Effect of phosphonium based ionic liquid on structural, electrochemical and thermal behaviour of polymer poly(ethylene oxide) containing salt lithium bis(trifluoromethylsulfonyl)imide

Abstract

Solid polymer electrolytes (SPEs) using polymer poly(ethylene oxide) (PEO), lithium salt bis(trifluoromethylsulfonyl)imide (LiTFSI) and ionic liquid (IL) trihexyltetradeylphosphonium bis(trifluoromethylsulfonyl)imide have been prepared. These prepared solid polymer electrolyte films have been characterised by using different experimental techniques: X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA), complex impedance spectroscopy, Fourier transform infrared spectroscopy (FTIR), an electrochemical analyser etc. Changes in crystallinity, melting temperature (T_m), glass transition temperature (T_g), thermal stability and ionic transport behaviour of the prepared polymer electrolyte have been observed when the LiTFSI salt and different concentrations of IL were incorporated in the pristine polymer PEO. Ionic conductivity of the prepared solid polymer electrolyte (PEO + 20 wt% LiTFSI) has been found to increase with increasing IL concentration in polymer electrolytes up to 20 wt% IL. Total ionic transference number >0.99 and cationic transference number ∼0.37 with an electrochemical window of ∼3.34 V has been observed for the optimized solid polymer electrolyte (PEO + 20 wt% LiTFSI + 20 wt% IL). Temperature dependant ionic conductivity obeys Arrhenius type thermally activated behaviour.

---

Introduction

Recent research efforts for energy storage systems are focused on solid polymer electrolytes for their application in rechargeable batteries. SPEs are an excellent choice for next generation electrochemical power sources because of their high energy density, good cyclability, flexible characteristics, high ionic conductivity, light weight and excellent thin film forming ability.^1,2 They also overcome the problems associated with liquid electrolytes such as leakage, corrosion, portability etc. Therefore, SPEs are used in many applications such as batteries, fuel cells, solar cells, double layer capacitors, electrochromic display devices etc. SPEs are prepared by using alkali salts like LiBF₄, LiTFSI, LiClO₄, LiPF₆, NaPF₆, NaTFSI etc. having low lattice energy so that they can easily dissociate into cations and anions. SPEs containing lithium salts play an important role in solid state electrochemical devices because of smaller size of Li cation, high energy density, high solubility and high electrochemical stability. SPEs are prepared by using salt and polymer have low room temperature ionic conductivity (∼10⁻⁷ S cm⁻¹ at 20 °C)² that limits their application in devices. In order to increase the ionic conductivity of the SPEs, variety of strategies are adopted such as (i) use of conventional plasticizers like EC (ethylene carbonate), PC (propylene carbonate), PEG (polyethylene glycol) etc., (ii) dispersion of nano/micro size inorganic ceramic fillers such as Al₂O₃, SiO₂, TiO₂ etc. (iii) copolymerization.^3–5 By adopting these approaches considerable increment in ionic conductivity at moderate temperatures has been achieved but it needs to be enhanced further for application in rechargeable batteries. Recently, a new class of materials termed as ionic liquids (ILs) is being used to prepare SPEs that provide good ionic conductivity value at room temperature along with high mechanical stability, wide electrochemical window and wide temperature range of operation.^6–9 ILs are the room temperature molten salts mainly composed of large organic cations and organic/inorganic anions. Because of the bulky and asymmetric structure of cation, interaction between cation and anion is very weak, so its melting point is below 100 °C.^10–12 Unlike other salts such as NaCl, KCl etc., it does not need any solvent for dissociating into cations and anions.¹² Several exotic properties associated with ILs such as non-flammability, non-volatility, excellent thermal stability, high ionic conductivity and recyclability make it very useful material for various applications.^12–15 Majority of studies have been done on imidazolium based ILs in SPEs due to its low viscosity. But presence of acidic proton at C2 position in imidazolium cation ring leads to electrode corrosion, when such electrolytes are used in application for rechargeable battery.¹⁶ Therefore, proton at C2 position was replaced by CH₃ and C₂H₅ group, or another ring. But these groups at C2 position in imidazolium cation ring have led to decrease in conductivity. So, the performance of batteries using SPEs based on these ILs also could not be improved much.¹⁶ Therefore, in the present study we used phosphonium based IL due to its high thermal and chemical stability. In order to make phosphonium based ILs suitable for electrochemical application; choice of anion should be noticed. TFSI is appropriate anion for low melting temperature and low viscosity.^17–19 In the present study, we have synthesized polymer electrolyte PEO + 20 wt% LiTFSI + X wt% IL (where X = 10, 20, 30, 40) and then characterized. In earlier studies on imidazolium based polymer electrolytes with dopant salt and IL having the same anion, it was found that the ionic conductivity monotonically increased with IL content.²⁰ But in the present study, it has been found that the conductivity first increases and then starts decreasing. This trend in ionic conductivity was explained by using the variation of percentage degree of crystallinity of SPEs and dielectric constant with IL content. It has been found that dielectric constant and hence the number of dissociated ions of the lithium salt first increases with IL content and then starts decreasing.²¹ Polymer electrolyte films were characterised by using various techniques such as scanning electron microscope (SEM), differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared (FTIR), impedance spectroscopy, linear sweep voltammetry (LSV), dc polarisation technique and combined ac/dc technique.

