density 2.26 kW/kg. The cyclic stability was also good and capacity retention was 94.63% after 10000 cycles at 2 A/g.
Another important electrolyte category is ‘redox-active electrolytes’ and are prepared by the addition of redox additives like methylene blue (MB), Iodide salts (KI, NaI), hydroquinone (HQ), p-diphenylamine, etc. [42]. Yadav et al. [43] prepared a GPE using 1-butyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide (BMITFSI) as IL, and sodium iodide (NaI) as redox additive with poly(vinyledeneflouride-co-hexaflouropropylene) (PVdF-HFP) as host polymer. The specific capacitance was 351 F/g at 5 mV/s for SC cell with redox additive and is higher than redox additive-free cell (128 F/g) (Figure 3.9a). The specific energy was 26.1 Wh/kg and power density of 15 kW/kg (Figure 3.9b). Inset shows the LED glow demonstration for 300 s by connecting 4 cells. The SC cell with redox additive demonstrates superior cyclic stability (5% initial fading) than redox additive-free SC cell (23% initial fading) for 10000 cycles.
Figure 3.8 (a) Cyclic voltammograms obtained at 50 mV/s, (b) Capacitance retention as a function of the number of cycles at the operating voltages of [Reproduced with permission from Ref. [39], © Elsevier 2019].
Figure 3.9 (a) Specific capacitance of Cell#1 and Cell#2 versus charge–discharge cycles measured at constant current density 0.84 A g-1, and (b) Ragone plots of Cell#1 and Cell#2 (inset shows glow of LED by four cells connected in series). [Reproduced with permission from Ref. [43], © Wiley 2019].
Another crucial approach to increase the electrochemical performance of SC cell is by developing composite materials (iongels) having two networks based on polarity [44]. Here, strongly polar (e.g., PEO, PVA, etc.) network provides superior electrochemical properties while, less polar network (e.g., NBR, natural rubber, PDMS, etc.) leads to enhanced mechanical properties. Based on this, Lu et al. [45] reported the preparation of iongels composite of PEO/NBR by in situ synthesis. The tensile modulus was 0.69 MPa and elongation break about 338% for iongel. The ionic conductivity was 2.4 mS/cm for 60% uptake of IL. Then using PEO/NBR iongels, an SC cell was fabricated using graphene electrodes and tested in the voltage window of 0-2.5 V. The specific capacitance was 2.8 F/g at 1A/g and decreases to 150 F/g at 10 A/g. The SC cell demonstrates good cyclic stability up to 10000 cycles (93.7% capacity retention) and negligible structural degradation as evidenced by XRD (after 10000 cycles). The energy density was very high 181 Wh/kg (comparable to commercial Lithiumion batteries) with a power density of 5.87 kWh/kg [46].
In solid polymer electrolytes (SPE) dielectric constant of the nanofiller also plays an important role in the enhancement of the ion transport parameters and hence the storage capacity of SC cell. So, to examine this Das et al. [47] investigated the effect of TiO2 (dielectric constant: 80) and ZnO (dielectric constant: 8.5) on polymer matrix of PVDF–HFP incorporating 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) as IL. The ionic conductivity of the prepared solid polymer electrolytes was 1.68 × 10−2 S/cm (nanofiller free), 2.57 × 10−2 S/cm (with ZnO) and 3.75 × 10−2 S/cm (with TiO2) at 303 K. The electrochemical stability window of solid polymer electrolytes was 4.57 V (nanofiller free), 5.55 V (with ZnO) and 5.98 V (with TiO2).
The ionic conductivity and voltage stability window were superior for TiO2 nanofiller based SPE. The specific capacitance for the SC cell [EDLC-I: nanofiller free, EDLC-II: with ZnO, EDLC-III: with TiO2] using AC as electrode obtained from impedance spectroscopy was 103.5 F/g (nanofiller free), 134.6 F/g (with ZnO) and 206.4 F/g (with TiO2). While specific capacitance for the SC cell from CV was 79.07 (nanofiller free), 93.07 (with ZnO) and 192.17 F/g (with TiO2) at 25 mV/s. The increase in specific capacitance for TiO2 was attributed to a high dielectric constant which supports salt dissociation. The value of specific capacitance for the SC cell from GCD was 104 F/g (nanofiller free), 131 F/g (with ZnO) and 239 F/g (with TiO2) at 1 A/g (Figure 3.10a, b). The TiO2 based SC cell shows an energy density of 33.19 Wh/ Kg and a power density of 1.17 kW/Kg. Also, all cells demonstrate a coulombic efficiency of 100 % after 2000 cycles.
Figure 3.10 (a) Comparison of charge–discharge curves at current for EDLC I, EDLC II, and EDLC III at a current density of 1 Ag−1. (b) Variation of the specific capacitance of EDLC I, EDLC II, and EDLC III cells at different constant current densities. (c) Specific capacitance of EDLC I, EDLC II, and EDLC III cells at current density of 2 Ag−1 shown as a function of charge–discharge cycles. (d) Two EDLC III cells in series to light up a yellow LED. [Reproduced with permission from Ref. [47], © Wiley 2020].
Another report from the same group examined the SC performance based on poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix, 1-propyl-3-methyleimidazolium bis(trifluromethylesulfonyl)-imide as ionic liquid with lithium bis (trifluoromethanesulfonyl)imide salt and plasticizer mixture (ethylene carbonate: propylene carbonate in the ratio 1:1) [32]. The highest specific capacitance from CV for cell 3 was 124.1 F/g at a scan rate of 10 mV/s with an energy density of 23.07 Wh/kg and power density of 0.5333 kW/ kg. Another report investigated the effect of cationic size and viscosity, dielectric constant of the ionic liquids on the electrochemical performance of SC cell [48]. Three polymer electrolytes were prepared: (i) BDMIMBF4-P(VdF-HFP) (BDMIMGPE-1), (ii) BMIMBF4-P(VdF-HFP) (BMIMGPE-2) and (iii) EMIMBF4-P(VdF-HFP) (EMIMGPE-3). The highest ionic conductivity was observed for the EMIMGPE-3 electrolyte and is 12.76 mS/cm which is attributed to the smaller cation size, high dielectric constant and low viscosity.
The value of specific capacitance as estimated from CV was 32.66 F/g (Cell-1), 49.1 F/g (cell-2), and 63.47 F/g (cell 3) at 10 mV/s. The highest specific capacitance for cell 3 is in correlation with ion conductivity results. The cell-3 demonstrates 74% capacity retention after 4000 cycles and 100% coulombic efficiency after 8000 cycles. The energy density and power density also superior to cell-3 (Figure 3.11).
Figure 3.11 Ragone plots (gravimetric specific energy versus specific power) for all EDL cells. [Reproduced with permission from Ref. [48], © Elsevier 2018].
Recently, Choi et al. [49] reported a novel strategy to achieve long-term cyclic stability and high energy density. The authors used the nanofiber cellulose incorporated nanomesh graphene-carbon nanotube hybrid buckypaper electrodes and ionic liquid-based solid
