Ionic Liquids: Applications in Rechargeable Lithium Batteries

Abstract

World is passing through the energy crises due to the rapid depletion of fossil fuels. To address this crisis and to fulfill the energy demands worldwide, development of energy storage devices have increased rapidly. Also, renewable energy resources are intermittent, and therefore nevertheless, this energy resources are not always available. In that context, rechargeable lithium batteries are most promising energy storage devices owing to high energy and power density. Although, the development of the component of rechargeable battery such as anode, cathode and electrolyte are in progress as they play major role in enhancing the electrochemical performance of lithium-ion battery. Among them, electrolyte plays crucial role as it provides the path for diffusion of Li+ ions between the electrodes. In that context, ionic liquid-based electrolytes are widely used as it acts as plasticizer and thus increases the conductivity of electrolyte considerably. In this chapter, we have discussed basics of ionic liquids and its application in electrolyte system. Also, in this chapter, we have discussed various properties of ionic liquid-based electrolytes and their application in rechargeable lithium battery.

1. Introduction

During the past decades, lithium-ion batteries (LIBs) have drawn more attention of researchers in the area of energy storage due to high energy density (∼250 Wh/kg) and high-power density, good cycle life etc. Also, LIBs are considered as the most efficient energy storage devices that can store the vast amount of energy from the renewable energy sources (such as wind, solar etc.) and thus help us in making the “Fossil free world”. Furthermore, in order to maximize the energy and power density, several industries and R&D groups have continuous worked hard on the improvement of electrochemical performance of electrode materials as well as electrolytes for LIBs [1, 2, 3, 4, 5]. Due to high power and energy density, good cycle life and bring light weight, lithium-ion batteries are widely used and industrialized in transportation devices as well as hybrid and electric vehicles [6] as shown in Figure 1. Commercial rechargeable LIBs have three components mainly anode, cathode and electrolyte. Each component of battery has significant role in enhancing the performance of LIBs. Among them, electrolyte provides the path for the diffusion of ions between the electrodes and also act as separator. Initially, organic based conventional electrolytes are commonly used in LIBs due to their high ionic conductivity (${10}^{-3}\text{to}{10}^{-2}\mathrm{s}/\mathrm{cm}$) [7]. Organic liquid electrolytes are usually lithium salts that are dissolved in organic solvents such as ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC) etc. However, they are highly flammable in nature, highly reactive towards the lithium electrode which causes the unwanted lithium metal electrode growth commonly knowns as lithium dendrite growth [8, 9, 10] as shown in the Figure 2. This inevitable lithium dendrite growth is the major problem of short circuiting of battery. Therefore, from the safety concern point, development of organic liquid-based electrolyte is restricted on large-scale applications. To address the above problem, solid polymer electrolytes are promising candidate as they have good mechanical strength, better thermal and chemical stability, good electrochemical stability window. Also, growth of lithium dendrite can be suppressed by such type of quasi-solid electrolytes.

Figure 1.

Schematic presentation of various application of Lithium-ion battery.

Figure 2.

Showing the lithium dendrite growth in rechargeable lithium batteries.

However, they suffer from low room temperature ionic conductivity (<${10}^{-6}\mathrm{s}/\mathrm{cm})$ [11, 12]. In order to enhance the ionic conductivity of solid polymer electrolyte, several strategies [13, 14] are adopted such as:

1. Use of ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC) as conventional plasticizer.

2. Use of additive/nano size filler such as Al₂O₃, SiO₂, TiO₂ etc.

3. Blending of polymer

4. Copolymerization.

By adopting this approach, considerable increase in the ionic conductivity of solid polymer electrolyte has been achieved but it needs to be increased further for Li-ion battery applications. For that, a new class of material known as the ionic liquid has been introduced in the solid polymer electrolyte that has flexible nature, good ionic conductivity, better mechanical stability and wide electrochemical window and ease of processibility as well as portability and is free from the corrosion and leakage problem [15, 16, 17, 18]. Ionic liquids have enormous effect on the research field due to its wide range of properties that have a good impact on the development of energy technologies. Ionic liquids are generally molten salts having large organic cation and organic/inorganic anion and therefore the ionic forces between them are weaker and thus low melting temperature. ILs have generally melting temperature less than 100°C and some ionic liquids are liquid at or below the room temperature usually known as room temperature ionic liquids (RTILs) [19, 20]. Also, RTILs have gained increasing attention in the electrochemical device due to their excellent properties such as nonvolatility & nonflammability and thermal stability. In addition, ILs have low vapor pressure, display wide electrochemical stability window (ESW), have high ionic conductivity and display high thermal and chemical stabilities. Few common anions and cations used in the formation of ILs [21, 22] are shown in the Figure 3. The choice of anions and cations plays a crucial role on the physical properties of ILs because these cations and anions can combine in millions of possible ways to give the IL having the specific properties for a particular interest. Due to these properties, ionic liquids are also known as “Designer Solvents” [23]. Therefore, due to their uniqueness in properties ionic liquids are widely used as plasticizer in solid polymer electrolytes. Ionic liquid based solid electrolytes are usually composed of polymer host matrix, lithium salts and ionic liquid. Ionic liquid-based polymer electrolytes not only have high ionic conductivity but have good electrode-electrolyte contact like liquid electrolyte, good mechanical strength, flexibility, wide electrochemical window and good thermal stability and chemical stability also. Therefore, in electrochemical devices especially, in lithium-ion batteries, ionic liquid-based electrolytes are widely used.

