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Electrolyte mixture based on acetonitrile and ethyl acetate for a wide temperature range performance of the supercapacitors
Abstract
This work presents the development of the electrolyte, ensuring stable operation of the supercapacitors (SC) in a wide temperature range from −60 °C to +60 °C and suitable for commercialization. SC cells based on this electrolyte have high cyclic stability of capacitance at +60 °C and provide high discharge current after a long dwelling time in a charged state at −60 °C. We have investigated electrochemical characteristics of SC based on various electrolyte mixtures optimized with co-solvents such as ethyl acetate, propyl acetate, and butyl acetate in a temperature range from −60 to +60 °C. Methyl triethylammonium tetrafluoroborate (CH3)(C2H5)3NBF4 (TEMA∙TFB) in various concentrations (0.7–1.2 M) was used as a basic salt for measurements. According to electrochemical impedance spectroscopy data, the time constant values grows with an increase of ethyl acetate concentration. We found that cells based on electrolyte, containing 30 vol% ethyl acetate and 1.2 M TEMA TFB, provides optimal capacitive behavior of SCs with the high-performance features in a wide temperature range from −60 up to +60 °C. The capacitance of SC based on the developed electrolytes at −60 °C differs from the corresponding values at room temperature by no more than 10%.
... For example, vinylene carbonate in LIB electrolytes is usually used as an additive to facilitate the formation of a solid electrolyte interphase layer (SEI). Previously the strategy for the selection of such modifiers was discussed in detail [13]. The compositions of low-temperature electrolytes for supercapacitors operating in the temperature range from − 60 • C to +60 • C have been proposed and their characteristics and properties have been described. ...
... The assembly of SC laboratory cells was carried out using a commercial electrode tape GMCC-61255 (GMCC, China), consisting of an aluminium foil (20 μm) and an active carbon layer (120 μm) applied to it. Detailed information about SC cell preparation and properties of electrode materials are described in the previous work [13]. ...
... As it has been shown in Ref. [13], esters are more suitable modifiers for the expansion of the operating temperature range of SC. ...
... In addition to the aqueous electrolytes, non-aqueous and IL electrolytes have also been investigated for low temperature supercapacitors. 77,78 For examples, Galimzyanov et al. developed low temperature supercapacitors based on various electrolyte mixtures in which triethylammonium tetrafluoroborate (TEMA-TFB) dissolved in ACN were used to mix with co-solvents such as ethyl acetate, propyl acetate, and butyl acetate. 77 and good ionic conductivity of 0.56 mS cm À1 at an ultra-low temperature of À50 C. 78 ...
... 77,78 For examples, Galimzyanov et al. developed low temperature supercapacitors based on various electrolyte mixtures in which triethylammonium tetrafluoroborate (TEMA-TFB) dissolved in ACN were used to mix with co-solvents such as ethyl acetate, propyl acetate, and butyl acetate. 77 and good ionic conductivity of 0.56 mS cm À1 at an ultra-low temperature of À50 C. 78 ...
With the increasing exhaustion of the traditional fossil energy and ongoing enhanced awareness of environment protection, research works on electrochemical energy storage (EES) devices have been indispensable. Now, a significant amount of works (design and fabrication of electrode materials, electrolytes, separators, etc.) devoted to improving energy and power density, safety, and service life of EES devices are under way to meet the demand for various applications. However, besides the intrinsic factors, the service environments of EES devices, such as ultra-low or ultra-high temperatures, external magnetic field, external stress, severe radiation, and other factors (electric field, light, etc.) in practical applications , will greatly affect their performance, particularly when applied to aerospace, submarine, polar scientific research, and so on. Yet, research works on EES devices in the extreme environments are limited, and enormous efforts are highly needed to overcome the existing fundamental and technological barriers. Herein, we mainly focus on the EES devices under particular service environments. On the one hand, we present a comprehensive analysis into the inherent effects of external service environments on electrochemical behaviors of EES devices and underlying effect mechanisms. On the other hand, a summary of recent progress in EES devices under particular service environments, including systematic experiments and simulations, is provided along with the well-established strategies/methodologies toward enhanced electro-chemical properties under these external environments. Finally, current challenges and future perspectives are proposed. The review is of enormous significance for the development of advanced EES devices especially under particular service environments.
... Furthermore, Galimzyanov et al. mixed AN with carboxylates and found that the AN-EA mixture shows expanded operating temperature range more significantly than counterparts containing propyl acetate and butyl acetate solvents. [165] Compared with nitriles, dinitriles are superior for their enhanced safety in terms of higher boiling points and flash points. Hirata et al. compared the low-temperature behaviors of LiFSI in 2methylglutaronitrile (MGN) and succinonitrile (SN). ...
... The change in shape of the CV curves is closely related to the equivalent series resistance (ESR) of the supercapacitor. The more like-rectangular the shape of the CV curves, the smaller the ESR and vice versa [16]. As shown in the Fig. 6a, the CV curves of the SP500, SP600, SP700 samples have the characteristic like-rectangular shape of an ideal capacitor and good reversibility but the samples SP400 and SP800 have not. ...
