Conventional Carbonate-based Electrolytes Research Articles

(Invited) Electrochemical Characterization of Lithium-Ion Cells Based on Li1.2Ni0.15Mn0.55Co0.1O2 and Li4Ti5O12

In this presentation we will detail the electrochemical characteristics of lithium-ion cells containing Li1.2Ni0.15Mn0.55Co0.1O2 (0.5Li2MnO3∙0.5LiNi0.375Mn0.375Co0.25O2)-based positive and Li4Ti5O12-based negative electrodes. The Li1.2Ni0.15Mn0.55Co0.1O2 compound is part of the lithium- and manganese- rich (LMR-NMC) family of layered oxides that display capacities exceeding 250 Ah-kg-1.1 To achieve these high capacities, the Li1.2Ni0.15Mn0.55Co0.1O2 is typically cycled beyond the activation plateau at ~4.45V vs. Li/Li+, which is associated with irreversible structural changes in the oxide. Li4Ti5O12 (LTO) negative electrodes are an alternative to graphite-based negative electrodes that are typically used in lithium-ion cells.2 LTO reversibly transforms to Li7Ti5O12, a rock salt-type compound during lithiation. The transformation occurs at ~1.55V vs. Li/Li+, well within the stability window of conventional carbonate-based electrolytes, and high enough to prevent Li-plating and allow use of aluminum current collectors. LTO-based electrodes show excellent good rate capability, enhanced thermal stability, minimal strain, and capacities to ~170 mAh-g-1. Cells with LTO electrodes coupled to LiCoO2, LiNi0.8Co0.15Al0.5O2, LiMn2O4, LiNi1/3Co1/3Mn1/3O2 and LiFePO4 demonstrate improved cycling stability over their graphite-based counterparts. All electrodes in this study are from the Cell Analysis, Modeling, and Prototyping (CAMP) facility at Argonne.3 The positive electrodes contained a coating of 92 wt% Li1.2Ni0.15Mn0.55Co0.1O2, 4 wt% C45 carbons and 4 wt% PVDF binder on an Al current collector. The negative electrodes contained a coating of 87 wt% Li4Ti5O12, 5 wt% C45 carbons and 8 wt% PVDF binder, also on an Al current collector. Electrochemical tests were conducted in 2032-type coin cells (1.6 cm2 electrodes) housed in a 30°C temperature chamber. All cells were assembled in an argon-atmosphere glove box and contained Celgard 2325 separator and EC:EMC (3:7 by wt.) + 1.2 M LiPF6 (aka Gen 2) electrolyte. The cells were cycled in 2 voltage ranges – 0.75-2.55 V (2.3-4.1 V vs. Li/Li+), which is below the oxide activation plateau, and 0.75-3.15 V (2.3-4.7 V vs. Li/Li+), which is above the oxide activation plateau. Cell capacity, voltage, energy and impedance data over 500 cycles were obtained during the testing period. Some observations from our experiments are as follows: The Li1.2Ni0.15Mn0.55Co0.1O2 is not ‘activated’ during the 0.75-2.55 V cycling. Therefore cell capacity is low, 70 mAh-g(oxide)-1, and cell impedance is high. After 500 cycles, cell capacity is mostly unchanged but cell impedance rises, exclusively at the positive electrode. Voltage fade, is any, is minimal even after long term cycling.The Li1.2Ni0.15Mn0.55Co0.1O2 is ‘activated’ during the 0.75-3.15 V cycling, and the cell yields a significantly higher capacity, 230 mAh-g(oxide)-1, which is mostly unchanged. However, cell impedance rises and significant voltage fade is observed after 500 cycles; both features can be attributed to the positive electrode. Additional experiments, including electrochemical measurements and physicochemical diagnostic studies on harvested electrodes and electrolytes will be detailed. The consequences of LTO-based negative electrodes on cell energy and performance degradation will be discussed during the presentation. Acknowledgements Support from the U.S. Department of Energy’s Office of Vehicle Technologies Program is gratefully acknowledged. We are grateful to S. Trask, B. Polzin, and A. Jansen from the CAMP Facility (Argonne), which is fully supported within the core funding of the Applied Battery Research (ABR) for Transportation Program. References Bettge et al., J. Electrochem. Soc. 160 (2013) A1-A10.Trask et al., J. Power Sources 259 (2014) 233-244.Abraham et al., Electrochimica Acta 51 (2005) 502-510. Figure 1

