Electrolyte For Electric Double-layer Capacitors Research Articles

A structural study on a specific Li-ion ordered complex in dimethyl carbonate-based dual-cation electrolytes.

Dimethyl carbonate (DMC) is a linear carbonate solvent commonly used as an electrolyte for electric double-layer capacitors (EDLCs) and Li-ion batteries. However, there are serious problems with the use of DMC as an electrolyte solvent: (1) low ionic conductivity when using Li salts (e.g. LiBF4) and (2) liquid-liquid phase separation when using spiro-type quaternary ammonium salts (e.g. SBPBF4). Dual-cation electrolytes, i.e., bi-salt (SBPBF4 and LiBF4) in DMC, are promising candidates to avoid the phase separation issue and to enhance the total and Li+ conductivities. Herein, we reported a specific Li-ion structure in DMC-based dual-cation electrolytes by combining high-energy X-ray total scattering (HEXTS) and all-atom molecular dynamics (MD) simulations. Quantitative radial distribution function analysis based on experimental and simulation results revealed that the phase-separated SBPBF4/DMC (i.e., the bottom phase of 1 M SBPBF4/DMC) forms long-range ion ordering based on the structured SBP+-BF4- ion pairs. When adding LiBF4 salt into SBPBF4/DMC (i.e., dual-cation electrolyte), the ordered SBP+-BF4- structure disappeared owing to the formation of Li-ion solvation complexes. We found that in the dual-cation electrolyte Li ions form multiple Li+-Li+ ordered complexes in spite of relatively low Li-salt concentration (1 M), being a promising Li+-conducting medium with reduced Li salt usage and low viscosity.

Understanding the effects of electrode meso-macropore structure and solvent polarity on electric double layer capacitors based on a continuum model

The structures of electrode meso-macropore and the solvent polarity are the crucial factors dominating the performance of the electric double layer capacitors (EDLCs), but their impacts are usually tangled and difficult to decouple and quantitate. Here the effects of electrode meso-macropore structure and solvent polarity on the specific capacitance of an EDLC are quantitatively investigated using a steady-state continuum model. The simulation results indicate the specific capacitances are significantly affected by the meso-macropore surface structure. The specific capacitances significantly decrease for both convex surface structures but obviously increase for both concave surface structures, with the increase of curvature radius from 1 to 20 nm. As for solvents, the polar solvent with high saturated dielectric permittivity improves the capacitance performance. Moreover, the electrode meso-macropore structure is of more concern compared with solvent polarity when aiming at enhancing the specific capacitance. These results provide fundamentals for the rational design of porous electrodes and polar electrolytes for EDLCs. © 2020 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd.. All rights reserved.

Sol-gel synthesis pathway and electrochemical performance of ionogels: A deeper look into the importance of alkoxysilane precursor

There is a great interest in the application of ionogels as electrolyte for energy storage devices due to their enhanced safety compared to conventional electrolytes. This work aims to shed light on an often-overlooked aspect of ionogel synthesis which is the impact of the precursors on the sol-gel reaction kinetics and the electrochemical properties of the resultant ionogels. Using time-resolved Raman spectroscopy the non-hydrolytic sol-gel synthesis of ionogels from four different alkoxysilane precursors were monitored. Their electrochemical performance as electrolyte for electric double-layer capacitors were characterised using cyclic voltammetry and electrochemical impedance spectroscopy. Our findings suggest that presence of ionic liquid does not impact the sol-gel process kinetics of all four formulations in the same manner. In addition, ionogel formulations with the longest gelation time demonstrated poor electrochemical properties caused by the further ingress of silica into the activated carbon electrodes. This was confirmed with EDX analysis.

Efficient Parametrization of Force Field for the Quantitative Prediction of the Physical Properties of Ionic Liquid Electrolytes.

