Graphitization of unburned carbon from oil-fired fly ash applied for anode materials of high power lithium ion batteries

Unburned carbon is an industrial waste product of oil-fired fly ash. From the viewpoint of waste recycling, using ground unburned carbon with an average size of 6 μm heat treatment at 2700 °C, and both charge and discharge at 0.1 C, yields the first columbic efficiency of about 93.6%. Moreover, after 50 cycles, the discharge capacity is 325.5 mAh g−1, and the capacity retention is about 97.5%. The two conditions tested for rate capability are as follows: First, a charge at 0.2 C rate and discharge at variable C-rates in which the unburned carbon after heat treatment at higher temperature exhibits a higher capacity in the 0.2–3 C region. However, unburned carbon with heat treatment at 2300 °C is a suitable material for the higher C-rate region (5–10 C). Second, for both charge and discharge at the same C-rate, the unburned carbon after heat treatment at higher temperature exhibits a higher capacity in the 0.2–0.5 C region; however, unburned carbon without graphitization treatment is a suitable material for the higher C-rate region (3–10 C). Based on TEM observations, the graphitization treatment samples had annular microstructure which has many active sites, improving the intercalation/deintercalation of lithium ions. Therefore, these results show that compared to natural graphite, graphitization of unburned carbon is more suitable for the anode materials of a high power battery.