Experimental

Materials

Polymer PEO (molecular weight = 6 × 10⁵), salt LiTFSI and ionic liquid (IL) trihexyltetradecylphosphonium bis trifluoromethylsulfonyl amide, (purity ∼99.99%) were obtained from Sigma Aldrich, Germany. Before preparing the polymer electrolyte films, all the materials were vacuum dried at 10⁻⁶ Torr for 2 days. Methanol (H₂O < 0.02%) and N-methyl-2-pyrrolidonel (NMP) (anhydrous, 99.5%) have been used as solvent for electrolyte and electrode preparation respectively.

Electrode preparation

Cathode material was prepared by using LiMn₂O₄ as active material, carbon black to enhance the electronic conductivity of electrode, polyvinylidene fluoride (PVdF) as a binder. NMP was used as a solvent for electrode preparation. LiMn₂O₄ (80%), carbon black (10%), and PVdF (10%) were mixed in NMP solvent until viscous solution was obtained. This slurry was coated on aluminium foil which was used as a current collector for cathode. After coating, cathode material was vacuum dried at 110 °C for 2 days. After that the prepared cathode was pressed under 3 ton pressure.

Preparation of polymer electrolyte films

Solution cast technique was used to prepare polymer electrolyte films of PEO + 20 wt% LiTFSI + X wt% IL (X = 0, 10, 20, 30, 40). Polymer PEO was first dissolved in dried methanol and stirred for 3–4 hours at 50 °C, after that, appropriate amount of LiTFSI salt was added into it and again stirred for 3–4 hours. After complete dissolution of the salt in the polymer PEO, desired amount of IL was added and again stirred for 4–5 hours for obtaining a complete homogeneous mixture. These viscous solutions were poured in polypropylene Petri dishes and solvent was allowed to evaporate slowly at room temperature for a week. After complete evaporation of the solvent, flexible, thin, free standing and semi-transparent polymer electrolyte films having thickness ∼200–400 μm were obtained. Prepared polymer electrolyte films were finally vacuum dried at ∼10⁻⁶ Torr for 2–3 days to remove any traces of moisture present in the films before characterization.

Characterization techniques

Scanning electron microscope model Quanta C-200 FEI was used to study the surface morphology of polymer electrolyte films. To record X-ray diffraction pattern of polymer electrolyte films, X-ray diffractometer model X'Pert PRO with CuK_α radiation (λ = 1.54 Å) in the range 2θ = 10 to 80 degree was used. FTIR spectra of polymeric membranes were recorded by using Perkin-Elmer FTIR Spectrometer (model RX 1) between 400–3200 cm⁻¹. The differential scanning calorimetry was done (under continue purging of N₂ gas) by using Mettler DSC 1 system in the temperature range −100 to 90 °C at a heating rate 10 °C min⁻¹. From DSC, melting heat of polymer electrolyte (ΔH_m) has been calculated by taking the area of melting peak observed in the graph of heat flow vs. temperature. Thermo gravimetric analysis (TGA) (Mettler DSC/TGA 1 system) was done under continuous purging of nitrogen gas. Ionic conductivity and dielectric studies of prepared SPEs were carried out using complex impedance spectroscopic technique using Novo control impedance analyser in the frequency range 1 Hz to 40 MHz. Electrical conductivity, σ, can be calculated from the relation:

σ = L/(R_bA) (1)

where L is the thickness of the sample; A is its cross section area and R_b is the bulk resistance.²² To obtain conductivity value of polymer electrolyte films, disc shaped prepared polymer film was placed between two stainless electrodes and whole assembly was placed in temperature controlled oven. For the determination of total ionic transference number (t_ion), dc polarization technique was used. In this process, prepared SPEs in disc shape were sandwiched between two stainless steel electrodes and 50 mV voltage was applied across it. The corresponding current was observed with respect to time. Cationic transference number was calculated by the combined ac/dc technique. In this technique, Li/PEO + 20 wt% LiTFSI + 20 wt% IL/Li cell, was polarised by applying 50 mV voltage for 1 hour at room temperature and corresponding current was recorded as function of time. For calculating cationic transference number of prepared SPEs, resistances of the cell were recorded before and after polarization. The linear sweep voltammetric study was done by using an electrochemical analyser with an AUTOLAB PGSTAT 302N controlled by NOVA 1.8 software version (Metrohm Lab) for observing electrochemical window of prepared SPEs. Prepared samples were handled in an argon filled glove box (mBRAun MB10 compact) with H₂O and O₂ ≤ 0.5 ppm.

