Application of sulfonium-, thiophenium-, and thioxonium-based salts as electric double-layer capacitor electrolytes

The purpose of this paper is to develop novel molten salts based on sulfonium, thiophenium, and thioxonium cations as electrolytes for EDLCs. We evaluated various kinds of the salts, with tetrafluoroborate (BF₄) and bis(trifluorometanesulfonyl)amide (TFSA) anions, including several kinds of ionic liquids. The cell using the electrolyte containing diethylmethylsulfonium (DEMS)-BF₄ salt had the higher capacitance and the lower DC-IR value than those containing conventional salts such as 1-ethyl-3-methylimidazolium (EMI)-BF₄ and N,N,N-triethyl-N-methylammonium (TEMA)-BF₄ at 243 K and 2 V. The capacitance and DC-IR at low temperature depended strongly on the structure, particularly on the size of each ion. We also examined the durability of cells by continuous charging at 333 K and 2.5 V. The stabilities of sulfonium-, thiophenium-, and thioxonium-based electrolytes were much inferior to that of EMI-BF₄.     

Application of Si-C-O glass-like compounds as negative electrode materials for lithium hybrid capacitors

The Si-C-O glass-like compound (a-SiCO) was applied to a negative electrode of a lithium hybrid capacitor (LHC) with activated carbon positive electrodes. The performance as a negative electrode (by a three-electrode system) and LHC (by a two-electrode system) was evaluated in LiClO₄ (EC-DEC) and LiBF₄ (PC) electrolytes. With a-SiCO reversible insertion/extraction of lithium ions at high current densities (0.5-2.0 A g⁻¹) was possible. By prior short-circuiting of the negative electrode with lithium metal in the electrolytes for appropriate periods, the charge/discharge performance of the assembled LHC compared favorably with an electric double layer capacitor (EDLC) made of the activated carbon used for LHC. The cycle performance of the LHC was better but the capacitance was smaller in the LiBF₄ (PC) electrolyte than in LiClO₄ (EC-DEC) electrolyte. Smaller capacitance is mainly due to lower electric conductivity and higher viscosity of LiBF₄ (PC) electrolyte than LiClO₄ (EC-DEC) electrolyte. The energy density of the assembled LHC reached a maximum of about three times that of EDLC, with the power density comparable to that of the EDLC.     

High/low temperature operation of electric double layer capacitor utilizing acidic cellulose-chitin hybrid gel electrolyte

An acidic cellulose-chitin hybrid gel electrolyte consisting of cellulose, chitin, 1-butyl-3-methylimidazolium, 1-allyl-3-methylimidazolium bromide, and an aqueous H₂SO₄ solution is investigated for electric double layer capacitors (EDLCs) with activated carbon fiber cloth electrodes. The acidic cellulose-chitin hybrid gel electrolyte shows a high ionic conductivity comparable to that for an aqueous 2 mol dm⁻³ H₂SO₄ solution at 0-80 °C. This system's temperature dependence in EDLC performance is investigated by galvanostatic charge-discharge measurement. An EDLC cell with the acidic hybrid gel electrolyte has higher capacitance than that with the aqueous H₂SO₄ solution in the range of operation temperatures (−10 to 60 °C). Moreover, the capacitance retention of the EDLC cell with the acidic hybrid gel electrolyte is better than that of a cell with the H₂SO₄ solution at 60 °C over 10,000 cycles. This suggests that the proposed acidic gel electrolyte has excellent stability in the presence of a strong acid, even at a high temperature of 60 °C.     

The effects of temperature on the performance of electrochemical double layer capacitors

An electrochemical double layer capacitor test cell containing activated carbon xerogel electrodes and ionic liquid electrolyte was tested at 15, 25 and 40 °C to examine the effect of temperature on electrolyte resistance (R_S) and equivalent series resistance (ESR) measured using impedance spectroscopy and capacitance using charge/discharge cycling. A commercial 10 F capacitor was used as a comparison. Viscosity, ionic self-diffusion coefficients and differential scanning calorimetry measurements were used to provide an insight into the behaviour of the 1,2-dimethyl-3-propylimdazolium electrolyte. Both R_S and ESR decreased with increasing temperature for both capacitors. Increasing the temperature also increased the capacitance for both the test cell and the commercial capacitor but proportionally more for the test cell. An increase in temperature decreased the ionic liquid electrolyte viscosity and increased the self-diffusion coefficients of both the anion and the cation indicating an increase in dissociation and increase in ionic mobility.     

