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

Abstract

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 1 1 1 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 ,7008C. The number of site I, which is negligible at ,7008C, steadily grows at the expense of site III to reach a maximum population at 1000–12008C 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 hetero-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 1 between the randomly oriented stacks of small constituent molecules. During the charging reaction, Li ions are stored at 1 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. Ó 2000 Elsevier Science Ltd. All rights reserved.