Ionic liquid-SnO2 nanoparticle hybrid electrolytes for secondary charge storage devices: Physicochemical and electrochemical studies

A series of nanoscale hybrid ionic fluids (NHIFs) has been prepared by tethering 1-ethyl-3-(3-trimethoxysilylpropyl)-imidazolium bis(trifluoromethanesulfonyl) imide ionic liquids to the surface of SnO₂ nanoparticle, at different grafting densities. Investigations reveal that SnO₂ nanocores with uniform particle size of 14–17 nm are uniformly dispersed in different NHIF matrices through strong covalent attachment. Thermal stability and mechanical properties of NHIFs are found to improve with increasing grafting density. Temperature dependent electrochemical cycling behaviour of NHIF at different grafting densities are thoroughly investigated. In comparison to pure ionic liquid, the hybrid ionic fluids show substantial enhancement in electrochemical properties, which further improve with increasing grating density. The electrochemical cell containing NHIFs as electrolytes show very good capacitance retentivity (>90%) and long term cyclic stability above room temperature. Results obtained from the study demonstrate the applicability of these hybrid ionic fluids as promising electrolytes in secondary charge storage devices.     

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.     

Lithium insertion in graphite from ternary ionic liquid-lithium salt electrolytes: I. Electrochemical characterization of the electrolytes

In this paper we report the results of chemical-physical investigation performed on ternary room temperature ionic liquid-lithium salt mixtures as electrolytes for lithium-ion battery systems. The ternary electrolytes were made by mixing N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl) imide (PYR₁₃FSI) and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (PYR₁₄TFSI) ionic liquids with lithium hexafluorophosphate (LiPF₆) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The mixtures were developed based on preliminary results on the cyclability of graphite electrodes in the IL-LiX binary electrolytes. The results clearly show the beneficial synergic effect of the two ionic liquids on the electrochemical properties of the mixtures.     

Solid polymer electrolytes with sulfur based ionic liquid for lithium batteries

The electrochemical properties of a solid hybrid polymer electrolyte for lithium batteries based upon tri-ethyl sulfonium bis(trifluorosulfonyl) imide (S₂TFSI), lithium TFSI, and poly(ethylene oxide) (PEO) is presented. We have synthesized homogenous freestanding films that possess low temperature ionic conductivity and wide electrochemical stability. The hybrid electrolyte has demonstrated ionic conductivity of 0.117 mS cm⁻¹ at 0 °C, and 1.20 mS cm⁻¹ at 25 °C. At slightly elevated temperature ionic conductivity is on the order of 10 mS cm⁻¹. The hybrid electrolyte has demonstrated reversible stability against metallic lithium at the anodic interface and >4.5 V vs. Li/Li⁺ at the cathodic interface.     

Synthesis and optimization of new polymeric ionic liquid poly(diallydimethylammonium) bis(trifluoromethane sulfonyl)imde based gel electrolyte films

Polymeric ionic liquid-based gel electrolyte films are a new generation of electrolyte materials for flexible energy storage device applications. In this work, Li-ion conducting gel electrolyte films are prepared with the polymeric ionic liquid poly(diallydimethylammonium) bis(trifluoromethane sulfonyl)imide (poly(DADMATFSI)) as the host polymer and the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt as the dopant. The crystalline nature, thermal stability, ionic conductivity and electrochemical stability window of the polymeric electrolyte films are analyzed by various characterization techniques. It is found that the polymeric electrolyte films exhibit high flexibility and excellent thermal stability. Their room-temperature electrical conductivity increases with increasing LiTFSI concentration and reaches a high value of 1.00 × 10^?3 S cm^?1 at 20 wt% LiTFSI. The ionic transference numbers of the polymeric electrolyte films are in the range of 0.98–0.99, indicating that they are perfect ion conductors. Finally, the electrochemical stability window of the 20 wt% LiTFSI-doped polymeric electrolyte film is determined as approximately 6 V, which is a promising value for flexible energy storage device applications.     

High‐performance solid PEO/PPC/LLTO‐nanowires polymer composite electrolyte for solid‐state lithium battery

Solid polymer composite electrolyte (SPCE) with good safety, easy processability, and high ionic conductivity was a promising solution to achieve the development of advanced solid‐state lithium battery. Herein, through electrospinning and subsequent calcination, the Li_0.33La_0.557TiO₃ nanowires (LLTO‐NWs) with high ionic conductivity were synthesized. They were utilized to prepare polymer composite electrolytes which were composed of poly (ethylene oxide) (PEO), poly (propylene carbonate) (PPC), lithium bis (fluorosulfonyl)imide (LiTFSI), and LLTO‐NWs. Their structures, thermal properties, ionic conductivities, ion transference number, electrochemical stability window, as well as their compatibility with lithium metal, were studied. The results displayed that the maximum ionic conductivities of SPCE containing 8 wt.% LLTO‐NWs were 5.66 × 10^?5 S cm^?1 and 4.72 × 10^?4 S cm^?1 at room temperature and 60°C, respectively. The solid‐state LiFePO₄/Li cells assembled with this novel SPCE exhibited an initial reversible discharge capacity of 135 mAh g^?1 and good cycling stability at a charge/discharge current density of 0.5 C at 60°C.     

