Electrospun cross-linked carbon nanofiber films as free-standing and binder-free anodes with superior rate performance and long-term cycling stability for sodium ion storage

A cross-linking strategy is proposed to design electrospun carbon nanofiber films towards free-standing and binder-free electrodes with high sodium-ion storage performance. Based on this strategy, electrospun cross-linked carbon nanofiber (CL-CNF) films are fabricated with polyvinylpyrrolidone solutions containing Cu(NO3)2. The formation of the cross-linked structure should be ascribed to the strong coordination of PVP with Cu(NO3)2, which acts as a cross-linking agent. The as-prepared CL-CNF film is flexible, and the average diameter of the nanofibers can be facilely adjusted by controlling the feeding rate. Remarkably, the CL-CNF film demonstrates an excellent rate performance and long cycle stability when used as a binder-free anode for SIBs, compared with the CNF film without a cross-linked structure. At 50 mA g−1, the CL-CNF film delivered a specific capacity of as high as 449 mA h g−1, which can rival the Na ion storage capacity of most of the reported electrospun nanofibers. At 5 and 10 A g−1, the CL-CNF film delivers an initial reversible capacity of 148 and 121 mA h g−1, respectively, and still maintains 126 and 111 mA h g−1 after 500 cycles without obvious capacity fading. The excellent electrochemical properties of the CNF film are attributed to the unique cross-linked structure that endows the CL-CNF film with fast electron transfer and ion diffusion kinetics as well as robust structural stability to bear the repeated impact of Na ions during the discharge/charge process. Therefore, this work opens up a new strategy to design high performance carbon nanofibers as free-standing electrodes for flexible energy sodium-ion storage devices.