Results and discussion

XRD analysis

XRD pattern of pure PEO, PEO + 20 wt% LiTFSI and PEO + 20 wt% LiTFSI + X wt% IL (where X = 10, 20 and 40) are shown in Fig. 1. Crystalline peaks of pure PEO consist of halo regions appearing between 2θ = 15° to 30° and the crystalline peaks are present at 2θ = 19.6°, 23.8°, 26.7°, and 27.4°.²² The simultaneous presence of halo region and crystalline peaks indicates the semi crystalline nature of PEO.^22,23 When 20 wt% salt was added in PEO, peaks related to the salt LiTFSI appeared at ∼14° and ∼16°.²⁴ These peaks may appear because LiTFSI salt may not completely mixed with polymer matrix PEO. On adding IL in (PEO + 20 wt% LiTFSI), two crystalline peaks at 2θ = 26.7° and 27.4° disappear and peaks at 19.6° and 23.8° remained. Broadening of halo region and increase in FWHM (full width half maxima) of crystalline peaks for PEO + 20 wt% LiTFSI + X wt% IL (X = 0, 10, 20) indicate the increase in amorphicity of the PEO which is responsible for the enhancement of conductivity of polymer electrolyte films. The peak positions are listed in Table 1.

Fig. 1 XRD pattern for (a) pure PEO and polymer electrolyte PEO + 20 wt% LiTFSI + X% IL (b) X = 0, (c) X = 10, (d) X = 20, (e) X = 40 at room temperature.

Table 1 XRD profile of PEO and polymer electrolyte PEO + 20 wt% LiTFSI + X wt% IL (X = 0, 10, 20, 40)

Polymer	2θ	FWHM
PEO	19.6	0.25
	23.8	0.10
	23.4	0.20
	26.7
	27.4
PEO + 20% LiTFSI	14	0.15
	16	0.43
	19.6	0.11
	23.8	0.23
PEO + 20% LiTFSI + 10% IL	19.6	0.06
	20.0	0.2
	24.0	0.3
	36.9	0.5
PEO + 20% LiTFSI + 20% IL	19.1	0.1
	22.9	0.1
	23.4	0.5
	39.7	0.6
PEO + 20% LiTFSI + 40% IL	14	0.63
	16	0.2
	19.4	0.2
	23.6	0.3
	35.5	0.4

---

It should be noticed that on further increasing the IL (40 wt%) content in polymer electrolyte film, FWHM and halo region decrease (see Fig. 1(e)) which shows the less amorphous nature of these electrolytes. At higher IL content (40 wt%) in polymer electrolyte film PEO + 20 wt% LiTFSI, again LiTFSI salt related peaks at ∼14° and ∼16° appeared again that indicate the recrystallization of the salt. Such recrystallization has also been seen in the SEM images of the samples PEO + 20 wt% LiTFSI + 40 wt% IL as discussed in the next section. The presence of LiTFSI salt crystallites can be confirmed by XRD pattern since LiTFSI salt has prominent peaks at 2θ = 14°, 16°, 18°, 19°, 22° (ref. 24) and some of these peaks (∼14° and 16°) have been observed in XRD pattern for polymer electrolyte film containing 40 wt% IL.

SEM study

In Fig. 2, SEM images of PEO and PEO + 20 wt% LiTFSI + X wt% IL (for X = 0, 10, 20 and 40) polymer electrolyte films are shown. From Fig. 2(a), it can be seen that spherulites are present indicating the semicrystalline nature of PEO. When salt LiTFSI is added in the polymer PEO (Fig. 2(b)), size of the spherulites decreases.²⁵ Upon incorporation of less amount of IL in polymer electrolyte film (10 and 20 wt% IL in PEO + 20 wt% LiTFSI), smoother morphology has been observed (see Fig. 2(c) and (d)). However, at high IL content (40 wt% IL), some white crystallites were observed in PEO + 20 wt% LiTFSI + 40 wt% IL as shown by arrows in Fig. 2(e). These white crystallites are not present in pure PEO and PEO + 20 wt% LiTFSI + X wt% IL (X = 10, 20) and are expected to occur due to the recrystallization of LiTFSI salt. Such recrystallization was also confirmed by XRD pattern discussed earlier. This might be taking place due to possible ion association. Since dopant salt as well as IL have the same anion (TFSI), therefore, at higher IL content (∼40 wt%), more anions are available from IL which facilitates ion association with Li salt making recrystallization to occur.

Fig. 2 SEM image for (a) pure PEO and PEO + 20 wt% LiTFSI + X% IL (b) X = 0, (c) X = 10, (d) X = 20, (e) X = 40.