Figure 3.

Some of cations and anions used in the formation of ionic liquids.

In this chapter, we discussed the physical, thermal, structural and electrochemical properties of IL-based polymer electrolytes and its application as electrolyte in lithium polymer batteries.

2. Different role of ionic liquids in lithium batteries

2.1 Pure Ionic liquids as electrolytes

Nowadays, room temperature ionic liquids are replaced by conventional organic carbonate-based electrolytes due to their unique features, such as (a) wide ECW, (b) non-flammability, (c) low vapor pressure (d) higher conductivity (e) wide operation range of temperature (f) non-toxicity (g) non-flammability. Surprisingly, some electrode materials show good performance with pure IL-based electrolytes, compromising of lithium salt and IL which are not working with the conventional organic carbonate-based electrolyte [24]. However, practical applications of ionic liquid electrolyte are limited due to their high viscosity. As the conductivity is affected by viscosity, higher the viscosity, it will be more difficult for Li⁺ ions to migrate. Among different types of ILs, 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) IL has highest conductivity (∼10⁻² S/cm at room temperature) due to smallest viscosity [25]. Ishikawa et al. [26] reported pure ionic liquid electrolytes such as the 0.8 M LiTFSI/EMI-TFSI, 0.8 M LiTFSI/EMI-FSI, or 0.8 M LiTFSI/P13-FSI and studied their electrochemical behavior using natural graphite as a negative electrode and Li as a counter electrode. They found that LiTFSI/EMI-TFSI shows outstanding performance than that of LiTFSI/EMI-FSI or LiTFSI/P13-FSI electrolyte and reversible capacity of a graphite negative electrode with LiTFSI/EMI-FSI as the electrolyte is ∼360 mAh g⁻¹ at C/5 rate remains stable during 30 cycles which favorably is comparable with that of the EC + DEC solvent.

Also, the Li-salt concentration greatly affects the conductivity of ionic liquid system. Shiro Seki et al. [27] have reported effect of lithium salt concentration in 1,2-dimethyl-3- propyl imidazole bis(trifluoromethylsulfonyl) imide ([DMP][ImTFSI])-based binary electrolyte on the electrolyte/electrode interfacial resistance, charge-discharge performance, and ionic conductivity. They found that ionic conductivity of DMPIMTFSI-LiTFSI mixed electrolyte is decreased with on decreasing LiTFSI concentration and also with optimized mixed binary electrolyte with LiCoO₂ cathode, Li-cell shows high reversibility during charge-discharge performance for more than 100 cycles.

2.2 Conventional carbonate-based electrolyte with added ILs

Due to strong ion-ion interaction, ionic liquids have high viscosity and low ionic conductivity which limits their application in batteries. In order to solve these mentioned problems, organic solvents such as ethylene carbonate (EC), diethylene carbonate (DEC), dimethyl carbonate (DMC) are added to ILs which significantly reduce the viscosity and enhance the ionic conductivity of system than that of pure ionic liquid electrolyte system [28]. Marco Agostini et al. [29] added ethylene carbonate (EC): dimethyl carbonate (DMC) in N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide, lithium bis(trifluoromethanesulfonyl)imide (Py_1,4 TFSI–LiTFSI) ionic liquid-based system and found that ionic conductivity as well as lithium-ion transference number increases from 10⁻³ to 10⁻² S/cm and 0.25 to 0.38 respectively. This increase in lithium-ion transference number and ionic conductivity is due to the solvation of Li⁺-ions and dissociation of Li-salts due to more conductive system.

