Li⁺ storage sites in non-graphitizable carbons prepared from methylnaphthalene-derived isotropic pitches

The reversible Li⁺ storage sites and storage-de-storage mechanisms are studied with the carbonaceous materials prepared from methylnaphthalene-derived isotropic pitches. Results of the electrochemical studies indicate that these carbons have at least three different Li⁺ storage sites: Li⁺ ions are de-stored from site I at 0.0-0.12 V (vs. Li/Li⁺), from site II at 0.12-0.8 V, and from site III at >0.8 V. Site III is the most prosperous among the three when the preparation temperature is <700 °C. The number of site I, which is negligible at <700 °C, steadily grows at the expense of site III to reach a maximum population at 1000-1200 °C and diminishes thereafter. Site I looks similar to site III in that its discharge potential profile appears as a plateau, but differs in that it shows a negligible hysteresis between the charging and discharging potential. A large potential hysteresis is observed with site III. Site III is likely to be the void spaces where some amount of hereto-atoms (H and O) or dangling bonds still exist, whereas site I is another type of void that is converted from site III by `molecular bridging' between the randomly oriented stacks of small constituent molecules. During the charging reaction, Li⁺ ions are stored at site II first, then at sites I and III in order. But Li⁺ de-storage takes place from site I first, then sites II and III.