Characterization of post-overoxidized PEDOT: PSS/PEO/ PVDF/ LITFSI composite as a solid polymer electrolyte for lithium-ion separator

The increasing demand for lithium-ion batteries (LIBs) presents challenges for efficient and sustainable recycling processes due to the depletion of the resource, as well as associated environmental challenges. This study explored the use of PEDOT: PSS as a conductive polymer in PEO/PVDF/LiTFSI composite membranes for lithium-ion separators in electrochemical recycling. The study focused on determining the optimum PEO formulation in PVDF/LiTFSI solutions, evaluating the effect of PEDOT: PSS on ionic conductivity and analysing the specific chemical, crystallinity, and morphology of the composite membranes. The membranes were developed by solvent casting with varying content (0 - 1.0 wt%) of PEDOT: PSS and were characterized using field emission scanning electron microscopy (FESEM), fourier transform infrared spectroscopy (FTIR), x-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS), and transference number (TN) study. The maximum amount of 1.0 wt% PEDOT: PSS resulted in the highest degree of morphological development with a completely formed IPN structure, which is highly fibrous in nature. Peaks were found to be very prominent using FTIR Spectroscopy which includes stretching of C-O-C ether group of PEO at approximately 1100 cm⁻¹, stretching of C=C thiophene ring of PEDOT at 1510 cm⁻¹, stretching of S=O of PSS in 1050–1180 cm⁻¹ range, bending of CF₂ groups of PVDF at 1180 cm⁻¹ and 840 cm⁻¹, stretching of SO₂ group at about 1350 cm⁻¹, and bending of CF₃ group of TFSI⁻ at 740 cm⁻¹ with considerable broadening and shift in peaks due to increasing concentration of PEDOT:PSS. XRD spectra showed a clear non-linear pattern in which sample (a) 0 wt% and sample (d) 0.7 wt% belong to one group that includes all the peaks for crystals, such as PEO (19.24°) and PVDF (23.49°), while sample (b) 0.1 wt%, sample (c) 0.4 wt%, and sample (e) 1.0 wt% belong to another group that has few peaks only at 38.44° and 44.68° but does not contain the PEO peak of 19.24° or any other high angle structure was maintained. The EIS results indicated that the peak ionic conductivity of 6.16 × 10⁻⁴ S/cm was at the 1.0 wt% of PEDOT: PSS due to the presence of conductive pathways induced and dissociation of salt. The TN analysis revealed dominant Li⁺ conduction (t₊ ≈ 0.98–0.99) at lower PEDOT: PSS loadings (0.4–0.7 wt%), but there were slight deviations at 1.0 wt% indicating small electronic contributions associated with PEDOT: PSS aggregation. This study provided confirmation of a clear enhancement of performance of PEO/PVDF/LiTFSI membranes with the addition of PEDOT: PSS into the system. This indicates a compromise in which 1.0 wt% optimizes conductivity for efficient charge transfer and 0.4-0.7 wt% optimizes ionic selectivity for purity of lithium extraction, whereas 0 wt% represents the control condition. Overall, these results are significant towards achieving a sustainable technique for membrane separator fabrication for LIBs recycling purposes. Future research should target the issue of long-term stability of such membranes as well as the possibility of large-scale PEDOT:PSS application in the process of fabrication.