Accelerating rate calorimetry studies of the reactions between ionic liquids and charged lithium ion battery electrode materials

Using accelerating rate calorimetry (ARC), the reactivity between six ionic liquids (with and without added LiPF₆) and charged electrode materials is compared to the reactivity of standard carbonate-based solvents and electrolytes with the same electrode materials. The charged electrode materials used were Li₁Si, Li₇Ti₄O₁₂ and Li_0.45CoO₂. The experiments showed that not all ionic liquids are safer than conventional electrolytes/solvents. Of the six ionic liquids tested, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI) shows the worst safety properties, and is much worse than conventional electrolyte. 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI) and 1-propyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (Py13-FSI) show similar reactivity to carbonate-based electrolyte. The three ionic liquids 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide (BMMI-TFSI), 1-butyl-1-methylpiperidinium bis(trifluoromethanesulfonyl)imide (Pp14-TFSI) and N-trimethyl-N-butylammonium bis(trifluoromethanesulfonyl)imide (TMBA-TFSI) show similar reactivity and are much safer than the conventional carbonate-based electrolyte. A comparison of the reactivity of ionic liquids with common anions and cations shows that ionic liquids with TFSI⁻ are safer than those with FSI⁻, and liquids with EMI⁺ are worse than those with BMMI⁺, Py13⁺, Pp14⁺ and TMBA⁺.     

The effects of Li salts on the performance of a polymer actuator based on single-walled carbon nanotube-ionic liquid gel

The effects of Li salts (Lithium tetrafluoroborate (Li[BF₄]) and Lithium bis(trifluoromethanesulfonyl)imide (Li[TFSI])) on the electrochemical and electromechanical properties of an actuator using a polymer-supported single-walled carbon nanotube (SWCNT)-ionic liquid (IL) gel electrode were investigated. The ionic conductivities of the gel electrolyte layers with molar ratios of Li[BF₄]/1-ethlyl-3-methylimidazolium tetrafluoroborate (EMI[BF₄]) = 0.1 and 0.5, and Li[TFSI]/1-ethlyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI[TFSI]) = 0.1 and 0.3 were higher than those containing only EMI[BF₄] and only EMI[TFSI], respectively. We found a large capacitance value 65-96 F/g at a slow sweep rate 1 mV s⁻¹. The actuator containing Li salt/IL performed much better than that containing only IL. It is considered that the higher ionic conductivity of the gel electrolyte layer containing Li salt/IL produces the quick response actuator, and that the large capacitance gives a large generated strain.     

Asymmetrical dicationic ionic liquids based on both imidazolium and aliphatic ammonium as potential electrolyte additives applied to lithium secondary batteries

Asymmetrical dicationic ionic liquids based on the combination of imidazolium and aliphatic ammonium cations with TFSI anion, MIC_nN₁₁₁-TFSI₂, have been synthesized for the first time, wherein MI represents imidazolium cation, N₁₁₁ represents trimethylammonium cation, and C_n represents spacer length. The physical and electrochemical properties of this family of ionic liquids were studied. 1-(3-Methylimidazolium-1-yl)ethane-(trimethylammonium) bi[bis(trifluoromethane-sulfonyl) imide] (MIC₂N₁₁₁-TFSI₂) shows solid-solid transition characteristics. 1-(3-Methylimidazolium-1-yl)pentane-(trimethylammonium) bi[bis(trifluoromethan-esulfonyl)imide] (MIC₅N₁₁₁-TFSI₂) has one of the lowest solid-liquid transformation temperatures among analogues, and belongs to the greatest thermal stable ionic liquids. Additionally, it has an order of conductivity of 10⁻¹ ms cm⁻¹, and electrochemical window of about 3.7 V at room temperature. To evaluate the potential of MIC₅N₁₁₁-TFSI₂ as an additive of electrolyte for lithium secondary batteries, cells composed of LiMn₂O₄ cathode/1 M LiPF₆ in EC:DMC (1:1, v/v) electrolytic solution containing 5 wt% of MIC₅N₁₁₁-TFSI₂/lithium metal anode have been prepared. The charge-discharge cycling test reveals that unlike the cases of Li/LiMn₂O₄ cells employing a conventional electrolyte with a monocationic ionic liquid, such as 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EtMeImTFSI) as an additive, the performances of Li/LiMn₂O₄ cells do not drop with the addition of MIC₅N₁₁₁-TFSI₂ at 1C rate, moreover, the cell exhibits better discharge capacity and cycle durability compared with the cell using the conventional electrolyte.     