The prediction of transport properties of room-temperature ionic liquids from nonpolarizable force field-based simulations has long been a challenge. The uniform charge scaling method has been widely used to improve the agreement with the experiment by incorporating the polarizability and charge transfer effects in an effective manner. While this method improves the performance of the force fields, this prescription is ad hoc in character; further, a quantitative prediction is still not guaranteed. In such cases, the nonbonded interaction parameters too need to be refined, which requires significant effort. In this work, we propose a three-step semiautomated refinement procedure based on (1) atomic site charges obtained from quantum calculations of the bulk condensed phase; (2) quenched Monte Carlo optimizer to shortlist suitable force field candidates, which are then tested using pilot simulations; and (3) manual refinement to further improve the accuracy of the force field. The strategy is designed in a sequential manner with each step improving the accuracy over the previous step, allowing the users to invest the effort commensurate with the desired accuracy of the refined force field. The refinement procedure is applied on N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), a front-runner as an electrolyte for electric double-layer capacitors and single-molecule-based devices. The transferability of the refined force field is tested on N,N-dimethyl-N-ethyl-N-methoxyethoxyethylammonium bis(trifluoromethanesulfonyl)imide (N112,2O2O1-TFSI). The refined force field is found to be better at predicting both structural and transport properties compared to the uniform charge scaling procedure, which showed a discrepancy in the X-ray structure factor. The refined force field showed quantitative agreement with structural (density and X-ray structure factor) and transport properties-diffusion coefficients, ionic conductivity, and shear viscosity over a wide temperature range, building a case for the wide adoption of the procedure.

Switching Response of Hydrogen-Terminated-Diamond-Based All-Solid-State Electric-Double-Layer Transistor

INTRODUCTION High-density electronic carrier control using an electric double layer (EDL) at electrolyte/electrode interfaces has been attracting attention for the application in various electronic devices. Specifically, the use of solid electrolyte is advantageous for practical use owing to its high compatibility with other peripheral devices.Recently, we have achieved an in situ modulation of hole density in hydrogen-terminated diamond (H-terminated diamond) using an all-solid-state electric double layer transistor (EDLT) [1]. The hole density increased from 1010 cm-2 to 1013 cm-2, a 3-order-of-magnitude difference, applying a negative gate voltage (V G). This indicates that the two-dimensional hole gas was electrostatically modulated by EDL charging at the solid electrolyte/H-terminated diamond interface. Then, we focus on a switching response speed, which is an important parameter for EDLTs. Towards further development of all-solid-state EDLTs for practical use, high switching response is desired. Because diamond is suitable for high-frequency electronic device owing to its high carrier mobility, diamond-based all-solid-state EDLT is a candidate to exceed the response speed of conventional all-solid-state EDLTs EXPERIMENTAL An H-terminated diamond thin film was homoepitaxially grown on the surface of an Ib-type HPHT diamond single crystal (100) using microwave plasma chemical vapor deposition. The channel area was fabricated by photolithography, and O2 plasma ashing. Source and drain electrodes were made of Pt/Pd. Gate electrode was made of LiCoO2 (LCO) and Pt films. We chose a Li+ conducting Li2O-ZrO2-SiO2 (LZSO) amorphous thin film as a solid-state-electrolyte. To compare response speed, we also fabricated EDLT with the same structure except for the gate electrode. The gate electrode of another EDLT was made of Au. Electrochemical measurements were performed using a Keithley 4200-SCS parameter analyzer and an atmosphere/temperature tunable prober system (Nagase). RESULTS AND DISCUSSION Figure 1 shows the switching characteristics of EDLTs measured at the drain voltage of 0.5 V. The measurements were performed in steps of 2 K. When a pulse (V G) is applied, drain current (i D) instantaneously decreases from the 10 microampere order to less than 10 nanoampere order. Thus, EDLTs are switched from an on-state to off-state. As explained in the introduction, this is attributed to the effect of EDL charging at the LZSO solid electrolyte/H-terminated diamond interface. In both EDLTs, the response time is shorter than 50 ms at 298 K. Because the speed is not affected even when the type of the gate electrode was changed, the effect of EDL charge and discharge at the LZSO/LCO interface can be ignored. Therefore, this response speed mainly resulted from EDL charge and discharge processes at the LZSO/H-terminated diamond interface. The response speed becomes faster with an increase in the temperature. This is attributed to an increase in the Li+ conductivity of LZSO. The conductivity of LZSO exhibited a thermally activated behavior with an activation energy of 0.66 eV; the conductivity of LZSO increased from 8×10-9 S/cm at 298 K to 2×10-7 S/cm at 340 K. Although the conductivity of the H-terminated diamond channel changed with an increase in the temperature, the variation was about 25%, which is sufficiently small compared to the variation in that of LZSO. Therefore, the rate of the response speed in EDLTs is determined by the conductivity of LZSO. At 340 K (Li+ conductivity: 2×10-7 S/cm), EDLTs show the response time of less than 800 μs. Thus, H-terminated diamond-based all-solid-state EDLT has a potential for ultrafast switching even at room temperature if an electrolyte with better conductivity is used. Since RbAg4I5 Ag+ superionic conductor (0.12 S/cm at RT [2]) was previously reported as an electrolyte for EDL capacitors [3], RbAg4I5 is a good candidate for ultrafast EDLTs, of which the response speed is expected to exceed 1 GHz.In the presentation, we will show the switching characteristics for H-terminated diamond-based all-solid-state EDLTs using various solid electrolytes. Acknowledgements This work was in part supported by JSPS KAKENHI Grant Number JP20H05816 (Grant-in-Aid for Scientific Research on Innovative Areas “Interface Ionics”), JP19K05279 and JP19J22244 (Grant-in-Aid for JSPS Fellows). REFERENCES [1] T. Tsuchiya, M. Takayanagi, M. Imura, Y. koide, T. Higuchi, and K. Terabe, Abstr. 22nd Intl. Conf. on Solid State Ionics, 2019, P-TUE-108.[2] J.N. Bradley and P.D. Greene, Trans. Faraday Soc., 63 (1967) 424.[3] J. E. Oxley, Proc. Power Sources Symp., 24, 20 (1970)Fig. 1. Switching characteristics of H-terminated diamond-based all-solid-state EDLTs in the temperature range of 298-340 K. The gate electrodes are (a) Pt/LCO and (b) Au. Figure 1

