Deep eutectic solvent-based supramolecular gel polymer electrolytes for high-performance electrochemical double layer capacitors

Gel polymer electrolytes (GPEs) can avoid the electrolyte leakage risk of electrochemical double layer capacitors (EDLCs). But aqueous GPEs often suffer from narrow electrochemical windows. Herein, a series of deep eutectic solvent (DES)-based supramolecular GPEs are firstly developed for carbon-based EDLCs with wide voltage windows. The as-fabricated DES-based GPE shows an ionic conductivity of ~58 mS cm^?1, which makes the stable voltage window of a carbon-based EDLC reach 2.4 V. The carbon-based EDLC exhibits a specific capacitance of 32.1 F g^?1, an energy density of 24.6 Wh kg^?1 and a capacitance retention of ~90% after 15,000 charge-discharge cycles. Moreover, when quinhydrone is added into the DES-based GPE, the specific capacitance and energy density of the corresponding EDLC can be further expanded to 60 F g^?1 and 43.6 Wh kg^?1, respectively. Therefore, our work may present a universal strategy to prepare novel supramolecular GPEs for high-performance EDLCs with wide voltage windows.     

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.     

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.     

Electrochemical characteristics of new electric double layer capacitor with acidic polymer hydrogel electrolyte

The acidic polymer hydrogel electrolyte was prepared from 1 M H₂SO₄ aqueous solution, poly(vinyl alcohol) (PVA) and glutaraldehyde (GA). A new electric double layer capacitor (EDLC) with the polymer hydrogel electrolyte was assembled, and its electrochemical characteristics were investigated. As a result, the EDLC cell with the polymer hydrogel electrolyte exhibited almost the same discharge capacitance and high-rate dischargeability as that with a 1 M H₂SO₄ aqueous solution as an electrolyte. It was also found that the self-discharge was remarkably suppressed by using the polymer hydrogel electrolyte.     

Mesoporous activated carbon fiber as electrode material for high-performance electrochemical double layer capacitors with ionic liquid electrolyte

Activated carbon fibers (ACFs) with super high surface area and well-developed small mesopores have been prepared by pyrolyzing polyacrylonitrile fibers and NaOH activation. Their capacitive performances at room and elevated temperatures are evaluated in electrochemical double layer capacitors (EDLCs) using ionic liquid (IL) electrolyte composed of lithium bis(trifluoromethane sulfone)imide (LiN(SO₂CF₃)₂) and 2-oxazolidinone (C₃H₅NO₂). The surface area of the ACF is as high as 3291 m² g⁻¹. The pore volume of the carbon reaches 2.162 cm³ g⁻¹, of which 66.7% is the contribution of the small mesopores of 2-5 nm. The unique microstructures enable the ACFs to have good compatibility with the IL electrolyte. The specific capacitance reaches 187 F g⁻¹ at room temperature with good cycling and self-discharge performances. As the temperature increases to 60 °C, the capacitance increases to 196 F g⁻¹, and the rate capability is dramatically improved. Therefore, the ACF can be a promising electrode material for high-performance EDLCs.     

Preparation of mesoporous carbons from amphiphilic carbonaceous material for high-performance electric double-layer capacitors

Amphiphilic carbonaceous material (ACM), with nanoscale dispersion in alkaline aqueous solutions, is synthesized from green needle coke. As a special precursor with small particle size, plenty of functional groups and widened d₀₀₂ simultaneously, ACM guarantees subsequent ACM-based activated carbons (AACs) with high specific surface area over 3000 m² g⁻¹ as well as well-developed mesoporous structure after KOH activation. Such pore properties enable AACs’ high performances as electrode materials for electric double-layer capacitors (EDLCs). In particular, surface area up to 3347 m² g⁻¹ together with notable mesopore proportion (26.9%) gives sample AAC814 outstanding EDLC behaviors during a series of electrochemical tests including galvanostatic charge/discharge, CV and electrochemical impedance spectroscopy. The electrode gets satisfactory gravimetric and volumetric specific capacitance at the current density of 50 mA g⁻¹, up to 348 F g⁻¹ and 162 F cm⁻³, respectively. Furthermore, for the mesoporosity, there is only a slight capacitance reduction for AAC814 as the current density reaches 1000 mA g⁻¹, indicating its good rate performance. It is all the ACM's unique characteristics that make AACs a sort of competitive EDLC electrode materials, both in terms of specific capacitance and rate capability.     