2.3 Quasi solid-state electrolytes containing ionic liquid

Quasi solid-state electrolyte containing ionic liquids are also known as ionic liquid-based electrolyte which have gained significantly more attention due to their high mechanical and chemical stability, good thermal and wide electrochemical stability, highly flexible in nature, non-flammability in nature and high ionic conductivity. In such system, ionic liquid is added into the polymer-based electrolyte as plasticizer which helps to improve the ionic conductivity, electrochemical window as well as Li-transference number. Singh et al. [30] added the 1-butyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]) ionic liquid into PEO + 20 wt.% of LiTFSI polymer electrolyte system and found the ionic conductivity of optimized ionic liquid-based polymer electrolyte (PEO + 20 wt.% LiTFSI) + 20 wt.% BMIMTFSI around ∼1.5 × 10⁻⁴ S/cm at 30°C. Furthermore, addition of ILs in to polymer-based electrolyte, increases the amorphicity as well as ionic conductivity of polymer electrolyte system due to the plasticizing effect of ILs. Simonetti et al. [31] have reported the PEO-based polymer electrolyte containing N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide ionic liquid and ionic conductivity values ∼3.46$\times$10⁻⁴ and 2.43$\times$10⁻³ S/cm at −20 and 20°C, respectively, with electrochemical stability window ∼4.5 V vs. Li/Li⁺. Furthermore, Kumar et al. [32] have also reported the effect of 1-ethyl 3-methyl imidazolium trifluoromethanesulfonate (EMITf) IL on the PEO and lithium trifluoromethanesulfonate (LiCF₃SO₃ or LiTf) (in the ratio ∼ 25) polymer electrolyte system. They found that PEO_25.LiTf +40 wt.% (EMITf) electrolyte system shows ionic conductivity of ∼3 × 10⁻⁴ S/cm at RT with wide electrochemical stability window (∼4.9 V vs. Li/Li⁺) and excellent thermal stability which proves the suitability of electrolyte in various energy storage/conversion devices. Also, such IL-based polymer electrolytes have freestanding and flexible nature along with excellent thermal and mechanical stabilities which proves the suitability of such electrolytes in Li-ion battery. Some of the IL-based polymer electrolyte systems are listed in Table 1 along with some properties such as ionic conductivity, Li-ion transference number and electrochemical stability window (ESW). From the Table 1, it can be seen that IL incorporation in the polymer and salt electrolyte system has increased the ionic conductivity as well ESW which proves the suitability of IL-based polymer electrolytes for Li-battery. In this chapter, we mainly discuss the physical, thermal, structural and electrochemical properties of IL-based polymer electrolyte system and its application in lithium metal battery (LMB).

IL based PEO-polymer system	Electrical conductivity (S.cm^−1)	Transference number (Ion/cation)	Electrochemical stability window	References
PEO + 20 wt.% LiFSI +20 wt.% BMPyTFSI	4.05 × 10^−5	0.99/0.37	4.2	[8]
PEO-LiTFSI- PYR13FSI	2.43 × 10^−3 at 20°C	—	4.5	[31]
(PEO)20LiTFSI + PYR13FSI	∼10^−4	—	—	[22]
PEO25.LiTf+40 wt.% EMITf	3.0 × 10^−4		4.9 V	[32]
PEO + 20 wt.% LiTFSI + [P6,6,6,14] [Ntf2]	4.2 × 10^−5	0.99/0.37	3.34	[33]
PEO + 20 wt.% LiTFSI +12.5 wt.% EMIMTFSI	2.08 × 10^−4	0.99/0.39	4.6	[34]
P(EO)20LiTFSI - BMPyTFSI	6.9 × 10^−4 at 40°C	—	4.8–5.3	[35]
P(EO)20LiTFSI + 1.27PP1.3TFSI	2.06 × 10^−4	0.339	4.5–4.7	[36]
PEO + 20wt%LiFSI+ 7.5wt%EMIMFSI	2.89 × 10^−4	0.99/0.28	3.8	[37]
PEO-NaMS- BMIM-MS	1.05 × 10^−4	0.99/0.46	4–5	[38]
PEO20LiDFOB + 40 wt.% IL EMImTFSI	∼$1.85\times {10}^{-4}$	—	2.5–4.0 V	[39]

Table 1.

Properties of Ionic liquid-based polymer electrolyte system.

