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).     

Novel polymer electrolytes based on thermoplastic polyurethane and ionic liquid/lithium bis(trifluoromethanesulfonyl)imide/propylene carbonate salt system

Polymer electrolytes were prepared from thermoplastic polyurethane with addition of mixture of ionic liquid N-ethyl(methylether)-N-methylpyrrolidinium trifluoromethanesulfonimmide (PYRA_12O1TFSI), lithium bis(trifluoromethanesulfoneimide) salt and propylene carbonate. The electrolytes characterization was performed by thermogravimetric analysis, differential scanning calorimetry and scanning electron microscopy. The electrical properties were investigated in detail by impedance spectroscopy with the aid of equivalent circuit fitting of the impedance spectra. A model describing temperature evolution of ionic conductivity and the properties of electrolyte/blocking electrode interface was developed. The electrochemical stability of the electrolytes was studied by linear voltammetry. Our results indicate that the studied electrolytes have good self-standing characteristics, and also a sufficient level of thermal stability and a fairly good electrochemical window. The ionic conductivity increases with increasing amount of mixture, and the character of temperature dependence of conductivity indicates decoupling of ion transport from polymer matrix. For studied system, the highest value of ionic conductivity measured at room temperature was 10⁻⁴ S cm⁻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.     

Electrical and structural properties of ionic liquid doped polymer gel electrolyte for dual energy storage devices

This paper reports the synthesis, characterization and dual electrochemical application of a new kind of ionic liquid (IL) based polymer electrolyte. The ionic liquid 1, 2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide (DMPImTFSI) and polymer Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) have been chosen for present study. The electrical conductivity measurement shows many fold enhancement of ionic conductivity by blending IL into polymer matrix. Scanning electron microscopy (SEM) image confirms the uniform surface morphology of the synthesized thin film and cross-section image shows the interface layer of polymer and electrode. We have fabricated an efficient dye sensitized solar cell (DSSC) and electric double layer capacitor (EDLC) using IL-polymer electrolyte (optimized maximum conductivity) system which further affirms that this material is highly stable and reliable for long duration in energy devices.     

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.     

Ionic conductivity studies of gel polyelectrolyte based on ionic liquid

Novel lithium polyelectrolyte-ionic liquids have been prepared and characterized of their properties. Poly(lithium 2-acrylamido-2-methyl propanesulfonate) (PAMPSLi) and its copolymer with N-vinyl formamide (VF) also has been prepared as a copolymer. 1-Ethyl-3-methylimidazolium tricyanomethanide (emImTCM) and N,N-dimethyl-N-propyl-N-butyl ammonium tricyanomethanide (N₁₁₃₄TCM) which are chosen because of the same with the anion of ionic liquid were prepared. The ionic conductivity of copolymer system (PAMPSLi/PVF/emImTCM: 5.43 × 10⁻³ S cm⁻¹ at 25 °C) exhibits about over four times higher than that of homopolymer system (PAMPSLi/emImTCM: 1.28 × 10⁻³ S cm⁻¹ at 25 °C). Introduction of vinyl formamide into the copolymer type can increase the dissociation of the lithium cations from the polymer backbone. The ionic conductivity of copolymer with emImTCM (PAMPSLi/PVF/emImTCM) exhibits the higher conductivity than that of PAMPSLi/PVF/N₁₁₃₄TCM (2.48 × 10⁻³ S cm⁻¹). Because of using the polymerizable anion it is seen to maintain high flexibility of imidazolium cation effectively to exhibit the higher conductivity. And also the viscosity of emImTCM (19.56 cP) is lower than that of N₁₁₃₄TCM (28.61 cP). Low viscosity leads to a fast rate of diffusion of redox species.     

Novel composite electrolyte membranes consisting of fluorohydrogenate ionic liquid and polymers for the unhumidified intermediate temperature fuel cell

Novel composite electrolyte membranes consisting of [EMIm](FH)_nF (EMIm = 1-ethyl-3-methylimidazolium, n = 1.3 and 2.3) ionic liquids and fluorinated polymers were synthesized and their physical and electrochemical properties were measured under unhumidified conditions for their application to the intermediate temperature fuel cells. The ionic conductivities of composite membrane, P(VdF-co-HFP)/s-DFBP-HFDP/[EMIm](FH)_2.3F (1/0.3/1.75 in weight ratio), were 11.3 and 34.7 mS cm⁻¹ at 25 and 130 °C, respectively. The open circuit voltage (OCV) observed for the single cell using [EMIm](FH)_2.3F composite electrolyte was ∼1.0 V at 130 °C for over 5 h. The maximum power density of 20.2 mW cm⁻² was observed under the current of 60.1 mA cm⁻² at 120 °C. From the high thermal stability and high ionic conductivity, the fluorohydrogenate ionic liquid composite membranes are regarded as promising candidates for the electrolytes of the unhumidified intermediate temperature fuel cells.     