H 3 PO 4-impregnated waste cotton was used as precursor to fabricate high porous activated carbon (AC) by the carbonization and activation processes with ultrahigh heating rate. The obtained activated carbon has unique physicochemical properties such as the structure mainly amorphous and ultrahigh specific surface area of 2769.7 m 2 /g for samples fabricated at carbonization temperature of 600 • C. The double-layer supercapacitors with activated carbon electrodes and electrolyte based on 1,1-dimethylpyrrolidinium tetraflu-oroborate solution in acetonitrile of 1 M concentration were fabricated. The specific capacitance of electrode material fabricated from AC obtained at carbonization temperature of 600 • C reached 110.8 F/g at the current density of 50 mA/g and 85.1 F/g at 1000 mA/g. At 1000 mA/g, the degradation was less than 25% after 5000 charge/discharge cycles. The carbonization temperature of 600 ◦C is considered as optimum for fabricating activated carbon and the obtained activated carbon can be used for supercapacitor electrode materials.
... Low-temperature electrolyte developments have shown considerable progress over the past two decades. Recent reviews continue to highlight promising strategies for IL eutectic systems, [27][28][29] salts dissolved in molecular solvents, [30][31][32] and design of IL/ molecular liquid mixtures for a variety of electrochemical applications down to −40 to −70°C. 33,34 For IL/ML electrolyte systems, popular solvent candidates such as carbonates, 35,36 carboxylates, 37 gamma-butyrolactone (GBL), 36 43 Dibutyl carbonate was selected to achieve a synergetic effect with ACN primarily due to its low melting point of −96°C and relatively low viscosity. ...
A designed low-temperature electrolyte of [BMIM][I]/BuCN/LiI extends the liquidus range down to −150 °C. The complex interactions between imidazolium/iodide ions and nitrile solvent molecule results in enhancement of thermal and transport properties.
... It is well known that electrolyte is an important component of SCs, which generally includes a combination of a salt and a solvent [8][9][10][11]. Desirable electrolytes are typically liquid with low viscosity, low density and high conductivity under a range of ambient temperature conditions [12][13][14][15]. They should also be commercially inexpensive, chemically and electrochemically stable and compatible with carbon. ...
Organic solvent-based electrolyte solutions are the main components of supercapacitors (SCs). Degradation and aging of the electrolyte affects the density energy of the SC. Therefore, in order to obtain more detailed knowledge about the degradation processes, and to identify the degradation products, relevant analytical techniques were used in this paper. The conformational changes of the tetraethylammonium ion (Et4N+) were determined by Raman spectroscopy after aging tests at 70 °C and for voltages of 2.7 and 2.8 V. The obtained results showed that the Et4N+ exhibits rotational isomerism between an all-trans conformation (tt.tt or Greek cross) and a trans-left conformation (tg.tg or Nordic cross). Moreover, the Raman spectra of both rotamers showed that (tt.tt) is more stable than (tg.tg), as well as the effect of aging temperature can be explained by the decrease of (tt.tt). In addition, a qualitative and quantitative analysis by 1H NMR spectroscopy reveal that more than 30% of the molar percentage corresponds to products obtained (triethylamine and methanol) by the degradation of the Et4N+ according to a proposed nucleophilic substitution reaction. Finally, a thermogravimetric analysis coupled with ATG-IR spectroscopy performed on the electrolyte of a damaged SC allowed to identify other chemical species (NH3, C2H2) as breakdown products.
In this study, the real‐time increase in pressure of the accumulated gases at the electrode/electrolyte interface serves as a safety criterion for four conductive electrolytes comprising acetonitrile (ACN) and organic salts. They include tetrafluoroborate as an anion and cyclic 1,1‐dimethylpyrrolidinium (Pyr11+), spiro‐(1,1′)‐bipyrrolidinium (SBP+), acyclic methyl triethyl ammonium (Et3MeN+) or standard tetraethylammonium (Et4N+) as cations. The main focus lies on the SPBF4/ACN system. While the concentrated Pyr11BF4/ACN exhibits a minimal pressure evolution (≈25 Pa) under ambient conditions at 3.0 V, its electrochemical stability is inferior to SPBF4 at high operating voltage. The electrolytes with acyclic tetrafluoroborate salts (1.0 mol L−1) reveal a 20‐fold increase in pressure due to the weak salt‐ACN interactions and the subsequent high solvent evaporation. The pressure evolution at the interface of activated carbon/electrolyte in electrochemical double layer capacitor (EDLCs) is merely related to the operating voltage and cation nature, viz. Pyr11+ < SBP+ < Et4N+ < Et3MeN+. The fixed specific capacity of 109 F g−1, volumetric capacity of 76 F cm−3, and moderate gas generation (≈190 Pa at 3.0 V, that shifts to ≈400 Pa at 3.4 V) confirm the safe character of the SPBF4/ACN electrolyte for such energy storage devices. Gas evolution characterization at the electrolytes/electrode interface for activated‐based supercapacitors serves as a safety criterion. This study shows that the pressure evolution in EDLC is related to the cation nature and that the moderate gas generation observed with SPBF4/ACN electrolyte confirms its safe character.