3. Structural, thermal and ionic transport properties of ionic liquid-based polymer electrolytes

3.1 Structural properties of ionic liquid-based polymer electrolytes

It is reported in the literature that on addition of IL in the PEO-based polymer electrolyte system, its amorphicity is increased and hence mobility of free ions which in turn increases the ionic conductivity of IL-based polymer electrolyte system. Such increase in the amorphicity is observed by the X-ray Diffraction study (XRD) of IL-based polymer electrolyte system. Meghnani et al. [8] have reported that when in the PEO + 20 wt.% LiFSI system, 1-butyl-3-methlypyridinium bis(trifluoromethylsulfonyl) imide (BMPyTFSI) IL is added, the amorphous nature of polymer electrolyte system increases which in turn increases the mobility of charge carrier due to free volume. Figure 4 shows the XRD pattern of pristine PEO and polymer electrolytes PEO + 20 wt.% LiFSI + X wt.% IL (b) X = 0, (c) X = 10, (d) X = 15, and (e) X = 20 at room temperature and it can be seen that on addition of ionic liquid in polymer electrolyte system, the halo region as well as FWHM increases which in turn decreases the crystallinity of the system. Gupta and Singh et al. [30, 33] observed the similar behavior, they reported the effect of addition of phosphonium based IL and BMIMTFSI IL in the PEO + 20 wt.% LiTFSI, and observed that the halo region of peak increases which makes the polymer system amorphous. This amorphous nature of polymer electrolyte system favors the Li ion diffusion and hence enhances the Li-ion conductivity.

Figure 4.

X-ray diffraction patterns of (a) PEO and PEO + 20 wt.% LiFSI + X wt.% IL (b) X = 0, (c) X = 10, (d) X = 15, and (e) X = 20 polymer electrolytes at RT.

Besides that, addition of ionic liquid does not only affect the XRD pattern of polymer electrolyte system, but also affects the surface morphology of it. It is reported in the literature that on addition of IL, surface of polymer electrolyte becomes smooth than that of without ionic liquid. We [40] have reported that addition of BMPyTFSI IL into PEO-based electrolyte system makes the surface of polymer electrolyte system smoother as shown in the Figure 5. Same behavior is also observed by Gupta et al. [33] They have reported the smoother surface morphology of IL based polymer electrolyte and found that addition of IL into polymer electrolyte system increases the amorphicity of the system and thus makes it surface smoother.

Figure 5.

SEM images of (a) Pristine PEO (b) PEO +20 wt.% LiFSI +20 wt.% IL.

3.2 Effect of IL on the thermal behavior of polymer electrolyte system

The effect of IL on the thermal stability of polymer electrolyte system is examined by Thermogravimetric and Differential scanning calorimetry analysis using Mettler Toledo DSC/TGA system (TGA and DSC study). For practical application of IL-based polymer electrolyte system in lithium batteries at high temperature, it is necessary to know the thermal stability as well as melting temperature of IL based polymer electrolyte system. Meghnani et al. [41] synthesized the IL-based polymer electrolytes PEO + 20 wt.% LiFSI + X wt.% N-methyl-N-propyl piperidinium bis(fluorosulfonyl)imide (PP₁₃FSI) (X = 10, 20, 30 and 40) by solution cast technique [8] and found the thermal stability ∼210°C as shown in the Figure 6(a). The authors have also reported the melting temperature as well as degree of crystallinity of IL-based polymer electrolytes by DSC technique. They found that on increasing the IL concentration in PEO + 20 wt.% LiFSI system, melting temperature (T_m) is shifted towards the lower temperature (∼47.6°C) and also area under the endothermic curve deceases due to decrease in the crystallinity of the system. This decrease in the crystallinity of the system is calculated by the formula [42]:

Figure 6.

(a) Thermal stability, (b) DSC thermograms of polymer electrolyte PEO + 20 wt.% LiFSI + X wt.% PP₁₃FSI (X = 0, 10, 20, 30, 40) and (c) TGA thermogram and (d) DSC thermograms of pristine PEO, polymer electrolytes films PEO + 20 wt.% LiFSI + X wt.% IL (X = 0, 10, 15, and 20).

$$\mathit{\text{Degree of Crystallinity}}=\frac{∆H_m}{∆H_m^0}\times 100\%$$E1

Where $∆H_m$ and $∆H_m^0$ are the enthalpy value of electrolyte and pristine PEO respectively. The value of $∆H_m$ is the area under the endothermic peak related to melting curve whereas the value of $∆H_m^0$ is 213.7 Jg⁻¹ for 100% crystalline polymer PEO. It is found that on addition of IL, the melting temperature as well as degree of crystallinity decrease and the lowest value is found to be for 40 wt.% IL containing polymer electrolyte (see Figure 6(b)). This is due to the enhancement of the amorphicity of the polymer electrolyte system which result in the increase of the segmental motion of polymeric chain.