Effects of Lewis-acid polymer on the electrochemical properties of alkylphosphate-based non-flammable gel electrolyte

Non-flammable polymer gel electrolytes (NPGE) consisting of 1.0 mol dm⁻³ (=M) LiBF₄/EC + DEC + TEP (55:25:20 volume ratio) + PVdF-HFP (EC: ethylene carbonate, DEC: diethyl carbonate, TEP: triethylphosphate, PVdF-HFP: poly(vinyledenefluoride-co-hexafluoropropylene)) have been developed for rechargeable lithium batteries. The effects of addition of Lewis-acid polymer (LAP) with different mole ratio in NPGE have been studied. The addition of LAP improved physico-chemical properties of NPGE, viz ionic conductivity and lithium ion transport number, as well as mechanical and thermal properties. The ionic conductivity of the gel electrolyte containing LAP reached that of the base solution electrolyte (1.0 M LiBF₄/EC + DEC + TEP (55:25:20)) along with better mechanical properties. Interfacial resistance at Li-metal electrode/NPGE was also improved by introducing LAP in the gel.     

Effects of casting solvent on microstructrue and ionic conductivity of anhydrous sulfonated poly(ether ether ketone)-inoic liquid composite membranes

As the key component of polymer electrolyte membrane fuel cells, the membrane has significant effect on the performance of fuel cells. The commonly used approach for preparation of membrane is solvent casting. In this paper, high temperature polymer electrolyte membranes consisting of sulfonated poly(ether ether ketone) and 1-butyl-3-methylimidazolium tetrafluoroborate were prepared using solvent casting process from N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone solutions to understand the solvent effect on the nature of formed membranes. It was found that solvents used for casting process strongly affect the microstructure and ionic conductivity of formed membranes. The composite membrane cast from DMF solution has clearly inter-connected ionic clusters with diameters of several hundreds nanometers to about 1.5 μm and exhibits the highest ionic conductivity, reaching 1.04 × 10⁻² S cm⁻¹ at 170 °C under anhydrous conditions.     

Application of bis(fluorosulfonyl)imide-based ionic liquid electrolyte to silicon-nickel-carbon composite anode for lithium-ion batteries

An ionic liquid electrolyte containing bis(fluorosulfonyl)imide (FSI) anion without any solvent is applied to a silicon-nickel-carbon (Si-Ni-carbon) composite anode for rechargeable lithium (Li)-ion batteries. The FSI-based ionic liquid electrolyte successfully provides a stable, reversible capacity for the Si-Ni-carbon anode, which is comparable to the performance observed in a typical commercialized solvent-based electrolyte, while a common ionic liquid electrolyte containing bis(trifluoromethanesulfonyl)imide (TFSI) anion without FSI presents no reversible capacity to the anode at all. Ac impedance analysis reveals that the FSI-based electrolyte provides very low interfacial and charge-transfer resistances at the Si-based composite anode, even when compared to the corresponding resistances observed in a typical solvent-based electrolyte. Galvanostatic cycling of the Si-based composite anode in the FSI-based electrolyte with a charge limitation of 800 mAh g⁻¹ is stable and provides a discharge capacity of 790 mAh g⁻¹ at the 50th cycle, corresponding to a cycle efficiency of 98.8%.     

Simultaneously enhancing the thermal stability and electrochemical performance of solid polymer electrolytes by incorporating rod-like Zn2(OH)BO3 particles

Solid polymer electrolytes provide high safety and good electrochemical stability in solid-state lithium batteries (SSLBs) compared with conventional liquid electrolytes. In this work, a novel solid polymer composite electrolyte based on poly (ethylene oxide) (PEO) filled with rod-like Zn₂(OH)BO₃ particles was prepared by a grinding process followed with a heating treatment process and a cold pressing process. The effect of the incorporation amount of rod-like Zn₂(OH)BO₃ particles on the ionic conductivity was investigated systemically. It is found that 10 mol% of rod-like Zn₂(OH)BO₃ particles addition resulted in a highest ionic conductivity of 2.78 × 10⁻⁵ at 30 °C and the improved ionic conductivity was considered to be caused by the reducing of PEO crystallinity and the increasing of Li ion migrating pathway on the interface between the Zn₂(OH)BO₃ and PEO. In addition, the optimum composite electrolyte exhibited a high electrochemical stability window of 4.51 V (vs. Li/Li⁺), good lithium stability and excellent thermal stability.     

