FSI-based Ionic Liquids Research Articles

High rate and long-term cycling of silicon anodes with phosphonium-based ionic liquids as electrolytes for lithium-ion batteries

This paper describes the effects of the length of the phosphonium alkyl chain in FSI-based ionic liquids (ILs) used as an electrolyte solvent with Si electrodes in a Li/Si half-cell. The electrochemical performance of Si nanoparticles at a high rate in the synthesized triethyl-n-butylphosphonium bis(fluorosulfonyl)imide) [P2224][FSI] and triethyl-n-octylphosphonium bis(fluorosulfonyl)imide) [P2228][FSI] ILs, which also contain 1 M LiFSI, is compared with that of a common organic electrolyte using either polyacrylic acid (PAA) or polyacrylonitrile (PAN) as a binder. For the three electrolytes, the highest performance was obtained with the PAN binder, while both electrodes in ILs exhibited superior performance compared to the conventional organic electrolyte. For the Si/PAN composite electrode, after 1000 cycles at 1 A.g−1, the delithiation capacities were 1344, 550 and 136 mAh.g−1 for [P2224][FSI] in 1 M LiFSI, [P2228][FSI] in 1 M LiFSI and the commercial organic electrolyte, which corresponded to capacity retentions of 61%, 50% and 5%, respectively. Our results indicate that the chemical structure of the phosphonium cation enabled the tuning of transport properties and significantly influences the electrochemical behavior of the Si anode in the presence of LiFSI/IL as an electrolyte.

Phase equilibria and thermophysical properties of aqueous solutions of two bis(fluorosulfonyl)imide-based ionic liquids

Ionic liquids comprising bis(fluorosulfonyl)imide (FSI) anion have been recognized to be well suited for technological applications as solvent and electrolyte media. In this work, we thoroughly investigated the thermodynamic and transport behavior of binary aqueous systems of two hydrophobic FSI-based ionic liquids, namely [EMIM][FSI] and [BMPYR][FSI]. Systematic measurements of phase equilibria, density, viscosity, and electric conductivity were conducted, our aim being to provide truly reliable information that would substantially upgrade that existing in the literature. Employing DSC measurements on the neat ILs and dynamic measurements of freezing or cloud point temperatures as well as static solubility measurements for aqueous ILs, phase diagrams for both these systems over a 100 K window were established. Rather large solubilities of water in these ILs allow for measurements of various other properties in the homogeneous IL-rich region. Extensive data thus obtained on water activity in this region were correlated for each system simultaneously with the mutual liquid-liquid solubilities, using an extended SSF (Sum of Symmetrical Functions) equation. This global excess Gibbs energy model not only accurately captures all the VLE, LLE, and SLE data we determined experimentally, but also discloses an enthalpy-driven system energetics with hydrophobic hydration effects that are more pronounced in the case of [BMPYR][FSI]. The measurements of density, viscosity, and conductivity covering the systems from the neat ILs to those almost saturated with water gave positive excess volumes and negative viscosity deviations that rise and diminish, respectively, as T increases. The conductivity increases monotonously with increasing water content and temperature. Accurate functional representations were also established for the volumetric, viscosity, and conductivity data, capturing simultaneously their composition and temperature dependence. The linkage between conductivity and viscosity was further analyzed by means of the fractional Walden rule which provided excellent fits for both isopleths and isotherms and indicated comparatively weaker coupling for the [BMPYR][FSI] system and the direction of the isothermal composition changes. Finally, comparing the FSI ILs with their TFSI analogs, the former were found to be clearly less hydrophobic.