Ionic liquid electrolytes in electric double layer capacitors

Electric double layer capacitors (EDLCs), which store free charges on the electrode surface via non-Faradaic process, balanced by the electric double layer on the electrolyte side, exhibit excellent cycle stability and high power density. Though EDLCs are considered as promising energy storage devices, the charges stored on the electrode surface in EDLCs are much lower than those in batteries. Ionic liquids (ILs), as a new type of electrolytes in EDLCs, are capable to deliver high energy density, due to their excellent physicochemical properties and wide electrochemical window. In this review, we focus on the widely studied IL electrolytes for EDLCs, including pure ILs, IL/IL binary electrolytes, IL/organic solvent mixtures, as well as functionalized ILs, with attention on the relationship between the structures of different IL-based electrolytes and the energy storage properties in EDLCs. For imidazolium- and ammonium-based IL electrolytes which are most widely studied in EDLCs, the former generally have higher gravimetric specific capacitance, while the latter exhibit wider electrochemical window. The modifications of functional group substituted can be an effective strategy to enhance the gravimetric specific capacitance of the latter and thus improve the energy density of EDLCs.

Ionic liquid-solvent mixture of propylene carbonate and 1,2-dimethoxyethane as electrolyte for electric double-layer capacitor

Ionic liquid EMIBF4 (EMI) was combined with propylene carbonate (PC) and/or 1,2-dimethoxyethane (DME) for electric double-layer capacitors (EDLCs). The effects of these support solvents on physicochemical properties of the electrolyte and capacitance performances of the capacitor were investigated. Compared to pure EMI, the existence of support solvent of PC or PC-DME provides better power characteristic mainly profited from improved ion conductivity, but sacrifices the stable potential window to some extent. The capacitor employing activated carbon and the electrolyte EMI-PC-DME yields an active mass normalized energy density of 38.5 Wh kg−1 at ~ 900 W kg−1 for the cut-off voltage of 3 V. It is a significant performance improvement compared to EMI and EMI-PC system at identical operation condition. This research guides an attractive direction to construct advanced 3 V based EDLC with excellent overall properties.