Novel anhydrous composite membranes based on sulfonated poly (ether ketone) and aprotic ionic liquids for high temperature polymer electrolyte membranes for fuel cell applications

Novel composite membranes were prepared using imidazolium type aprotic ionic liquids and sulfonated poly (ether ketone) (SPEK) as polymer matrix by solution casting process. All the prepared membranes were characterized for their thermal stability, mechanical properties, ion exchange capacity, proton conductivity and leaching out of ionic liquids in presence of water. Ionic liquid based membranes were more flexible than neat SPEK membrane due to the plasticization effect of ionic liquids. The interactions and compatibility occurring among components were investigated by vibration spectroscopy (FTIR ATR) and scanning electron microscopy respectively. The thermal stability of composite membranes was higher than unmodified membranes. The ion conductivity of composite membranes under anhydrous conditions was found to be dependent on temperature, type and concentration of ionic liquid in SPEK matrix. Ion conductivities of composite membranes under anhydrous condition were found to be up to two orders (∼100 times) higher than neat SPEK membrane and it was found to be ∼5 mS/cm at 140 °C for SPEK/OTf-70. These composite membranes can be successfully operated at temperatures ranging from 40 °C to 140 °C under anhydrous conditions.     

Performance of carbon-carbon supercapacitors based on organic, aqueous and ionic liquid electrolytes

Properties of capacitors working with the same carbon electrodes (activated carbon cloth) and three types of electrolytes: aqueous, organic and ionic liquids were compared. Capacitors filled with ionic liquids worked at a potential difference of 3.5 V, their solutions in AN and PC were charged up to the potential difference of 3 V, classical organic systems to 2.5 V and aqueous to 1 V. Cyclic voltammetry, galvanostatic charging/discharging and impedance spectroscopy were used to characterize these capacitors. The highest specific energy was recorded for the device working with ionic liquids, while the highest power is characteristic for the device filled with aqueous H₂SO₄ electrolyte. Aqueous electrolytes led to energy density an order of magnitude lower in comparison to that characteristic of ionic liquids.     

Analysis of SEI formed with cyano-containing imidazolium-based ionic liquid electrolyte in lithium secondary batteries

Two kinds of cyano-containing imidazolium-based ionic liquid, 1-cyanopropyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)imide (CpMI-TFSI) and 1-cyanomethyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)imide (CmMI-TFSI), each of which contained 20 wt% dissolved LiTFSI, were used as electrolytes for lithium secondary batteries. Compared with 1-ethyl-3-methylimidazolium-bis(trifluoromethane-sulfonyl)imide (EMI-TFSI) electrolyte, a reversible lithium deposition/dissolution on a stainless-steel working electrode was observed during CV measurements in these cyano-containing electrolytes, which indicated that a passivation layer (solid electrolyte interphase, SEI) was formed during potential scanning. The morphology of the working electrode with each electrolyte system was studied by SEM. Different dentrite forms were found on the electrodes with each electrolyte. The SEI that formed in CpMI-TFSI electrolyte showed the best passivating effect, while the deposited film formed in EMI-TFSI electrolyte showed no passivating effect. The chemical characteristics of the deposited films on the working electrodes were compared by XPS measurements. A component with a cyano group was found in SEIs in CpMI-TFSI and CmMI-TFSI electrolytes. The introduction of a cyano functional group suppressed the decomposition of electrolyte and improved the cathodic stability of the imidazolium-based ionic liquid. The reduction reaction route of imidazolium-based ionic liquid was considered to be different depending on whether or not the molecular structure contained a cyano functional group.     

UV-cured polymer electrolytes encompassing hydrophobic room temperature ionic liquid for lithium batteries

We demonstrate herewith the application of in situ one-shot free radical photo-polymerisation (UV-curing) process to incorporate room temperature ionic liquids (RTILs) into polymer membranes which can be used as electrolytes for lithium-based batteries. The reactive formulation for the preparation of the polymer membranes was based on a dimethacrylic oligomer (BEMA). The polymer electrolyte membranes were synthesized by UV radiating a mixture of BEMA and a proper radical photo-initiator with different compositions of 1-ethyl-3-methylimidazolium bis(perfluoroethylsulfonyl)imide [EMIPFSI] and, additionally, LiTFSI as lithium source.Stable and flexible polymer films with good mechanical integrity can easily be produced with varying the EMIPFSI content by using this method. Remarkable values of ionic conductivity were obtained even at ambient temperature. Galvanostatic charge/discharge cyclability tests were performed on the polymer electrolyte membranes by constructing a cell using LiFePO₄ as cathode and Li metal as anode. The preliminary results are interesting, exhibiting good reversibility and cyclability.     

