Core–shell-structured, ultrafine SnOx/carbon nanofiber (CNF)/carbon nanotube composite films are in situ synthesized by electrospinning through a dual nozzle. The carbon shell layer functions as a buffer to prevent the separation of SnOx particles from the CNF core, allowing full utilization of high-capacity SnOx in both Li-ion and Na-ion batteries. The composite electrodes reveal an anomalous Li- and Na-ion storage mechanism where all the intermediate phases, like LixSn and NaxSn alloys, maintain amorphous states during the entire charge/discharge process. The uniform dispersion on an atomic scale and the amorphous state of the SnOx particles remain intact in the carbon matrix without growth or crystallization even after 300 cycles, which is responsible for sustaining excellent capacity retention of the electrodes. These discoveries not only shed new insights into fundamental understanding of the electrochemical behavior of SnOx electrodes but also offer a potential strategy to improve the cyclic stability of other types of alloy anodes that suffer from rapid capacity decays due to large volume changes.