Excellent long-term cycling stability and safety are the basic qualities of electrode materials in electrochemical energy storage equipment. Nevertheless, the inferior cycling stability and device safety caused by impurity ions constrain their practical application. Thus, it is essential to purify biomass-based carbon materials when they are used as electrodes in the field of energy storage. Herein, four purification methods are studied, including water-washing, acid-pickling, ion exchange and ultrasonic wave, respectively. Particularly, trace elements are detected by inductive coupled plasma emission spectrometer and ion chromatography to explore the impurity removal effects. The results suggest that ion exchange possesses the best impurity-cleaning effect among the four methods. In addition, the supercapacitor employing the biochar electrode material with minimum impurities exhibits excellent ultra-long cycling stability. In symmetric supercapacitors, the capacitance retentions are 86.7% after 120000 cycles using 6 M KOH electrolyte and 70% after 30000 cycles using 1 M MeEt3NBF4/PC electrolyte at 4 A g⁻¹. Furthermore, we reveal the influence mechanism of metal ion impurities on cycling stability and impedance. In summary, the elaborate study provides a feasible method to solve the problem of impurity removal and designs suitable biomass carbon materials for commercial application in the energy storage field.
To achieve desirable performance of lithium-ion batteries at low temperatures, using co-solvents with low melting points, especially carboxylic esters, is an efficient strategy and thus, there is a general question: does the co-solvent with a lower melting point correspond to the composite electrolytes with better performance? Herein, we chose two constitutional isomers with 22 °C of difference in melting point, ethyl propionate and propyl acetate, as the co-solvent in the formula of two low-temperature electrolytes for comparison. At low temperatures, the Li||LiNi0.8Co0.1Mn0.1O2 coin cells and graphite||LiNi0.8Co0.1Mn0.1O2 pouch cells using the two electrolytes show close capacity retention, suggesting that the co-solvent with a lower melting point does not necessarily result in better low-temperature performance. Molecular dynamics simulations indicate that the similar solvation structure of the contact ion pair for the two electrolytes is mainly responsible for their similar low-temperature performance. Furthermore, the Li||LiNi0.8Co0.1Mn0.1O2 and graphite||LiNi0.8Co0.1Mn0.1O2 cells with the two electrolytes at ambient temperature show capacity retention of 84.1༅, 84.3༅ after 200 cycles and 88.2%, 89.6% after 360 cycles, respectively, demonstrating the reliability of the electrolytes. The results of this study provide general guidance for the development of low-temperature electrolyte regarding the choice of co-solvents.
In the quest for new electrolytes on electrochemical energy storage devices, we present, characterize and test a simple organic electrolyte based on cesium bis (trifluoromethanesulfonyl) imide (CsTFSI) for supercapacitor (SC) application. The physicochemical properties of CsTFSI-based electrolytes in acetonitrile (ACN) and propylene carbonate (PC), alone and mixed with LiTFSI as a co-salt salt, were characterized under various concentrations and temperatures and revealed a different solution organization system, depending on the solvent and nature of the salt. The solvation radii based on the Jones-Dole-Kaminsky model indicated that ACN forms a single sphere of solvation around the cation Cs+, unlike PC, which does not solvate the large and polarizable Cs cation. The CsTFSI -ACN (1 mol L⁻¹) electrolyte presents a suitable viscosity (<1 mPa•s) and density (36 mS cm⁻¹) for SC application, ionic mobility within an extended temperature range (-40 to 60°C), and its molecular level is compatible with the porosity of activated carbon (AC) electrodes. The nanopores of the ACs display good electronic conductivity and ion transport capability, characteristic of electrochemical double-layer capacitors EDLC within an electrochemical window of 3 V and a wide temperature span, i.e., -20 to 40°C. In-situ measurements of accumulated gas pressure during typical galvanostatic charge-discharge protocols exposed a minimum pressure at 40°C (2.6 × 10⁸ Pa for CsTFSI-ACN, 1 mol L⁻¹) compared to standard electrolytes (Et4BF4-ACN, 1 mol L⁻¹) and yielded specific capacitances (Csp) of 115 F g⁻¹ at a normalized current density of 2 A g⁻¹. Standard accelerated ageing floating tests confirmed the stability of the SCs (>100 h → 12% decrease on Csp). Finally, the suitability of CsTFSI as a co-salt (with lithium bis(trifluoromethanesulfonyl)imide, LiTFSI) was demonstrated for SC devices, aiming to boost Csp (i.e.,164 F g⁻¹ for equimolar amounts of LiTFSI and CsTFSI). The above results attest to the functionality of CsTFSI as a promising salt for energy storage applications.