A new DMF-derived ionic liquid with ultra-high conductivity for high-capacitance electrolyte in electric double-layer capacitor

In this study, a new DMF-derived ionic liquid electrolyte [EDMF]BF4 was prepared, characterized and applied as a high-capacitance electrolyte for electric double-layer capacitors (EDLCs). [EDMF]BF4 was measured to have very high ionic conductivity and low viscosity (24.8 mS cm−1 and 21.6 mPa s at 25 °C), as well as a comparable electrochemical window (4.4 V, on a GC electrode) with [EMIm]BF4, which is one of the most widely studied ionic liquid electrolytes with high performance. Therefore, [EDMF]BF4 was used as a non-volatile electrolyte for activated carbon-based EDLCs. The [EDMF]BF4-based EDLCs are capable to operate stably over a voltage of 2.7 V, and the specific capacitance of activated carbon was determined to be 165 F g−1 (100th cycle at 1 mA cm−2 and 30 °C), which is much larger than in the case of [EMIm]BF4 (126 F g−1). Besides, [EDMF]BF4 showed lower ohmic resistance and double layer resistance than [EMIm]BF4 in EDLCs. The high performance of [EDMF]BF4 electrolyte should be related to the quasi-linear [EDMF] cation of moderate size, which is presumably more suitable for enhanced ion mobility and compact electric double layer.

TetraPEG Network Formation via a Michael Addition Reaction in an Ionic Liquid: Application to Polymer Gel Electrolyte for Electric Double-layer Capacitors

Polymer network formation via Michael addition of two tetra-arm poly(ethylene glycol) (TetraPEG) prepolymers possessing maleimide (MA) and thiol (SH) terminals was investigated in a typical aprotic...

Fast-cure ionogel electrolytes with improved ion transport kinetics at room temperature

Fast-cure 1-ethyl-3-methylimidazolium trifluoromethanesulfonate-based ionogels have been realised for the first time. The influence of curing temperature on the structure of ionogels and their performance as the electrolyte for electric double-layer capacitors (EDLCs) has been investigated. Hybrid ionogels were synthesised via a non-hydrolytic sol-gel route and were fully gelled post heat-treating at 125, 150, 175 and 200 °C for 60 min with minimal shrinkage. Charge-transfer resistance (a rate-limiting parameter in cell kinetics during charge/discharge cycles) was reduced by ∼80% by increasing the heat-treatment temperature; this was partially attributed to the interlocking effect facilitated by high curing temperature. We report a maximum areal capacitance of 95 mF cm−2. Due to ∼40% increase in the penetrability coefficient of the ionic liquid, the electrode 'full' wetting time dropped from 48 to 5 h when the curing temperature was increased above 150 °C. These results were supported by SEM and Raman spectroscopy to characterise the effect of high temperature heat-treatment on the electrode-ionogel interface and the degree of electrode wetting by the ionic liquid. The fast-cure fabrication process for ionogels removes one of the major hurdles in their industrial application while the improved room temperature ion transport kinetics expands the potential application of ionic liquid-based electrochemical systems.

Low-Temperature Performance of Vinylene Carbonate Additive Containing Electrolyte for Electric Double-Layer Capacitors