Anodic behavior of Al current collector in 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] amide ionic liquid electrolytes

The anodic behaviors of aluminum current collector for lithium ion batteries were investigated in a series of 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] amide room temperature ionic liquids (RTILs) and EC + DMC electrolytes. It was found that the aluminum corrosion, which occurred in EC + DMC electrolytes containing LiTFSI, was not observed in the RTIL electrolytes. Further research showed that a passive film with amide compounds as main components formed firmly on aluminum surface during the anodic polarization in the RTIL electrolytes, which inhabited the aluminum corrosion. In addition, the additives generally used in the batteries, such as ethylene carbonate, ethylene sulfite and vinyl carbonate, as well as temperature did not obviously affect the aluminum passive film, the oxidation of the RTILs increased at the elevated temperature, which only resulted in the corrosion potential of aluminum in the RTIL electrolytes shifted to more negative potential, a passive film still firmly formed on the aluminum surface to surpassed the further oxidation of the aluminum current collector. Those results lead to a potential for the practical use of LiTFSI salt in the room temperature ionic liquid electrolytes for lithium ion batteries.     

On the addition of conducting ceramic nanoparticles in solvent-free ionic liquid electrolyte for dye-sensitized solar cells

Titanium carbide (TiC) is an extremely hard conducting ceramic material often used as a coating for titanium alloys as well as steel and aluminum components to improve their surface properties. In this study, conducting ceramic nanoparticles (CCNPs) have been used, for the first time, in dye-sensitized solar cells (DSSCs), and the incorporation of TiC nanoparticles in a binary ionic liquid electrolyte on the cell performance has been investigated.Cell conversion efficiency with 0.6 wt% TiC reached 1.68%, which was higher than that without adding TiC (1.18%); however, cell efficiency decreased when the TiC content reached 1.0 wt%. The electrochemical impedance spectroscopy (EIS) technique was employed to analyze the interfacial resistance in DSSCs, and it was found that the resistance of the charge-transfer process at the Pt counter electrode (R_ct1) decreased when up to 1.0 wt% TiC was added. Presumably, this was due to the formation of the extended electron transfer surface (EETS) which facilitates electron transfer to the bulk electrolyte, resulting in a decrease of the dark current, whereby the open-circuit potential (V_OC) could be improved. Furthermore, a significant increase in the fill factor (FF) for all TiC additions was related to the decrease in the series resistance (R_S) of the DSSCs. However, at 1.0 wt% TiC, the largest charge-transfer resistance at the TiO₂/dye/electrolyte interface was observed and resulted from the poor penetration of the electrolyte into the porous TiO₂. The long-term stability of DSSCs with a binary ionic liquid electrolyte, which is superior to that of an organic solvent-based electrolyte, was also studied.     

Stable dye-sensitized solar cells based on organic chromophores and ionic liquid electrolyte

A series of polyene-diphenylaniline based organic dyes (coded as D5, D7, D9 and D11) have been reported for the application in ionic liquid electrolyte based dye-sensitized solar cells. The effects of substitution of organic dyes on the photovoltaic performance have been investigated, which show addition of methoxy groups on the triphenylamine donor group increases short-circuit current, open-circuit voltage and photovoltaic performance. A power conversion efficiency of 6.5% under AM 1.5 sunlight at 100 mW/cm² has been obtained with D11 dye in combination with a binary ionic liquid electrolyte, which when subjected to accelerated testing under one sun light soaking at 60 °C, the efficiency remained 90% of initial efficiency.     

Limiting current density in bis(trifluoromethylsulfonyl)amide-based ionic liquid for lithium batteries

The physicochemical and electrochemical properties of the binary ionic liquid (IL), lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) dissolved in N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)amide (DEMETFSA), were investigated. The ionic conductivity of the binary IL decreased with an increase in LiTFSA concentration. The self-diffusion coefficients of Li⁺, DEME⁺, and TFSA⁻ dissolved in the IL were measured by using the pulsed-field-gradient spin-echo (PGSE) NMR method. The self-diffusion coefficient of each ionic species was also found to decrease with increasing concentration of LiTFSA. The limiting current density in the IL electrolyte was evaluated by chronoamperometry using symmetric Li|IL|Li cell. The results suggest that the diffusion process of Li(I) in the IL dominates the limiting current density in the cell. The highest limiting current density is achieved at a concentration of 0.64 mol dm⁻³ of LiTFSA.