In this work, we present a study on the physical and electrochemical properties of three new Deep Eutectic Solvents (DESs) based on N-methylacetamide (MAc) and a lithium salt (LiX, with X = bis[(trifluoromethyl)sulfonyl]imide, TFSI; hexafluorophosphate, PF6; or nitrate, NO3). Based on DSC measurements, it appears that these systems are liquid at room temperature for a lithium salt mole fraction ranging from 0.10 to 0.35. The temperature dependences of the ionic conductivity and the viscosity of these DESs are correctly described by using the Vogel-Tammann-Fulcher (VTF) type fitting equation, due to the strong interactions between Li(+), X(-) and MAc in solution. Furthermore, these electrolytes possess quite large electrochemical stability windows up to 4.7-5 V on Pt, and demonstrate also a passivating behavior toward the aluminum collector at room temperature. Based on these interesting electrochemical properties, these selected DESs can be classified as potential and promising electrolytes for lithium-ion batteries (LIBs). For this purpose, a test cell was then constructed and tested at 25 °C, 60 °C and 80 °C by using each selected DES as an electrolyte and LiFePO4 (LFP) material as a cathode. The results show a good compatibility between each DES and LFP electrode material. A capacity of up to 160 mA h g(-1) with a good efficiency (99%) is observed in the DES based on the LiNO3 salt at 60 °C despite the presence of residual water in the electrolyte. Finally preliminary tests using a LFP/DES/LTO (lithium titanate) full cell at room temperature clearly show that LiTFSI-based DES can be successfully introduced into LIBs. Considering the beneficial properties, especially, the cost of these electrolytes, such introduction could represent an important contribution for the realization of safer and environmentally friendly LIBs.

No abstract

This study describes the preparation, characterization and application of [Et(3)NH][TFSA], either neat or mixed with acetonitrile, as an electrolyte for supercapacitors. Thermal and transport properties were evaluated for the neat [Et(3)NH][TFSA], and the temperature dependence of viscosity and conductivity can be described by the VTF equation. The evolution of conductivity with the addition of acetonitrile rendered it possible to determine the optimal mixture at 25 °C, with a weight fraction of acetonitrile of 0.5. This mixture was also evaluated for transport properties, and showed a Newtonian behavior, as the neat PIL. An electrochemical study demonstrated, at first, a passivation on Al after the second cyclic voltammogram. Subsequently, the electrochemical window was estimated using a three-electrode cell to 4 V on a platinum electrode, and to 2.5 V on activated carbon. Finally, the neat PIL was found to exhibit good performances as promising electrolyte for supercapacitor applications.

No abstract

This study describes the utilization of deep eutectic solvents (DESs) based on the mixture of the N-methylacetamide (MAc) with a lithium salt (LiX, with X = bis[(trifluoromethyl)sulfonyl]imide, TFSI; hexafluorophosphate, PF 6 ; or nitrate, NO 3 ) as electrolytes for carbon-based supercapacitors at 80°C. The investigated DESs were formulated by mixing a LiX with the MAc (at x Li = 0.25). All DESs show the typical eutectic characteristic with eutectic points localized in the temperature range from −85 to −52°C. Using thermal properties measured by differential scanning calorimetry (DSC), solid−liquid equilibrium phase diagrams of investigated LiX−MAc mixtures were then depicted and also compared with those predicted by using the COSMOThermX software. However, the transport properties of selected DESs (such as the conductivity (σ) and the fluidity (η −1 )) are not very interesting at ambient temperature, while by increasing the temperature up to 80°C, these properties become more favorable for electrochemical applications, as shown by the calculated Walden products: w = ση −1 (mS cm −1 Pa −1 s −1 ) (7 < w < 16 at 25°C and 513 < w < 649 at 80°C). This "superionicity" behavior of selected DESs used as electrolytes explains their good cycling ability, which was determined herein by cyclic voltammetry and galvanostic charge−discharge methods, with high capacities up to 380 F g −1 at elevated voltage and temperature, i.e., ΔE = 2.8 V and 80°C for the LiTFSI−MAc mixture at x Li = 0.25, for example. The electrochemical resistances ESR (equivalent series resistance) and EDR (equivalent diffusion resistance) evaluated using electrochemical impedance spectroscopy (EIS) measurements clearly demonstrate that according to the nature of anion, the mechanism of ions adsorption can be described by pure double-layer adsorption at the specific surface or by the insertion of desolvated ions into the ultramicropores of the activated carbon material. The insertion of lithium ions is observed by the presence of two reversible peaks in the CVs when the operating voltage exceeds 2 V. Finally, the efficiency and capacitance of symmetric AC/AC systems were then evaluated to show the imbalance carbon electrodes caused by important lithium insertion at the negative and by the saturation of the positive by anions, both mechanisms prevent in fact the system to be operational. Considering the promising properties, especially their cost, hazard, and risks of these DESs series, their introduction as safer electrolytes could represent an important challenge for the realization of environmentally friendly EDLCs operating at high temperature.

