From stress to charge: investigating the piezoelectric response of solvate ionic liquid in structural energy storage composites

Abstract

Solvate ionic liquids (SILs) are a class of ionic liquids where the liquid-state salt is chelated by a coordinating solvent, and of interest due to their advantageous properties such as low vapour pressure and superb thermal and chemical stability for energy storage applications. The electromechanical and piezoelectric effect were studied in lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solvated by triethylene glycol dimethyl ether (triglyme, G3), forming [Li-G3]TFSI. These effects were also investigated in full solid polymer electrolyte (SPE) used in energy storage devices, consisting of [Li-G3]TFSI paired with an epoxy-based resin system. The SIL's electromechanical response was first established in isolation, as well as within the SPE. Experimental data demonstrates the effect of a major part of the SPE contributing to the electrical potential generation during application of force and subsequent pressurisation as well as depressurisation, underlined by a direct piezoelectric effect. SPE response to applied load is explored after the recent discovery of liquid-to-crystalline phase transition following pressurisation in pure ionic liquids. This finding has the potential to ameliorate the performance of energy storage composites via additional effects of charging such a device by subjecting it to stress, leading to increased efficiency. Results to date show a bulk potential difference across the SIL of up to 150 mV, while the SPE potential response is scaled down due to a significantly lower volume of SIL at the interface (∼30 mV). Nevertheless, such findings can still significantly affect the performance of carbon fibre (CF)-based structural supercapacitors and batteries that are able to store and release electrical energy whilst simultaneously contributing to load-bearing performance.