Solid-State Lithium and Li-Ion Batteries with Solid Nano-Composite Electrolytes

Solid electrolytes with a Li-ion conductivity exceeding 1 mS/cm are a prerequisite to enable solid-state Li-ion batteries. In solid composite electrolytes (SCEs), the interface between an ionic conductor and a dielectric matrix can be engineered to enhance this ionic conductivity. The solid composite electrolyte developed at imec contains a monolithic, mesoporous, silica matrix filled with a non-volatile ionic liquid and an organic Li-salt. This composite material is made by a sol-gel process, similar to that for ionogels, with that difference that no acid is used but water. The resulting aqueous gel is carefully dried from water and solvents, resulting in the solid nano-composite electrolyte where the ionic liquid and the lithium salt are confined in the pores and channels of the mesoporous silica matrix. The slow sol-gel reaction and drying allows the adsorption of an ordered molecular layer on the fully hydrolyzed silica surface. Interfacial ice layers induce strong adsorption and ordering of the ionic liquid molecules through H-bonding, rendering them immobile and solid-like as for the interfacial ice layer itself. The dipole over the adsorbate results in solvation of the Li+ ions for enhanced conduction. We demonstrate that when the silica surface is appropriately hydroxylated, the Li-ion conductivity of the nano-SCE can be several times higher than that of the pure ionic liquid electrolyte itself. By this process solid nano-SCEs with conductivities between 0.4 and 8 mS/cm have been synthesized, using TFSI- and FSI-based ionic liquid electrolytes. Battery cells can be made by impregnating the liquid sol-gel precursor solution inside the powder-based electrodes, very similar to the application of liquid electrolytes. As our precursor solution does not contain corrosive acid compounds such as formic acid, which is typically proposed as catalyst in literature recipes, we can deposit the solution directly on and into the porous electrodes. The sol-gel reaction and drying take place in-situ inside the electrodes allowing the nano-SCE to fill the empty spaces and providing an all-around contact with the active material. As such, we are able to realize functional solid-state cells with the TFSI- and FSI-based nano-SCE. Not only low voltage cells with LFP and LTO electrodes, but also high voltage cells with NCA and Li metal or Li alloy electrodes are demonstrated (Fig 1a and b). C-rate and cycling performance of these solid-state cells with the nano-SCE are given. Figure 1

(Invited) Fsi-Based Ionic Liquid Electrolyte and Lifsi-Based Solvent Electrolyte for Excellent Lib Performance

The application of ionic liquids (ILs) in Li-ion batteries (LIBs) would ensure safety taking advantage of their lower flammability than conventional organic electrolytes. However, decomposition of ILs on a negative electrode and intercalation of an organic cation, which lead to irreversible Li+ insertion/extraction, had been a serious problem of losing their charge-discharge capacity. Nevertheless, we successfully applied bis(fluorosulfonyl)imide anion (FSI)-based ILs to electrolytes for LIBs, which provide reversible Li-ion insertion/extraction and high efficiency for a graphitized anode without any additives [1-6]. We also tested the performance of some positive electrodes in FSI-based IL electrolytes: two kinds of active material, LiNi1/3Mn1/3Co1/3O2 (NMC) and LiNi0.5Mn1.5O4 (NM). According to 1.0/1.0 C-rate charge/discharge measurements in a potential range of 3.0–4.5 V vs. Li, the NMC cathode in EMImFSI with a Li salt (LiTFSI) maintains 95% of the initial discharge capacity even after 100 cycles, suggesting that the FSI-based IL provides relatively stable operation of the NMC cathode without any additives. Another remarkable point is that the NMC cathode in our FSI-based ionic liquids represents an excellent rate capability of ca. 70% even at 10 C-rate. Surprisingly, the value is rather better than that obtained in a conventional electrolyte of LiPF6/EC+DMC. We also tested Li-ion batteries containing FSI ILs as a “full cell”. A Li-ion battery with NMC as cathode material maintained over 80% of its initial capacity in 2000 cycles at 100% DOD [7]. As our IL battery technology have been developed very well, we applied our FSI-based IL-LIBs to a rechargeable power source on a small satellite in space. The Hodoyoshi-3 satellite with our IL-LIBs was launched into space on June 2014. Our space batteries can be still operated on the Hodoyoshi-3 in an earth orbital [8]. Moreover, we are also applying LiFSI-based solvent-type electrolytes to LIBs. We achieved significant reduction of a reaction resistance at the de-solvation process of EC molecule from Li ion by a novel electrolyte design focusing on the solvation state of Li ion by using the LiFSI salt.

