Lithium electrolytes based on modified imidazolium ionic liquids

In this study, new electrolytes for Li-ion batteries in the form of lithium salt solutions in room temperature imidazolium ionic liquids (RTIL) are reported. The ionic liquids applied, for higher reduction potential stability, were substituted at position C2 with oligooxyethylene groups of various length ([Im nEO]⁺X⁻; where: n = 0, 3, 7, 20 and X = Cl, BF₄, N(CF₃SO₂)₂). It was found that they are good solvents for lithium salts (LiBF₄, LiN(CF₃SO₂)₂, {[CH₃(OCH₂CH₂)₃O]₃BC₄H₉}Li) forming liquid solutions of low glass transition temperature (T_g in the −70 to −40 °C range). Ionic conductivity depends on the length of oxyethylene substituent in Im nEO and on the concentration of the salt applied, for 10 mol%, σ_RT is of the order of 10⁻⁴ S cm⁻¹. On the basis of polarization measurements by the variable-current method, the proportion of lithium cations in electric charge transfer (t₊) has been determined. The values obtained (typical for ionic liquids) are low and depend on n and lithium salt concentration but do not exceed a dozen or so percent.     

Ionic liquids as electrolytes for Li-ion batteries—An overview of electrochemical studies

The paper reviews properties of room temperature ionic liquids (RTILs) as electrolytes for lithium and lithium-ion batteries. It has been shown that the formation of the solid electrolyte interface (SEI) on the anode surface is critical to the correct operation of secondary lithium-ion batteries, including those working with ionic liquids as electrolytes. The SEI layer may be formed by electrochemical transformation of (i) a molecular additive, (ii) RTIL cations or (iii) RTIL anions. Such properties of RTIL electrolytes as viscosity, conductivity, vapour pressure and lithium-ion transport numbers are also discussed from the point of view of their influence on battery performance.     

Ionic liquid electrolytes based on 1-vinyl-3-alkylimidazolium iodides for dye-sensitized solar cells

Five new ionic liquids of 1-vinyl-3-alkylimidazolium iodide were synthesized to develop novel electrolytes for dye-sensitized solar cells. The effects of photovoltaic characteristics of the cell and the ionic liquid features such as viscosity and ionic conductivity were described. The 1-vinyl-3-alkylimidazolium cation volume was calculated by quantum chemistry method. The linear dependence of photon-to-current conversion efficiency on the non-solvated cation volume was revealed. After lithium iodide was added to 1-vinyl-3-alkylimidazolium salts as electrolytes, except the photovoltage, the photocurrent, fill factor and photon-to-current efficiency were improved correspondingly.     

Performance of PEMFC with new polyvinyl-ionic liquids based membranes as electrolytes

Polymer Electrolyte Membrane Fuel Cells (PEMFC) represent a key technology for sustainable energy production due to their high efficiency and low environmental impact. The use of task specific protic ionic liquids as electrolytes is gaining interest due to their high conductivity and thermal and electrochemical stability under anhydrous conditions. Ionic liquids with the imidazolium cation exhibit a high electrochemical stability, besides sulfonic groups can be incorporated to the cation as side chains acting as carriers in order to facilitate the proton transport. Moreover suitable anions such as Tf₂N and OTf provide high ionic conductivity. In this work, two different types of membranes based on protic ionic liquids have been tested in PEMFC under anhydrous conditions i) Nafion membranes impregnated with the protic ionic liquids 1-methyl-3-(4-sulfobutyl)-imidazolium bis(trifluoromethylsulfonyl)-imide ([HSO₃-BMIm][NTf₂]) and 1-butyl-3-(4-sulfobutyl)-imidazolium trifluoromethanesulfonate ([HSO₃-BBIm][OTf]) and, ii) membranes based on the polymerization of the specifically designed ionic liquid 1-(4-sulfobutyl)-3-vinylimidazolium trifluoromethanesulfonate ([HSO₃-BVIm][OTf]). The influence of different operation variables such as cell temperature, gas humidity and membrane thickness on the performance of the PEMFC has been analyzed, and the resistance exerted by the electrolyte was determined using electrical impedance spectroscopy. Nafion membranes impregnated with [HSO₃-BBIm][OTf] achieve current densities of 217 mA/cm² under anhydrous conditions at 25 °C whereas [HSO₃-BVIm][OTf] polymerized electrolytes provide current densities of 154 mA/cm² at the same conditions. This is the first report that describes the application of designed polymerized protic ionic liquids membranes for fuel cells. Although some improvements in terms of thermal and mechanical stability should be achieved, this first approach presents a promising electrolyte with challenging characteristics.     