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.     

Cresyl diphenyl phosphate as flame retardant additive for lithium-ion batteries

To improve the safety of lithium-ion batteries, cresyl diphenyl phosphate (CDP) was used as a flame retardant additive in a LiPF₆ electrolyte solution. The flammability of the electrolytes containing CDP and the electrochemical performances of the cells, LiCoO₂/Li, graphite/Li and the battery LiCoO₂/graphite with these electrolytes, were studied by measuring the self-extinguishing time of the electrolytes, the variation of surface temperature of the battery and the charge/discharge curve of the cells or battery. It is found that the addition of CDP to the electrolyte provides a significant suppression in the flammability of the electrolyte and an improvement in the thermal stability of battery. On the other hand, the electrochemical performances of the cells become slightly worse due to the application of CDP in the electrolyte. This alleviated trade-off between the flammability and thermal stability and cell performances provides a possibility to formulate a nonflammable electrolyte by using CDP.     

Improved thermal stability of lithium ion battery by using cresyl diphenyl phosphate as an electrolyte additive

To enhance the safety of lithium ion battery, cresyl diphenyl phosphate (CDP) is explored as an additive in 1.0 M LiPF₆/ethylene carbonate (EC) + diethyl carbonate (DEC) (1:1 wt.). The electrochemical performances of LiCoO₂/CDP-electrolyte/C cells are tested. At the thermal aspect, the thermal stability of the electrolyte with CDP is detected firstly by using a C80 micro-calorimeter, and then the charged LiCoO₂/CDP-electrolyte/C cells are disassembled and wrapped to detect the thermal behaviors. The results indicate that CDP-containing electrolyte enhances the thermal stabilities of electrolyte and lithium ion battery, and the electrochemical performances of LiCoO₂/CDP-electrolyte/C cell become slightly worse by using CDP in the electrolyte. Furthermore, the cell with 10% (wt.) CDP-containing electrolyte shows better cycle efficiency than that of other CDP-containing electrolyte, such as containing 5% (wt.) CDP and 15% (wt.) CDP. This maybe because that the mass ratio between CDP and electrolyte is close to the reaction stoichiometric ratio in the 10% (wt.) CDP-containing electrolyte, where stable solid electrolyte interphase (SEI) is formed. Therefore, 10% CDP-containing electrolyte improves the safety of lithium ion battery and keeps its electrochemical performance.     

Fabrication of high-power electric double-layer capacitors

The electrochemical behavior of activated carbon/carbon (AC/C) composite electrodes was investigated for high-power electric doublelayer capacitors (EDLCs). It was found that high-rate charge/discharge characteristics are affected by the resistance of the electrolyte phase in the pores of the electrode. The charge/discharge characteristics were improved by optimizing the pore-size distribution of the electrodes. The size and total volume of the macro-pores in the electrodes were controlled by mixing and burning out polymer spheres. A high-power EDLC (15V, 470 F), which can discharge as much as 500 A, was fabricated by using improved AC/C composite electrodes.     

The synthesis and performance analysis of various biomass-based carbon materials for electric double-layer capacitors: A review

Biomass is one of the most promising clean energy sources. The porous carbon materials prepared by biomass as electrode materials of electric double-layer capacitors (EDLCs) are easily available at a low price, which would greatly reduce the cost of the production. However, carbon materials made with biomass generally have many disadvantages such as low specific surface area (SSA), poor pore size structure, and difficulty to control the pore diameter, which results in the poor EDLC performance. In this paper, the prime purpose is to expose the recent progress of biomass carbon in the fields of electrode materials of EDLC. The review provides a comprehensive literature review that is focused on EDLC electrodes derived from biochar of the evidence of 181 publications published over a period of 30 years from 1989 to 2019. Various carbon materials derived from different biomass for electrode of EDLC are discussed. The most promising methods for the preparation of several biomass carbons are described in detail. Some factors such as SSA, pore size structure, surface functional groups, and electrolyte are further analyzed to discuss the effects on the electrochemical performance of the EDLC. Notably, current deficiencies and possible solutions of preparation methods of biomass carbon as electrode materials are outlined. And the future research trends in this field are prospected.     