A rechargeable lithium/quinone battery using a commercial polymer electrolyte

The present study reports superior electrochemical performance with capacity doubled for organic positive electrodes based on a small redox-active molecule when using the Lithium Metal Polymer (LMP) technology. Particularly, the simple use of the regular solid polymer electrolyte currently employed in commercial LMP cells allows obtaining for the first time an efficient two-electron cycling of tetramethoxy-p-benzoquinone with high-rate capability at temperatures as high as 100 °C. With no optimization, the restored capacity represents 80% of the theoretical value (190 mAh/g) after 20 cycles operated at a C rate. On the contrary, when cycled in conventional carbonate-based electrolytes, this quinone compound exhibits quite poor electrochemical features such as a very limited depth of discharge (~ 50% of the theoretical capacity in the first cycle) followed by rapid capacity decay. After cycling, preliminary post-mortem characterizations did not evidence any obvious degradation in the cell. Although the adverse effect of the diffusion of the active material is not fully inhibited, the coulombic efficiency is close to 100% while the Li/electrolyte interface appears stable.

In Situ Investigation on Spatiotemporal Changes of SEI Properties By SECM

In our society Li-ion batteries are widely used, especially for small-scale devices. During the first charging process the solid-electrolyte-interface (SEI) between the electrode and electrolyte is established. This layer passivates the surface against electron transfer reaction but allows transfer of Li-ions. In order to realize high-energy Li-ion batteries in conventional carbonate-based electrolytes, the formation of the SEI is required.[1] Content and structure of the SEI are highly discussed.[2, 3] The SEI represents a complex chemical system, where several solid, liquid and gaseous compounds establish interdependent equilibria. Characterization of the SEI is a difficult challenge, because of the similar chemical identity of SEI components and electrolyte molecules. Furthermore, ex situ analysis of the SEI requires separation and isolation of the SEI, which may change the content and the structure of the SEI.[3] In situ analysis is highly sought after to circumvent this problem.In our approach we use the scanning electrochemical microscopy (SECM) to investigate in situ the electron transport across the SEI. For this purpose, a redox mediator is added to the electrolyte, which communicates between the battery electrode and the positionable ultra-microelectrode (UME). Our results by SECM demonstrate the dynamic of SEI formation and dissolution. Furthermore, the swelling behavior of practical graphite anodes in standard battery electrolytes is characterized.