Pyrrolidinium-based polymeric ionic liquids as mechanically and electrochemically stable polymer electrolytes

A new family of polymeric ionic liquids having pyrrolidinium cation pendant units was synthesized from commercially available poly(diallyldimethylammonium) chloride. A simple anion exchange procedure was applied to the poly(diallyldimethylammonium) chloride using different salts such as LiTFSI, KPF₆, LiBF₄ and NaDBSA. The anion exchange reaction was quantitative as confirmed by NMR, FT-IR and titration experiments. Among these polymers, poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (TFSI) showed excellent performance as polymer matrix for polymer electrolyte compositions together with pyrrolidinium ionic liquid and lithium salt having a similar TFSI counter-anion. In this sense, free standing mechanically stable transparent polymer films showing an ionic conductivity higher than 10⁻⁴ S cm⁻¹ at room temperature were prepared and characterized. Furthermore, the polymer electrolytes presented a wide electrochemical stability window (7.0 V) which makes them interesting candidates for solid-state lithium batteries.     

Ionic liquids as diathermic fluids for solar trough collectors’ technology: A corrosion study

The AISI 304 and AISI 1018 steels (frequently used in solar collectors’ plants) in contact with four different ionic liquids (ILs) suitable as diathermic fluids, were studied. Immersion tests were performed at 220 °C (the working temperature in such plants) for 10 days. The corrosion morphologies of the steels were investigated by scanning electron microscopy coupled with energy dispersive X-ray (EDX) microanalysis and the content of metals in the solution were detected via ICP-OES. The tests showed that the most performing IL is the ethyl-dimethyl-propyl-ammonium-bis(trifluoromethylsulphonyl)imide. The corrosion properties of the two alloys in contact with such IL were investigated by means of Tafel plots and resistance polarization at room temperature in open-to-air vessels.     

Proton-conducting membranes based on protic ionic liquids

New classes of proton-conducting membranes formed by incorporating Brönsted acid–base ionic liquids in a poly(vinyldenefluoride-co-hexafluoropropylene) PVdF polymer matrix, are here reported and discussed. We show that these membranes are characterized by high, thermally stable proton conductivity. However, this favourable property is in part contrasted by the release of the ionic liquid component, which may affect the long-term stability of the membranes. Various strategies are underway in our laboratory to solve this issue, and in this work we describe one of them, based on the dispersion of selected ceramic fillers in the polymer matrix. We show that this approach, while successful in enhancing the conductivity of the membranes, is not much effective in preventing the release of the ionic liquid component and thus, that other roads have to be explored to reach a satisfactory improvement of the integrity of the membrane.     

Functionalized ionic liquids based on guanidinium cations with two ether groups as new electrolytes for lithium battery

Two new functionalized ionic liquids (ILs) based on guanidinium cation with two ether groups and TFSA⁻ anion are synthesized and characterized. Their physicochemical and electrochemical properties, including melting point, thermal stability, density, viscosity, conductivity, and electrochemical window are determined. Both the ILs are liquids at room temperature, and the viscosities are about 60 mPa s at 25 °C. Behavior of lithium redox, chemical stability against lithium metal and charge-discharge characteristics of lithium battery, are also examined for the two ILs as electrolyte with 0.6 mol kg⁻¹ LiTFSA. Though the cathodic limiting potentials of the two ILs are higher than 0 V versus Li/Li⁺, the lithium plating and stripping on Ni electrode can be observed in the two IL electrolytes, indicating their good chemical stability against lithium metal. Li/LiFePO₄ cells using the two IL electrolytes without any additive showed good cycle property at the current rate of 0.2 C at 25 °C and 55 °C.     

New functionalized ionic liquids based on pyrrolidinium and piperidinium cations with two ether groups as electrolytes for lithium battery

Four new functionalized ILs based on piperidinium and pyrrolidinium cations with two ether groups and TFSI⁻ anion are synthesized and characterized. Physical and electrochemical properties of these ILs, including melting point, thermal stability, viscosity, conductivity and electrochemical stability, are investigated. All the ILs are liquids at room temperature, and the viscosities of P(2o1)₂-TFSI and P(2o1)(2o2)-TFSI are 55 and 53 mPa s at 25 °C, respectively. Behavior of lithium redox, chemical stability against lithium metal and charge-discharge characteristics of lithium batteries, are also investigated for these IL electrolytes with 0.6 mol kg⁻¹ LiTFSI. Though the cathodic limiting potentials of these ILs are 0.4 V versus Li/Li⁺, the lithium plating and striping on Ni electrode can be observed for these IL electrolytes, and these IL electrolytes show good chemical stability against lithium metal. Li/LiFePO₄ cells using these IL electrolytes without additives have good capacity and cycle property at the current rate of 0.1 C, and the cell using the P(2o1)(2o2)-TFSI electrolyte owns good rate property.     