Density, rheological properties, and conductivity of a homologous series of ammonium-based ionic liquids N-alkyl-triethylammonium bis{(trifluoromethyl)sulfonyl}imide were studied at atmospheric pressure as a function of alkyl chain length on the cation, as well as of the temperature from (293.15 to 363.15) K. From these investigations, the effect of the cation structure was quantified on each studied properties, which demonstrated, as expected, a decrease of the density and conductivity, a contrario of an increase of the viscosity with the alkyl chain length on the ammonium cation. Furthermore, rheological properties were measured for both pure and water-saturated ionic liquids. The studied ionic liquids were found to be Newtonian and non-Arrhenius. Additionally, the effect of water content in the studied ionic liquids on their viscosity was investigated by adding water until they were saturated at 293.15 K. By comparing the viscosity of pure ionic liquids with the data measured in water-saturated samples, it appears that the presence of water decreases dramatically the viscosity of ionic liquids by up to three times. An analysis of involved transport properties leads us to a classification of the studied ionic liquids in terms of their ionicity using the Walden plot, from which it is evident that they can be classified as "good" ionic liquids. Finally, from measured density data, different volumetric properties, that is, molar volumes and thermal expansion coefficients were determined as a function of temperature and of cationic structure. Based on these volumetric properties, an extension of Jacquemin's group contribution model has been then established and tested for alkylammonium-based ionic liquids within a relatively good uncertainty close to 0.1 %.

International audienceNa-doped Birnessite-type manganese oxide (δ-MnO2) has been synthesized using the chemical method and characterized through X-ray diffraction and SEM, showing the lamellar structure and high crystal structure. A comparative study of the electrochemical performances of this material with those of the commercial Cryptomelane-type MnO2 has then been undertaken in ten neutral aqueous electrolytes for supercapacitor applications. Aqueous electrolytes, containing a lithium salt, LiX (where X = SO42-, NO3-, CH3CO2-, CH3SO3-, ClO4-, C7H15CO2-, TFSI-, Beti-, BOB-, or Lact-), have been first prepared under neutral pH conditions to reach the salt concentration, providing the maximum in conductivity. Their transport properties are then investigated through conductivities, viscosities, and self-diffusion coefficient measurements. Second, the thermal behaviors of these electrolytic aqueous solutions are then evaluated by using a differential scanning calorimeter from (213.15 to 473.15) K in order to access their liquid range temperatures. Cyclic voltammograms (CV) in three electrode configurations are thereafter investigated using Na Birnessite and Cryptomelane as working electrode material from (−0.05 to 1.5) V versus Ag/AgCl at various sweep rates from (2 to 100) mV*s-1. According to anion nature/structure and manganese oxide material type, different CV responses are observed, presenting a pure capacitive profile for Beti- or C6H13CO2- and an additional pseudocapacitive signal for the smallest anions, such as ClO4- and NO3-. The capacitances, energies, and efficiencies are finally calculated. These results indicate clearly that electrolytes based on a mineral lithium salt under neutral pH condition and high salt concentration (up to 5 mol*L-1) have better electrochemical performances than organic ones, up to 1.4 V with good material stability and capacity retention. The relationship between transport properties, electrostatic and steric hindrance considerations of hydrated ions, and their electrochemical performances is discussed in order to understand further the lithium intercalation-deintercalation processes in the lamellar or tunnel structure of investigated MnO2

This study describes the use of the ionic liquid trimethylsulfonium bis(trifluorosulfonimide) [Me 3 S][TFSI] as an electrolyte for carbon-based supercapacitors at temperatures up to 80 °C. [Me 3 S][TFSI] is synthesized by a metathesis reaction, possesses a low melting point (T m = 45.5 °C), and presents crystal plastic behavior indicated by a strong organizational structure with many possible conformations corresponding to several solid−solid phase transitions T S−S . Furthermore, [Me 3 S][TFSI] is thermally stable up to T = 280 °C and presents a high conductivity up to 20.42 mS cm −1 at 80 °C and a low viscosity, 3 mPa s, at the same temperature. The combination of a good cycling ability with high capacities up to 150 F g −1 at elevated voltage and temperature, i.e., ΔE = 3 V, T = 80 °C, enables the realization of supercapacitors with high specific energies at high temperatures.