Exploration of cobalt selenite–carbon composite porous nanofibers as anode for sodium-ion batteries and unveiling their conversion reaction mechanism

• Cobalt selenite–carbon porous nanofibers were studied as anode material for sodium-ion batteries. • Conversion reaction mechanism of cobalt selenite for sodium-ion storage was studied. • Unique structured cobalt selenite–carbon nanofibers showed excellent electrochemical properties. Efforts have been made to develop a promising anode material with a novel composition for sodium-ion batteries (SIBs). In this study, the sodium-ion storage mechanism of transition metal selenite that comprises transition metal cation coupled with two anions is studied. Amorphous cobalt selenite (CoSeO 3 )–carbon composite nanofibers containing numerous pores are synthesized via electrospinning process. Upon heat treatment of the electrospun nanofibers containing selenium, CoSe 2 nanoclusters are formed. During the subsequent oxidation, CoSe 2 transformed into amorphous CoSeO 3 and some part of carbon was oxidized into CO 2 , leaving the pores inside the nanofiber. To unveil the electrochemical reaction mechanism, analytical methods including cyclic voltammetry, ex-situ X-ray photoelectron spectroscopy, ex-situ transmission electron microscopy, and in-situ electrochemical impedance spectroscopy techniques were adopted. Based on the analyses, the following conversion reaction from the second cycle onward is suggested: CoO + x SeO 2 + (1 - x )Se + 4( x + 1)Na + + 4( x + 1)e − ↔ Co + (2 x + 1)Na 2 O + Na 2 Se. Furthermore, the electrochemical properties of porous CoSeO 3 –carbon composite nanofibers are analyzed in detail. The anode material exhibited stable cycle stability up to 200 cycles at 0.5 A g -1 and high rate performance up to 5 A g -1 .