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

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 (CoSeO₃)–carbon composite nanofibers containing numerous pores are synthesized via electrospinning process. Upon heat treatment of the electrospun nanofibers containing selenium, CoSe₂ nanoclusters are formed. During the subsequent oxidation, CoSe₂ transformed into amorphous CoSeO₃ and some part of carbon was oxidized into CO₂, 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 + xSeO₂ + (1 - x)Se + 4(x + 1)Na⁺ + 4(x + 1)e⁻ ↔ Co + (2x + 1)Na₂O + Na₂Se. Furthermore, the electrochemical properties of porous CoSeO₃–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.