Synthesis and characterization of ionic liquid electrolytes for use in electrical energy storage devices at elevated temperatures
Embargo Date
2023-04-08
OA Version
Citation
Abstract
Electric energy storage (EES) devices like lithium ion batteries and supercapacitors have limited operational temperatures between 0 and 40 oC, preventing their use in demanding, high temperature applications. The principal limitation of these systems is the flammable and volatile carbonate-based solvent electrolyte. A significant barrier hindering the replacement of carbonate-based solvents is the inability to identify thermally stable solvents that display equivalent properties. Room temperature ionic liquids (RTILs) are ideal candidates to replace carbonate-based solvents to expand operational temperatures to 60 °C and above. RTILs are nonvolatile, nonflammable, thermally and electrochemically stable but most RTILs do not meet current conductivity or viscosity standards to function effectively.
To this end, we generated a library of RTIL electrolytes to discover a structure-function relationship and to enable the design of RTILs with precise physiochemical properties. Modifications were made to a base RTIL structure and the resulting thermal and physical properties were evaluated. Specifically, the identity of the cation was altered, an electronegative moiety was included, the alkyl chain lengths were varied, or different anionic components were utilized. It was found that a small anion with a delocalized negative charge when paired with an asymmetric cation generally produced a RTIL with the desired physicochemical properties.
Furthermore, RTILs were paired with anions with a delocalized negative charge and then binary, ternary, and quaternary RTIL and salt mixtures were created. The electrochemical behavior of the formulations was assessed via cyclic voltammetry and the relationship between the cation and anion of the RTIL and lithium salts was studied. The coordination strength between the anion of the lithium salt and the cation of the RTIL strongly dictated the electrochemical stability of the formulations. This highlighted the novel feature of RTILs to act both as an inert solvent and a mobile ion in an EES device.
Using the developed structure-function relationship, we fabricated two systems of supercapacitors that utilized RTIL electrolytes. The first is a piperidinium based electrolyte with a fluorinated lithium salt: the supercapacitor displayed sustained cycling for over 10,000 cycles at 100 oC with a discharge capacity of 28 F/g, while a traditional system failed after 3200 cycles. The second is a lithium free dicyanamide anion based RTIL electrolyte equipped supercapacitor. This system relies on the cations and anions of the RTIL to act as the medium of transport and as the mobile ions of the system to generate an electric double layer during charging/discharging. Our system cycled over 10,000 times at 60 oC with an average discharge capacity of 10 F/g for the last 5,000 cycles.