Meghnani et al. [8] also reported the effect of BMPyTFSI IL on the thermal stability of PEO-based electrolyte system. They have found that that IL based polymer electrolyte PEO + 20 wt.% LiTFSI + X wt.% (X = 10, 15, 20) shows three step decomposition and thermal stability is found in the range 200–220°C as shown in Figure 6(c). Also, on increasing the IL concentration, melting temperature shifted towards lower temperature side (see Figure 6(d)). Therefore, IL based polymer electrolyte is more suitable electrolyte for Li-battery applications at high temperature due to its good thermal stability.

3.3 Effect of IL on the ionic conductivity of polymer electrolyte system

One of the crucial effects of IL on the polymer electrolyte system is ionic conductivity which is usually examined by the complex impedance spectroscopy technique. It has been reported in literature that on addition of IL into the polymer electrolyte system, the conductivity of parent system increases because IL acts as plasticizer which enhances flexibility of polymer electrolyte system and thus ionic conductivity of the system. The ionic conductivity of IL-based polymer electrolyte system is calculated by the equation:

$$\sigma =\frac{1}{R_b}\frac{L}{A}$$E2

Where L and A are the thickness and area of polymer electrolyte respectively and R_b is the bulk resistance of electrolyte.

Meghnani et al. [41] have reported that on increasing the IL concentration the bulk resistance of polymer electrolyte PEO + 20 wt.% LiFSI + X wt.% PP₁₃FSI (X = 0, 10, 20, 30, 40) decreases and the lowest bulk resistance or highest ionic conductivity is found to be for 40 wt.% IL containing polymer electrolyte as shown in Figure 7(a,b).

Figure 7.

(a) Nyquist plot, (b) variation of conductivity with IL concentration at 30°C and 40°C and (c) Arrhenius plot of the cell SS/PEO + 20 wt.% LiFSI + X wt.% PP₁₃FSI/SS (X = 0, 10, 20, 30, 40) (d) temperature-dependent conductivity of polymer electrolytes PEO + 20 wt.% LiFSI + X wt.% IL (X = 10, 15, and 20) and (e) variation of conductivity (σ), and activation energy (E_a) with IL concentration.

This decreasing trend may be due to the plasticizing nature of IL which enhances the flexibility of polymer chain and thus ionic conductivity of polymer electrolyte. They have also studied the temperature dependent conductivity and found that on increasing the temperature, ionic conductivity of the system increases and show Arrhenius type thermally activated behavior:

$$\sigma ={\sigma }_0{e}^{-\frac{E_a}{\mathit{KT}}}$$E3

Where E_a, K and T are the activation energy, Boltzmann constant and temperature respectively$\text{and}{\sigma }_0$ is pre-exponential factor. It is found that activation energy of electrolyte system decreases with increasing the IL concentration as seen from Figure 7(c). In another study Meghnani et al. [8] have reported the conductivity of PEO + 20 wt.% LiFSI + X wt.% BMPyTFSI (X = 10, 15, 20) system and found that on increasing IL content, conductivity of system increases and follows Arrhenius type thermally activated behavior as shown in the Figure 7(d). Also, they have observed that conductivity and activation energy both are inverse in nature (see Figure 7(e)). Balo et al. [43] also observed almost similar behavior in PEO + 20 wt.% LiTFSI + X wt.% EMIMFSI (x = 0, 2.5, 5, 7.5, 10, 12.5, and 15) polymer electrolyte system. They found that on increasing the IL concentration upto 10 wt.%, ionic conductivity of system increases and thereafter it is decreases. It is because beyond this IL concertation, formation of ion pairs starts which reduces the ionic conductivity of the system.

3.4 Effect of IL on the Li⁺ diffusion coefficient $\left(D_{L{i}^{+}}\right)$ and Li-ion ionic conductivity of polymer electrolyte system