Carbon fibers have high surface areas and rich functionalities for interacting with ions, molecules, and particles. However, the control over their porosity remains challenging. Conventional syntheses rely on blending poly-acrylonitrile with sacrificial additives, which macrophase-separate and result in poorly controlled pores after pyrolysis. Here, we use block copolymer microphase separation, a fundamentally disparate approach to synthesizing porous carbon fibers (PCFs) with well-controlled mesopores (~10 nm) and micropores (~0.5 nm). Without infiltrating any carbon precursors or dopants, poly(acrylonitrile-block-methyl methacrylate) is directly converted to nitrogen and oxygen dual-doped PCFs. Owing to the interconnected network and the highly optimal bimodal pores, PCFs exhibit substantially reduced ion transport resistance and an ultrahigh capacitance of 66 mF cm −2 (6.6 times that of activated carbon). The approach of using block copolymer precursors revolutionizes the synthesis of PCFs. The advanced electrochemical properties signify that PCFs represent a new platform material for electrochemical energy storage.
Fast-charging lithium-ion cells require electrolyte solutions that balance high ionic conductivity and chemical stability. The introduction of an organic ester co-solvent is one route that can improve the rate capability of a cell. Several new co-solvent candidates were identified based on viscosity, permittivity (dielectric constant), and DFT-calculated electrochemical stability windows. Several formate, nitrile, ketone, and amide co-solvents are shown to increase the ionic conductivity of lithium hexafluorophosphate in conventional organic-carbonate-based solutions. Based on gas production during the first formation cycle in Li[Ni 1-x-y Co x Al y ]O 2 /graphite-SiO pouch cells, five candidates were identified: methyl formate (MF), ethyl formate (EF), propionitrile (PN), isobutyronitrile (iBN), and dimethyl formamide (DMF). High temperature storage (60 • C), long-term cycling, and ultrahigh-precision coulometry results indicate that MF offers the greatest balance between conductivity increase and cell lifetime. Future work is encouraged to develop more stable solution chemistries that incorporate MF. PN may prove useful for low temperature (< 40 • C) applications.
Electrolytes have been identified as some of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) is discussed. The challenges in producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim of improving ESs' energy density without sacrificing existing advantages (e.g., a high power density and a long cycle-life) (507 references).
Supercapacitors are considered to be promising candidates for power devices in future generations. These devices are expected to find many future applications in hybrid electric vehicles and other power devices and systems. For supercapacitors to realize their promise, it is important that their energy and power densities be maximized. An important way to address this is to develop advanced electrode materials and methods to fabricate these materials. The recent years have seen enormous interest in the research of numerous materials and methods for their synthesis for applications in supercapacitor electrode technology. In the constantly changing technological landscape, it is relevant to review the various aspects of supercapacitor devices. This review paper gives an overview of the types of supercapacitors. It describes the advanced materials and fabrication methods for these devices, including recent developments in these areas, and their implications on the future of supercapacitor technology. The paper also addresses the principal technological challenges facing the development efforts in the future.
This paper presents the results obtained on the electrochemical behavior of electrochemical capacitors assembled in nonaqueous electrolyte. The first part is devoted to the electrochemical characterization of carbon-carbon 4 cm(2) cells systems in terms of capacitance, resistance, and cyclability. The second part is focused on the electrochemical impedance spectroscopy study of the cells. Nyquist plots are presented and the impedance of the supercapacitors is discussed in terms of complex capacitance and complex power. This allows the determination of a relaxation time constant of the systems, and the real and the imaginary part of the complex power vs. the frequency plots give information on the supercapacitor cells frequency behavior. The complex impedance plots for both a supercapacitor and a tantalum dielectric capacitor cells are compared. (C) 2003 The Electrochemical Society.
The self-discharge of an electrochemical capacitor, also referred to as a supercapacitor, is an important factor in determining the duration of maintaining stored energy, especially in low-duty-cycle applications. The study of self-discharge is conducted as follows: first, the self-discharge is characterized by measuring the decline of open-circuit voltage of the electrochemical capacitor. Second, the mechanisms of self-discharge, leakage current, and diffusion of ions at the electrode-electrolyte interfaces are modeled by an electrical equivalent circuit. The equivalent circuit elements are experimentally determined according to the self-discharge time behavior. In addition, the dependence of the self-discharge parameters on both temperature and initial voltage across the electrochemical capacitor is described in detail.
Rapid storage and efficient delivery of electrical energy in heavy-duty applications are being enabled by electrochemical capacitors.
We use molecular dynamics (MD) simulations to investigate the effects of solvent concentration on the bulk properties of an ion liquid electrolyte and the electrochemical performance on carbon- based electrodes, including pristine graphene, oxidized graphene, graphene armchair edge, graphene zigzag edge, onion-like carbon, and slit-pore carbon. We find that diluting the electrolyte reduces the number of ion pairs in the bulk, and improves ion dynamics. The capacitance of the two edge electrodes decreases monotonically as the solvent concentration increases, while the capacitance of other non-edge electrodes exhibits non-monotonic behavior and a capacitance maximum is observed. Further analyses on the electric double layer reveals two competing factors: solvation reduces the charge overscreening effect, but it also causes the dilution of absorbed ion concentration. While the former increases the capacitance in the high ion concentration regime, the later decreases the capacitance in the low ion concentration regime. In addition, the dilution also significantly improves the ion dynamics at the interface. Our simulation results demonstrate that diluting an ionic liquid electrolyte could potentially boost the power density, while maintaining or even slightly increasing the energy density with a careful selection of solvent concentrations on a non-edge carbon electrode.
