Natural Polymer Nanocomposites Reinforced with Ceramic Nanofillers for Flexible Energy Storage: A Review

The growing demand for flexible and wearable electronics has stimulated the development of sustainable energy-storage materials capable of maintaining electrochemical performance under mechanical deformation. Natural polymers are attractive candidates owing to their renewability, low toxicity, biodegradability, and structural versatility; however, their limited electrical conductivity and electrochemical activity often require functional reinforcement. This review critically examines natural polymer–ceramic nanocomposites developed for flexible batteries and supercapacitors, considering their use as electrodes, electrolytes, and separators. A systematic literature search identified 41 studies in which the natural polymer remained a constituent of the final functional composite, while systems employing natural polymers solely as sacrificial templates were excluded. Cellulose emerges as the most widely explored matrix, combined with a broad range of ceramic materials, including MnO2, ZnO, SnO2, TiO2, Fe3O4, BaTiO3, and other functional oxides and inorganic compounds. Ceramic incorporation generally improves electrochemical activity, ion transport, thermal stability, or mechanical integrity, although the reported performance strongly depends on composition, architecture, and device configuration. A major limitation across the literature is the lack of standardized mechanical and flexibility testing, which hinders meaningful comparison between systems. Future progress will require the development of multifunctional, genuinely sustainable architectures together with standardized electrochemical–mechanical testing, biodegradability and end-of-life assessments, and scalable manufacturing strategies.