Application of Ionic Liquid Electrolyte to Lithium Ion Capacitor Based on Electrodes with Porous Three-Dimensional Current Collector

A lithium ion capacitor (LIC) is a new storage device which combines an electric double-layer capacitor (EDLC) with a lithium ion battery (LIB). Namely, LIC consists of an activated carbon as positive electrode and lithium-ion-intercalating carbon material such as hard carbon as negative electrode. LIC also contains functionalities derived from both EDLC and LIB. During charge and discharge of LIC, ion adsorption/desorption occurs on the surface of the positive electrode, while lithium ion intercalation/de-intercalation occurs at the negative electrode. Although LIC features an excellent power density like EDLC, the energy density of LIC is lower than that of LIB. Therefore, improvement of energy density is required for LIC. In the present study, to achieve high energy density of LIC, porous 3-dimensinal (3D) current collector is applied to LIC electrodes. It is possible to increase the packing density of active material and make a LIC cell lighter. It is, however, expected that diffusion of carriers is limited with conventional organic electrolytes because electrodes using porous 3D current collector become massive; the typical thickness is ca. 1 mm. As a result, LIC using porous 3D current collector may deliver limited power. Thus, ionic liquids which have high carrier density would be useful as electrolyte of LIC to maintain power. The purpose of this study is applying an ionic liquid electrolyte to LICs with porous 3D current collector and investigating the basic operating characteristics.We assembled a three-electrode cell. A positive electrode using aluminum porous 3D current collector was made from activated carbon (AC, 87 wt.%), acetylene black (AB, 3 wt.%) and polyvinylidene di-fruoride (PVdF, 10 wt.%). A negative electrode using copper porous 3D current collector was made from hard carbon (HC, 87 wt.%), AB (8 wt.%) and PVdF (5 wt.%). A lithium foil was used as a counter electrode and reference electrode. The electrolyte was 1.5 mol dm−3 LiFSI/EMImFSI. The cell was galvanostatically charged and discharged for 3000 cycles at 1.0 C-rate in a voltage range of 2.0 – 3.8 V after a pre-doping process under predetermined conditions. We also evaluated rate performances of LIC cells with various electrolytes by rapid charging and discharging test. 1.5 mol dm−3 LiFSI/EMImFSI was used as standard ionic liquid electrolyte. 1.0 mol dm−3 LiPF6 / EC : DMC = 1 : 1 (v/v) and 1.0 mol dm−3 LiBF4 / EC : DMC = 1 : 1 (v/v) were used as organic electrolytes for comparison. C-rates for the power performance test were varied from 0.1 C to 30 C every 5 cycles after 100 pre-cycles.Fig. 1 shows potential profiles of positive and negative electrodes during charge and discharge at 2, 1000, 3000 cycles. It turns out that the LIC cell containing ionic liquid electrolytes is possible to charge and discharge reversibly as well as stably. Even though it was long-term cycling such as 3000 cycles, its capacity has not significantly decreased. Therefore, long-term cycle stability is high. Fig. 2 compares rete performances with the ionic liquid electrolyte and two organic electrolytes. Although ionic liquids have high viscosity [1], it is shown that FSI-based ionic liquid electrolyte has high rete performances comparable to a LiPF6-based organic electrolyte. This may be ascribed to high carrier density of ionic liquids. Rate performances of the FSI-based ionic liquid electrolyte and LiPF6-based organic electrolyte are superior to that of the LiBF4-based organic electrolyte. This is attributed to lower ionic conductivity of LiBF4. These results suggest that applying the ionic liquid electrolyte to a LIC electrode with porous 3D current collector is promising. FSI-based ionic liquid electrolytes may replace conventional organic electrolytes as electrolyte of LIC because of their resulting high cycle stability and high energy density without power loss.

