Series-stacked, double-layer carbon capacitors are slated to be used in electric vehicles for power management as well as in consumer electronics for memory backup and burst power. Nonaqueous electrolytes are preferred over aqueous electrolytes, since a wider voltage window can be accessed in the former electrolytes, thereby requiring fewer cells in the series stack. However, it has historically been difficult to assess whether the organic solvent and/or the supporting electrolyte determine the anodic limit. We have eliminated this ambiguity by using solvent-free ionic liquids where the source of anodic oxidation may be ascribed to the anion alone. Even though the new ionic liquids manifested high oxidation limits, we found that when used in practical capacitors comprising high-surface-area carbon cloth electrodes, a much lower capacitance (compared to smooth electrodes) was achieved. To understand whether the observed decrease in capacitance might be due to the microporosity of the carbon cloth electrode or to practical limitation of the device itself, we first measured the differential capacitance (Cdl) at a Hg/1-ethyl-3-methyl imidazolium imide. The integral capacitance at the Hg interface was then calculated and compared with that of a smooth glassy carbon electrode, a carbon yarn, and a cloth electrode. In addition, the effect of (CF3502)3C, (CF3502)2N, CF350, and BF on Cdl were interpreted based on existing theories of double-layer structure.

Electrolytes based on 1-ethyl-3-methylimidazolium cation (EMI + ) and either the hexafluorophosphate (EMIPF 6 ) or tetrafluorborate (EMIBF 4 ) anion in organic alkyl carbonate solvents have been evaluated for use in electrochemical capacitors. The conductivity, capacitance, limiting oxidation and reduction potentials, and thermal stability were assessed. High conductivity and capacitance values were found regardless of whether cyclic (high viscosity/high dielectric constant) or acyclic (low viscosity/low dielectric constant) alkyl carbonates were used. The best correlation with conductivity for the EMIPF, salt was found to be the molecular weight (K-1/Mw) and to a lesser degree the viscosity (Kc1/i) of the solvent. The high specific capacitance (130 F/g) and excellent stability (>3.5 V, >130°C) make these electrolytes well suited for use in electrochemical double-layer capacitors.

Methyl propyl carbonate (MPC) solutions containing Li salts can be used as a single‐solvent electrolyte without addition of ethylene carbonate (EC). Graphite electrodes can be cycled at high reversible capacity in MPC solutions containing LiPF6 and LiAsF6 . The use of acyclic, unsymmetric alkyl carbonate solvents, such as ethyl methyl carbonate (EMC) and MPC in Li‐ion based electrolytes, increases the stability of the graphite electrode. Whereas a small amount of EC is still needed as cosolvent in EMC solutions to obtain stable surface films on graphite electrodes, we show here that the surface films produced on graphite in MPC solutions (without added EC) are highly stable, allowing reversible Li‐ion intercalation. To understand this trend, we investigated the surface chemistry developed on lithium and carbon electrodes in MPC solutions in conjugation with electrochemical studies.

The electrochemical behavior of graphite electrodes cycled galvanostatically versus Li metal in electrolyte solutions containing LiPF6, LiC(SO2CF3)3, and L1N(S02C2F5)2 in ethyl and methyl alkyl carbonates was studied. The solvents include ethyl methyl, ethyl propyl, methyl propyl, isopropyl methyl, and isopropyl ethyl carbonates. The use of asymmetric, aliphatic alkyl methyl carbonates is shown to be essential to achieve both high capacity and long cycle life with graphite electrodes in Li-ion batteries.