Invited Presentation: Electrolyte Breakthrough to Advance Flexible Batteries

Flexible batteries have garnered a great deal of attention as a promising power source for versatile-shaped electronic devices including bendable smart phones, roll-up displays and wearable electronics, which play crucial roles in facilitating the advent of wearable electronics era. One formidable challenge for development of the flexible batteries arises from difficulties in securing solid-state electrolytes with reliable electrochemical/mechanical properties. Currently available liquid electrolytes show good electrochemical performance suitable for practical applications, however, they often suffer from safety failures due to unwanted leakage problems and flammable characteristics. Moreover, liquid electrolytes limit choices in cell design due to their fluidic attributes, which essentially require the use of separator membranes and packaging canisters with fixed dimension in cell assembly. These shortcomings of liquid electrolytes motivate us to develop advanced solid-state electrolytes that can easily conform to complex-structured substrates such as three-dimensional (3D) electrodes and also ensure sufficient mechanical deformability for reliable use.Herein, as electrolyte breakthrough to advance flexible batteries, a new class of bendable, shape-conformable and thermally-stable composite polymer electrolyte is demonstrated and also its potential application to flexible batteries featuring shape diversity is explored. More notably, the composite polymer electrolyte can be directly writable and printable onto complicated/contoured electrodes, without the aid of processing solvents. The unique structural/compositional designs and well-tuned rheological properties, in collaboration with direct UV-assisted nanoimprint lithography, allow for successful fabrication of solid-state composite polymer electrolytes in geometries that lie far beyond those accessible with conventional electrolytes. As an alternative electrolyte to exceed conventional carbonate-based liquid electrolytes, plastic crystal electrolytes (PCEs) have been investigated. The PCEs, which are composed of lithium salts and plastic crystals bearing good solvation capability, are characterized with unusual thermal stability and ionic transport behaviors. Among various plastic crystal candidates, succinonitrile (SN) is known to show excellence in thermal stability and ionic transport due to high boiling point and structural defects (i.e., trans-gauche isomerism) of plastic crystal phase. Despite of these advantageous characteristics, application of the PCEs to flexible batteries is staggered due to their poor mechanical properties. Most of PCEs are excessively plastic and even more show liquid-like behavior at room temperature. In this talk, we present a wide variety of material strategies to improve the physicochemical properties (in particular, focusing on mechanical flexibility) of SN-based PCEs, which are mainly devoted to integrating with functional polymer matrix that serves as a mechanical framework. The structural design concept demonstrated herein for the synthesis of plastic crystal polymer electrolytes can be suggested as a versatile and scalable electrolyte platform to bring unprecedented opportunities in progress of next-generation flexible energy storage/conversion devices. References Keun-Ho Choi, Sung-Ju Cho, Se-Hee Kim, Yo Han Kwon, Je Young Kim, Sang-Young Lee, "Thin, deformable, and safety-reinforced plastic crystal polymer electrolytes for high-performance flexible lithium-ion batteries", Adv. Funct. Mater. online published.Sang-Young Lee, Keun-Ho Choi, Yo Han Kwon, Hye-Ran Jung, Heon-Cheol Shin, Je Young Kim, "Progress in flexible energy storage and conversion systems, with a focus on cable-type lithium ion battery", Energy Environ. Sci. 2013, 6, 2414. (featured as a back cover image).Eun-Hye Kil, Keun-Ho Choi, Hyo-Jeong Ha, Sheng Xu, John A. Rogers, Mi Ri Kim, Young-Gi Lee, Kwang-Man Kim, Kuk Young Cho, Sang-Young Lee, "Imprintable, bendable, and shape-conformable polymer electrolytes for versatile-shaped lithium-ion batteries", Adv. Mater. 2013, 25, 1395. (featured as a back cover image).

An Alternative Electrolyte Based on Sebaconitrile for Thermally-Stable Lithium-Ion Batteries

Volatile, flammable carbonate-based liquid electrolytes, although they show good electrochemical performance suitable for practical use, have posed a critical threat to safeties of lithium-ion batteries. This safety concern becomes more serious in newly-emerging application fields such as (hybrid) electric vehicles and grid scale energy storage systems, which employ large-sized, high-capacity lithium-ion batteries. One promising solution to address the liquid electrolyte-induced safety issues is to replace with nonflammable, thermally-stable electrolytes. Here, we demonstrate a new electrolyte system composed of 1M LiTFSI (lithium bis-trifluoromethanesulphonimide) in sebaconitrile (SBN). The SBN is featured with high boiling temperature (~ 375 oC) and nonflammability, which are expected to enable significant improvements in high-temperature performance of SBN-based electrolytes. Based on the characterization of thermal/electrochemical properties of SBN-based electrolytes, their application to lithium-ion batteries is explored as a function of operating temperature. Notably, the cell incorporating SBN-based electrolytes show stable cycling performance at 80 oC. This advantageous effect of SBN-based electrolytes on thermal stability of cell, in comparison to conventional carbonate-based liquid electrolytes, is discussed by scrutinizing the variation in AC impedance of cells and ionic conductivity of the electrolytes as a function of temperature.