Improving the low-temperature tolerance and increasing the energy density of supercapacitors are currently desirable for real-life applications. Commercially available non-aqueous supercapacitors are typically limited in operation to −40 °C or higher, and they usually exhibit poor performance at lower than room temperature. In this work, we investigate symmetric supercapacitors using the 1-butyl-3-methylimidazolium tetrafluoroborate (EMIMBF 4 ) ionic liquid modified with organic solvents as the electrolyte and activated polyaniline-derived carbon (APDC) as the active material. The symmetric coin cells tests are performed in the range of temperatures from −100 °C to 25 °C. Interestingly, with decreasing temperature, the working voltage window of these supercapacitors gradually increases from 2.5 V to 4.0 V, whereas the energy density initially increases and then decreases. The optimal combination of an APDC electrode and the EMIMBF 4 /acetonitrile/methyl acetate-based electrolyte enables such a supercapacitor to work with a high potential window of 3.5 V and deliver a maximum energy density of 80 Wh kg ⁻¹ (based on the total mass of the two-electrode material), a maximum power density of 26 kW kg ⁻¹ , and a long-term cycle life of 10000 cycles at a low temperature of −50 °C.
Electrochemical impedance spectroscopy (EIS) consists of plotting Nyquist plots representing the imaginary versus the real parts of the complex impedance of individual electrodes or electrochemical cells. To date, interpretations of Nyquist plots have been based on physical intuition and/or on the use of equivalent RC circuits. However, physical interpretations of Nyquist plots and equivalent RC circuits are not unique and have often been inconsistent in the literature. This study aims to provide unequivocal physical interpretations of electrochemical impedance spectroscopy (EIS) results for electric double layer capacitor (EDLC) electrodes and devices. To do so, a physicochemical transport model was used for numerically reproducing Nyquist plots accounting for (i) electric double layer (EDL) formation at the electrode/electrolyte interface, (ii) charge transport in the electrode, and (iii) ion electrodiffusion in binary and symmetric electrolytes. The Nyquist plots of EDLC electrodes were numerically generated for different electrode conductivity and thickness, electrolyte domain thickness, as well as ion diameter, diffusion coefficient, and concentrations. The electrode resistance, electrolyte resistance, and the equilibrium differential capacitance were identified from Nyquist plots without relying on equivalent RC circuits. The internal resistance retrieved from the numerically generated Nyquist plots was comparable to that retrieved from the "IR drop" in numerically simulated galvanostatic cycling. Furthermore, EIS simulations were performed for EDLC devices and similar interpretations of Nyquist plots were obtained. Finally, these results and interpretations were confirmed experimentally using EDLC devices consisting of two identical activated-carbon electrodes in both aqueous and non-aqueous electrolytes.
We use molecular dynamics simulations in a constant potential ensemble to study the effects of solution composition on the electrochemical response of a double layer capacitor. We find that the capacitance first increases with ion concentration following its expected ideal solution behavior but decreases upon approaching a pure ionic liquid. The non-monotonic behavior of the capacitance as a function of ion concentration results from the competition between the independent motion of solvated ions in the dilute regime and solvation fluctuations in the concentrated regime. When charge fluctuations induced by correlated ion-solvent fluctuations are large relative to those induced by the pure ionic liquid, such non-monotonic behavior is expected to be generic.
A novel electrolyte of electric double-layer capacitors (EDLCs) for ultra-low temperature applications based on spiro-(1,1')-bipyrolidinium tetrafluoroborate (SBP-BF4) dissolved in acetonitrile (AN) and AN + dibutyl carbonate (DBC) mixtures has been developed. The physic-chemical properties and electrochemical characterizations at low temperature have been demonstrated on pure AN and AN/DBC mixed electrolytes. From 20 to −50 °C, AN electrolyte shows a slight superiority of the capacitance, but the novel DBC-mixed electrolytes exhibit much higher ion conductivity and better electrochemical performance at −60 °C, while AN electrolyte does not cycle at all due to the crystallization of AN and precipitation of SBP-BF4. The specific discharge capacitance of DBC mixtures maintain 92 F g−1 even at a high current density of 5 A g−1. At −60 °C, from electrochemical impedance spectroscopy measurement, the whole resistance for AN electrolyte rapidly increased about 60 times higher than that of 20 °C, while those for DBC mixtures remain relatively low within the whole temperature range. The cycle performance test indicates the DBC mixtures exhibit extremely stable capacitance, especially at ultra-low temperature. The capacitance for DBC-20% electrolyte is 95 F g−1 even after 10000 cycles, which maintained 83% of the capacity of 20 °C.