The weakest link in modern electric double-layer capacitors (EDLCs) is the solvents’ low thermodynamic stability. In practical applications, acetonitrile enables voltages up to 2.85 V, but is also extremely toxic. On the other hand, safer solvents like propylene carbonate (PC) are much more viscous, which hinders low-temperature performance. Therefore, much work is devoted on studying solvent mixtures [1,2] which are inspired form the fields of electrosynthesis and battery research [3,4], since the applied voltages extend from 4.2 V in the case of lithium-ion batteries and up to 25 V in electrosynthesis [5]. Herein, we have investigated the electrochemical properties of ternary solvent mixture featuring PC, ethyl acetate (EA) in 1:1 volume ratio with a small volume percentage (0.5 – 5)% of vinylene carbonate (VC). 1M NaPF6 was chosen as the salt due to sodium’s high abundance [6], better performance in EDLCs compared to widely used NaClO4 [1] and to provide potential electrolytes for the emerging chemistry of sodium-ion batteries [6]. EA (η = 0.423 mPa s) was chosen to lower the viscosity of rather viscous PC (η = 2.53 mPa s) in the mixture. VC (ε = 126) was added to enhance electrolyte conductivity. In the first part of the study, we measured electrolyte conductivities for various solvent systems (using 1M NaPF6): PC, PC + 5% VC, PC:EA (1:1), PC:EA (1:1) + 0.25% VC, PC:EA (1:1) + 0.5% VC, PC:EA (1:1) + 1% VC, PC:EA (1:1) + 2% VC, PC:EA (1:1) + 5% VC. In the second part, we carried out room-temperature electrochemical measurements of the test-cells with beforementioned solvent systems using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and constant current charge-discharge (CCCD) methods. 1M NaPF6in PC:EA (1:1) + 0.5% VC electrolyte showed best overall performance and was therefore chosen as the best candidate for wide-temperature (−40 to 60 °C) electrochemical studies. Again CV, EIS and CCCD were used. Figure 1 shows cyclic voltammograms measured at −30 °C. At low scan rates the system exhibits almost ideal capacitive behaviour, however at 50 mV s-1 the shape of the CVs starts to deviate due to increased viscosity at such low temperatures. Still, its high-rate performance exceeds that of purely PC-based EDLCs. Results of rigorous equivalent circuit modelling will be shown. Acknowledgements The present study was supported by the Estonian Centre of Excellence in Science project 3.2.0101.11-0030, Estonian Energy Technology Program project 3.2.0501.10-0015, Material Technology Program project 3.2.1101.12-0019, Project of European Structure Funds 3.2.0601.11-0001, Estonian target research project IUT20–13, NAMUR project 3.2.0304.12-0397 and projects 3.2.0302.10-0169 and 3.2.0302.10-0165.

Preparation of spiro-type quaternary ammonium salt via economical and efficient synthetic route as electrolyte for electric double-layer capacitor

A spiro-type quaternary ammonium salt, spiro-(1,1′)-bipyrrolidinium tetrafluoroborate (SBP-BF4) was successfully prepared by an economical and efficient three-step process comprising the cyclization reaction of 1,4-dibromobutane and pyrrolidine, and subsequent ion exchange pathway with KOH followed by neutralization reaction via HBF4 in the system of ethanol solution. 1H NMR, 13C NMR, FI-IR and XPS analyses showed the structure of SBP-BF4. The as-obtained SBP-BF4 was dissolved in AN and used as the electrolyte for supercapacitor. Electrochemical measurements demonstrate that, compared with commercial electrolyte TEMA-BF4/AN, SBP-BF4/AN exhibits high ionic conductivity, lower resistance and improved cycling performance, which is due to its smaller ion size and stable symmetry structure.

Outstanding features of alginate-based gel electrolyte with ionic liquid for electric double layer capacitors

An alginate-based gel electrolyte with an ionic liquid (Alg/IL) is investigated for electric double-layer capacitors (EDLCs) by using physicochemical and electrochemical measurements. The Alg/EMImBF4 (EMImBF4 = 1-ethyl-3-methylimidazolium tetrafluoroborate) gel electrolyte is thermally stable up to 280 °C, where EMImBF4 decomposes. Furthermore, the EDLC with the gel electrolyte can be operated even at high temperature. The cell containing Alg/EMImBF4 is also electrochemically stable even under high voltage (∼3.5 V) operation. Thus, the alginate is a suitable host polymer for the gel electrolyte for EDLCs. According to the result of charge–discharge characteristics, the voltage drop in the charge−discharge curve for the cell with Alg/EMImBF4 gel electrolyte is considerably smaller than that with liquid-phase EMImBF4 electrolyte. To clarify the effect of Alg in contact with the activated carbon electrode, we also prepared an Alg-containing ACFC electrode (Alg + ACFC), and evaluated its EDLC characteristics in liquid EMImBF4. The results prove that the presence of Alg close to the active materials significantly reduces the internal resistance of the EDLC cell, which may be attributed to the high affinity of Alg to activated carbon.