High performance C/S composite cathodes with conventional carbonate-based electrolytes in Li-S battery.

High stable C/S composites are fabricated by a novel high-temperature sulfur infusion into micro-mesoporous carbon method following with solvent cleaning treatment. The C/S composite cathodes show high Coulombic efficiency, long cycling stability and good rate capability in the electrolyte of 1.0 M LiPF6 + EC/DEC (1:1 v/v), for instance, the reversible capacity of the treated C/S-50 (50% S) cathode retains around 860 mAh/g even after 500 cycles and the Coulombic efficiency is close to 100%, which demonstrates the best electrochemical performance of carbon-sulfur composite cathodes using the carbonate-based electrolyte reported to date. It is believed that the chemical bond of C-S is responsible for the superior electrochemical properties in Li-S battery, that is, the strong interaction between S and carbon matrix significantly improves the conductivity of S, effectively buffers the structural strain/stress caused by the large volume change during lithiation/delithiation, completely eliminates the formation of high-order polysulfide intermediates, and substantially avoids the shuttle reaction and the side reaction between polysulfide anions and carbonate solvent, and thus enables the C/S cathode to use conventional carbonate-based electrolytes and achieve outstanding electrochemical properties in Li-S battery. The results may substantially contribute to the progress of the Li-S battery technology.

A shape-deformable and thermally stable solid-state electrolyte based on a plastic crystal composite polymer electrolyte for flexible/safer lithium-ion batteries

A solid-state electrolyte with reliable electrochemical performance, mechanical robustness and safety features is strongly pursued to facilitate the progress of flexible batteries. Here, we demonstrate a shape-deformable and thermally stable plastic crystal composite polymer electrolyte (denoted as “PC-CPE”) as a new class of solid-state electrolyte to achieve this challenging goal. The PC-CPE is composed of UV (ultraviolet)-cured ethoxylated trimethylolpropane triacrylate (ETPTA) macromer/close-packed Al2O3 nanoparticles (acting as the mechanical framework) and succinonitrile-mediated plastic crystal electrolyte (serving as the ionic transport channel). This chemical/structural uniqueness of the PC-CPE brings remarkable improvement in mechanical flexibility and thermal stability, as compared to conventional carbonate-based liquid electrolytes that are fluidic and volatile. In addition, the PC-CPE precursor mixture (i.e., prior to UV irradiation) with well-adjusted rheological properties, via collaboration with a UV-assisted imprint lithography technique, produces the micropatterned PC-CPE with tunable dimensions. Notably, the cell incorporating the self-standing PC-CPE, which acts as a thermally stable electrolyte and also a separator membrane, maintains stable charge/discharge behavior even after exposure to thermal shock condition (=130 °C/0.5 h), while a control cell assembled with a carbonate-based liquid electrolyte and a polyethylene separator membrane loses electrochemical activity.

Embedding Sulfur in MOF‐Derived Microporous Carbon Polyhedrons for Lithium–Sulfur Batteries