DSC analysis

Differential Scanning Calorimetry (DSC) has been done to determine phase transition temperatures: glass transition (T_g) and melting temperatures (T_m) are shown in Fig. 3. From Fig. 3, it can be seen that glass transition temperature (T_g) decreases on incorporation of IL concentration (up to 30 wt%) in polymer electrolyte film (PEO + 20 wt% LiTFSI) as given in Table 2. The decrease in T_g of polymer electrolyte film shows enhanced flexibility of polymer chain except for PEO + 20 wt% LiTFSI + 40 wt% IL. This can be due to the accumulation of ions in polymer electrolyte film, due to which motion of polymer chain is hindered. When 20 wt% LiTFSI salt was incorporated in PEO, melting temperature of PEO decreases. This occurs because of the presence of strong electron withdrawing group (SO₂CF₃) in salt LiTFSI and high flexibility of (–SO₂–N–SO₂–) of TFSI anion, due to which, salt dissociation occurs.²⁶ When IL was added in PEO and PEO + 20 wt% LiTFSI, shift in melting temperature of PEO (T_m1) and melting temperature corresponding to IL (T_m2) occurred. T_g and T_m of polymer electrolyte films have been given in Table 2. From Table 2, it can be observed that T_m1 decreases up to 30 wt% IL and above 30 wt% IL content it starts increasing. T_m2 remains almost constant irrespective of IL content in PEO + 20 wt% LiTFSI. These phase transition temperatures (i.e. T_m and T_g) are related to the chain flexibility or segmental motion of the polymer chain. Therefore, the intermolecular force between IL and C–O–C group of polymer PEO was reduced upon addition of IL in polymer electrolyte which is responsible for increase in conductivity.

Fig. 3 DSC thermo gram for (a) pure PEO and polymer electrolyte PEO + 20 wt% LiTFSI + X% IL (b) X = 0, (c) X = 10, (d) X = 20, (e) X = 30, (f) X = 40.

Table 2 Melting (T_m1) and glass transition (T_g) temperature of polymer electrolyte PEO and PEO + 20 wt% LiTFSI + X wt% IL (X = 0, 10, 20, 30, 40) with melting temperature of IL (T_m2) (in °C) and calculated value of degree of crystallinity (X_c)

Sample	Tm1 (PEO) (°C)	Tm2 (IL) (°C)	Tg (PEO) (°C)	Xc (%)	ΔHm (J g^−1)
Pure PEO	71.0		−66.23	80	−171
PEO + 20% LiTFSI	67.6		−41.2	66.9	−143
PEO + 20% LiTFSI + 10% IL	63.0	−71.0	−44.8	26.9	−57.51
PEO + 20% LiTFSI + 20% IL	61.5	−69.9	−46.3	25.4	−54.32
PEO + 20% LiTFSI + 30% IL	59.9	−69.4	−47.3	22.7	−48.49
PEO + 20% LiTFSI + 40% IL	61.5	−69.9	−44.3	25.1	−53.63

---

Degree of crystallinity (X_c) of the sample can also be determined using DSC thermogram and it was expected that the degree of crystallinity (X_c) will decrease when salt and IL were added to PEO polymer.²⁷ Degree of crystallinity can be calculated from the relation shown below.²⁷

X_c = (ΔH_m/ΔH^°_m) × 100 (2)

where, ΔH_m is the melting heat of polymer electrolyte film; ΔH^°_m is the melting heat of 100% crystalline PEO phase. The value ΔH^°_m for pure PEO is 213.7 J g⁻¹.²⁷ Values of ΔH_m and X_c are given in Table 2. It is observed that X_c for PEO + 20 wt% LiTFSI + X wt% IL decreases up to X = 30. This shows the increment in amorphous nature of polymer and hence conductivity of polymer is expected to show similar behaviour with IL content. But upon increasing the IL content further (i.e. above 30 wt% IL), X_c increases. This could be due to the accumulation of LiTFSI salt in the polymer electrolyte containing 40 wt% IL, restraining the polymer segmental motion. So, when 40 wt% IL was incorporated in PEO + 20 wt% LiTFSI, its amorphicity decreased. As a consequence, conductivity of the polymer electrolyte film decreased when 40 wt% IL was incorporated in the sample (PEO + 20 wt% LiTFSI) as discussed in electrical conductivity section.