A binary ionic liquid system composed of N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide and lithium bis(trifluoromethanesulfonyl)imide: A new promising electrolyte for lithium batteries

Room temperature ionic liquids are nowadays the most appealing research target in the field of liquid electrolytes for lithium batteries, due to their high thermal stability, ionic conductivity and wide electrochemical windows. The cation structure of such solvents strictly influences their physical and chemical properties, in particular the viscosity and conductivity.In this paper we report on the preparation and characterization of a complete series of solutions between lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and the promising N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide (PY_1,2O1) ionic liquid. A wide molality range has been explored in order to identify the optimal compositions in terms of conductivity and electrochemical stability. Our thermal results show that the solutions are amorphous independently on the LiTFSI content. Up to salt concentration of 0.4 mol kg⁻¹ the solutions have a very low viscosity (η ∼ 36 cP), a high ionic conductivity, even at temperatures below 0 °C, and a good electrochemical stability. Cations transport numbers ranging between 0.05 and 0.39 have been determined as a function of LiTFSI content. The combination of these properties makes the PY_1,2O1-based solutions potentially attractive liquid electrolytes for lithium batteries.     

Electrode materials for ionic liquid-based supercapacitors

The use of ionic liquid (IL) electrolytes is a promising strategy to enhance the performance of supercapacitors above room temperature. In this paper we present the results of a study on optimization of electrode materials for IL-based supercapacitors featuring a hybrid configuration with carbon negative electrode and poly(3-methylthiophene) (pMeT) as positive operating at 60 °C with the ILs N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) and 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI). As it concerns the carbon electrode two routes have been pursued: (i) surface modification of commercial activated carbon and (ii) synthesis of mesoporous cryo- and xerogel carbons. Pore size distribution and electrochemical characterization data are related and suggest that the second route should be the most promising for carbons of high specific capacitance and low time constant in IL. For the polymer electrode the nature of the galvanostatic polymerization bath plays a crucial role to provide pMeT of high specific capacitance and the best results may be obtained when pMeT is electropolymerized in the same IL used for the capacitance tests. The strategy of using the acid additive trifluoromethanesulfonimide in IL-based polymerization baths is also described in some detail. This strategy that provides pMeT featuring more than 200 F g⁻¹ in IL is a clean procedure which prevents consumption of the ionic liquid with great advantage in terms of costs.     

LiTFSI-BEPyTFSI as an improved ionic liquid electrolyte for rechargeable lithium batteries

We report an electrochemical study of solutions of lithium bis(trifluoromethanesulfonyl)imide, LiTFSI, in a N-n-butyl-N-ethylpyrrolidinium bis(trifluoromethanesulfonyl)imide, BEPyTFSI. We show that these ionic liquid solutions have stability towards lithium metal electrode which allows various electrochemical tests, including impedance spectroscopy and voltammetry. The ionic conductivity and lithium transference number, of the order of 10⁻³ S cm⁻¹ and 0.4, respectively, make these solutions suitable for application as electrolytes in advanced lithium batteries. A prototype of these batteries, having lithium iron phosphate as the cathode, showed good performance in terms of charge–discharge efficiency and rate capability. The results reported in this work, although preliminary, are encouraging in supporting the practical interest of this LiTFSI-BEPyTFSI class of lithium conducting ionic liquids.     

Are ionic liquids based on pyrrolidinium imide able to wet separators and electrodes used for Li-ion batteries?

Surface free energy and contact angle measurements were conducted with a series of room temperature ionic liquids (RTILs) based on N,N′-alkyl-pyrrolidinium imide. Wetting characteristics of various separators (Celgard^® and Separion^®) and electrodes (LiCoO₂, Li₄Ti₅O₁₂ and graphite), commonly used in Li-ion batteries, were performed. Initially, the free surface energies were determined for both smooth polymeric materials, constituent of the separators, and pyrrolidinium RTILs. Experimental results and calculations show that (i) N-methyl-N-pentyl pyrrolidinium imide is the most wetting RTIL whatever the separator used, and that (ii) the separator wettability is one of the most important factor to take into account in electrochemical devices.     

