Quasi-reversible conversion reaction of CoSe2/nitrogen-doped carbon nanofibers towards long-lifetime anode materials for sodium-ion batteries

Developing electrode materials with a full conversion reaction is an effective route to break through the capacity barriers, but this goal remains a great challenge. In this work, we report the synthesis of CoSe2 anodes with a quasi-reversible conversion reaction by uniformly encapsulating CoSe2 nanoparticles in nitrogen-doped carbon nanofibers (CoSe2/N-CNFs). The space-confined effect of CNFs effectively inhibits pulverization and amorphization of CoSe2 during charge and discharge and thus remarkably ensures the integrity of the crystal structure. Meanwhile, pseudocapacitive contributed by the size effect of CoSe2 nanodots and excellent conductivity due to doped nitrogen heteroatoms give the electrode outstanding electrochemical properties. When used as an anode, the CoSe2/N-CNFs achieved a reversible capacity of 371.8 mA h g−1 after 500 cycles at 0.2 A g−1. Even at a high current density of 2 A g−1, the discharge capacity of the CoSe2/N-CNFs still reached 308 mA h g−1 after 1000 cycles with excellent rate performance. The first principles calculations demonstrate that the heterointerfaces between CoSe2 and nitrogen-doped carbon can significantly enhance the stability of the structure and improve the storage/diffusion capability of Na+. Moreover, ex situ high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) reveal that the unique structure enables quasi-reversible crystalline-phase transformation of CoSe2, which leads to the superior cycling stability of the battery with high capacity retention.