TGA analysis

TGA thermograms along with their first derivatives of TGA (DTGA) of PEO, IL and PEO + 20 wt% LiTFSI + X wt% IL (X = 0, 10, 20, 30, 40) polymer electrolyte films are given in Fig. 4. Fig. 4(a) and (c) show the weight loss peaks (T_d) due to the decomposition of pristine PEO and IL occurring at 399.9 °C and 456.7 °C respectively. The decomposition peak of LiTFSI salt has been reported at 340 °C.^28,29 When 20 wt% salt LiTFSI was added in polymer PEO, the decomposition peak of PEO was shifted to 403 °C and an extra peak was observed at 424 °C as shown in Fig. 4(b). The peak found at 424 °C may have arisen due to the complexation of LiTFSI salt with C–O–C group of polymer PEO matrix. On incorporating 10 wt% IL in PEO + 20 wt% LiTFSI, the decomposition peak (T_d) of polymer PEO and polymer–salt complexation shifted at 402 °C and 413 °C respectively. A new peak was also observed at 446 °C. The peak at 446 °C may be obtained due to the uncomplexed IL in polymer electrolyte PEO + 20 wt% LiTFSI + 10 wt% IL. Since the amount of IL is small, hence the peak associated with uncomplexed IL was found at 446 °C, which is negligibly small in intensity as shown in Fig. 4(d). By increasing amount of IL (20 wt%) in polymer electrolyte PEO + 20 wt% LiTFSI system, the decomposition peak of PEO, complexation peak of polymer–salt and uncomplexed IL related peaks shifted to 398 °C, 415 °C, 448 °C respectively as depicted in Fig. 4(e). Polymer–IL complexation has also been shown by FT-IR analysis (see ESI†). When large amount of IL (30 and 40 wt%) was added in the polymer electrolyte film, PEO + 20 wt% LiTFSI, the intensity of uncomplexed IL peak was dominated and intensity of polymer–salt complexation related peak diminished. The dominant peak intensity of uncomplexed IL shows that the amount of uncomplexed IL increases with increasing the IL concentration in polymer matrix. Similarly decrease in intensity of polymer–salt complaxation related peak signifies that the complexation of LiTFSI with C–O–C group of polymer PEO is reduced. By observing Fig. 4(f) and (g), it can be seen that there are three peaks at 402.9 °C, 372 °C, 451.9 °C and 394.3 °C, 373 °C, 455.8 °C respectively related to polymer electrolyte films PEO + 20 wt% LiTFSI + X wt% IL for X = 30, 40 respectively. Samples are thermally stable up to ∼350 °C, because before this temperature no weight loss occurs. Since area of the IL related peaks increases with IL content and appears to be distinct, this shows that the IL is not complexed with PEO and still acts as plasticizer in the polymer electrolyte film.

Fig. 4 TGA and DTGA curve for (a) pure PEO, (b) PEO + 20 wt% LiTFSI, (c) pure IL and PEO + 20 wt% LiTFSI + X% IL (d) X = 10, (e) X = 20, (f) X = 30, (g) X = 40.

Electrical conductivity

Fig. 5 shows the composition dependant conductivity (σ) graph. From Fig. 5, it can be seen that the conductivity first increases with IL content (up to 20 wt% IL) and then it shows decreasing trend at higher amount of added IL. The conductivity of these systems is given as follows:³⁰

σ = neμ (3)

where n is number of free charge carriers, e is the charge on ions and μ is the mobility. Conductivity (σ) depends on the ions mobility and number of free mobile ions (n). n can be represented as:³¹

n = n₀ exp(−E/ε′kT) (4)

where, k is the Boltzmann constant, T is temperature, E is activation energy and ε′ is dielectric constant or real part of permittivity.

Fig. 5 Variation of conductivity with respect to IL concentration at room temperature. Solid line has been drawn as visual to the eye guide.

Thus, from the above relation, it can be observed that n depends on ε′ and from the experimental data it was found that ε′ increases up to 20 wt% IL and later on decreases as shown in Fig. 6. Hence, it can be concluded that the number of charge carriers (n) and hence the conductivity (σ) increases up to 20 wt% IL (∼4.2 × 10⁻⁵ S cm⁻¹). Later on for 30 and 40 wt% of IL incorporated in PEO + 20 wt% LiTFSI, conductivity decreases. At lower IL concentration (10 and 20 wt%) in polymer electrolyte PEO + 20 wt% LiTFSI, IL acts as a plasticizer and provides free charge carriers also. Due to this, conductivity of SPEs increased. But at higher concentration of IL, number of TFSI⁻ anion increased and the intermolecular spacing between the ions got reduce which lead to the formation of ion pair (Li⁺ and TFSI⁻) instead of participating in interaction with ether oxygen of PEO. Due to the formation of ion pairs at higher IL content, number of free ions reduced which is responsible for the decrement in the ionic conductivity.

Fig. 6 Variation of dielectric constant (ε′) with respect to IL concentration. Solid line has been drawn as visual to the eye guide.