As a promising rechargeable battery system, lithium– sulfur (Li–S) batteries can deliver an exceptionally high theoretical specific capacity of 1672 mAhg 1 and an energy density of 2500 Whkg 1 with the low-cost and environmentfriendly sulfur as the cathode material. Although the potential use of sulfur as a cathode material has long been discovered, several severe drawbacks have hindered the realization of Li–S batteries. One limitation is the insulating nature of sulfur with a very low conductivity of 5 10 30 Scm , which results in low utilization of sulfur. Another well-known problem is associated with the easy dissolution of polysulfides, the intermediate products formed during the electrochemical reaction, in organic electrolytes. The dissolved polysulfides “shuttle” between the electrodes, leading to the low Coulombic efficiency and deposition of a highly resistive layer on the surface of electrodes. These detrimental issues result in unsatisfactory electrochemical performance with rapid fading of capacity. Several approaches have been proposed to overcome the above-mentioned challenges in Li–S batteries, such as developing novel electrolytes and electrode materials. Among these efforts, using sulfur-containing composites instead of pure sulfur as the cathode materials has been demonstrated as an effective way towards high-performance Li–S batteries. Polymers and porous carbons are the common candidates to form composites with sulfur, which immobilize the loaded sulfur, and probably also the derived polysulfides via physical and/or chemical interactions. In addition, the electrical conductivity of composite materials is also better than that obtained with pristine sulfur. In particular, porous carbon materials have attracted intensive attention due to their good compatibility with sulfur, easy accessibility, and the abundance of candidates with diverse porosity and structures. Mesoporous carbon materials have been widely studied as the host materials to confine sulfur. For example, nanocomposites consisting of sulfur and ordered mesoporous carbon or mesoporous hollow carbon spheres have shown improved sulfur utilization and cycling stability. Nonetheless, continuous capacity fading upon prolonged cycling is still commonly observed, and the use of optimized ether-based electrolytes seems to be indispensable. Recent reports on carbon materials with rich micropores have revealed distinct characteristics. 25] Sulfur embedded in microporous carbon shows a pronounced discharge plateau at a lower potential of about 1.8 V versus Li/Li, which is different from the two plateaus of a typical sulfur cathode. More importantly, these microporous carbon/sulfur nanocomposites generally show outstanding capacity retention upon cycling and good compatibility with conventional carbonate-based electrolytes. However, the origins of the unusual characteristics of microporous carbon are not fully understood yet. In recently years, syntheses of porous carbon materials from metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) have attracted growing attention due to the facile preparation procedures, high carbon yield, and unique porous structures. For example, carbonization of MOF-5 with furfuryl alcohol results in nanoporous carbon, which shows excellent supercapacitive performance. The carbon materials with fiber-like morphology prepared from Al-based PCPs exhibit remarkably high porosity. In particular, MOFs and PCPs are very attractive as both the template and the precursor for the fabrication of microporous carbon. Compared with many other highly porous carbon materials, such as those prepared by post-activation processes, the porous carbon derived from MOFs and PCPs exhibits highly uniform porosity, largely originating from the ordered crystalline structures of the MOFs and PCPs. However, the interesting application of these carbon materials derived from MOFs and PCPs for Li–S batteries needs to be further explored. Herein, we report the facile synthesis of microporous carbon polyhedrons (MPCPs) using unique MOF polyhedrons as both the template and precursor, and their use as carbon host to incorporate sulfur for Li–S batteries. The asprepared MPCPs with abundant and uniform micropores serve as an ideal model system for investigating the electrochemical behaviors of sulfur embedded in microporous [a] H. B. Wu, S. Wei, Dr. L. Zhang, Prof. R. Xu, Prof. X. W. Lou School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, Singapore 637459 (Singapore) E-mail : rxu@ntu.edu.sg xwlou@ntu.edu.sg Homepage: http://www.ntu.edu.sg/home/xwlou [b] H. B. Wu, Prof. H. H. Hng School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798 (Singapore) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201301689.

Electrochemical properties of ether-based electrolytes for lithium/sulfur rechargeable batteries

The lithium/sulfur (Li/S) battery is a promising electrochemical system that has a high theoretical capacity of 1675mAhg−1. However, the system suffers from several drawbacks: poor active material conductivity, active material dissolution, and use of the highly reactive lithium metal electrode. In this study, we investigated the electrolyte effects on electrochemical performances of the Li/S cell, by acting on the solvent composition. As conventional carbonate-based electrolytes turned out to be unusable in Li/S cells, alternative ether solvents had to be considered. Different kinds of solvent structures were investigated by changing the ether/alkyl moieties ratio to vary the lithium polysulfide solubility. This allowed to point out the importance of the solvent solvation ability on the discharge capacity. As the end of discharge is linked to the positive electrode passivation, an electrolyte having high solvation ability reduces the polysulfide precipitation and delays the positive electrode passivation.

