Core–shell structured SiO₂ particles with different core diameters were synthesized by radical polymerization of 4-styrenesulfonic acid sodium salt with vinyl-functionalized SiO₂ core particles and were used as Li⁺ ion-conducting fillers in composite polymer electrolytes. Composite polymer electrolytes prepared with core–shell SiO₂ particles exhibited high ionic conductivity exceeding 10⁻³ S cm⁻¹ at room temperature and good mechanical properties, allowing the preparation of a free-standing film with a thickness of 30 μm. Lithium-ion polymer cells composed of graphite negative electrode, composite polymer electrolyte and LiFePO₄ positive electrode were assembled, and their cycling performance was evaluated. Cells assembled with a composite polymer electrolyte containing core–shell SiO₂ particles with a core diameter of 250 nm exhibited good cycling performance in terms of discharge capacity, capacity retention and rate capability.