Thin films of Sn-Cu alloys with various compositions were prepared by annealing tin-coated copper foils at 463 K for 0-45 h. 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 363 K. A Sn-Cu film annealed for 4 h 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. (C) 2014 Elsevier Ltd. All rights reserved.