For application of IL-based polymer electrolyte system in Li-battery, it is important to know the Li⁺ diffusion coefficient $\left(D_{L{i}^{+}}\right)$ and Li-ion ionic conductivity. Li⁺ diffusion coefficient is usually estimated by restricted diffusion method [44]. In this method, polymer electrolyte is sandwich between two non-blocking electrodes and constant potential is applied for a fixed period of time to set up constant current. Thereafter, potential is interrupted and drop of potential is recorded with respect to time. By determining the slope of this curve, diffusion coefficient of electrolyte is calculated using the formula [44]:

$$\mathit{\text{Slope}}=\frac{-{\pi }^{2}D}{L^2}$$E4

Where L is the thickness of the electrolyte. Meghnani et al. [41] have estimated the Li⁺ diffusion coefficient $\left(D_{L{i}^{+}}\right)$ for PEO + 20 wt.% LiFSI + X wt.% (0, 10, 40) PP₁₃FSI system (see Figure 8(a,b)). It is found that $D_{L{i}^{+}}$ value for PEO + 20 wt.% LiFSI system is ∼$3.51\times 10^{-9}\mathrm{c}{\mathrm{m}}^2{\mathrm{s}}^{-1}$ at room temperature (RT) while when IL is added in the PEO + 20 wt.% LiFSI system, $D_{L{i}^{+}}$ increases progressively and its maximum value is observed ∼$1.32\times 10^{-8}\mathrm{c}{\mathrm{m}}^2{\mathrm{s}}^{-1}$ for 40 wt.% IL containing polymer electrolyte as shown in the Figure 8(c). This increase in $D_{L{i}^{+}}$ value confirms the higher lithium-ion conductivity which makes IL-based electrolyte system more suitable for Li-battery applications.

Figure 8.

(a-b) Li⁺ ion Diffusion coefficient measurement using RDM of the polarized cell Li/PEO + 20 wt.% LiFSI + X wt.% (0, 10, 40) PP₁₃FSI/Li shown by (e) Variation of Li⁺ ion diffusion coefficient with respect to IL concentration.

Besides that, IL also affects the lithium-ion transference number of polymer electrolyte system. Meghnani et al. [8] have determined the Li-ion transference no by dc polarization technique. In this technique, IL-based polymer electrolyte is sandwiched between two Li-metal foils (Li/IL-based-PE/Li) and a small constant potential (0.1 V) is applied across this cell configuration for a fixed period of time and corresponding current is recorded with respect to time, as shown in Figure 9(a). Also, the impedance plot is recorded before and after polarization (see Figure 9(b)). Authors have calculated the transference number of Li + ion using the Bruce and Vincent’ formula [45]:

Figure 9.

(a) dc polarization curve of cell (Li/20 wt.% IL containing PEs/Li) at the voltage 0.05 V, (b) Nyquist plot of the cell (Li/20 wt.% IL containing PEs/Li) before and after polarization and (c) Variation of Li⁺ transference number and Li⁺ ion conductivity with IL concentration.

$$t_{{\mathit{Li}}^{+}}=\frac{I_{\mathit{ss}}\left(∆V-I_0{R}_0\right)}{I_0\left(∆V-I_{\mathit{SS}}{R}_{\mathit{SS}}\right)}$$E5

Where I_SS and R_SS are the steady- state current and passive layer resistance after polarization and $I_0\&R_0$ are the initial current and passive layer resistance before polarization respectively.

From these studies they found that $t_{{\mathit{Li}}^{+}}$ as well Li-ion conductivity both are increased with IL concentration and the maximum $t_{{\mathit{Li}}^{+}}$ value is ∼0.37 for 20 wt.% IL-containing polymer electrolyte as seen from Figure 9(c).

Besides that, Balo et al. [34] also reported the same result for PEO + 20 wt.% LiTFSI + x wt.% EMIMTFSI (x = 0, 2.5, 5, 7.5, 10 and 12.5) IL-based polymer electrolyte system. They found that for PEO + 20 wt.% LiTFSI, $t_{{\mathit{Li}}^{+}}$ is around 0.11 while on addition of IL it is reached to 0.16 for PEO + 20 wt.% LiTFSI +2.5 wt.% EMIMTFSI and increases gradually on increasing the IL concentration and maximum value is found to be ∼0.39 for PEO + 20 wt.% LiTFSI +12.5 wt.% EMIMTFSI.

3.5 Effect of IL on the electrochemical stability window of polymer electrolyte

For application of IL-based polymer electrolyte in Li-battery, it is important to know the electrochemical stability window (ESW) of electrolyte. For this motive, Linear sweep voltammetry (LSV) technique is used. In this technique electrolyte is sandwiched between stainless steel electrode which work as reference electrode and Li-electrode which act as working electrode and current is record with respect to voltage. Meghnani et al. [41] have observed the effect of IL on the ESW (Figure 10) and found that for polymer system PEO + 20 wt.% LiFSI without IL, the ESW is ∼3.67 V vs. Li/Li⁺ [while for 40 wt.% IL containing polymer electrolyte, electrochemically stable window ∼4.72 V vs. Li/Li⁺. From the above discussion, it can be seen that the presence of IL improves the electrochemical stability window of polymer system significantly which is good for high voltage Li-battery application.

