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

is not appropriate for comparing [EMI][I]/[EMI][BF<sup>4</sup>

231.9 Wh kg−1 at 0.5 A g−1, when the power density is 2705.6 W kg−1.

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

ILs adopted polymer structure are described in **Figure 10**.

] [73]. As shown in **Figure 8**, noticeable increase in specific power and power den-

] mixture and neat [EMI][BF<sup>4</sup>

Ionic Liquid for High Voltage Supercapacitor http://dx.doi.org/10.5772/intechopen.73053 35

].

]. For this

[BF<sup>4</sup>

[EMI][BF<sup>4</sup>

A new strategy has been explored to increase the capacitance of SCs by inducing the pseudocapacitive contribution from the redox-active electrolytes [67]. The Faradaic reactions occur in the electrolyte, which can contribute extra capacitance to the SCs. In this case, the pseudocapacitance is not only contributed by the pseudocapacitive electrode materials but can also be contributed from the electrolyte. The introduced redox mediator was halide (mostly iodide), vanadium (IV) oxide sulfate (VOSO<sup>4</sup> ), metal cation (Cu2+), heteropoly acids, factionalized arene, and quinode-benzoide [67–71]. As mentioned previously, most of redox couple were applied with aqueous medium. However, to achieve higher cell voltage and thus a higher energy density, a number of nonaqueous electrolytes including organic and IL-based electrolytes have been studied and reported. In the case of ILs, halide ion species was used as

**Figure 8.** (a) Specific energy vs. specific power and (b) energy density vs. power density plots for EDLCs based on microporous-mesoporous carbon electrodes in EMImBF4 + 5 wt% EMImI mixture (triangles), in neat EMImBF<sup>4</sup> (circles), obtained from constant power tests within the cell potential range from 2.4 to 0.4 V and for comparison for EDLCs based on TiC-CDC carbon electrodes in EMImBF<sup>4</sup> (squares), and obtained from constant power tests within the cell potential range from 3.0 to 1.5 V. (EMImBF<sup>4</sup> : [EMI][BF<sup>4</sup> ] and EMImI: [EMI][I]) [74].

**Figure 9.** Cyclo-voltammogram of [EMI][TFSI] and [EMI][I and Br]/[EMI][TFSI] mixture at 1 mV s−1.

*MnO*2−*<sup>x</sup>* [*DCA*]

34 Supercapacitors - Theoretical and Practical Solutions

on TiC-CDC carbon electrodes in EMImBF<sup>4</sup>

: [EMI][BF<sup>4</sup>

range from 3.0 to 1.5 V. (EMImBF<sup>4</sup>

iodide), vanadium (IV) oxide sulfate (VOSO<sup>4</sup>

<sup>2</sup>*<sup>x</sup>* + 2 *x*<sup>−</sup> ↔ *MnO*<sup>2</sup> + 2*x* [*DCA*]<sup>−</sup> (*x* ≤ 0.5) (3)

), metal cation (Cu2+), heteropoly acids, factional-

(circles),

A new strategy has been explored to increase the capacitance of SCs by inducing the pseudocapacitive contribution from the redox-active electrolytes [67]. The Faradaic reactions occur in the electrolyte, which can contribute extra capacitance to the SCs. In this case, the pseudocapacitance is not only contributed by the pseudocapacitive electrode materials but can also be contributed from the electrolyte. The introduced redox mediator was halide (mostly

ized arene, and quinode-benzoide [67–71]. As mentioned previously, most of redox couple were applied with aqueous medium. However, to achieve higher cell voltage and thus a higher energy density, a number of nonaqueous electrolytes including organic and IL-based electrolytes have been studied and reported. In the case of ILs, halide ion species was used as

**Figure 8.** (a) Specific energy vs. specific power and (b) energy density vs. power density plots for EDLCs based on microporous-mesoporous carbon electrodes in EMImBF4 + 5 wt% EMImI mixture (triangles), in neat EMImBF<sup>4</sup>

obtained from constant power tests within the cell potential range from 2.4 to 0.4 V and for comparison for EDLCs based

] and EMImI: [EMI][I]) [74].

(squares), and obtained from constant power tests within the cell potential

redox active couple. Taniki et al. reported that the N-ethyl-N-methylpyrrolidinium ([EMPyr)) fluorohydrogenate ([(FH)2.3F]) could significantly contribute extra specific capacitance to the SCs through the redox reaction of the electrolyte [72]. In the case of charging up to 2.5 V, the EDL capacitance of the positive electrode is 140 F g−1, and the redox capacitance is 150 F g−1, both of which contribute to the total capacitance. Correspondingly, the negative capacitance is the sum of the double-layer capacitance of 130 F g−1 and the redox capacitance of 116 F g−1. In 2014, Tooming et al. reported redox active electrolyte using 5 wt% [EMI] iodide into [EMI] [BF<sup>4</sup> ], and they archive a nearly 50% increase in specific capacitance in comparison to bare [EMI][BF<sup>4</sup> ] [73]. As shown in **Figure 8**, noticeable increase in specific power and power density (∼30% at P ∼ 1 kW kg−1/ 1 kW dm−3) as well as in specific energy and energy density (∼60% at P ∼ 1 Wh kg−1 / 1 Wh dm−3) has been achieved by 5 wt% addition of [EMI][I] into [EMI][BF<sup>4</sup> ].

However, EDL capacitance is proportional to number of the ions (charges). Thus, weight ratio is not appropriate for comparing [EMI][I]/[EMI][BF<sup>4</sup> ] mixture and neat [EMI][BF<sup>4</sup> ]. For this reason, our group suggested redox active electrolyte, which have same number of ions. We choose bromide and iodide as redox mediator. The specific capacitances were increased by amount of [EMI] halide (**Figure 9**). The SCs adopting 0.12 mole fraction of [EMI][I] show the highest specific capacity of 176.1 F g−1 with 3.5 V of working voltage. The energy density is 231.9 Wh kg−1 at 0.5 A g−1, when the power density is 2705.6 W kg−1.