Ionic Liquid Electrolytes for Li–Air Batteries: Lithium Metal Cycling

In this work, the electrochemical stability and lithium plating/stripping performance of N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr₁₄TFSI) are reported, by investigating the behavior of Li metal electrodes in symmetrical Li/electrolyte/Li cells. Electrochemical impedance spectroscopy measurements and galvanostatic cycling at different temperatures are performed to analyze the influence of temperature on the stabilization of the solid electrolyte interphase (SEI), showing that TFSI-based ionic liquids (ILs) rank among the best candidates for long-lasting Li–air cells.     

Mixtures of ionic liquid and organic carbonate as electrolyte with improved safety and performance for rechargeable lithium batteries

In this paper we report the results of physical–chemical and electrochemical investigations performed on ternary mixtures of the room temperature ionic liquid (IL) N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI), propylene carbonate (PC), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as electrolyte for lithium-ion batteries. The thermal stability, ionic conductivity, viscosity and electrochemical stability windows of all considered mixtures were investigated and compared with those of electrolytes based on the pure PYR₁₄TFSI and PC. The mixtures were also used as electrolyte in combination with LiFePO₄-based electrodes. The specific capacity and cycling stability of these systems were investigated at different C-rates, both at room temperature and 60 °C.     

Methacrylate-based ionic liquid: radical polymerization/copolymerization with methyl methacrylate and evaluation of molecular weight of the obtained homopolymers

Azobisisobutyronitlite (AIBN)-induced free radical polymerization of a methacrylate-based ionic liquid monomer, 1-(2-methacryloxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Met-IL) was carried out in a common organic solvent, N,N-dimethylformamide (DMF), and an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI). The molecular weight of the obtained poly(Met-IL) was evaluated by transforming it to non-ionic poly(methyl methacrylate) with hydrolysis of the imidazolium-salt-substituted pendant ester groups and methyl esterification. Radical copolymerization with methyl methacrylate (MMA) was also carried out in both DMF and EMImTFSI. Analysis of copolymer composition revealed that the reactivity of Met-IL was lower than that of MMA in both DMF and EMImTFSI solutions.     

Effect of water and oxygen traces on the cathodic stability of N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide

Although research in the field of ionic liquids for electrochemical applications has led to a deeper knowledge in their electrochemical properties, doubts in the interpretation of the experimental results are still encountered in the literature due to the poor control of the experimental conditions and/or to the limited number of experiments conducted. In this work, the effect of water and oxygen traces on the cathodic stability window of hydrophobic, air-stable ionic liquids composed of N-alkyl-N-methylpyrrolidinium (PYR_1A⁺) cations and bis(trifluoromethanesulfonyl)imide (TFSI⁻) anion, is reported. The extensive investigation performed by linear sweep voltammetry (LSV) and cyclic voltammetry (CV) indicates that the TFSI⁻ anion is cathodically stable if the ionic liquid is pure and dry. The N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids investigated showed featureless cathodic linear sweep voltammetry curves before the massive cation decomposition took place at very low potentials.     

Electropolymerization of poly(3-methylthiophene) in pyrrolidinium-based ionic liquids for hybrid supercapacitors

The ionic liquids (ILs) N-butyl-N-methyl-pyrrolidinium trifluoromethanesulfonate (PYR₁₄Tf) and N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide (PYR₁₃FSI) are investigated as electropolymerization media for poly(3-methylthiophene) (pMeT) in view of their use in carbon/IL/pMeT hybrid supercapacitors. Data on the viscosity, solvent polarity, conductivity and electrochemical stability of PYR₁₄Tf and PYR₁₃FSI as well as the effect of their properties on the electropolymerization and electrochemical performance of pMeT, which features >200 Fg⁻¹ at 60 °C when prepared and tested in such ILs, are reported and discussed; the results of the electrochemical characterization in N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide of the so-obtained pMeT are also given, for comparison.     