UV-curable semi-interpenetrating polymer network-integrated, highly bendable plastic crystal composite electrolytes for shape-conformable all-solid-state lithium ion batteries

Download options Please wait... Article information DOI https://doi.org/10.1039/C2EE03025J Article type Paper Submitted 31 Oct 2011 Accepted 17 Jan 2012 First published 17 Feb 2012 Download Citation Energy Environ. Sci., 2012,5, 6491-6499 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions UV-curable semi-interpenetrating polymer network-integrated, highly bendable plastic crystal composite electrolytes for shape-conformable all-solid-state lithium ion batteries H. Ha, E. Kil, Y. H. Kwon, J. Y. Kim, C. K. Lee and S. Lee, Energy Environ. Sci., 2012, 5, 6491 DOI: 10.1039/C2EE03025J To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Hyo-Jeong Ha Eun-Hye Kil Yo Han Kwon Je Young Kim Chang Kee Lee Sang-Young Lee

Study of the LiMn1.5Ni0.5O4/Electrolyte Interface at Room Temperature and 60°C

The surface layer (Cathode-Electrolyte Interface; CEI) on LiMn1.5Ni0.5O4, a promising, high voltage positive electrode for Li-ion batteries, was studied by XPS, AC impedance spectroscopy and FTIR spectroscopy. Half cells and full cells with LiMn1.5Ni0.5O4 as positive electrode material and Li4Ti5O12 as a negative electrode material were assembled in conventional carbonate-based electrolytes with LiPF6 or LiBF4 as the salt, and the effect of cycling at different operating conditions (short and long storage time, state of charge and temperature) on the surface layer composition was assessed. Capacities reaching near the theoretical value of 140 mAh g−1 were obtained in half cells cycled at C/2 and room temperature, with 85% of the capacity being retained after 100 cycles. Cycling at 60°C leads to a decrease in capacity and coulombic efficiency. The surface analysis by XPS revealed that the CEI is composed of inorganic species such as LiF and LixPFyOz or LixBFyOz as well as organic species such as polyethers and carbonates. Generally, it was found that cycling or storing the material at 60°C with an electrolyte using LiPF6 as a salt yield more organic species and less LiF at the surface than the one with LiBF4.

Novel electrolytes based on aliphatic oligoether dendrons

A novel electrolyte system having well structurally controlled aliphatic oligoether dendrons with a carbonate core is developed for lithium ion batteries. The synthetic dendrons have high boiling point, much higher dielectric constant up to 7.4–8.7 and better stability against oxidation than the conventional linear carbonates such as DEC. The ionic conductivities of the electrolyte with 80 wt% of dendrons and 20 wt% of LiTFSI are 0.11–0.61 mS cm −1 at 20 °C. A 463443-type prismatic battery having the electrolyte solution with 20 wt% dendrons was prepared and its battery performance such as capacity and cycleability was investigated. The prismatic battery with dendron-based electrolyte has the same level of capacity to that with the conventional carbonate-based electrolyte, and shows good cycleability, suggesting a high possibility to use as a kind of cosolvents for lithium ion batteries.

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 LiPF 6) 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 Li 1Si, Li 7Ti 4O 12 and Li 0.45CoO 2. 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 +.

Charge–discharge and high temperature reaction of LiCoO 2 in ionic liquid electrolytes based on cyano-substituted quaternary ammonium cation

The charge–discharge performance of LiCoO 2 positive electrode was observed in a mixed electrolyte system consisting of two ionic liquids: cyano-substituted quaternary ammonium bis(trifluoromethane)sulfoneimide (TFSI) and a same-anion salt of 1-ethyl-3-methyl imidazolium (EMI). The positive electrode exhibited a discharge capacity rather close to the theoretical one when N, N, N, N-cyanoethyl trimethyl ammonium salt was applied. Differential scanning calorimetry (DSC) studies revealed that these electrolytes exhibited exotherm only around 260 °C, 50 °C higher than conventional carbonate-based electrolytes. This is the first attempt to reveal the thermal stability of ionic liquid electrolyte under a practical situation.