Electrochemical double layer capacitor and lithium-ion capacitor based on carbon black

In this paper we report the physical investigation and the electrochemical performance of the carbon black SC3 from Cabot Corporation. The SC3 carbon black was investigated in terms of BET surface area, pore size distribution, resistivity and morphology. Composite electrodes containing SC3 as active material were prepared and used for the realization of electrochemical double layer capacitor (EDLC) and lithium-ion capacitor (LIC). In EDLC, at 5 mA cm⁻² charge-discharge currents, the carbon black displays a specific capacity of 40 mAh g⁻¹ and a specific capacitance of 115 F g⁻¹. It also displays a very good cycling stability for over 50,000 cycles and excellent performance retention at currents up to 50 mA cm⁻². The performance retention at high currents outstandingly differentiates this carbon black from a few commercially available EDLC-grade activated carbons. Because of the high specific capacity of SC3, the carbon black electrodes were also used in combination with LiFePO₄ electrodes in LIC. The results of this study indicate that SC3 carbon black is an interesting carbonaceous candidate for the realization of LIC.     

Charge storage mechanism of manganese dioxide for capacitor application: Effect of the mild electrolytes containing alkaline and alkaline-earth metal cations

Effect of the electrolyte particularities on manganese dioxide in the neutral electrolytes containing alkaline (Li⁺, Na⁺, or K⁺) and alkaline-earth (Mg²⁺, Ca²⁺, or Ba²⁺) metal cations for electrochemical capacitor is investigated by cyclic voltammetry and electrochemical impedance spectroscopy. A very high capacitance of 357.9 F g⁻¹ is measured when Ca²⁺ ion is used as electrolyte cation. The results show that cations of the electrolyte instead of proton or anions are responsible for the pseudo-capacitive behavior of manganese dioxide. The capacitance of manganese dioxide is found to depend strongly on the electrolyte particularities, for example, pH value, cation and anion species, and salt concentrations. A logarithmic dependence of capacitance of MnO₂ on cation activity is obtained for all alkaline and alkaline-earth metal cations. The capacitance of MnO₂ can be doubled by replacing univalent cation with bivalent cation at a close cation activity.     

KPF6 dissolved in propylene carbonate as an electrolyte for activated carbon/graphite capacitors

KPF₆ dissolved in propylene carbonate (PC) has been proposed as an electrolyte for activated carbon (AC)/graphite capacitors. The electrochemical performance of AC/graphite capacitor has been tested in XPF₆-PC or XBF₄-PC electrolytes (X stands for alkali or quaternary alkyl ammonium cations). The AC/graphite capacitor using KPF₆-PC electrolyte shows an excellent cycle-ability compared with other electrolytes containing alkali ions. The big decomposition of the PC solvent at the AC negative electrode is considerably suppressed in the case of KPF₆-PC, which fact has been correlated with the mild solvation of K⁺ by PC solvent. The relationship between the ionic radius of cation and the corresponding specific capacitance of AC negative electrode also proves that PC-solvated K⁺ ions are adsorbed on AC electrode instead of naked K⁺ ions.     

Electrochromic devices employing methacrylate-based polymer electrolytes

Poly(ethyl methacrylate) (PEMA)- and poly(2-ethoxyethyl methacrylate) (PEOEMA)-based polymer gel electrolytes with entrapped solutions of lithium perchlorate in propylene carbonate (PC) were prepared by direct, UV-initiated polymerization. The electrolytes were studied using electrochemical methods and they exhibit good ionic conductivity (up to 0.7 mS cm⁻¹ at 20 °C) as well as electrochemical stability up to 2.5 V vs. Cd/Cd²⁺ (5.1 V vs. Li/Li⁺) on gold electrode. The electrolytes have thermal stability up to 125 °C. The electrolytes were successfully tested as ionic conductors in the electrochromic device FTO/WO₃/Li⁺-electrolyte/V₂O₅/FTO using coupled optoelectrochemical methods to discuss the relationship between the electrolyte composition and parameters such as change of transmittance, response time and stability. The transmittance change Δτ was found to be 30-45% at 634 nm.     