This article aims to offer a critical overview of selected literature on capacitive and non-capacitive faradaic charge storage. It is particularly relevant to the concept of pseudocapacitance that is generally described as a result of fast surface faradaic processes. In general, faradaic processes represent electron transfer reactions at the interface between an electrode and its contacting solid or liquid electrolyte phase that is able to accept or donate electrons. Obviously, not all faradaic processes can be associated with pseudocapacitance. The question is how to differentiate pseudocapacitance related faradaic charge storage from the others. Therefore, attempts have been made to apply the band model for semiconductors to account qualitatively for the origin of pseudocapacitance. Capacitive and non-capacitive faradaic processes are then proposed to define and differentiate different charge storage mechanisms in supercapacitor and battery. On the other hand, the unequal electrode capacitance approach and the use of Ca2+ in aqueous electrolytes are discussed in relation with enhanced energy capacity of supercapacitors. In addition, the principle of supercapattery as a hybrid device is explained with recent literature examples.
The ability to quickly store and deliver a significant amount of electrical energy at ultralow temperatures is critical for the energy-efficient operation of high altitude aircraft and spacecraft, exploration of natural resources in polar regions and extreme altitudes, and astronomical observatories exposed to ultralow temperatures. Commercial high-power electrochemical capacitors fail to operate at temperatures below –40 °C. According to conventional wisdom, mesoporous electrochemical capacitor electrodes with pores large enough to accommodate fully solvated ions are needed for sufficiently rapid ion transport at lower temperatures. It is demonstrated that strictly microporous carbon electrodes with much higher volumetric capacitance can be efficiently used at temperatures as low as –70 °C. The critical parameters, with respect to electrolyte properties and electrode porosity and microstructure, needed for achieving both rapid ion transport and efficient ion electroadsorption in porous carbons are discussed. As an example, the fabrication of an electrochemical capacitor with an outstanding performance at temperatures as low as –60 and –70 °C is demonstrated. At such low temperatures the capacitance of the synthesized electrodes is up to 123 F g−1 (≈76 F cm−3), which is 50–100% higher than that of the most common commercial electrochemical capacitor electrode at room temperature. At –60 °C selected cells based on ≈0.2 mm electrodes exhibited characteristic charge–discharge time constants of less than 9 s, which is faster than the majority of commercial devices at room temperature. The achieved combination of high energy and power densities at such ultralow temperatures is unprecedented and extremely promising for the advancement of energy storage systems.
To investigate the degradation mechanisms of electric double-layer capacitor (EDLC) components using 1.0 M triethylmethylammonium (TEMA) tetrafluoroborate (BF(4)) in propylene carbonate (PC), the failure-mode processes of positive and negative electrodes were characterized as a function of the applied voltage (2.5-4.0 V). When the cell voltage ranges below 3.0 V, no impedance spectra or surface morphology changes were observed, indicating that no side reactions occur in this case. In the voltage range from 3.0 to 3.7 V, the exfoliation of graphene layers in activated carbon (AC) and the formation of cracks were observed in the positive electrode over 4.9 V vs Li/Li(+) possibly due to the gasification of surface functional groups with adsorbed water. On the negative electrode, the adsorbed water is electrochemically reduced to H(2) gas and OH(-). The generated OH-induces the Hoffman elimination of TEMA(+) and activates the hydrolysis of PC. These water-induced side reactions could be the most critical factors for higher voltage operation. In the higher voltage range (over 3.7 V), the accumulation of solid electrolyte interface films by electrochemical oxidation and the reduction of PC were observed for both electrodes, indicating that the electrochemical oxidation and the reduction of PC on the AC surfaces occur above 5.2 V and below 1.5 V vs Li/Li(+), respectively.
Double-layer capacitor electrolytes employing 1,3-dioxolane as a cosolvent with acetonitrile have been evaluated in coin cells using electrochemical impedance spectroscopy and dc charging and discharging tests. Addition of the lower-melting-point 1,3-dioxolane to the standard acetonitrile solvent was found to extend the low-temperature operational range of test cells beyond that of commercially available cells. By adjusting the concentration of the tetraethylammonium tetrafluoroborate salt used, the equivalent series resistance can be minimized to enable optimal power delivery at a given temperature. (C) 2008 The Electrochemical Society.
The electrolytic conductivities and limiting reduction and oxidation potentials for various organic liquid electrolytes based on quaternary onium salts have been measured to find better electrolytes for electrical double-layer capacitors. An electrolyte composed of tetraethylammonium cation, tetrafluoroborate anion, and propylene carbonate solvent showed well-balanced performance of high electrolytic conductivity, a wide stable potential window and resistance to hydrolysis. Among quaternary onium salts, triethylmethylammonium, ethylmethylpyrrolidinium, and tetramethylenepyrrolidinium tetrafluoroborate salts exhibited higher electrolytic conductivity than the conventional tetraethylammonium salt due to their much greater solubility.