The Importance of Electrolyte Composition to the Charge-Discharge Performance of Lithium-Ion Batteries Using Ionic Liquids

Ionic liquids such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) are still attractive candidates for use as electrolytes in electrochemical energy storage devices, because of their relatively low flammability and reactivity, and because of their electrochemical stability, wide liquidus temperature range, and acceptable ionic conductivity. We have reported a promising ionic liquid based on bis(fluorosulfonyl)imide (FSI-) for lithium-ion batteries since 2006. [1] This liquid with a lithium salt offers incredible charge-discharge performances; despite the relatively high viscosity of the electrolyte, it affords not only reversible and stable cycling but also rapid charge-discharge performance for both negative and positive electrodes. [2] In particular, we are interested in the mechanism of the high rate capability and high stability of the negative electrode of a LIB in FSI-based ionic liquids. By applying several electrochemical techniques, we also revealed the specific electrode–electrolyte interface formed in the presence of both Li+ and FSI- in the ionic liquid electrolyte [3]. On the negatively polarized carbon electrode, minimal Li+ can exist dominantly at the primary layer because of the moderately strong interaction between Li+ and FSI-, leading to the exclusion of EMIm+ away from the electrode. As a result, the presence of FSI-apparently improves the cathodic stability of the electrolyte. Herein, we report the results from charge-discharge tests, focusing on the effect of the electrolyte composition; we investigated how the high concentrated ionic liquid electrolyte including the lithium salts promotes the more stable and rapid charging-discharging of lithium-ion cell. To clarify the effect of concentration of LiFSI on the dependencies of the discharge capacity retention of a graphite electrode in EMImFSI and EMImTFSI (EMIm+ = 1-ethyl-3-methylimidazolium) on the charge-discharge C-rate, which were evaluated by determining the 5-cycle durability for each C-rate operation after primary charge-discharge cycling at the 0.1/0.1 C-rate, are summarized in Figure 1. Here, the discharge capacity retention was calculated as Cx /C 2, where Cx and C 2 are the specific discharge capacities for each cycle (x) and for the second cycle (1.0 C-rate), respectively. The remarkable aspect in this figure is that the high concentration (1.46 mol dm-3) of LiFSI improves the output characteristics in comparison with 0.43 mol dm-3 LiFSI/EMImFSI despite the electrolyte possessing a lower ionic conductivity, while 1.46 mol dm-3 LiTFSI/EMImFSI decreases the rate capability of the graphite compared with 0.43 mol dm-3LiTFSI/EMImFSI.

A Study on Rate Capacity and Safety of Ionic Liquid-Based Lithium Ion Battery

Lithium ion battery (LIB) is expected to be applied to large-format energy storage system and the cell performance with higher safety along with high energy density and high power density is demanded in industry applications. In order to improve the safety performance of LIB, much attention has been paid to an ionic liquid, instead of an organic solvent, as an electrolyte. LIB with the ionic liquid-based electrolyte, however, shows relatively poor rate capability compared to LIB with the organic solvent. Recently, several papers have revealed that LIB with bis(fluorosulfonyl)imide (FSI) anion-based ionic electrolyte shows good rate capability1,2). The reason is because of the reduction of charge transfer resistance between FSI anion-based ionic liquid electrolyte and electrode. The point of further development of the LIB using the ionic liquid-based electrolyte is to achieve both the improvement in the rate capability and safety by optimizing cell design including electrode, separator and others. In this study, influence of the electrode design and the nonwoven fabric separator on rate capability of a LIB with N-methyl-N-propylpyrrolidinium (Py13)/FSI-based ionic liquid electrolyte was investigated. The safety behavior of the LIB using Py13/FSI-based ionic liquid electrolyte was also evaluated by the overcharging test and nail penetrating test, comparing with the LIB using organic electrolyte. The rate capability of the LIB using Py13/FSI-based ionic liquid electrolyte was improved by optimizing the electrode design with higher porosity and thinner thickness and by using the nonwoven separator with more than 80 % of porosity. It is suggested that the enhancement of lithium ion mobility in both the porous electrode and the porous separator filled with the ionic liquid electrolyte is more effective to improve the rate capability, comparing the organic electrolyte. As a result of evaluation of the safety behavior, LiNi0.5Mn1.5O4/Li4Ti5O12cell using Py13/FSI-based ionic liquid electrolyte, as compared with the organic electrolyte, exhibited a superior safety performance in the nail penetrating test. The heat generation was also drastically suppressed by the high thermal stability ionic liquid electrolyte. The heat generation was drastically suppressed in comparison with the organic electrolyte due to high thermal stability of the ionic liquid electrolyte. References 1) H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono , J. Power Sources., 160, 1308 (2006) 2) T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono, M. Ishikawa, J. Power Sources., 183, 436 (2008)