Physical Properties of a New Deep Eutectic Solvent Based on a Sulfonium Ionic Liquid as a Suitable Electrolyte for Electric Double-Layer Capacitors

We present in this study the physical properties of two deep eutectic solvent (DES) mixtures based on solid sulfonium bis[(trifluoromethyl)sulfonyl]imide (S111TFSI) aprotic ionic liquid and two different H-bond donors, formamide (FMD) and trifluoroamide (TFA), according to temperature. First, we investigated their thermal properties by differential scanning calorimetry , and the results revealed the formation of a deep eutectic solvent giving a wide liquid range from −40 to 270 °C for these mixtures which froze at a much lower temperature than either of the individual components. The densities, ionic conductivities, and viscosities of these DESs were measured according to temperature and then discussed by applying Arrhenius or Vogel–Tamman–Fulcher (VTF) equations, as well as the Walden classification. Thanks to their favorable transport properties, both S111TFSI/TFA and S111TFSI/FMD mixtures contribute to the formulation of the electrolytes with 1 mol·L–1 LiTFSI. The performances of these electrolytes were then estimated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge for activated carbon electrochemical double layer capacitor applications at 80 °C. The results showed that the selected H-bond donors allowed ion dissociation without solvation, increasing micropore accessibility and giving high capacitance values up to 350 F·g–1 in the case of formamide-based DESs. These unusual performances of the activated carbon material are debated with regards to the activation energy barrier to access the microporosity by ions in sulfonium-amides DESs.

Analysis of Ion Transport in Solution Electrolytes for Electrochemical Capacitors

