Anomalous improvement of the electrochemical properties of mesocarbon microbeads by Ar–H2–SF6 thermal plasma treatment

Abstract Highly graphitized mesocarbon microbead (MCMB) powders, which consist of spherical artificial graphite particles, were treated in Ar–H2–SF6 radio-frequency inductively coupled thermal plasma. The results of X-ray diffractometry, scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and element analysis showed that the plasma treatment greatly affected the surface structure and chemical composition of the MCMB powders. Sulfur and fluorine could be incorporated into the particle by treating them with plasma containing SF6, and this plasma modification improved the electrochemical properties of the powders. A lithium-ion rechargeable cell with a negative electrode of plasma-treated powder had a discharge capacity as much as 13% greater than that of a cell with a negative electrode of untreated powder, while its irreversible capacity loss (ICL) was as much as 9% less. The specific ICL calculated by dividing the ICL by surface area decreased as the BET surface area increased. Furthermore, a new plateau near 2 V was added to the discharge curve by treating the MCMB powder with SF6 plasma but not by treating it with Ar–H2, Ar–N2, Ar–H2–CO2, or Ar–N2–CO2 plasmas. The incorporation of sulfur and fluorine should be a key factor for the improvement of negative electrode characteristics.