<p>Composite polymer electrolytes (CPEs) hold significant potential for enhancing the safety of lithium-ion batteries. This study innovatively utilized phosphogypsum (PG) solid waste as a raw material to prepare modified calcium sulfate whiskers (mCSW), which were incorporated into poly(ethylene oxide) (PEO)-based composite electrolyte films. The results demonstrated that the addition of mCSW significantly enhanced the thermal stability of the composite electrolyte, reduced its crystallinity, and yielded higher ionic conductivity and lithium-ion transference numbers compared to conventional PEO electrolytes. The electrolyte containing 10 wt% mCSW exhibited an ionic conductivity as high as 1.9 × 10⁻⁵ S cm⁻¹ at 60°C. It demonstrated excellent compatibility with lithium iron phosphate cathodes and achieved stable cycling performance at a 0.2 C rate under 60°C conditions. This research not only provides a novel approach for the high-value utilization of phosphogypsum but also opens new avenues for the development of high-performance all-solid-state lithium-ion batteries.</p>