Surface modification of carbon anodes for secondary lithium battery by fluorination

Recent results on the surface modification of petroleum cokes and their electrochemical properties as anodes of secondary lithium batteries are summarized. The surface of petroleum coke and those heat-treated at 1860–2800 °C were fluorinated by elemental fluorine (F2), chlorine trifluoride (ClF3) and nitrogen trifluoride (NF3). No surface fluorine was found except only one sample when ClF3 and NF3 were used as fluorinating agents while surface region of petroleum coke was fluorinated when F2 was used. Transmission electron microscopic (TEM) observation revealed that closed edge of graphitized petroleum coke was destroyed and opened by surface fluorination. Raman spectra showed that surface fluorination increased the surface disorder of petroleum cokes. Main effect of surface fluorination with F2 is the increase in the first coulombic efficiencies of petroleum cokes graphitized at 2300–2800 °C by 12.1–18.2% at 60 mA/g and by 13.3–25.8% at 150 mA/g in 1 mol/dm3 LiClO4–ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1, v/v). On the other hand, main effect of the fluorination with ClF3 and NF3 is the increase in the first discharge capacities of graphitized petroleum cokes by ∼63 mAh/g (∼29.5%) at 150 mA/g in 1 mol/dm3 LiClO4–EC/DEC.