Synthesis of ionic liquids (IL) based on oxygen‐containing spirobipyrrolidinium salts with BF4, BF3C2F5, and NTf2 as counterions was undertaken. Their physical and electrochemical properties were evaluated for suitability for Room Temperature Ionic Liquids (RTIL) application. Reduction in melting point occurred upon exchange of C(2) by an O‐atom of spirobipyrrolidinium, without sacrificing the electrochemical stability; while introduction of alkyl groups between the N‐ and O‐atoms led to incorporation of asymmetry, and hence reduced the melting points, and viscosity.
Eutectic mixtures of oxygen-containing spirobipyrrolidiniumbased ionic liquids were investigated as prospective room temperature ionic liquid electrolytes (RTILs) for supercapacitor application. These in-house developed novel ionic liquids (IL) possess wide voltage window (6-7 V) and solubility (3-5.5 M) in conventional solvents, such as acetonitrile and propylene carbonate. The potential window of mixed RTIL was about 5.5 V (at 50 μA/cm 2 threshold). The RTIL mixtures exhibit higher breakdown potential (> 4.6 V) as compared to conventional molecular solvent (acetonitrile)-based cells (~4.0 V) in supercapacitor tests. However, these RTIL mixtures (neat, i.e. w/o any solvent addition) lead to high equivalent series resistance (ESR) initiating around 3.9 V and lower capacitance. Cycling of the negative electrode at 4.6 V led to degradation of the active carbon material, while the positive electrode maintained its integrity.
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