In recent years, electric double layer capacitors (EDLCs) have attracted much attention not only for small electric devices as smart phone and tablet PC, but also for large power sources in electric vehicles. The advantage of EDLCs is a high-rate capability and good cycleability, which are due to the fast charge/discharge mechanism by using electric double layer regardless of Faradaic reaction with electrodes. Also, lithium ion capacitors (LICs) with Faradaic reaction electrodes such as graphite anode on the one side have been intensively developed to solve the small energy density of EDLCs. The performances of capacitors (EDLCs and LICs) strongly depend on the electrolyte properties, i.e. the ionic conductivity and electrochemical stability. In this study, we separately measured self-diffusion coefficients D of ions and solvent in the solution electrolytes for EDLCs and LICs by a pulse gradient spin-echo (PGSE) NMR. The viscosity and ionic conductivity were also examined to discuss the relationship with D values. Moreover, the interactions between ions and ion-solvent were evaluated from the DFT calculations.1 M solutions of (C2H5)4NBF4(TEABF4) and LiBF4 in propylene carbonate (PC) were obtained from Kishida Chemical Co, Ltd. TEAPF6 (99%, Aldrich), TEATfO (98%, Wako Pure Chemicals) and (C2H5)3CH3NBF4 (TEMABF4) (98%, Tokyo Chemical Industry Co., Ltd.) were used as supporting salts, and dissolved in PC (Wako Pure Chemicals) as a solvent in an Ar-filled dry box to prepare their 1 M solutions. The solution electrolytes were subjected to the PGSE-NMR for measuring the D values of cations (Li+(7Li), TEA+(1H), TEMA+(1H)), anions (BF4 -(9F), PF6 -(9F)) and solvent (PC(1H)) [1]. The viscosity and ionic conductivity of electrolytes were evaluated by a viscometer (anton paar) and an AC impedance analyzer (Bio-Logic), respectively. The interaction energies between cation-anion and ion-solvent were estimated by DFT calculations with Gaussian09.Fig. 1 plots the D values of ions at 30oC against the ion radius, where the D value of Li+ was also plotted at the point of solvated radius by PC [2]. For the EDLC electrolytes, the smaller size of anions (BF4 -, PF6 -, TfO-) showed relatively higher D values than the larger cations (TEA+, TEMA+). In addition, as for the kind of cation and anion, smaller size of TEMA+ and BF4 - were also exhibited a higher D values than the other larger ions. These facts indicate that there is a clear dependence of ion radius for the EDLC electrolytes. As a result, it was found that the TEMABF4 was the best supporting salt among them. On the other hand, for the LIC electrolytes smaller size of Li+ exhibited lower D value than the larger BF4 -. Moreover, considering the solvated radius of Li+ by PC suggested the reasonable results on the transport behaviors as well as the EDLC electrolytes. This implies that Li+ in the LIC electrolytes was strongly solvated by PC, and suppressed the smooth transport compared with BF4 -. In contrast, the PC solvation of TEA+ in the EDLC electrolytes is suggested to be weaker than that of Li+ in the LIC ones. Fig. 2 shows the concentration dependence of D values of ions in the representative electrolytes for EDLC (TEABF4/PC) and LIC (TEABF4/PC) at 30oC. For all the concentration, the EDLC electrolyte exhibited higher D values than the LIC one, and the trend observed in Fig. 1 was not changed by the concentration. For both electrolytes, the D values became higher with a decrease in the concentration. This is due to a decrease in the viscosity of electrolytes. In addition, for the LIC electrolyte, promotion of the dissociation of LiBF4 salt was also suggested to enhance the D values. To elucidate the detail, ion radius against PC (r ion/r PC) in the solution electrolytes at 30oC were estimated by using the Stokes-Einstein equation (Fig. 3) [1]. The r ion/r PC values of Li+ and BF4 - for the LIC electrolyte were higher than those of ions in the EDLC electrolytes. This implies that Li+ in the LIC electrolyte strongly interacts with PC and BF4 - to form large solvated ions and ion pairs, respectively. Especially in comparison of the r ion/r PC value of BF4 -, it was found that the LiBF4 was more difficult to dissociate in PC than the TEABF4 and the dissociation of LiBF4 was promoted by the decrease in the concentration. Further analysis of Dvalues and comparison with the other parameters of viscosity, ionic conductivity, etc. will be discussed in the meeting.This study was supported by JST “A Tenure-track Program” from MEXT, Japan.[1] M. Saito et al., s in ACEPS-7, 2P-32 (2013). [2] M. Ue, J. Electrochem. Soc., 141, 3336 (1994).

Physical properties of a new Deep Eutectic Solvent based on lithium bis[(trifluoromethyl)sulfonyl]imide and N-methylacetamide as superionic suitable electrolyte for lithium ion batteries and electric double layer capacitors

Herein we present a study on the physical/chemical properties of a new Deep Eutectic Solvent (DES) based on N-methylacetamide (MAc) and lithium bis[(trifluoromethyl)sulfonyl]imide (LiTFSI). Due to its interesting properties, such as wide liquid-phase range from −60°C to 280°C, low vapor pressure, and high ionic conductivity up to 28.4mScm−1 at 150°C and at xLiTFSI=1/4, this solution can be practically used as electrolyte for electrochemical storage systems such as electric double-layer capacitors (EDLCs) and/or lithium ion batteries (LiBs). Firstly, relationships between its transport properties (conductivity and viscosity) as a function of composition and temperature were discussed through Arrhenius’ Law and Vogel–Tamman–Fulcher (VTF) equations, as well as by using the Walden classification. From this investigation, it appears that this complex electrolyte possesses a number of excellent transport properties, like a superionic character for example. Based on which, we then evaluated its electrochemical performances as electrolyte for EDLCs and LiBs applications by using activated carbon (AC) and lithium iron phosphate (LiFePO4) electrodes, respectively. These results demonstrate that this electrolyte has a good compatibility with both electrodes (AC and LiFePO4) in each testing cell driven also by excellent electrochemical properties in specific capacitance, rate and cycling performances, indicating that the LiTFSI/MAc DES can be a promising electrolyte for EDLCs and LiBs applications especially for those requiring high safety and stability.

