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

Abstract

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.