A joint theoretical/experimental study of the structure, dynamics, and Li+ transport in bis([tri]fluoro[methane]sulfonyl)imide [T]FSI-based ionic liquids

The structure and dynamics of N-butyl-N-methyl pyrrolidinium(+) bis([tri]fluoro[methane]sulfonyl)imide(-) (PYR14(+)-[T]FSI(-)) ionic liquids doped with Li(T)FSI are investigated by combining experimental measurements to molecular dynamics simulations. The polarizable force field calculations indicate that the lithium cations are coordinated by (T)FSI anion oxygens forming lithium adducts stabilized over a large temperature range by strong Li-O bonds. Lithium aggregation is found to be negligible at the doping level considered here (10% mole fraction), and Li(+) diffusion occurs primarily by exchanging the (T)FSI anions in their first coordination shell. The resulting calculated transport properties are in good agreement with the corresponding nuclear magnetic resonance data.

FSI-Based Ionic Liquid Electrolyte and Its Specific Effects with Other Component Materials on Li Battery Performance

We evaluated charge-discharge behavior of four graphite electrodes—raw synthetic graphite (SG), shaped natural graphite (NG), soft carbon-coated natural graphite (SCNG), and hard carbon-coated natural graphite (HCNG)—in a 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMImFSI) ionic liquid containing lithium bis(trifluoromethylsulfonyl)imide (LiTFSI). According to charge-discharge curves at the first cycle in LiTFSI/EMImFSI, a capacity loss by an irreversible reaction was observed for cells with SG and SCNG, whereas the capacity loss for cells with NG and HCNG was relatively small. Coulombic efficiency with the cell cycling also proved that charge-discharge behavior of the graphite electrodes in LiTFSI/EMImFSI is strongly affected by surface structure of the graphite active materials and that basal-plane-oriented graphite, such as NG and HCNG, can reduce the loss of primary charge capacity.

Charge–discharge behavior of graphite negative electrodes in bis(fluorosulfonyl)imide-based ionic liquid and structural aspects of their electrode/electrolyte interfaces

We evaluated the charge–discharge behavior of four types of graphite electrode, including raw synthetic graphite (SG), shaped natural graphite (NG), soft carbon-coated natural graphite (SCNG), and hard carbon-coated natural graphite (HCNG), in a 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMImFSI) ionic liquid. Based on the charge–discharge curves for the first cycle in LiTFSI/EMImFSI, capacity loss arising from an irreversible reaction on the graphite anodes was clearly observed for the cells consisting of SG and SCNG, whereas the capacity loss for the cells containing NG and HCNG was relatively low. The coulombic efficiency during cell cycling also confirmed that the charge–discharge behavior of graphite in LiTFSI/EMImFSI is strongly affected by the surface structure of the graphite and that basal-plane-oriented graphite, such as NG and HCNG, can reduce the primary charge capacity loss. The advantages of the stable charge–discharge behavior provided by FSI-based ionic liquids for lithium ion batteries are also discussed based on the application of voltammetric and electrochemical impedance techniques to model carbon electrodes in ionic liquids. Based on these analyses, a mechanism involving a double-layer-based interface (not a solid electrolyte interface (SEI)) is proposed to stabilize the surface of graphite negative electrodes.