Ionic liquid doped PEO-based solid polymer electrolytes for lithium-ion polymer batteries

The influence of adding the room-temperature ionic liquid 1-ethyl-3-methyllimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) to poly(ethylene oxide) (PEO)–lithium difluoro(oxalato)borate (LiDFOB) solid polymer electrolyte and the use of these electrolytes in solid-state Li/LiFePO₄ batteries has been investigated. Different structural, thermal, electrical and electrochemical studies exhibit promising characteristics of these polymer electrolyte membranes, suitable as electrolytes in rechargeable lithium-ion batteries. The crystallinity decreased significantly due to the incorporation of ionic liquid, investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The ion–polymer interaction, particularly the interaction of cations in LiDFOB and ionic liquid with ether oxygen atom of PEO chains, has been evidenced by FT-IR studies. The polymer electrolyte with ～40 wt% of ionic liquid offers a maximum ionic conductivity of ～1.85 × 10^?4 S/cm at 30 °C with improved electrochemical stabilities. The Li/PEO-LiDFOB-40 wt% EMImTFSI/LiFePO₄ coin-typed cell cycled at 0.1 C shows the 1st discharge capacity about 155 mAh g^?1, and remains 134.2 mAh g^?1 on the 50th cycle. The addition of the ionic liquid to PEO₂₀-LiDFOB polymer electrolyte has resulted in a very promising improvement in performance of the lithium polymer batteries.     

Ternary polymer electrolytes containing pyrrolidinium-based polymeric ionic liquids for lithium batteries

The electrochemical properties of solvent-free, ternary polymer electrolytes based on a novel poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide polymeric ionic liquid (PIL) as polymer host and incorporating PYR₁₄TFSI ionic liquid and LiTFSI salt are reported. The PIL-LiTFSI-PYR₁₄TFSI electrolyte membranes were found to be chemically stable even after prolonged storage times in contact with lithium anode and thermally stable up to 300 °C. Particularly, the PIL-based electrolytes exhibited acceptable room temperature conductivity with wide electrochemical stability window, time-stable interfacial resistance values and good lithium stripping/plating performance. Preliminary battery tests have shown that Li/LiFePO₄ solid-state cells are capable to deliver above 140 mAh g⁻¹ at 40 °C with very good capacity retention up to medium rates.     

Lithium bis(fluorosulfonyl)imide-PYR14TFSI ionic liquid electrolyte compatible with graphite

Lithium bis(fluorosulfonyl)imide (LiFSI) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) was successfully tested as an electrolyte for graphite composite anodes at elevated temperature of 55 °C. The graphite anode showed a good cyclability during the galvanostatic testing at C/10 rate and 55 °C with the capacity close to theoretical. The formation of SEI in different electrolytes was the subject of study using impedance spectroscopy on symmetrical cells containing two lithium electrodes. The 0.7 m LiFSI in PYR₁₄TFSI exhibits a good ionic conductivity (5.9 mS cm⁻¹ at 55 °C) along with high electrochemical stability and high thermal stability. These properties allow their potential application in large-scale lithium ion batteries with improved safety.     

Electrochemical and thermal properties of graphite electrodes with imidazolium- and piperidinium-based ionic liquids

The electrochemical and thermal properties of graphite electrodes with electrolytes containing 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) and N-methyl,N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (MPPpTFSI) ionic liquids are investigated. The ionic liquids undergo extensive reductive decomposition on a graphite electrode during the first charge. The effect of a fluoroethylene carbonate (FEC) additive on the reductive decomposition of the ionic liquids is examined by electrochemical, scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis. Thermal reactions between a lithiated graphite electrode and an ionic liquid-containing electrolyte are investigated with differential scanning calorimetry (DSC). The introduction of an ionic liquid can effectively reduce the exothermic heat evolution from the thermal reactions between a lithiated graphite electrode and an electrolyte.     

Comparative study of EC/DMC LiTFSI and LiPF6 electrolytes for electrochemical storage

Lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) salt are potentially a good alternative to LiPF₆ since it could both improve the chemical and thermal stability as salt for electrolyte. This work presents a systematic comparative study between LiPF₆ and LiTFSI in a mixture of EC/DMC on the basis of some of their physicochemical properties. Transport properties (viscosity and conductivity) are compared at various temperatures from −20 to 80 °C. Using Walden rule, we have demonstrated that LiTFSI 1 M in EC/DMC is more ionic than LiPF₆ 1 M in the same binary solvent. Moreover, the electrochemical storage properties of an activated carbon electrode were investigated in EC/DMC mixture containing LiTFSI or LiPF₆. The specific capacitance C_s of activated carbon was determined from the Galvanostatic charge-discharge curve between 2 and 3.7 V, at low current densities. The capacitance values were found to be 100 and 90 F g⁻¹ respectively for LiTFSI and LiPF₆ electrolytes at 2 mA g⁻¹. On the basis of the physicochemical and electrochemical measurements, we have correlated the improvement of the specific capacitance with activated carbon to the increase of the ionicity of the LiTFSI salt in EC/DMC binary system. The drawback concerning the corrosion of aluminium collectors was resolved by adding a few percentage of LiPF₆ (1%) in the binary electrolyte. Finally, we have studied the electrochemical behavior of intercalation-deintercalation of lithium in the graphite electrode with EC/DMC + LiTFSI as electrolyte. Results of this study indicate that the realization of asymmetric graphite/activated carbon supercapacitors with LiFTSI based electrolyte is possible.     

Lithium insertion in graphite from ternary ionic liquid-lithium salt electrolytes: II. Evaluation of specific capacity and cycling efficiency and stability at room temperature

In this paper we report the results about the use of ternary room temperature ionic liquid-lithium salt mixtures as electrolytes for lithium-ion battery systems. Mixtures of N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl) imide, PYR₁₃FSI, and N-butyl-N-methylpyrrolidinium bis(trifluoromethansulfonyl) imide, PYR₁₄TFSI, with lithium hexafluorophosphate, LiPF₆ and lithium bis(trifluoromethansulfonyl) imide, LiTFSI, containing 5 wt.% of vinylene carbonate (VC) as additive, have been used in combination with a commercial graphite, KS6 TIMCAL. The performance of the graphite electrodes has been considered in term of specific capacity, cycling efficiency and cycling stability. The results clearly show the advantage of the use of ternary mixtures on the performance of the graphite electrode.     

An imidazolium based ionic liquid electrolyte for lithium batteries

An electrolyte for lithium batteries based on the ionic liquid 3-methy-1-propylimidazolium bis(trifluoromethysulfony)imide (PMIMTFSI) complexed with lithium bis(trifluoromethysulfony)imide (LiTFSI) at a molar ratio of 1:1 has been investigated. The electrolyte shows a high ionic conductivity (∼1.2 × 10⁻³ S cm⁻¹) at room temperature. Over the whole investigated temperature range the ionic conductivity is more than one order of magnitude higher than for an analogue electrolyte based on N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (Py₁₄TFSI) complexed with LiTFSI and used here as a benchmark. Raman results indicate furthermore that the degree of lithium coordinated TFSI is slightly lower in the electrolyte based on PMIMTFSI and thus that the Li⁺ charge carriers should be higher than in electrolytes based on Py₁₄TFSI. An ionic liquid gel electrolyte membrane was obtained by soaking a fibrous fully interconnected membrane, made of electrospun P(VdF-HFP), in the electrolyte. The gel electrolyte was cycled in Li/ionic liquid polymer electrolyte/Li cells over 15 days and in Li/LiFePO₄ cells demonstrating good interfacial stability and highly stable discharge capacities with a retention of >96% after 50 cycles (∼146 mAh g⁻¹).     

Electrochemical performance of gel polymer electrolyte with ionic liquid and PUA/PMMA prepared by ultraviolet curing technology for lithium-ion battery

A series of diethylethyletherylmethanamine bis(trifluoromethanesulfonyl)imide (DEEYTFSI) ionic liquid gel polymer electrolyte based polyurethane acrylate (PUA)/poly(methyl methacryltae) (PMMA) matrix with different contents of DEEYTFSI, PUA and LiTFSI were prepared via ultraviolet (UV) curing system. Electrochemical performances of the gel polymer were studied by electrochemical station and charge–discharge system. The gel polymer electrolyte with 19 wt.% DEEYTFSI obtained a maximum conductivity σ of 2.76 × 10^?4 S cm^?1 and the transference number t_Li+ of ～0.22 at room temperature. 19 wt.% DEEYTFSI caused the easier transferring of lithium ions due to less apparent activation energy E_a of 21.1 kJ mol^?1. The DEEYTFSI/LiTFSI/PUA/PMMA electrolyte had good compatibility with LiFePO₄ cathode. The DEEYTFSI/LiTFSI/PUA/PMMA electrolyte with the electrochemical window of 4.70 V was enough stability for being the electrolyte material of lithium battery. The Li/19 wt.% DEEYTFSI–LiTFSI–PUA–PMMA/LiFePO₄ coin-typed cell cycled at 0.1 C presented 95% efficiency on the 50th cycle.     

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.