Electrochemical cell studies based on non-aqueous magnesium electrolyte for electric double layer capacitor applications

Performances of electric double layer capacitors (EDLCs) based on an activated carbon electrode with acetonitrile (ACN), propylene carbonate (PC), or a ternary electrolyte, i.e., PC:ethylene carbonate (EC):diethyl carbonate (DEC), at 1 mol dm⁻³ of magnesium perchlorate [Mg(ClO₄)₂] salt have been investigated. The electrochemical responses were studied by impedance spectroscopy, cyclic voltammetry, and galvanostatic charge-discharge experiments at 25 °C in a three-electrode configuration. For a comparative evaluation, lithium perchlorate (LiClO₄) salt-based systems were also evaluated. All the observed results showed typical EDLC characteristics within the potential range between 0 and 1 V vs. an Ag/Ag⁺ reference electrode. The Mg-based systems exhibited similar or rather better performances than the corresponding Li-based electrolytes; in particular, the rate capability of Mg-based ACN and PC electrolytes was much better than the corresponding Li-based electrolytes, indicating the high accessibility and utility of activated carbon pores by solvated Mg ions.     

A binary ionic liquid system composed of N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide and lithium bis(trifluoromethanesulfonyl)imide: A new promising electrolyte for lithium batteries

Room temperature ionic liquids are nowadays the most appealing research target in the field of liquid electrolytes for lithium batteries, due to their high thermal stability, ionic conductivity and wide electrochemical windows. The cation structure of such solvents strictly influences their physical and chemical properties, in particular the viscosity and conductivity.In this paper we report on the preparation and characterization of a complete series of solutions between lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and the promising N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide (PY_1,2O1) ionic liquid. A wide molality range has been explored in order to identify the optimal compositions in terms of conductivity and electrochemical stability. Our thermal results show that the solutions are amorphous independently on the LiTFSI content. Up to salt concentration of 0.4 mol kg⁻¹ the solutions have a very low viscosity (η ∼ 36 cP), a high ionic conductivity, even at temperatures below 0 °C, and a good electrochemical stability. Cations transport numbers ranging between 0.05 and 0.39 have been determined as a function of LiTFSI content. The combination of these properties makes the PY_1,2O1-based solutions potentially attractive liquid electrolytes for lithium batteries.     

Effect of electrolyte on electrochemical characteristics of MmNi3.55Co0.72Al0.3Mn0.43 alloy electrode for hydrogen storage

A series of experiments have been performed to investigate the effects of three electrolytes of different compositions (EO, EA and EM) on the electrochemical characteristics of MmNi_3.55Co_0.72Al_0.3Mn_0.43 hydrogen storage alloy electrode. Electrolytes EA and EM were obtained by adding appropriate amounts of Al₂(SO₄)₃ and MnSO₄ to the original electrolyte EO (6 M KOH + 1 wt% LiOH), respectively. Electrode activation, maximum capacity, cycle life, self-discharge and high-rate discharge characteristics have been studied. It was found that a maximum capacity of about 260 mA h/g has been obtained for the alloy electrodes in all these electrolytes after 5–7 cycles of charging/discharging. The alloy electrodes have a good durability in electrolytes EA and EM, especially after 175 cycles. Using the capacity retention as an indication of self-discharge resistance, almost identical degree of capacity retention (82% after 4 days and 45% after 16 days) has been observed at 298 K, regardless of the electrolytes used. When tested at higher temperature, however, a higher capacity retention (46% after 3 days) at 333 K has been observed for electrodes in electrolyte EA, and about 32% for electrodes in both electrolytes EO and EM. As to high-rate discharge behavior of the results of high-rate discharge tests indicated that about 50% of discharge efficiencies were obtained in the three electrolytes at 333 K by continuous-model high-rate discharge method, at a discharge rate of 7C, and 22% in 298 K. The alloy electrode in electrolyte EM has the best durability, in which about 50% of discharge efficiency at DC = 9C was obtained by step-model high-rate discharge method at 333 K, which was even higher than that at 298 K.     

Pyridinium-based protic ionic liquids as electrolytes for RuO2 electrochemical capacitors

A series of pyridinium-based room temperature protic ionic liquids has been synthesized and their key properties, including dynamic viscosity, density, ionic conductivity, glass transition temperature, and potential window on Pt and on glassy carbon have been determined. In order to evaluate the PILs performance as electrolytes in metal-oxide based electrochemical capacitor, the specific capacitance (from 40 to 50 F g⁻¹) and the specific charge (from 23 to 48 C g⁻¹) of a thermally prepared RuO₂ electrode have been measured. This class of protic ionic liquids was found to exhibit very low viscosities (from 2.77 to 87.9 cP) along with conductivities ranging from 1.10 to 11.25 mS cm⁻¹, which are properties of the highest importance for applications in energy-storage devices. Structure-property relationships, including the effect of the alkyl chain length of the cation's substituent and of its position, are reported and should improve the success of the rational design of protic ionic liquids for specific applications in the future.     