Figure 10.

Linear sweep voltammetry measurement of Li/PEO + 20 wt.% LiFSI + X wt.% (0, 40) PP₁₃FSI/SS at scan rate ∼ 0.05 mV/s.

4. Application of IL-based polymer electrolyte in Li-metal battery

In Li-battery, charging and discharging take place by means of electrochemical redox reaction. For Li-ion insertion and extraction during charging-discharging process, electrolyte is required. Electrolyte plays the crucial role in determining the Li-battery performance. Among the different type of electrolytes, IL-based polymer electrolytes have gained more attention due to its some unique properties such as high flexibility, wide ECW, good thermal and mechanical stability as well as high ionic conductivity.

IL-based polymer electrolyte which is most commonly known as gel-polymer electrolyte is widely used in Li-battery carries the hybrid properties of solid electrolyte with an embedded liquid electrolyte that is liquid electrolyte is embedded in the polymer matrix. Due to immobilization of electrolyte, enhanced ionic conductivity as well as wide ESW is achieved when compared to solid and liquid electrolytes. Also, it reduces the probability of short-circuiting due to stable solid-interphase formation (SEI) which reduces in turn the lithium-dendrite growth, there is ease of portability, safety issue of battery enhances due to no electrolyte leakage and absence of volatile reaction. Therefore, IL-based polymer electrolyte is attractive choice for Li-batteries. Among the alternative clear energy resources, Li-metal battery have gained more attention. They are widely used in the market such as portable electronic devices, electric vehicles (EVs), hybrid electric vehicles (HEVs), grid energy storage system and so on [46, 47, 48].

Furthermore, from safety point of view specially in lithium-metal battery (LMB), IL -based polymer electrolyte is widely used but still there needs to improve some parameters like internal resistance of the cell should be low and also cell must deliver maximum capacity with 100% coulombic efficiency and better capacity retention. For that purpose, IL-based polymer electrolytes having high ionic conductivity must be used in LMBs so that cell resistance as well as SEI layer resistance would be low. The performance of some of LMBs with IL-based polymer electrolytes are listed in Table 2.

IL-based polymer electrolyte	Cathode	Current rate (C-rate)	Number of cycles	Specific discharge capacity (mAh/g)	Coulombic efficiency (%)	Reference
Poly(ionic liquid)s-based polymer electrolyte	LiNi0.8Co0.15Al0.05O2	0.05 C	20	123	>96	[49]
PEO-LiTFSI-Pyr14TFSI	LiNi0.8Co0.15Al0.05O2	C/15	35	125	∼99.75	[50]
PEO + 20 wt.% LiFSI +40 wt.% PP13FSI	BiPO4@NCA	0.2C	150	110	∼99.96	[41]
PEO + LiFSI+7.5 wt.% EMIMFSI	LiFePO4	C/20	100	143	∼100	[37]
PEO + 20 wt.% LiFSI+10 wt.% PYR13FSI	GO coated LiFePO4	C/10	100	163	∼99	[51]
PEO + 20 wt.% LiTFSI +10 wt.% EMIMFSI	LiFePO4	C/10	200	145	∼100	[43]
PEO + 20 wt.% LiTFSI +10 wt.% EMIMFSI	NCA	C/10	200	175	∼100	[43]
PEO + 20 wt.% LiTFSI+30 wt.% BMPyTFSI	LiFePO4	C/10	35	106	∼100	[52]
P(EO)10LiTFSI–PYR14TFSI	LiFePO4	C/10	180	170 at 40 °C	∼100	[53]
PEO20LiTFS [Pyr14TFSI]	LiFePO4	C/10	450	140 at 40°C	∼100	[54]
PEO20LiTFSI [Pip1.101TFSI]	Li4Ti5O12	C/20	40	150	∼100	[55]
PEO20LiTFSI [Pip1.101TFSI]	LiFePO4	C/20	35	120	∼100	[55]
PEO + 20 wt.% LiFSI +20 wt.% BMPyTFSI	NCA	C/5	125	137	∼96	[8]
P(EO)20LiTFSI + 1.27PP1.3LiTFSI	LiFePO4	C/10	20	120	∼99	[36]
PEO + 20 wt.% LiTFSI+12.5 wt.% EMIMTFSI	LiMn2O4	C/10	100	120	∼100	[34]
PEO + 20 wt.% LiTFSI+20 wt.% BMIMTFSI	LiMn2O4	C/10	25	90	∼100	[30]

Table 2.