The electrochemical behavior of 1,3-dioxolane (DN)—LiClO4 solutions with lithium and noble metal (eg gold, platinum) electrodes was investigated using surface sensitive FT-ir, scanning electron microscopy (SEM), X-ray microanalysis and electrochemical techniques.It was found that the salt and the solvent react with lithium or noble metal electrodes at low potentials to form surface films which control the electrochemical behavior of these systems. Their structure and mechanism of formation, as well as correlation between surface chemistry, surface morphology, and cycling efficiency of lithium electrodes in these solutions are discussed in detail.
The electrochemical behaviour of 2-methyltetrahydrofuran (2Me-THF) solutions with lithium and noble metal electrodes was investigated. Surface-sensitive FTIR, X-ray microanalysis and scanning electron microscopy were applied in conjunction with electrochemical techniques in order to characterize the electrode surfaces in solutions and to correlate the cycling efficiency and surface morphology of lithium electrodes with their surface chemistry in solutions. The influence of a variety of contaminants, including oxygen, water, CO2, nitrogen, furans and paraffin oil, on the Li cycling efficiency, morphology and surface chemistry was also studied. It was found that the surface films formed on lithium in 2Me-THF contained several types of alkoxides. The presence of trace O2, water, CO2; and furans modifies the structure of these surface films and influences both the morphology and the Li cycling efficiency.The best efficiency of Li electrodes in charge-discharge cycling was obtained with uncontaminated LiAsF6 + 2Me-THF solutions.
This work describes the design and testing of organic electrolyte systems that extend the low temperature operational limit of double-layer capacitors (also known as supercapacitors) beyond that of typical commercially available components. Electrolytes were based on a tetraethylammonium tetrafluoroborate/acetonitrile system, modified with low melting co-solvents (such as formates, esters and cyclic ethers) to enable charging and discharging of test cells to as low as −75°C. Cell capacitance exhibited little dependence on the electrolyte salt concentration or the nature of the co-solvent used, however, both variables strongly influenced the cell equivalent series resistance (ESR). Minimizing the increase in ESR posed the greatest design challenge, which limited realistic operation of these test cells to −55°C (still improved relative to the typical rated limit of −40°C for commercially available non-aqueous cells).
Double layer capacitors made with activated carbon electrodes and filled with nonaqueous electrolytes at different salt concentrations
have been studied. It was found that the performance of the capacitor is strongly dependent on the salt concentration in the
electrolyte. For electrolytes with high salt concentrations, the maximum energy stored in the capacitor is limited by the
capacitance of the electrode material. For electrolytes with low salt concentrations, the maximum operating voltage as well
as the maximum energy decreases with decreasing salt concentration. It has been demonstrated from ac impedance measurements
that the decay of the maximum energy is due to the depletion of free ions in the electrolyte. The maximum energy storage in
double layer capacitors with electrolytes at different salt concentrations was measured and was found to agree with the theory
developed previously. From the study of dc charge and discharge cycles at different constant current rates, it was found that
the power performance of the capacitor is also strongly dependent on the salt concentration in the electrolyte.
The aging behavior of electrochemical double layer capacitors (EDLCs) based on activated carbon electrodes bound with poly(tetrafluoroethylene) (PTFE) was tested in electrolyte solutions based on acetonitrile (AN) and propylene carbonate (PC) at a constant elevated cell voltage of 3.5V. The aging was quantified in terms of capacitance loss and resistance increase for the full cell and the individual electrodes. It is shown that the enhanced aging rate of symmetric EDLCs in either solvent at elevated voltages is dominated by the aging of a single electrode, and that the polarity of this limiting electrode depends directly on the solvent. In AN, the positive electrode ages much more rapidly than the negative, while in PC the negative electrode exhibits faster aging than the positive. After aging, the electrodes were investigated by nitrogen adsorption and X-ray photoelectron spectroscopy, revealing significant modifications of the electrode surface and providing clear evidence for the deposition of electrolyte degradation products on the electrodes.
For hybrid electric vehicle traction applications, energy storage devices with high power density and energy efficiency are required. A primary attribute of supercapacitors is that they retain their high power density and energy efficiency even at −30 °C, the lowest temperature at which unassisted starting must be provided to customers. More abuse-tolerant electrolytes are preferred to the high-conductivity acetonitrile-based systems commonly employed. Propylene carbonate based electrolytes are a promising alternative. In this work, we compare the electrochemical performance of two high-power density electrical double layer supercapacitors employing acetonitrile and propylene carbonate as solvents. From this study, we are able to elucidate phenomena that control the resistance of supercapacitor at lower temperatures, and quantify the difference in performance associated with the two electrolytes.
