Closing the Loop: Recyclable Solvate Ionic Liquids in Solid Polymer Electrolytes for Circular Economy

This paper demonstrates a pioneering method aimed at furthering the principles of a circular economy within energy storage systems. Specifically, it focuses on the reclamation and recycling of solvate ionic liquids (SILs), lithium in tetra-glyme (G4) with the TFSI/TFSA/NTf2 (bis(trifluoromethanesulfonyl)imide) anion, from epoxy-based solid polymer electrolytes (SPEs). The bicontinuous SPE matrix of G4:LiTFSI 7:3 epoxy resin is evaluated for its recyclability. The approach involves the extraction of SILs from SPEs and their subsequent reincorporation back into SPEs, thereby promoting a sustainable loop of resource utilization within the energy storage ecosystem. The recycling process, employing ethanol as an environmentally friendly solvent and basic physical separation (blending/shredding/comminution), yielded a remarkable recovery rate of up to 92.8% SIL from the SPEs. The recycling process was repeated 5 times, with an average SIL recovery rate of 83% (ranging from 92.8% to 81.1%), to demonstrate the robustness of the process. Characterization studies, including nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), ionic conductivity, and flexural testing, revealed that the properties of SPEs incorporating recycled SILs closely mirrored those of SPEs containing pristine SILs. Blended resin pulp was shown to be a promising filler for epoxy-based resin systems. With 10% (w/w) filler, mechanical properties improved by 11% for flexural modulus (1678 vs 1507 MPa) and 23% for flexural strength (35.8 vs 29.1 MPa) compared to the resin control, thereby demonstrating the full recyclability of the SPE. These findings underscore the viability of SIL recycling, maintaining essential electrochemical and mechanical properties that are crucial for energy storage applications. The implementation of the recycling process was calculated to result in a significant cost reduction of 66.51%, lowering the manufacturing cost of 1 kg of SPE from $1081.22 (USD) to $362.06 (USD) over five recycling cycles. Overall, the successful integration and recycling of SILs in SPEs represent a significant advancement toward closing the loop in energy storage systems, fostering a more sustainable and circular approach to material utilization in the field of energy technology. This work represents the first report detailing both the yield and purity of an electrolyte recovered from an SPE, establishing a new benchmark for transparency in reporting critical metrics in the field of energy storage material recycling.