The effects of overoxidation on the morphological and electrical properties of polyaniline (PANI) – based solid polymer electrolyte (SPE)

This study investigates the impact of overoxidation on polyaniline (PANI)-based solid polymer electrolytes (SPEs) by examining changes in their structural, electrical, and morphological properties. SPEs have recently gained significant attention for solid-state lithium-ion batteries (LIBs) due to their flexibility, safety, and compatibility with electrodes. Unlike conventional liquid electrolytes, SPEs eliminate issues such as leakage and flammability, but they remain vulnerable to degradation under excessive oxidative conditions. Addressing this gap, this work presents the first investigation into the overoxidation behaviour of PANI:PEO:PVDF:LiTFSI composites, thereby contributing new insights into the stability of conducting-polymer-based electrolytes. Composite SPE films were fabricated via solvent casting with varying PANI loadings (1 wt. %, 3 wt. %, and 5 wt. %) alongside polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Overoxidation was induced using cyclic voltammetry (CV), while structural and electrochemical changes were analysed via Fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and electrochemical impedance spectroscopy (EIS). The 1 wt.% PANI composite exhibited the highest initial ionic conductivity of 1.76 × 10-5 S/cm, which decreased to 1.79 × 10-6 S/cm after overoxidation. In contrast, the 3 wt.% and 5 wt.% samples displayed lower conductivities (1.41 × 10-6 S/cm and 1.25 × 10-6 S/cm, respectively), which further declined to 4.17 × 10-7 S/cm and 1.79 × 10-7 S/cm post-treatment. FTIR confirmed the generation of carbonyl and hydroxyl groups, while FESEM revealed disrupted interpenetrating polymer networks (IPN), both correlating with conductivity loss and material degradation. The findings demonstrate that overoxidation significantly compromises the electrochemical and morphological stability of PANI-based SPEs, limiting their long-term efficiency in LIB applications. The novelty of this study lies in introducing the first PANI:PEO:PVDF:LiTFSI composition and providing the first systematic analysis of its overoxidation effects. These insights are crucial for designing more stable polymer electrolytes and developing mitigation strategies such as protective coatings or electrolyte modifications. Ultimately, this research advances the understanding of polymer degradation mechanisms, contributing toward safer and longer-lasting solid-state batteries for applications in electric vehicles, portable electronics, and renewable energy storage.