Anti-freezing aqueous electrolytes for electric double-layer capacitors

Different types of alcohol have been proposed as the anti-freezing additives for aqueous electrolytes in double-layer capacitors (EDLCs). The influence of ion solvation on the capacitive behavior of porous carbon electrode in the anti-freezing aqueous electrolytes has been studied. The low-temperature performance of EDLCs composed of either micro- or meso-porous carbon in these electrolytes has been fully investigated. Deduced from cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge measurements, it has been discovered that the anti-freezing electrolyte of methanol/water-concentrated NaClO4 possesses low equivalent series resistance at −40°C. EDLCs using the electrolytes of methanol/water-concentrated NaClO4 solutions deliver improved energy density and power density at the temperature as low as −40°C.

Concentrated NaClO 4 aqueous solutions as promising electrolytes for electric double-layer capacitors

Concentrated NaClO 4 aqueous solutions have been proposed as the electrolytes in electric double-layer capacitors. The advantages of this kind of electrolytes have been addressed in the terms of enlarged specific capacitance, enhanced rate capability and elevated low-temperature performance of porous carbon electrodes. Thermal analysis, ionic conductivity measurement and Raman spectroscopic investigations have been performed on the NaClO 4 aqueous solutions in conjunction with the electrochemical study of porous carbon electrodes in this kind of electrolytes. The correlation between the hydration number of ions in the solutions and capacitive behavior of porous carbon has been clarified. The rate performance improvement in porous carbon electrode has also been connected to the increase in ionic conductivity of the electrolytes. The enhanced capacitance retention of porous carbon electrode at low temperatures in concentrated solutions has been ascribed to a fall of freezing point.

Allyl-functionalized ionic liquids as electrolytes for electric double-layer capacitors

Double-layer capacitor electrolytes employing allyl-functionalized ionic liquids as electrolytes with solvents have been evaluated. Imidazolium cations with allyl groups enabled the high capacitances and low resistances of electric double-layer capacitor (EDLC) cells at a wide range of temperature in spite of the large cation sizes and low ionic conductivities of the electrolytes compared to imidazolium with saturated alkyl groups, 1-ethyl-3-methylimidazolium (EMIm). The improvement of EDLC performance was noted particularly in the case of diallylimidazolium (DAIm) cation and TFSA anion. The substitution of the vinyl group increased the high capacitance only at 298 K and decreased the capacitance at low temperature and direct current resistance (DC-IR) at 243 and 298 K. The butenyl group deteriorated the capacitance and DC-IR at 243 and 298 K. The stability of EDLC cell of DAIm–BF 4/PC was inferior to that of EMIm–BF 4/PC. The addition of DMC to PC improved the stability.

Quantitative Evaluation of Electrochemical Potential Windows of Electrolytes for Electric Double-Layer Capacitors Using Ab Initio Calculations

The electrochemical potential windows of seven organic liquid electrolytes for electric double-layer capacitors calculated using ab initio molecular orbital theory are reported. Four types of models were used to investigate the effect of intermolecular interactions: (i) a single-ion in vacuo model, (ii) a single-ion-in-solvent model, (iii) an ion-pair in vacuo model, and (iv) an ion-pair-in-solvent model. For all the calculations, the Hartree–Fock level of theory using the basis set was used. Solute–ion interactions were treated by considering several cation–anion pair confirmations, and solute–solvent interactions were introduced by applying the isodensity polarizable continuum model. The ion-pair-in-solvent model quantitatively reproduced the experimental electrochemical potential windows with high accuracy. This demonstrates that in actual electrolytes intermolecular interactions, particularly cation–anion and solute–solvent, play an important role in determining electrochemical potential windows.