For all the prepared samples, conductivity was measured at different temperatures in the range 30 to 80 °C as shown in Fig. 7. It can be observed that the conductivity (σ) of polymer electrolyte at a particular concentration increases with temperature. This occurs because on increasing the temperature, empty space increases and free volume is available for the segmental motion of polymer chain. Now polymeric chain becomes more flexible. As a result, the mobility of ions is increased and the effect of ionic cloud at the electrode–electrolyte interface decreases. Variation in the conductivity, σ over 1000/T shows an Arrhenius type thermally activated behaviour given as:³²

σ = σ_o exp(−E_a/kT) (5)

where, σ_o is pre exponential factor, E_a is activation energy, k is Boltzmann constant and T is temperature. A phenomenological model has been given in Fig. 8 to explain the change in polymer chain flexibility and hence mobility of ions upon increasing the IL content. At lower IL contents (10 wt% of IL) in polymer electrolyte film, polymer chain becomes more flexible and unfolds due to the plasticization action of IL as given in Fig. 8(a). As IL content is increased to 20 wt%, polymer chain becomes more flexible and chain unfolding becomes faster as shown in Fig. 8(b). However, as the amount of IL is further increased in the polymer electrolyte, ions crowd and ion association starts taking place. As a result, Li cation associates with the anion of the salt, as depicted in Fig. 8(c). Unlike free cations and anions, recrystallised LiTFSI hinders polymer chain segmental motion and thereby amorphicity gets reduced resulting decreased conductivity at higher IL content.

Fig. 7 Temperature dependant conductivity of polymer electrolyte PEO + 20 wt% LITFSI + X% IL (a) X = 0, (b) X = 10, (c) X = 20, (d) X = 30, (e) X = 40.

Fig. 8 Schematic representation of flexibility of polymer chain PEO + 20 wt% LiTFSI + X% IL (a) X = 10, (b) X = 20, (c) X = 30. (a) shows at lower IL content phosphonium anions, (b) at enhance IL content (20 wt%), crowding of anions occurs. This crowding of anions makes the polymer segmental motion difficult. (c) Thus conductivity decreases at higher IL content.

Frequency dependent conductivity plot has been used to explain ion dynamics process of polymer–salt complexes. Fig. 9(a) shows the frequency dependent conductivity plot for polymer electrolyte PEO + 20 wt% LiTFSI + 20 wt% IL (this is the optimized composition with highest ionic conductivity). The behaviour of conductivity spectra follows Jonscher's universal power law:³¹

σ(ω) = σ_dc + Aωⁿ (6)

where, σ_dc is frequency independent conductivity, A is temperature dependant parameter, ω is frequency and n is power law exponent (0 < n < 1). Exponent n explains the ion conduction process that mostly occurrs in disordered materials. Jonscher's universal power law is generally applicable for disordered materials such as polymers, glasses and ceramics etc. Value of exponent n varies from 0 to 1. As n > 1, this corresponds to the nearly constant loss. Fig. 9(a) shows the frequency dependant conductivity plot at different temperatures for polymer electrolyte PEO + 20 wt% LiTFSI + 20 wt% IL. In lower frequency region, electrode polarisation effect occurs. Due to the polarisation of space charges at corresponding electrodes.^33,34 In low frequency region, conductivity is almost independent of frequency. This region is related to the translation motion of mobile ions. So, the long range translation occurs at low frequency region. However, as frequency increases, dispersion nature of conductivity has been observed. Frequency independent to frequency dependant region movement shows the onset of relaxation process. High frequency region leads to the back and forth motion of ions. From scaling of ac conductivity data, further information of ions dynamical process can be obtained. Scaling of PEO + 20 wt% LiTFSI + 20 wt% IL conductivity data has been shown in Fig. 9(b). It is observed that the conductivity graph at different temperatures can be scaled into single master curve as given in Fig. 9(b). This shows that the charge carriers in polymer electrolyte PEO + 20 wt% LiTFSI + 20 wt% IL follow common relaxation mechanism at all temperature.^20,35–37

Fig. 9 Frequency dependant real part of ionic conductivity (σ′) for (a) PEO + 20 wt% LiTFSI + 20 wt% IL at different temperature. (b) Scaling of conductivity spectra obtained from conductivity vs. frequency plot of (a) at different temperature as indicated in figure.

Transference number

Fig. 10 shows the dc polarisation curve of PEO + 20 wt% LiTFSI + 20 wt% IL sandwiched between two stainless steel electrode with applied voltage 50 mV.⁴ Total ionic transport number (t_ion) has been calculated by the equation:⁴

t_ion = (i_t − i_e)/i_t (7)

where i_t is the total current i_e is the steady or residual current. The value of t_ion has been found ∼0.99. This indicates that the ionic conductivity in prepared polymer electrolyte PEO + 20 wt% LiTFSI + 20 wt% IL mainly occurrs due to the presence of ions (Li⁺, TFSI⁻, IL cation) in polymer electrolyte. Three different ions (Li⁺, TFSI⁻, IL cation) are present in PEO + 20 wt% LiTFSI + 20 wt% IL. The main focus is to find the contribution of Li⁺ transport in polymer electrolyte PEO + 20 wt% LiTFSI + 20 wt% IL. The transference number of Li⁺ in polymer electrolyte, PEO + 20 wt% LiTFSI + 20 wt% IL has been calculated by combined ac/dc technique. In this process, Li/PEO + 20 wt% LiTFSI + 20 wt% IL/Li cell was polarized by applying 50 mV potential across the cell for 1 hour and corresponding current was recorded as function of time as shown in Fig. 10.

