Vinyl-functionalized SiO2 nanoparticles were synthesized and uniformly dispersed on the surface of fibrous polyacrylonitrile (PAN) membrane for use as cross-linking sites. Composite polymer electrolyte was prepared by in-situ cross-linking between vinyl-functionalized SiO2 particles on the PAN membrane and electrolyte precursor containing tri(ethylene glycol) diacrylate. The cross-linked composite polymer electrolyte effectively encapsulated electrolyte solution without leakage. It exhibited good thermal stability as well as favorable interfacial characteristics toward electrodes. Lithium-ion polymer cells composed of a graphite negative electrode and a LiNi0.8Co0.15Al0.05O2 positive electrode were assembled with the in-situ cross-linked composite polymer electrolyte. The cells with cross-linked composite polymer electrolytes using fibrous PAN membrane and vinyl-functionalized SiO2 particles exhibited high discharge capacity and good capacity retention at both ambient temperature and elevated temperature.