Sulfur-mesoporous carbon composites in conjunction with a novel ionic liquid electrolyte for lithium rechargeable batteries

Sulfur coated mesoporous carbon (S-C) composites have been synthesized and physically characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and thermogravimetric analysis. Firstly, the electrochemical properties of the S-C composite cathode materials were tested in a conventional electrolyte consisting of 1 mol/L lithium bistrifluoromethanesulfonimidate in poly(ethylene glycol) dimethyl ether to compare them with pure sulfur electrode. The capacity and cyclic stability of the S-C composite were improved. Then the S-C composites were tested in a novel ionic liquid electrolyte consisting of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and lithium bistrifluoromethanesulfonimidate. The capacity and cyclic stability of the S-C composite using the ionic liquid electrolyte were much better than for the sample tested in a conventional organic solvent electrolyte.     

Properties of LiMn2O4 cathode in electrolyte based on N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide

LiMn₂O₄ was examined as a cathode material for lithium-ion batteries, working together with a room temperature ionic liquid electrolyte, obtained by dissolution of solid lithium bis(trifluoromethanesulfonyl)imide (LiNTf₂) in liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (MePrPipNTf₂), with the formation of a liquid LiNTf₂-MePrPipNTf₂ system. The Li/LiMn₂O₄ cell was tested by galvanostatic charging/discharging and by impedance spectroscopy. The LiMn₂O₄ cathode showed good cyclability and Coulombic efficiency in the presence of 10 wt.% of vinylene carbonate (VC) as an additive to the ionic liquid. The flash point of the LiNTf₂-MePrPipNTf₂-VC(10%) electrolyte was estimated to be above 300 °C.     

Electrochemical properties of novel ionic liquids for electric double layer capacitor applications

An aliphatic quaternary ammonium salt which has a methoxyethyl group on the nitrogen atom formed an ionic liquid (room temperature molten salt) when combined with the tetrafluoroborate (BF₄⁻) and bis(trifluoromethylsulfonyl)imide [TFSI; (CF₃SO₂)₂N⁻] anions. The limiting oxidation and reduction potentials, specific conductivity, and some other physicochemical properties of the novel ionic liquids, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF₄) and DEME-TFSI have been evaluated and compared with those of 1-ethyl-3-methylimidazolium tetrafluoroborate. DEME-BF₄ is a practically useful ionic liquid for electrochemical capacitors as it has a quite wide potential window (6.0 V) and high ionic conductivity (4.8 mS cm⁻¹ at 25 °C). We prepared an electric double layer capacitor (EDLC) composed of a pair of activated carbon electrodes and DEME-BF₄ as the electrolyte. This EDLC (working voltage ∼2.5 V) has both, a higher capacity above room temperature and a better charge-discharge cycle durability at 100 °C when compared to a conventional EDLC using an organic liquid electrolyte such as a tetraethylammonium tetrafluoroborate in propylene carbonate.     

Cycling stability of a hybrid activated carbon//poly(3-methylthiophene) supercapacitor with N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid as electrolyte

A long cycle-life, high-voltage supercapacitor featuring an activated carbon//poly(3-methylthiophene) hybrid configuration with N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid, a solvent-free green electrolyte, was developed. The cyclability of a laboratory scale cell with electrode mass loading sized for practical uses was tested at 60 °C over 16,000 galvanostatic charge-discharge cycles at 10 mA cm⁻² in the 1.5 and 3.6 V voltage range. The reported average and maximum specific energy and power, specific capacitance and capacity, equivalent series resistance and coulombic efficiency over cycling demonstrate the long-term viability of this ionic liquid as green electrolyte for high-voltage hybrid supercapacitors.     