Fig. 10 DC polarisation curve of polymer electrolyte PEO + 20 wt% LiTFSI + 20 wt% IL sandwich between two symmetry stainless steel electrodes with applied voltage 50 mV.

Cationic transference number (t_Li) can be calculated by the expression given below:⁴

t_Li = I_ss(Δv − I_oR_o)/I_o(Δv − I_ssR_ss) (8)

where I_ss is the steady state current, Δv is the applied voltage required, I_o is the initial current, R_o and R_ss are the resistances across the cell before and after polarization respectively at room temperature as shown in Fig. 11. Cationic transference number (t_Li) of polymer electrolyte PEO + 20 wt% LiTFSI + 20 wt% IL was found to be ∼0.37. The resistances of the cell Li/PEO + 20 wt% LiTFSI + 20 wt% IL/Li before and after polarization have been obtained using impedance plots as shown in inset of Fig. 11. This shows that 37% conductivity of polymer electrolyte PEO + 20 wt% LiTFSI + 20 wt% IL is contributed by Li⁺ cation transport and rest of the conductivity occurs due to cation and anion of salt and IL (TFSI⁻).^38–40

Fig. 11 DC polarisation curve for polymer electrolyte PEO + 20 wt% LiTFSI + 20 wt% IL at applied voltage 50 mV and inset of figure is the ac impedance plot before and after polarization of cell (Li/PEO + 20 wt% LiTFSI + 20 wt% IL/Li) at room temperature.

Electrochemical characterization

To obtain the electrochemical stability of prepared polymer electrolyte film, linear sweep voltammetry has been used. The electrochemical window of prepared polymer electrolyte film PEO + 20 wt% LiTFSI + 20 wt% IL was observed at room temperature. This optimized polymer electrolyte film PEO + 20 wt% LiTFSI + 20 wt% IL has been chosen, since highest ionic conductivity was observed at room temperature for this polymer electrolyte film. Fig. 12 shows the linear sweep voltammetric trace for the polymer electrolyte film (PEO + 20 wt% LiTFSI + 20 wt% IL) sandwiched between two non – blocking electrodes Li/PEO + 20 wt% LiTFSI + 20 wt% IL/LiMn₂O₄ at room temperature with scan rate 0.5 mV s⁻¹. The electrochemical stability of polymer electrolyte film PEO + 20 wt% LiTFSI + 20 wt% IL has been found ∼3.34 V, which is the voltage limit of the prepared polymer electrolyte film. So it is found that the prepared polymer electrolyte film PEO + 20 wt% LiTFSI + 20 wt% IL is electrochemically stable up to ∼3.34 V.

Fig. 12 Linear sweep voltametry of the polymer electrolyte PEO + 20 wt% LiTFSI + 20 wt% IL sandwiched between two non-blocking electrodes Li/polymer electrolyte/LiMn₂O₄ at room temperature with scan rate 0.5 mV s⁻¹.

Conclusions

Free standing polymer films PEO + 20 wt% LiTFSI + X wt% IL (X = 0, 10, 20, 30, 40) are prepared and characterised using different characterization techniques like XRD, SEM, DSC, TGA, conductivity, FTIR etc. It is found that the conductivity of the prepared film increases up to X = 20 wt% IL incorporated in the sample. X_c (degree of crystallinity) also decreases up to X = 20 wt%. DTGA and FTIR results show the presence of complexed and uncomplexed IL in the polymer film. From the study of SEM, XRD and DSC, it can be shown that PEO + 20 wt% LiTFSI + X wt% IL (X = 30, 40) is not completely mixed in the polymer membrane due to accumulation of the salt in the polymer system and from the TGA thermograms, it can be concluded that the samples are thermally stable up to 350 °C. Polymer electrolyte containing 20 wt% IL has been found to have the highest conductivity (∼4.2 × 10⁻⁵ S cm⁻¹ at room temperature) and obeys Arrhenius type thermally activated process. Linear sweep voltametry shows electrochemical potential window ∼3.34 V. The anionic transference number is found >0.99 with cationic transference number ∼0.37.

Acknowledgements

One of us RKS gratefully acknowledged financial support from BRNS-DAE, Mumbai and DST, New Delhi, India to carry out this work.