Fast Charge/Discharge of Li Metal Batteries Using an Ionic Liquid Electrolyte

Because of their potentially superior safety characteristics, room temperature ionic liquids (RTILs or ILs) have been vigorously researched as a potential replacement for current commercial lithium battery electrolytes, which are based on volatile and flammable organic carbonates. However, relatively poor battery performance, which is a consequence of the higher viscosity and lower conductivity of these materials, has prevented them becoming mainstream electrolytes for commercial lithium batteries. Amongst various RTILs, those containing the bis(fluorosulfonyl)imide (FSI) anion exhibit high conductivities and diffusivities, making them interesting potential electrolytes for lithium metal batteries. Here, we evaluate the electrochemical stability, lithium electrochemistry, and Li+ transference numbers of FSI-based ionic liquid electrolytes intended for use in rechargeable Li metal batteries. We show that ILs containing high concentrations of lithium, up to 3.2 mol.kg−1 in C3mpyr FSI, have excellent rate capability (higher than that of standard battery electrolytes) with both the lithium metal electrode and LiCoO2 cathode, in spite of their significantly higher viscosities and lower conductivities. This unusual behavior is ascribed to the concomitant increase in transference number with increasing Li-salt concentration.

Highly Stable Cycling of Si-C Composite Anode for Lithium-Ion Batteries by Using FSI-Based Ionic Liquid

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FSI-Based Ionic Liquid Electrolyte and Its Specific Effects with Other Component Materials on Li Battery Performance

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Charge-Discharge Characteristics of a LiNi1/3Mn1/3Co1/3O2 Cathode in FSI-based Ionic Liquids

Charge-Discharge Characteristics of a LiNi1/3Mn1/3Co1/3O2 Cathode in FSI-based Ionic Liquids

Li-Ion Battery Performance with FSI-Based Ionic Liquid Electrolyte and Fluorinated Solvent-Based Electrolyte

We evaluated the electrochemical behavior of a LiNi1/3Mn1/3Co1/3O2 (NMC) electrode in a neat ionic liquid, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMImFSI), electrolyte. According to charge/discharge measurements, the discharge capacity of the NMC cathode in EMImFSI at 1 C in a voltage range of 3.0-4.5 (vs. Li/Li+) shows 165 mAh g-1, which is higher than that in a conventional electrolyte, EC+DMC. The rate capability of a cell with EMImFSI exceeds that with EC+DMC, especially at high rates, probably due to a low resistance at the electrode/electrolyte interface. These results suggest that EMImFSI is a suitable electrolyte for Li-ion batteries utilizing a NMC cathode. Also, we applied fluorine-containing solvents (FCSs) to an electrolyte for Li-ion batteries with a cobalt oxide (LiCoO2) cathode to enhance the upper voltage limit and investigated its electrochemical properties in a proposed FCS-based electrolyte. A test cell containing a FCS-based electrolyte exhibits high and stable discharge capacity for 30 cycles even with a high upper cutoff voltage of 4.5 V when compared with a cell containing a conventional electrolyte.

Li-Ion Battery Performance with FSI-Based Ionic Liquid Electrolyte and Fluorinated Solvent-Based Electrolyte

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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%.

Electrochemical Lithium Insertion/Extraction for Carbon Electrodes in FSI-based Ionic Liquids

Introduction of bis(fluorosulfonyl)imide (FSI) as an anion to an ambient-temperature ionic liquid electrolyte based on 1-ethyl-3-methylimidazolium and lithium (Li) as cations can provide a reversible Li intercalation into a graphitized electrode, while the intercalation is completely irreversible without FSI. Laser desorption/ionization mass spectroscopy and ac impedance spectroscopy suggest that FSI-coordinated Li+ is facilely mobile even without any solvents and is reversibly accessible to a graphite interior.

Electrochemical Lithium Insertion/Extraction for Carbon Electrodes in FSI-based Ionic Liquids

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