电化学阴极剥离制备少层石墨烯及其微型超级电容器

以石墨为原料高效、绿色、低成本制备少层石墨烯,对石墨烯的规模化生产和应用具有非常重要的意义。电化学阴极剥离法是一种高效制备少层石墨烯的方法,但已有的报道均采用有机溶液体系,成本高且不够绿色环保。开发了一种绿色的水溶液电化学剥离方法,在6 mol·L^-1 KOH溶液中,将石墨作为阴极进行快速剥离制备出少层石墨烯。获得的少层石墨烯具有含氧量低[1.27%(质量)]、缺陷少(I_D/I_G < 0.035)、片径尺寸为5~10 μm、高电导率(大于200 S·cm^-1)以及良好溶液可加性等特点。基于此,采用叉指型掩模板辅助过滤的方法可以高效制备出图案化石墨烯基平面微电极,在硫酸-聚乙烯醇凝胶电解液中,构筑的准固态微型电容器在没有金属集流体存在的情况下,表现出高扫描速率,达到了100000 mV·s^-1,弛豫时间常数低至24 ms;以1-乙基-3甲基-咪唑双(三氟甲基磺酰基)亚胺和双(三氟甲基磺酰基)亚胺锂盐的混合液为电解液,所构建的微型超级电容器的工作电压达4.0 V,体积能量密度为113 mW·h·cm^-3,远高于目前报道的微型超级电容器的电化学性能(<50 mW·h·cm^-3)。     

The influence of air and its components on the cathodic stability of N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide

Although water- and air-stable ionic liquids have been in use for some years, experiments found in the literature are still performed in inert gas with ppm levels of oxygen and water. In this study, the influence of different environments (vacuum, argon, nitrogen, air and oxygen and water) on the cathodic electrochemical window of the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) is reported and compared with investigations and processes found in the literature. The investigation indicates that this ionic liquid is highly stable in a vacuum and under argon flow. However, its cathodic stability is reduced in nitrogen and dry air. The simultaneous presence of water and air strongly affected the useful electrochemical window, as seen previously for imidazolium-based ionic liquids.     

The electrodeposition of polypyrrole on Al alloy from room temperature ionic liquids

The direct electrodeposition of conjugated polymers onto active metals such as aluminum and its alloys is complicated by the concomitant oxidation of the metal that occurs at the positive potential required for polymer formation/deposition. We previously described an approach that uses electron transfer mediation to reduce the deposition potential of polypyrrole (PPy) on aluminum and aluminum alloy by nearly 500 mV, permitting film deposition from aqueous solution with nearly 100% current efficiency. In this report, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM⁺TFSI⁻) has been successfully employed both as the growth medium and the supporting electrolyte for directly depositing uniform and conductive PPy coatings onto Al alloy 2024-T3 surface via a potentiodynamic technique. The depositions of PPy were carried out under cyclic voltammetric conditions from 0.3 M pyrrole in ionic liquid solutions. Film morphology was characterized by atomic force microscopy, optical microscopy, and scanning electron microscopy (SEM). Energy dispersive X-ray analysis and X-ray photoelectron spectroscopy verified that the TFSI⁻ anion was incorporated into the polymer as the dopant ion. Thickness of the film was measured by SEM and film conductivity was determined by both a four-point probe technique and by conducting atomic force microscopy. Electrochemical activity of the film was assessed by cyclic voltammetry. Results from these preliminary studies will be reported. Presented at the 2007 FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in Toronto, Ont., Canada, on October 3–5, 2007.     

Preparation of networked polymer electrolytes by copolymerization of a methacrylate with an imidazolium salt structure and an ethyleneglycol dimethacrylate in the presence of lithium bis(trifluoromethanesulfonyl)imide

Ionic networked polymers containing ionic liquids were synthesized by radical copolymerization of 1-(2-methacryloyl)ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Met-IL) and difunctional or trifunctional methacrylate cross-linkers, ethylene glycol dimethacrylate (EGDMA), or trimethylolpropane trimethacrylate (TMPTMA) in the presence of an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI). For comparison, nonionic networked polymers containing the ionic liquid were prepared by radical copolymerization of methyl methacrylate (MMA) with EGDMA in the presence of EMImTFSI. Met-IL/EGDMA/EMImTFSI(50) showed higher ion conductivity and ionic liquid holding ability than the corresponding MMA/EGDMA/EMImTFSI(50), while Met-IL/TMPTMA/EMImTFSI(50) showed lower ionic conductivity with higher holding ability than MMA/EGDMA/EMImTFSI(50).     

