Thin films of Sn–Cu alloys with various compositions were prepared by annealing tin-coated copper foils at 463K for 0–45h. Their charge–discharge behaviors as negative electrodes for sodium secondary batteries were investigated using an inorganic ionic liquid electrolyte, i.e., NaFSA–KFSA (FSA=bis(fluorosulfonyl)amide), at 363K. A Sn–Cu film annealed for 4h exhibited a stable reversible capacity of approximately 100 mAh (g-Sn)−1. The improvement in the cyclability is discussed based on X-ray diffraction, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy results.