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

Abstract

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.