A non-aqueous electrolyte for the operation of Li/air battery in ambient environment

In this work we report a non-aqueous electrolyte that supports long-term operation of the Li/air battery in dry ambient environments based on a non-hydrolytic LiSO₃CF₃ salt and a low volatility propylene carbonate (PC)/tris(2,2,2-trifluoroethyl) phosphate (TFP) solvent blend. By measuring and analyzing the viscosity of PC/TFP solvent blends, the ionic conductivity of electrolytes, and the discharge performance of Li/air cells as a function of the PC/TFP weight ratio, we determined the best composition of the electrolyte is 0.2 m (molality) LiSO₃CF₃ 7:3 wt. PC/TFP for Li/O₂ cells and 0.2 m LiSO₃CF₃ 3:2 wt. PC/TFP for Li/air cells. Discharge results indicate that Li/air cells with the optimized electrolyte are significantly superior in specific capacity and rate capability to those with baseline electrolytes. More interestingly, the improvement in discharge performance becomes more significant as the discharge current increases or the oxygen partial pressure decreases. These results agree neither with the viscosity of the solvent blends nor the ionic conductivity of the electrolytes. We consider that the most likely reason for the performance improvement is due to the increased dissolution kinetics and solubility of oxygen in TFP-containing electrolytes. In addition, the electrolyte has a 5.15 V electrochemical window, which is suitable for use in rechargeable Li/air batteries.     

Application of activated carbon/DNA composite electrodes to aqueous electric double layer capacitors

A novel capacitor electrode auxiliary, deoxyribonucleic acid (DNA), is applied to an electric double layer capacitor (EDLC) containing an aqueous 3.5 M NaBr electrolyte. The present electrode is composed of activated carbon (95 wt.%) and DNA (2.5 wt.%) with polytetrafluoroethylene (PTFE) as a binder (2.5 wt.%). An EDLC cell with the DNA-loading electrodes exhibits improved rate capability and discharge capacitance. An EDLC cell with DNA-free electrodes cannot discharge above a current density of 3000 mA g⁻¹ (of the electrode), while a cell with the DNA-loading electrodes can work at least up to 6000 mA g⁻¹. Moreover, an open-circuit potential (OCP) of the DNA-loading electrode sifts negatively with ca. 0.2 V from an OCP of the corresponding electrode without DNA. It is noteworthy that a small amount of DNA loading (2.5 wt.%) to the activated carbon electrode not only improves the rate capability but also adjusts the working potential of the electrode to a more stable region.     

Ion-conducting lithium bis(oxalato)borate-based polymer electrolytes

Poly(2-ethoxyethyl methacrylate) polymer gel electrolytes containing immobilised lithium bis(oxalato)borate in aprotic carbonates: propylene carbonate (PC), propylene carbonate–ethylene carbonate (PC–EC 50:50 vol.%) and diethyl carbonate–ethylene carbonate (DEC–EC 50:50 vol.%) were prepared by a direct radical polymerisation. The electrolyte composition was optimised to achieve suitable ionic conductivity 0.5 and 2.4 mS cm⁻¹ at 25 and 70 °C respectively along with good mechanical properties. The electrochemical stability up to 5.1 V vs. Li/Li⁺ was determined on gold electrode by voltammetrical measurements. The polymer electrolytes with high-boiling solvents (PC and PC/EC) showed higher thermal stability (up to 110–120 °C) compared to the liquid electrolytes. The proposed area of application is in the lithium-ion batteries with cathodes operating at elevated temperatures of 70 °C, where higher electrochemical stability of the polymer electrolytes is employed.     

Li/LiFePO4 batteries with gel polymer electrolytes incorporating a guanidinium-based ionic liquid cycled at room temperature and 50 °C

Gel polymer electrolytes composed of PVdF-HFP microporous membrane incorporating a guanidinium-based ionic liquid with 0.8 mol kg⁻¹ lithium bis(trifluoromethanesulfonylimide) are characterized as the electrolytes in Li/LiFePO₄ batteries. The ionic conductivity of these gel polymer electrolytes is 3.16 × 10⁻⁴ and 8.32 × 10⁻⁴ S cm⁻¹ at 25 and 50 °C, respectively. The electrolytes show good interfacial stability towards lithium metal and high oxidation stability, and the decomposition potential reaches 5.3 and 4.6 V (vs. Li/Li⁺) at 25 and 50 °C, respectively. Li/LiFePO₄ cells using the PVdF-HFP/1g13TFSI-LiTFSI electrolytes show good discharge capacity and cycle stability, and no significant loss in discharge capacity of the battery is observed over 100 cycles. The cells deliver the capacity of 142 and 150 mAh g⁻¹ at the 100th cycling at 25 and 50 °C, respectively.     

