New Conducting Salts for Non-Aqueous High-Voltage Supercapacitors

Carbon based electrochemical double layer capacitors (EDLCs) display high specific power and have an extremely high cycle life (>500,000). Because of these characteristics, EDLCs are conveniently used in a large number of applications where rapid charge-discharge capability and reliability are required. [1,2]

Since the most effective strategy to increase power as well as energy of an EDLC is to raise the operating voltage [3], many research efforts have been focused on the introduction of high voltage electrolytes.

Among the electrolytes proposed so far, those containing mixtures of ionic liquids (ILs) and organic electrolytes, e.g. N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) and Propylene Carbonate (PC), appear particular promising as they display low viscosity, high conductivity and electrochemical stability [4]. EDLCs containing the mixture PC-PYR₁₄TFSI as electrolyte display high operative voltage (3.5 V) and a remarkable cycling stability [4]. It is interesting to notice that in these mixtures the IL merely acts as a conductive salt. Taking this point into account, it appears evident that the properties of the conducting salt, particularly its electrochemical stability and its solubility in the solvent, might play a crucial role for the development of high performance (and high voltage) EDLCs since they affect the physical-chemical properties of the electrolyte as well as double layer formation [5, 6,7].

In recent work, we proposed the use of a new conducting salt, N-butyl-N-methylpyrrolidinium tetrafluoroborate (PYR₁₄BF₄). We showed that the electrolyte 2.3 M PYR₁₄BF₄ in PC displays a conductivity and viscosity comparable to conventional Et₄NBF₄ in PC, but exhibits a much broader electrochemical stability window, which makes the realization of EDLCs with an operative voltage of 3.2 V possible [8].

In this work we reports an extended study of the use of PYR₁₄BF₄ among other compounds as conductive salts in EDLC electrolytes.

The electrochemical properties of the considered electrolytes were determined by Cyclovoltametry (CV), electrochemical impedance spectroscopy (EIS), float voltage tests and long term charge discharge experiments.

We found that several of the electrolyte’s electrochemical properties - especially the electrochemical stability – are influenced by the choice of salt. The use of PYR₁₄BF₄ as conductive salt results in higher operative voltages and thus higher values for energy and power compared to conventional EDLC systems.

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[8]        S. Pohlmann, A. Balducci, A New Conducting Salt for High-Voltage Propylene Carbonate-Based Electrochemical Double Layer Capacitors, Electrochim. Acta, 110, 2013, 221-227.