Cresyl diphenyl phosphate effect on the thermal stabilities and electrochemical performances of electrodes in lithium ion battery

To improve the safety of lithium ion battery, cresyl diphenyl phosphate (CDP) is used as a flame-retardant additive in a LiPF₆ based electrolyte. The electrochemical performances of LiCoO₂/CDP-electrolyte/Li and Li/CDP-electrolyte/C half cells are evaluated. The thermal behaviors of Li_0.5CoO₂ and Li_0.5CoO₂-CDP-electrolyte, and Li_xC₆ and Li_xC₆-CDP-electrolyte are examined using a C80 micro-calorimeter. For the LiCoO₂/CDP-electrolyte/Li cells, the onset temperature of single Li_0.5CoO₂ is put off and the heat generation is decreased greatly except the one corresponding to 5% CDP-containing electrolyte. When Li_0.5CoO₂ coexists with CDP-electrolyte, the thermal stability is enhanced. CDP improves the thermal stability of lithiated graphite anode effectively and the addition of 5% CDP inhibits the decomposition of solid electrolyte interphase (SEI) films significantly. The electrochemical tests on LiCoO₂/CDP-electrolyte/Li and Li/CDP-electrolyte/C cells show that when less than 15% CDP is added to the electrolyte, the electrochemical performances are not worsen too much. Therefore, the addition of 5-15% CDP to the electrolyte almost does not worsen the electrochemical performance of LiCoO₂ cathode and graphite anode, and improves theirs thermal stability significantly; thus, it is a possible choice for electrolyte additive.     

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.     

Hybrid capacitor with activated carbon electrode,Ni(OH)2 electrode and polymer hydrogel electrolyte

A new hybrid capacitor (HC) cell was assembled using an activated carbon (AC) negative electrode, an Ni(OH)₂ positive electrode and a polymer hydrogel electrolyte prepared from crosslinked potassium poly(acrylate) (PAAK) and KOH aqueous solution. The HC cell was characterized compared with an electric double layer capacitor (EDLC) using two AC electrodes and the polymer hydrogel electrolyte. It was found that the HC cell successfully worked in the larger voltage range and exhibited ca. 2.4 times higher capacitance than the EDLC cell. High-rate dischargeability of the HC cell was also superior to that of the EDLC cell. These improved characteristics strongly suggest that the HC cell can be a promising system of capacitors with high energy and power densities.     

Possible use of non-flammable phosphonate ethers as pure electrolyte solvent for lithium batteries

Dimethyl methyl phosphonate (DMMP) was selected and tested as a non-flammable solvent for primary and secondary lithium batteries, because of its non-flammability, good solvency of lithium salts and appropriate liquidus properties. Experimental results demonstrated that DMMP can solvate considerable amount of commonly used lithium salts to form non-flammable and Li⁺-conducting electrolyte, which has very wide electrochemical window (>5 V vs. Li) and excellent electrochemical compatibility with metallic lithium anode and oxide cathodes. Primary Li–MnO₂ cells using DMMP-based electrolyte showed almost the same discharge performances as those using organic carbonate electrolytes, and also, Li–LiMn₂O₄ cells using DMMP electrolyte exhibited greatly improved cycleability and dischargeability, suggesting a feasible application of this new electrolyte for constructing high performance and non-flammable lithium batteries.     

Mathematical functions for optimisation of conducting polymer/activated carbon asymmetric supercapacitors

Equations routinely used to describe the properties of conventional symmetric electrochemical double-layer capacitors (EDLCs) are expanded to develop straightforward mathematical functions that can effectively describe the performance characteristics of asymmetric supercapacitors based on electrically conducting polymer and activated carbon (ECP–AC) electrodes. Formulae are developed to describe cell parameters (based on total active material mass) such as maximum specific capacitance (F g⁻¹), maximum specific energy (Wh kg⁻¹), and optimum electrode mass ratios that can be used for maximising the specific energy of asymmetric cells. The electrode mass ratios are found to have a significant impact on the swing voltages across the positive and negative electrodes. Illustrative EDLC and ECP–AC devices are explored and employed to verify the derived equations that serve to predict essential parameters of both symmetric and asymmetric systems, irrespective of electrolyte ion concentration, solvent or species and independent of voltage. The utility of the equations is demonstrated by predicting cell parameters for a number of theoretical asymmetric ECP–AC systems and used to correlate experimentally obtained parameters.     