Novel hydrophobic ionic liquids electrolyte based on cyclic sulfonium used in dye-sensitized solar cells

A novel series of hydrophobic room temperature ionic liquids based on six cyclic sulfonium cations were first time synthesized and applied in dye-sensitized solar cells as pure solvents for electrolyte system. The chronoamperograms result showed that the length of substituent on sulfonium cations could inhibit the diffusion and the five-ring structure of sulfonium was benefit for fast triiodide ion diffusion. The electrochemical impendence spectra measurement of dye-sensitized solar cells with these ionic liquid electrolytes was carried out and the result indicated that the cations’ structure had indeed influence on the cells’ performance especially for the fill factor, which was further proved by the measurement result of I-V curves of these dye-sensitized solar cells. The conclusion was obtained that the electron exchange reaction on Pt counter electrode/electrolyte interface dominated the cells’ performance for these ionic liquid electrolyte-based DSCs.     

New membranes based on ionic liquids for PEM fuel cells at elevated temperatures

Proton exchange membrane (PEM) fuel cells operating at elevated temperature, above 120 °C, will yield significant benefits but face big challenges for the development of suitable PEMs. The objectives of this research are to demonstrate the feasibility of the concept and realize [acid/ionic liquid/polymer] composite gel-type membranes as such PEMs. Novel membranes consisting of anhydrous proton solvent H₃PO₄, the protic ionic liquid PMIH₂PO₄, and polybenzimidazole (PBI) as a matrix have been prepared and characterized for PEM fuel cells intended for operation at elevated temperature (120–150 °C). Physical and electrochemical analyses have demonstrated promising characteristics of these H₃PO₄/PMIH₂PO₄/PBI membranes at elevated temperature. The proton transport mechanism in these new membranes has been investigated by Fourier transform infrared and nuclear magnetic resonance spectroscopic methods.     

Fast cycling of Li/LiCoO2 cell with low-viscosity ionic liquids based on bis(fluorosulfonyl)imide [FSI]

A charge–discharge cycling test of a Li/LiCoO₂ cell containing ionic liquids based on bis(fluorosulfonyl)imide ([FSI]⁻) as the electrolyte media, revealed significantly better rate properties compared to those of cells using conventional ionic liquids. The use of an 1-ethyl-3-methylimidazolium (EMI⁺) salt permitted the retention of 70% of the discharge capacity at a 4 C current rate. In contrast, similar performance of cells containing N-methyl-N-propylpyrrolidinium (Py₁₃⁺) and N-methyl-N-propylpiperidinium (PP₁₃⁺) salts of [FSI]⁻ was limited to operation at 2 and 1 C current rates, respectively. However, the charge/discharge cycling stability of the cell with Py₁₃[FSI] was much better than that of the cell using EMI[FSI].     

A comparative study of gel polymer electrolytes based on PVDF-HFP and liquid electrolytes, containing imidazolinium ionic liquids of different carbon chain lengths in DSSCs

The photoelectrochemical characteristics of titanium dioxide (TiO₂)-based dye-sensitized solar cells (DSSCs) containing gel polymer electrolyte (GPE) and organic liquid electrolyte (OLE) were studied in detail. GPE was prepared by adding poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP) to imidazolinium ionic liquids (IILs) of the type, 1-methyl-3-alkyl imidazolinium iodides (alkyl is C_nH_2n+1, where n=3–10) in methoxy propionitrile (MPN) and the OLE contained the above molten salt in MPN. The IILs were synthesized in the laboratory and characterized by ¹H nuclear magnetic resonance spectroscopy (NMR). The conductivities (σ) of both GPE and OLE decrease with increase in chain length (n) of the alkyl group of IILs; however, the effect is more drastic in the former case. The performance of the DSSCs containing OLE increases with the increase in alkyl chain length of IIL from C3 to C7, whereas, there is a linear decrease in the efficiency of the DSSCs incorporated with GPE containing IIL of alkyl chain length from C3 to C10. The change in short circuit current density (J_SC) determines the cell efficiency as the V_OC of the DSSCs remains almost the same with increase of alkyl chain length of IILs for both the electrolytes. The change in J_SC values and the consistency of the V_OC of the DSSCs for both the electrolytes may be explained on the basis of increase in viscosity of IILs from C3 to C10 and the dominating role of the 4-tertiary butyl pyridine (TBP), respectively, on the phenomenon of charge recombination.     

Electropolymerization of high stable poly(3,4-ethylenedioxythiophene) in ionic liquids and its potential applications in electrochemical capacitor

Poly(3,4-ethylenedioxythiophene) (PEDOT) has been successfully electropolymerized using a purified 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) as both the growth medium and the supporting electrolyte. The electrochemical performance of the PEDOT thin film was investigated in 1 mol L⁻¹ H₂SO₄ solution. It possesses nearly ideal capacitive property, and its specific capacitance is about 130 F g⁻¹. Compared with other conducting polymers, enhanced cycling lifetime (up to 70,000 cycles), which is close to that of active carbon materials, was observed on repetitive redox cycling.     