Activation of an ionic liquid electrolyte for electric double layer capacitors by addition of BaTiO3 to carbon electrodes

To increase the capacitance of an electric double layer capacitor (EDLC) containing 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄) as an ionic liquid electrolyte, barium titanium oxide (BaTiO₃) as a ferroelectric material is incorporated into activated carbon electrodes. An EDLC composed of BaTiO₃ powder/activated carbon composite electrodes and the ionic liquid electrolyte EMIBF₄ exhibits a large capacitance when compared to the corresponding EDLC without BaTiO₃. Moreover, the rate performance of the EDLC containing BaTiO₃ is improved when the electrodes are prepared by an improved method, i.e., vacuum penetration of BaTiO₃-dispersed EMIBF₄ into the activated carbon electrodes. No positive addition effect of BaTiO₃ is observed for the corresponding EDLC with a typical organic electrolyte composed of a tetraalkylammonium salt and propylene carbonate as a solvent. These results suggest that BaTiO₃ incorporated into the electrodes would improve the dissociation of the ionic liquid EMIBF₄, and hence increase the carrier concentration available for the formation of an electric double layer.     

Electrochemical capacitance of NiO/Ru0.35V0.65O2 asymmetric electrochemical capacitor

A designed asymmetric hybrid electrochemical capacitor was presented where NiO and Ru_0.35V_0.65O₂ as the positive and negative electrode, respectively, both stored charge through reversible faradic pseudocapacitive reactions of the anions (OH⁻) with electroactive materials. And the two electrodes had been individually tested in 1 M KOH aqueous electrolyte to define the adequate balance of the active materials in the hybrid system as well as the working voltage of the capacitor based on them. The electrochemical tests demonstrated that the maximum specific capacitance and energy density of the asymmetric hybrid electrochemical capacitor were 102.6 F g⁻¹ and 41.2 Wh kg⁻¹, respectively, delivered at a current density of 7.5 A cm⁻². And the specific energy density decreased to 23.0 Wh kg⁻¹ when the specific power density increased up to 1416.7 W kg⁻¹. The hybrid electrochemical capacitor also exhibited a good electrochemical stability with 83.5% of the initial capacitance over consecutive 1500 cycle numbers.     

Effects of the cationic structures of fluorohydrogenate ionic liquid electrolytes on the electric double layer capacitance

Electric double layer capacitance of an activated carbon electrode has been measured for fluorohydrogenate ionic liquids (FHILs) based on five different cations (1,3-dimethylimidazolium (DMIm⁺), 1-ethyl-3-methylimidazolium (EMIm⁺), 1-butyl-3-methylimidazolium (BMIm⁺), 1-ethyl-1-methylpyrrolidinium (EMPyr⁺), and 1-methoxymethyl-1-methylpyrrolidinium (MOMMPyr⁺)) at 25 °C. For all the FHILs, the capacitance increases with increase in charging voltage, and exhibits the maximum value around 2.7 V. The capacitances for FHILs are higher than those for EMImBF₄ or 1 M tetraethylammonium tetrafluoroborate in propylene carbonate (TEABF₄/PC) in the measured range (1.0 < V < 3.2). For the three imidazolium-based FHILs, the maximum capacitance decreases with increase in the size of the cation in the order, DMIm(FH)_2.3F (178 F g⁻¹) > EMIm(FH)_2.3F (162 F g⁻¹) > BMIm(FH)_2.3F (135 F g⁻¹). On the other hand, the maximum capacitance observed for MOMMPyr(FH)_2.3F (152 F g⁻¹) is larger than that for EMPyr(FH)_2.3F (134 F g⁻¹) in spite of the larger size of MOMMPyr⁺ than EMPyr⁺, which is derived from introduction of the methoxy group. Some FHILs with low melting points exhibit a sufficient capacitance even at −40 °C (64 F g⁻¹ for EMIm(FH)_2.3F).