References

1. Shalu, S. K. Chaurasia and R. K. Singh, RSC Adv., 2014, 4, 50914–50924.
2. A. K. Shukla and T. P. Kumar, Curr. Sci., 2008, 94, 314–331.
3. J. Y. Song, Y. Y. Wang and C. C. Wan, J. Power Sources, 1999, 77, 183–197.
4. Shalu, V. K. Singh and R. K. Singh, J. Mater. Chem. C, 2015, 3, 7305–7318.
5. A. L. Saroj, R. K. Singh and S. Chandra, J. Phys. Chem. Solids, 2014, 75, 849–857.
6. R. Meziane, J. P. Bonnet, M. Courty, K. Djellab and M. Armand, Electrochim. Acta, 2011, 57, 14–19.
7. A. M. Stepha and K. S. Nahm, Polymer, 2006, 47, 5952–5964.
8. J. Ding, D. Zhou, G. Spinks, G. Wallace, S. Forsyth, M. Forsyth and D. MacFarlane, Chem. Mater., 2003, 15, 2392–2398.
9. S. Ahmad, Ionics, 2009, 15, 309–321.
10. J. S. Wilkes, Green Chem., 2002, 4, 73–80.
11. R. Hagiwara and Y. Ito, J. Fluorine Chem., 2000, 105, 221–227.
12. C. H. Lochmüller, Anal. Chem., 2009, 1–456.
13. J. H. Shin, W. A. Henderson and S. Passerini, Electrochem. Commun., 2003, 5, 1016–1020.
14. B. Rupp, M. Schmuck, A. Balducci, M. Winter and W. Kern, Eur. Polym. J., 2008, 44, 2986–2990.
15. P. P. Soo, B. Huang, Y. Jang, Y. M. Chiang, D. R. Sadoway and A. M. Mayes, J. Electrochem. Soc., 1999, 146, 32–37.
16. S. Fang, Y. Tang, X. Tai, L. Yang, K. Tachibana and K. Kamijima, J. Power Sources, 2011, 196, 1433–1441.
17. K. Tsunashima, A. Kawabata, M. Matsumiya, S. Kodama, R. Enomoto, M. Sugiya and Y. Kunugi, Electrochem. Commun., 2011, 13, 178–181.
18. K. Tsunashima and M. Sugiya, Electrochemistry, 2007, 75, 734–736.
19. C. P. Gonzalo, A. A. J. Torriero, M. Forsyth, D. R. MacFarlane and P. C. Howlett, J. Phys. Chem. Lett., 2013, 4, 1834–1837.
20. S. K. Chaurasia, A. L. Saroj, Shalu, V. K. Singh, A. K. Tripathi, A. K. Gupta, Y. L. Verma and R. K. Singh, AIP Adv., 2015, 5, 1–12.
21. S. K. Chaurasia, S. Chandra and R. K. Singh, J. Polym. Sci., Part B: Polym. Phys., 2011, 49, 291–300.
22. Shalu, S. K. Chaurasia, R. K. Singh and S. Chandra, J. Phys. Chem. B, 2013, 117, 897–906.
23. H. Cheng, C. Zhu, B. Huang, M. Lu and Y. Yang, Electrochim. Acta, 2007, 52, 5789.
24. N. N. Sa'adun, R. Subramaniam and R. Kasi, Sci. World J., 2014, 2014, 1–7.
25. A. G. B. Pereira, R. F. Gouveia, G. M. de Carvalho, A. F. Rubira and E. C. Muniz, Mater. Sci. Eng., C, 2009, 29, 499–504.
26. Z. Xue, D. He and X. Xie, J. Mater. Chem. A, 2015, 3, 19218–19253.
27. S. K. Chaurasia, R. K. Singh and S. Chandra, Solid State Ionics, 2011, 183, 32–39.
28. Z. Lu, L. Yang and Y. Guo, J. Power Sources, 2006, 156, 555–559.
29. V. A. Aravindan, J. Gnanaraj, S. Madhavi and H. K. Li, Chem.–Eur. J., 2011, 17, 14326–14346.
30. M. Watanabe, K. Sanui and N. Ogata, J. Appl. Phys., 1985, 57, 123–128.
31. N. Shukla, A. K. Thakur, A. Shukla and D. T. Marx, Int. J. Electrochem. Sci., 2014, 9, 7644–7659.
32. D. P. Almond, C. C. Hunter and A. R. West, J. Mater. Sci., 1984, 19, 3236–3248.
33. A. K. Nath and A. Kumar, Electrochim. Acta, 2014, 129, 177–186.
34. N. K. Karan, D. K. Pradhan, R. Thomas, B. Natesan and R. S. Katiyar, Solid State Ionics, 2008, 179, 689–696.
35. A. Ghosh and A. Pan, Phys. Rev. Lett., 2000, 84, 2188–2190.
36. S. Summerfield, Philos. Mag. B, 1985, 52, 9–22.
37. M. P. Singh, R. K. Singh and S. Chandra, Prog. Mater. Sci., 2014, 64, 73–120.
38. Y. T. Kim and E. S. Smotkin, Solid State Ionics, 2002, 149, 29–37.
39. M. Doyle and J. Newman, J. Electrochem. Soc., 1995, 142, 3465–3468.
40. E. Quartarone and P. Mustarelli, Chem. Soc. Rev., 2011, 40, 2525–2540.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20393k

---