Efficiency and stability of transition metal electrocatalysts for the hydrogen evolution reaction using ionic liquids as electrolytes

Different electrodes (nickel, molybdenum, and iron alloys containing chromium, manganese, and nickel) were tested as cathodes for the hydrogen evolution reaction (HER) using 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF₄) ionic liquid (IL) as the electrolyte. HERs were conducted at room temperature, at a cathodic potential of −1.7 V (PtQRE) using 10 vol.% aqueous BMI.BF₄ solutions. Reactions performed in a thermostated Hoffman cell gave current densities between 14.6 and 77.5 mA cm⁻² and efficiencies in the range 97.0–99.2%. Mo electrocatalysts in IL have been shown to be better than Pt, contrary to the classic behavior observed in an aqueous KOH medium. The electrochemical properties of molybdenum, as well as its resistance to corrosion (studied by Tafel plots and observed using SEM) indicate the potential use of this material as a cathode in an IL medium, which can lead to many attractive technological applications.     

Lithium ion conducting PVdF-HFP composite gel electrolytes based on N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide ionic liquid

Blends of PVdF-HFP and ionic liquids (ILs) are interesting for application as electrolytes in plastic Li batteries. They combine the advantages of the gel polymer electrolytes (GPEs) swollen by conventional organic liquid electrolytes with the nonflammability, and high thermal and electrochemical stability of ILs.In this work we prepared and characterized PVdF-HFP composite membranes swollen with a solution of LiTFSI in ether-functionalized pyrrolidinium-imide ionic liquid (PYRA_12O1TFSI). The membranes were filled in with two different types of silica: (i) mesoporous SiO₂ (SBA-15) and (ii) a commercial nano-size one (HiSil™ T700). The ionic conductivity and the electrochemical properties of the gel electrolytes were studied in terms of the nature of the filler.The thermal and the transport properties of the composite membranes are similar. In particular, room temperature ionic conductivities higher than 0.25 mS cm⁻¹ are easily obtained at defined filler contents. However, the mesoporous filler guarantees higher lithium transference numbers, a more stable electrochemical interface and better cycling performances. Contrary to the HiSil™-based membrane, the Li/LiFePO₄ cells with PVdF-HFP/PYRA_12O1TFSI-LiTFSI films containing 10 wt% of SBA-15 show good charge/discharge capacity, columbic efficiency close to unity, and low capacity losses at medium C-rates during 180 cycles.     

Application of ionic liquids as an electrolyte additive on the electrochemical behavior of lead acid battery

Ionic liquids (ILs) belong to new branch of salts with unique properties which their applications have been increasing in electrochemical systems especially lithium-ion batteries. In the present work, for the first time, the effects of four ionic liquids as an electrolyte additive in battery's electrolyte were studied on the hydrogen and oxygen evolution overpotential and anodic layer formation on lead–antimony–tin grid alloy of lead acid battery. Cyclic and linear sweep voltammetric methods were used for this study in aqueous sulfuric acid solution. The morphology of grid surface after cyclic redox reaction was studied using scanning electron microscopy. The results show that most of added ionic liquids increase hydrogen overpotential and whereas they have no significant effect on oxygen overpotential. Furthermore ionic liquids increase antimony dissolution that might be related to interaction between Sb³⁺ and ionic liquids. Crystalline structure of PbSO₄ layer changed with presence of ionic liquids and larger PbSO₄ crystals were formed with some of them. These additives decrease the porosity of PbSO₄ perm selective membrane layer at the surface of electrode. Also cyclic voltammogram on carbon–PbO paste electrode shows that with the presence of ionic liquids, oxidation and reduction peak current intensively increased.     

Ionic liquid electrolytes compatible with graphitized carbon negative without additive and their effects on interfacial properties

Introduction of bis(fluorosulfonyl)imide (FSI) as an anion to an ambient-temperature ionic liquid electrolyte based on 1-ethyl-3-methylimidazolium (EMI) as well as lithium (Li) cations can provide a reversible Li intercalation into a graphitized electrode, while such intercalation is completely irreversible without FSI. The surface-layer components on the graphitized electrodes, cycled in the ionic liquid electrolytes with and without FSI, were found to be chemically similar based on X-ray photoelectron spectroscopy. Ac impedance spectroscopy revealed that the resistance of the electrode charged with FSI was much lower even than that charged in a solvent electrolyte system containing ethylene carbonate (EC) and dimethyl carbonate (DMC). On the basis of these physicochemical analyses, the origins of cycleability in the presence of FSI are discussed.