Reliable characterization of polymer electrolytes for next-generation energy storage: misinterpretations, challenges, and methodological solutions.

Polymer electrolytes (PEs) are integral to safer, flexible, high-energy solid-state lithium and sodium-ion batteries and supercapacitors. Yet, their characterization is often misinterpreted due to semi-crystalline polymer matrices, salt-polymer coordination, plasticizer effects, filler interactions, and non-standardized experimental or data-processing methods. These challenges can yield ambiguous conclusions regarding crystallinity, ion dissociation, thermal transitions, degradation, morphology, surface chemistry, and mechanical integrity. This review critically examines structural, thermal, morphological, surface, and mechanical characterization of PEs using techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning and field-emission electron microscopy (SEM/FESEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and mechanical testing. Particular emphasis is placed on assumptions, limitations, and interpretive difficulties, especially for systems containing blends, plasticizers, inorganic fillers, and salts. The review underscores the need for complementary cross-validation rather than stand-alone measurements. Reliable interpretation requires instrument calibration, transparent data-processing, statistically meaningful datasets, and correlation between structure, ion transport, electrochemical performance, and mechanical stability. By identifying pitfalls and offering best-practice recommendations, this work aims to enhance reliability, reproducibility, and comparability of PE studies, thereby advancing understanding of ion-transport mechanisms and guiding rational design of next-generation energy-storage devices.