Electrochemical performance of lithium metal batteries (LMBs) with IL-based polymer electrolytes.

Meghnani et al. [41] have reported the performance of lithium metal polymer battery (LMPB) using the synthesized IL-based polymer electrolyte, PEO + 20 wt.% LiFSI +40 wt.% PP₁₃FSI in the cell Li/PEO + 20 wt.% LiFSI +40 wt.% PP₁₃FSI/ Pristine NCA and Li/PEO + 20 wt.% LiFSI +40 wt.% PP₁₃FSI/BiPO₄@NCA configuration. In the cell configuration Li/pristine NCA, cell delivers a discharging capacity ∼150 mAh.g⁻¹ at C/10 rate while Li/BiPO₄@NCA cell delivers the discharge capacity ∼164 mAh.g⁻¹ at C/10 rate as shown in Figure 11(a,b). Also, the coulombic efficiency of the cell Li/BiPO₄@NCA is found to be ∼99.96% after 150 cycles and for the cell Li/pristine NCA, it is around ∼91.68% as shown in Figure 11(c,d).

Figure 11.

(a,b) charge-discharge curve and (c,d) cyclic stability of cell Li/PEO + 20 wt.% LiFSI +40 wt.% PP₁₃FSI/Pristine NCA and Li/PEO + 20 wt.% LiFSI +40 wt.% PP₁₃FSI/BiPO₄@NCA respectively.

Balo et al.[43] have studied the electrochemical performance of Li-battery using the IL based polymer electrolyte, PEO + 20 wt.% LiTFSI+10 wt.% EMIMFSI in the cell Li/ PEO + 20 wt.% LiTFSI+10 wt.% EMIMFSI /GO-LiFePO₄ and Li/PEO + 20 wt.% LiTFSI+10 wt.% EMIMFSI/NCA configuration. They found that the cell Li/GO-LiFePO₄ delivers the maximum discharge capacity 145, 83, 38, 20 mAh/g at C/10, C/5 and 1 C and 2 C rate respectively (see Figure 12(a)). While another cell configuration Li/NCA delivers maximum discharge capacity 175, 168, 157, 150 mAh/g at C/10, C/5, 1 C and 2 C rate respectively (Figure 12(c)). When these cells are cycled at C/10 upto 200 cycles it is found that for cell Li/GO-LiFePO₄ in the initial few cycles discharge capacity is lower and it reaches to around ∼145 mAh/g within the 10^th cycle (see Figure 12(b)). However, 142 mAh/g capacity remains after 200th cycle. They have also calculated capacity fading per cycle (inset of Figure 12(b)). It is seen that it shows linear behavior and there is only 0.01% capacity loss per cycle. From the cyclic performance of the cell Li/NCA it was inferred that the discharge capacity is low (∼126 mAh/g) in initial 6–7 cycles thereafter it value increases gradually and attains the maximum capacity ∼175 mAh/g (see Figure 12(d)). However, after that, the capacity remains constant during the cycling process. The maximum discharge capacity and better cyclic stability with good coulombic efficiency of these two cells may be consistent with the higher lithium-ion conductivity of ionic liquid-based polymer electrolyte and lower interfacial resistance due to better contact with the electrode.

Figure 12.

(a, c) Charge-discharge curve and (b, d) cyclic performance of the cell Li/PEO + 20 wt.% LiTFSI+10 wt.% EMIMFSI/GO-LiFePO₄ and Li/PEO + 20 wt.% LiTFSI+10 wt.% EMIMFSI/NCA.

From the above discussion, it can be concluded that IL-based polymer electrolytes are most promising candidate for Li-metal polymer batteries.

5. Conclusion

As we can see that electrolyte plays the crucial role in batteries performance. It not only acts as a separator between electrodes but also provides the path for ion transportation. Therefore, electrolyte should possess high ionic conductivity, wide electrochemical window, good thermal and mechanical stability. All these properties are embedded in ionic liquid-based gel polymer electrolyte. IL-based polymer electrolytes not only have high ionic conductivity but also are free from leakage problem, portability issue and short-circuiting problem which is usually faced in case of liquid electrolytes. The unique properties of IL-based polymer electrolytes proves its suitability for battery applications. For this reason, IL-based polymer electrolytes are widely used in Li-battery and it is found that IL-based polymer electrolyte with Li-metal shows better cyclic stability as well good coulombic efficiency. Thus, IL-based polymer electrolytes are most attractive choice for lithium metal polymer battery.