Water/ionic liquid/organic three-phase interfacial synthesis of coral-like polypyrrole toward enhanced electrochemical capacitance

Two interfacial synthesis strategies are proposed to synthesize polypyrrole samples for electrochemical capacitors (ECs). In contrast to water/organic two-phase route, unique water/ionic liquid (IL)/organic three-phase interface strategy is first performed to prepare coral-like polypyrrole with even better electrochemical capacitance, where 1-Ethyl-3-methylimidazolium tetrafluoroborate IL, as a “buffering zone”, is set between the water and organic phases to control the morphology and micro-structure of the polypyrrole phase during polymerization. The polypyrrole synthesized by three-phase interfacial route owns more ordered structure, less charge transfer resistance and better electronic conductivity, compared with two-phase method, and delivers larger specific capacitance, higher rate performance and better electrochemical stability at large current densities in 3 M KCl aqueous electrolyte.     

The effects of LiBOB additive for stable SEI formation of PP13TFSI-organic mixed electrolyte in lithium ion batteries

A safe electrolyte system is prepared from N-methyl-N-propylpiperidinium bis (trifluoromethanesulfonyl) imide (PP13TFSI), organic electrolyte (1 mol L⁻¹ LiPF₆/EC-DEC) and lithium bis (oxalato) borate (LiBOB). The additive of LiBOB enhances the stability of interface between electrolyte and anode. The LiBOB-containing mixed electrolytes show non-flammability and good compatibility with active materials. The performance of anode for lithium ion battery is successfully improved by LiBOB-containing mixed electrolytes, which shows 200 mA h g⁻¹ reversible capacities at 0.3 C rate. The ionic conductivity and the lithium ion transference number in LiBOB-containing mixed electrolytes system also suits to application for lithium ion battery.     

Synthesis and properties of methacrylate-based ionic networked polymers containing ionic liquids: comparison of ionic and nonionic networked polymers

Ionic networked polymers containing ionic liquids were synthesized by radical copolymerization of 1-(2-methacryloyl)ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Met-IL) and difunctional or trifunctional methacrylate cross-linkers, ethylene glycol dimethacrylate (EGDMA), or trimethylolpropane trimethacrylate (TMPTMA) in the presence of an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI). For comparison, nonionic networked polymers containing the ionic liquid were prepared by radical copolymerization of methyl methacrylate (MMA) with EGDMA in the presence of EMImTFSI. Met-IL/EGDMA/EMImTFSI(50) showed higher ion conductivity and ionic liquid holding ability than the corresponding MMA/EGDMA/EMImTFSI(50), while Met-IL/TMPTMA/EMImTFSI(50) showed lower ionic conductivity with higher holding ability than MMA/EGDMA/EMImTFSI(50).     

Hydrogen bioelectrooxidation in ionic liquids: From cytochrome c3 redox behavior to hydrogenase activity

Electrochemistry of polyheme bacterial cytochrome c₃ and catalytic oxidation of hydrogen by two different bacterial [NiFe] hydrogenases were investigated for the first time in pure room-temperature ionic liquids (RTILs) as electrolyte. Direct electrochemical response of Desulfovibrio vulgaris Hildenborough cytochrome c₃ (DvH cytc₃) adsorbed at a pyrolytic graphite (PG) electrode was observed in the RTILs used in this work: 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4), 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EmimNTf2). The electrochemical signal differed however from that obtained in aqueous buffer, and depended on the type of RTIL. UV–vis measurements as well as transfer experiments from aqueous buffer to RTILs or RTILs to aqueous buffer strongly suggested that the protein was not denatured in the presence of RTILs. EmimNTf2, as a hydrophobic non-water-miscible RTIL, was demonstrated to stabilize the native form of DvH cytc₃. Moreover it allowed an amount of electroactive DvH cytc₃ 30-fold higher than observed in aqueous buffer. Catalytic oxidation of H₂ via Desulfovibrio fructosovorans [NiFe] hydrogenase (Df Hase) mediated by DvH cytc₃ failed however. Further investigation suggested that Df Hase could be inhibited in the presence of RTILs. Reasons for such an inhibition were explored, including the blocking up of the substrate channels. By using hyperthermophilic [NiFe] membrane-bound hydrogenase from Aquifex aeolicus (Aa Hase) an efficient direct catalytic oxidation process was obtained in mixed aqueous buffer/RTILs electrolytes, although direct H₂ oxidation was not observed in pure RTIL. Chronoamperometric experiments showed that Aa Hase could afford 80% RTILs in aqueous buffer, thus giving the opportunity of future electrolytes with uncommon and variable properties for biofuel cell design.