This study reports on new electrolyte systems utilizing linear sulfones. The characteristics of linear sulfones have been evaluated with the primary focus on higher withstand voltage. Investigations have been made on eight different types of linear sulfones with relatively low molecular weights. They were subjected to screening with regard to the melting and boiling point, dielectric con-stant, viscosity and the solubility of electrolyte salts. The results of the investigations were that Ethyl isopropyl sulfone (EiPS) and Ethyl isobutyl sulfone (EiBS) emerged as solvents with great potential. The EiPS has a relatively low melting point (8C) and a high boiling point (265C), higher than that (242C) of propylene carbonate (PC). Also, it was possible to dissolve the electrolyte salt in EiPS at 1.5 mol l1 or higher. The EiPS system showed a high withstand voltage (3.3–3.7 V), exceeding that of PC (2.5–2.7 V). This high withstand voltage was found to have been caused by the high stability of EiPS at the interface between the activated carbon electrode and the electrolyte. By taking into consideration the degradation mechanisms of the EiPS system, we were able to clarify that one of the reasons for the high stability was the low reactivity between EiPS and H2O. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3593001] All rights reserved. Manuscript submitted March 8, 2011; revised manuscript received April 28, 2011. Published June 3, 2011. To increase the market size of the following energy devices, it is essential that their performance and durability be improved: electro-lytic capacitors; electrochemical capacitors such as electric double

This study describes new electrolyte systems that utilize alkylated cyclic carbonates, with a primary focus on getting a higher withstand voltage for electric double-layer capacitors (EDLCs). We attempted to increase the oxidative durability of carbonate solvents by protecting the 4th and/or 5th positions of the five-membered carbonate ring; protection was achieved by substituting those positions with small alkyl group(s). We investigates six different types of cyclic carbonates, viz., ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), 2,3-butylene carbonate (2,3BC), isobutylene carbonate (iBC), and pentylene carbonate (PlC) have been investigated with regard to the electrochemical stability, as well as the melting point, boiling point, dielectric constant, viscosity, and the solubility of electrolyte salts. As a result, 2,3BC remained as the best potential candidate for an alternative solvent for EDLCs. 2,3BC has a high boiling point (243 • C) that is comparable to PC (242 • C) and dissolved spirobipyrrolidinium tetrafluoroborate (SBP-BF 4 ). A SBP-BF 4 /2,3BC system showed a stabilized capacitance within wider voltage windows ( V = 3.5 V) that far exceeded that of conventional PC based systems ( V = 2.7 V). This high withstand voltage is caused mainly by the outstanding oxidative durability of 2,3BC. To meet urgent demands from HEV, smart grid, and energy harvesting applications it is essential to increase the availability of the electrochemical energy storage devices by enhancing their energy and power performance as well as improving their cycle durability. New electrolyte systems that are more durable, or, specifically, have a wide operational voltage, are a common requirement for all devices, including electrolytic capacitors, electrochemical capacitors and EDLCs, lithium ion batteries (LIBs), hybrid devices, and dye-sensitized solar cells (DSSCs).EDLCs are capable of delivering a very fast power supply (high power density) with an almost unlimited cycle life, and also require zero maintenance. However, the market demand for EDLCs is currently limited as they have relatively low energy density (E), below 10 Wh L −1 . Major efforts have been devoted to increase the E value up to a target in the vicinity of 20-30 Wh L −1 . Here, electrolytes play an important role in improving the E. Practically, the voltage window of the electrolyte has to either be widened, or a higher withstand voltage V has to be achieved. For EDLCs, because the E is expressed as E = 1/2CV 2 , an increase in V can contribute significantly to an increase in E. The V depends mainly on the oxidative and reductive stability (in both thermodynamic and kinetic senses) of the solvents and so to increase V, stable solvents have been studied extensively. Most of such studies have focused on propylene carbonate (PC) (V = 2.5-2.7), 1 nitriles (V = 2.5-2.7), 1 ionic liquids (V = ca. 4.0), 2 and cyclic sulfones (V = ca. 3.3).3, 4

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Dual-cation electrolyte systems, which contain two cations [Li + and spiro-1,1′-bipyrrolidinium (SBP + ), are proposed to enhance the power capability of hybrid capacitors composed of thick Li 4 Ti 5 O 12 (LTO) negative (200 μm) and activated carbon (AC) positive electrodes (400 μm), which thus reduces the resistive overvoltage in the system. Detailed studies of the mass transport properties based on the combination of spectroscopy and electrochemical analysis have shown that the presence of SBP + , despite slower Li + transport in the electrolyte bulk, further reduces overvoltage associated with migration limitation in the thick LTO electrode macropores. This study on the dual-cation electrolyte quantifies the influence of the addition of a supporting electrolyte and shows interest in SBPBF 4 addition for increasing the output power density of hybrid capacitors with a thick electrode configuration.

To achieve lithium ion-based energy storage devices having high power densities and stabilities, the use of a low-viscosity and low-dielectric-constant solvent [such as dimethyl carbonate (DMC)] as well as a chemically stable BF4 –-based salt (such as LiBF4 or quaternary ammonium salts [spiro-(1,1)-bipyrrolidinium tetrafluoroborate (SBPBF4))] is promising for application in next-generation electrolytes. However, these combinations are impractical for several reasons, including the low ionic conductivity of LiBF4/DMC and the phase separation of SBPBF4/DMC systems. Thus, we developed a DMC-based dual-cation system (1 M LiBF4 + 1 M SBPBF4/DMC) possessing a higher ionic conductivity (5.7 mS cm–1) than that of single-cation systems (1 M LiBF4/DMC, 0.5 mS cm–1) and realizing a stable single-phase solution. Raman measurements suggest that the dual-cation system constitutes one DMC and two or three BF4 – complexes (not neutral-charged states), which should result in high ionic conductivity. Furthermore, the DMC-based dual-cation system exhibited a higher power performance in a Li4Ti5O12//activated carbon hybrid capacitor than the single-cation system (88 and 13% capacity retention at 50 mA cm–2, respectively) and demonstrated high Li+ conductivity (dual-cation: 1.6 mS cm–1, single-cation: 0.2 mS cm–1). Therefore, the dual-cation strategy could aid the development of diverse electrolyte combinations involving salts and solvents that have been considered impracticable.