Hydroxyapatite in solid polymer electrolytes: A comparative review of synthetic and bio-derived fillers for energy storage

The solid polymer electrolytes (SPEs) have been extensively studied as the prospective outlook for solid-state batteries due to safety and versatility, yet due to low ionic conductivity at room temperature (10 −3 mScm⁻¹ in pure polyethylene oxide [PEO]) and mechanical weakness, they cannot be put into practice. This review aims to critically analyze hydroxyapatite (HAp) as a ceramic of calcium phosphate as a functional filler in addressing these issues, and the key role of comparing the performance of synthetic sources and biowaste sources as sources. Arranged on the polymer support (PEO, poly [vinylidene fluoride-co-hexafluoropropylene], poly [methyl methacrylate]), the review demonstrates that HAp can allow the dissociation of salts and decrease their crystallinity, as well as increase the mechanical strength, with obtaining room-temperature conductivities of 10⁻² to 1.7 mS cm⁻¹ and supplying Li⁺ transference figures between 0.2 and 0.6–0.7. In addition, mechanical parameters also increase, e.g., the tensile modulus of PEO-based systems increased to 15.8 MPa using 10 wt% HAp. There is high purity and reproducibility of synthetic HAp, with the morphologies predefined (nanowires), further facilitating cation flow (tLi-0.69). In comparison, a low-cost, low-carbon bioderived HAp using bones, shells, or eggshells can provide similar conductivity enhancement (∼0.1 mS.cm⁻¹ at 70 °C in PEO-NaTFSI) and the same improvement in mechanical reinforcement but is typically more variable due to dependency on the source of crystallinity. Such a comparative model emphasizes trade-offs between surface modification and sustainability, placing an emphasis on how future progress in the development of surface modification and dispersion shall be the central key to moving HAp-filled SPEs from lab-scale prototypes to scalable and environmentally conscious solid-state batteries. Further developments in the future remain in need of better filler dispersion, bio-sourcing, and scalable processing paths to turn HAp-reinforced SPEs into long-lasting and sustainable electrolytes that can be used in the high-performance solid-state battery.