Poly(lactic acid) - PLA as sustainable building block to enhance structural stability and performance of PEO-based electrolytes

In the framework of sustainable solid polymer electrolyte development, here we investigate poly(lactic acid) (PLA) as a bio-derived reinforcing component for PEO-based electrolytes, exploiting both enantiopure PLA blocks (namely, PLLA as the L-form, and PDLA as the D-form) and their mix (with consequent stereocomplex formation). Two PLA-PEO-PLA triblock copolymers, specifically PLLA-PEO-PLLA and PDLA-PEO-PDLA, were synthesized by ring-opening polymerization (ROP) of lactides, whereas the stereocomplex supramolecular structure was obtained by mixing the two copolymers in solution. The resulting copolymers were dry-blended with high molecular weight PEO and LiTFSI and then processed into self-standing SPEs by hot-pressing, thus enabling a solvent-free membrane fabrication procedure. The electrochemical performance benefits from the combined effect of PLA-containing copolymers and solvent-free, time/energy saving UV-induced crosslinking. Actually, introducing PLA and, particularly, stereocomplex PLA in the UV-cured SPEs, led to improved electrochemical stability while maintaining effective ion transport in PEO-based SPEs. Thorough physicochemical characterization is carried out by NMR, GPC, DSC, TGA, and XRD, while results of EIS, chronoamperometry, and galvanostatic Li plating/stripping tests support their potential use as polymer electrolytes for solid-state lithium metal batteries. In this respect, full cells with a LiFePO 4 cathode were assembled and tested to assess the compatibility of the developed SPEs with commercial cathodes and Li-metal anodes.