Enhanced electrochemical properties of PEO-based composite polymer electrolyte with shape-selective molecular sieves

ZSM-5 molecular sieves, usually known as shape-selective catalyst in a great deal of catalysis fields, due to its special pore size and two-dimensional interconnect channels. In this work, a novel PEO-based composite polymer electrolyte by using ZSM-5 as the filler has been developed. The interactions between ZSM-5 and PEO matrix are studied by DSC and SEM techniques. The effects of ZSM-5 on the electrochemical properties of the PEO-based electrolyte, such as ionic conductivity, lithium ion transference number, and interfacial stability with lithium electrode are studied by electrochemical impedance spectroscopy and steady-state current method. The experiment results show that ZSM-5 can enhance the ionic conductivity and increase the lithium ion transference number of PEO-based electrolyte more effectively comparing with traditional ceramic fillers such as SiO₂ and Al₂O₃, resulting from its special framework topology structure. The excellent performances such as high ionic conductivity, good compatibility with lithium metal electrode, and broad electrochemical stability window suggesting that PEO–LiClO₄/ZSM-5 composite polymer electrolyte can be used as candidate electrolyte materials for lithium polymer batteries.     

Gelled composite electrolyte comprising thermoplastic polyurethane and poly(ethylene oxide) for lithium batteries

Composite polymer electrolyte films consisting of poly(ethylene glycol) based thermoplastic polyurethane blended with poly(ethylene oxide) (denoted as TPU(PEG)/PEO) incorporating LiClO₄–PC have been prepared and their electrochemical properties were studied. The thermal analysis of the composite films were performed to demonstrate the miscibility of the polymer blend by using differential scanning calorimeter (DSC). TPU(PEG)/PEO based polymer electrolyte shows ionic conductivity of the order 6.4×10⁻⁴ S/cm at room temperature, irrespective of time evolution. Cyclic voltammogram shows that this composite electrolyte has good electrochemical stability in the working voltage ranging from 2 to 4.5 V. Cycling performances of Li/polymer electrolyte/LiCoO₂ cells are also followed. From AC impedance results, the recharging ability of the cells is proved to be dominated by the passive layer formation at Li electrode–polymer electrolyte interface.     

Dimethyl methyl phosphate: A new nonflammable electrolyte solvent for lithium-ion batteries

A new fire retardant-dimethyl methyl phosphate (DMMP) was tested as a nonflammable electrolyte solvent for Li-ion batteries. It is found that in the addition of chloro-ethylene carbonate (Cl-EC) as an electrolyte additive, the electrochemical reduction of DMMP molecules can be completely suppressed and the graphite anode can be cycled very well with high initial columbic efficiency (∼84%) and excellent cycling stability in the DMMP electrolyte. The prismatic C/LiCoO₂ batteries using 1.0 mol L⁻¹ LiClO₄ + 10% Cl-EC + DMMP electrolyte exhibited almost the same charge and discharge performances as those using conventional carbonate electrolytes, suggesting a feasible use of this new electrolyte for constructing nonflammable Li⁺-ion batteries.     

Novel agarose polymer electrolyte for quasi-solid state dye-sensitized solar cell

Quasi-solid state dye-sensitized solar cells (DSSCs) are fabricated with a novel polysaccharide gel electrolyte composed of agarose in 1-methyl-2-pyrrolidinone (NMP) as polymer matrix, lithium iodide (LiI)/iodine (I₂) as redox couple and titania nanoparticles as fillers. The polysaccharide electrolyte with different agarose concentrations (1-5 wt%) and various inorganic filler TiO₂ concentrations (0-10 wt%) are studied systematically by differential scanning calorimetry (DSC) and the AC impedance spectra. The electrochemical and photoelectric performances of DSSCs with these electrolytes are also investigated. It is found that increasing agarose and inorganic filler concentration leads to a decrease in T_g in the range of 1-2 wt% for agarose and 0-2.5 wt% for TiO₂ changed electrolytes, which results in high conductivity in these electrolytes. From the electrochemical analysis, it is observed that the electron lifetime in TiO₂ of DSSCs increases with agarose, while decreases with inorganic filler contents. The prolonged electron lifetime in DSSCs is advantageous to improve open-circuit voltage (V_oc). Based on these results, the cell with the electrolyte of 2 wt% agarose shows the optimized energy conversion efficiency of 4.14%. The optimized efficiency of the DSSC with added titania is 4.74% at 2.5 wt% titania concentration.