Electrical double layer capacitors based on ideally polarizable nanoporous carbon electrodes in acetonitrile, γ-butyrolactone and propylene carbonate as the solvents for the 1.0 M (C2H5)3CH3NBF4 electrolyte have been tested by cyclic voltammetry and electrochemical impedance methods. The limits of ideal polarizability, low-frequency limiting capacitance and series resistance, time constant, complex power components and other characteristics have been discussed. The applicability limits of the Srinivasan and Weidner model have been tested and the very strong influence of the solvent nature (viscosity and molar conductivity of the solution) on the characteristics of the nanoporous carbon electrode cells has been established.
The electrical characteristics of the electrical double layer capacitors based on the nanoporous carbon|1 M triethylmethylammonium tetrafluoroborate in ethylene carbonate mixed with dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl formate, methyl acetate and ethyl acetate in the 1:1 volume ratio have been studied using the cyclic voltammetry and the electrochemical impedance spectroscopy methods. The specific series and parallel capacitances, phase angle, time constant, series and parallel resistances dependent on the binary solvent system used have been established. The region of ideal polarisability of nanoporous carbon electrodes ΔE ⩾ 3.0 V for 1 M (C2H5)3CH3NBF4 in various binary non-aqueous solvent systems has been achieved. Specific conductivity values for 1 M TEMA solution in various organic carbonate-based electrolytes have been obtained at −40 °C < T < 50 °C and compared with the electrochemical data.
The electrochemical characteristics of the electrical double layer capacitor (EDLC) single cell based on the nanoporous carbon electrode in 1 M (C2H5)3CH3NBF4 (TEMA) solution in various non-aqueous organic carbonate and organic ester binary, ternary and quaternary solvent systems (ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl formate (MF), methyl acetate (MA) and ethyl acetate (EA)) mixed in the x:y, x:y:z and x:y:z:f volume ratios, respectively) have been studied using the cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) methods. The specific capacitance, phase angle, series and parallel resistance values dependent on the solvent system used have been calculated. The region of ideal polarisability of nanoporous carbon electrodes ΔE ⩾ 3.0 V for 1 M TEMA in various binary, ternary and quaternary non-aqueous solvent systems has been achieved. Specific conductivity values for 1 M TEMA solution in various organic carbonate – organic ester based electrolytes have been obtained at −40 °C < T < 50 °C and compared with electrochemistry data.
A mathematical model was developed which simulates the self-discharge capacity losses in the carbon anode for a SONY 18650 lithium-ion battery. The model determines the capacity loss during storage on the basis of a continuous reduction of organic solvent and de-intercalation of lithium at the carbon/electrolyte interface. The state of charge, open circuit potential, capacity loss and film resistance on the carbon electrode were calculated as a function of storage time using different values of rate constant governing the solvent reduction reaction.
In order to understand what causes supercapacitors ageing in an organic electrolyte (tetraethylammonium tetrafluoroborate-Et4NBF4 -1 mol L-1 in acetonitrile), the activated carbon electrodes were characterized before and after prolonged floating (4000-7000 h) at an imposed voltage of 2.5 V After ageing, the positive and negative electrodes were extensively washed with pure acetonitrile in neutral atmosphere to eliminate the physisorbed species. Then, the carbon materials were dried and transferred without any contact with air to be studied by XPS, F-19 NMR, B-11 NMR and Na-23 NMR. Decomposition products have been found in the electrodes after ageing. The amount of products depends on the kind of activated carbon and electrode polarity, which suggests redox reactions of the electrolyte with the active surface functionality. Nitrogen adsorption measurements at 77 K on the used electrodes showed a decrease of accessible porosity, due to trapping of the decomposition products in the pores. Hence, the evolution of the supercapacitor performance with time of operation, i.e. the capacity decrease and the resistance increase, are due to the decomposition of the organic electrolyte on the active surface of the carbon substrate, forming products which block a part of porosity. The concentration of surface groups and their nature were found to have an important influence on the performance fading of supercapacitors. (c) 2007 Elsevier B.V. All rights reserved.
Electrochemical capacitors (EC) also called ‘supercapacitors’ or ‘ultracapacitors’ store the energy in the electric field of the electrochemical double-layer. Use of high surface-area electrodes result in extremely large capacitance. Single cell voltage of ECs is typically limited to 1–3 V depending on the electrolyte used. Small electrochemical capacitors for low-voltage electronic applications have been commercially available for many years. Different applications demanding large ECs with high voltage and improved energy and power density are under discussion. Fundamental principles, performance, characteristics, present and future applications of electrochemical capacitors are presented in this communication.
Degradation processes, leading to the generation of gas in a deep polarization of supercapacitors with organic electrolytes
- Kalashnik
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Degradation processes, leading to the generation of gas in a deep polarization of supercapacitors with organic electrolytes
- A T Kalashnik
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- M Kundu
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A.T. Kalashnik, R.R. Galimzyanov, S.V. Stakhanova, O.V. Zaitseva, I.S. Krechetov,
A.A. Klimont, M. Kundu, M.V. Astakhov, Degradation processes, leading to the
generation of gas in a deep polarization of supercapacitors with organic
electrolytes, Rev. Adv. Mater. Sci. 50 (2017) 62-68.