ANODIC PERFORMANCE AND MECHANISM OF MESOPHASE-PITCH-DERIVED CARBONS IN LITHIUM ION BATTERIES

The anodic performance of soft carbons prepared from synthetic mesopha se pitches by heat-treatment at 500 to 1200 degrees C are investigated in order to clarify their mechanism for the insertion of Lithium ions . It is found that the insertion mechanism for soft carbon heat-treate d at low temperatures is divided into the following three cases: (i) l ithium ions partially charge transferred on the surface of hexagonal p lanes or in the unstacked carbon layers to be charged and discharged a t 0.25 to 0.8 V (Type I); (ii) intercalated into carbon layers up to a higher stage to be charged and discharged at 0.0 to 0.25 V;(Type II); (iii) inserted into the microspaces located at the edges of carbon cl usters to be charged at 0.0 to 0.1 V and discharged at 0.8 to 2.0 V (T ype III). Lithium ions of Types I and II are charged and discharged re versibly, hence, the capacity is stable with cycling. By contrast, the capacity of Type III ions decreases gradually with cycle number. The irreversible charge-discharge and poor cycle stability of Type III ion s suggest some chemical reactions during charge-discharge that increas e the discharge potential and modify the carbon structure. Bonding of carbon planes at facing edges in the anisotropic carbon may be respons ible for the poor cycle stability. The capacity of Type IJ ions Increa ses gradually with heat-treatment which graphitizes carbon to allow in tercalation. By contrast, the capacities of Types I and III ions are d ecreased gradually and sharply, respectively, by heat-treatment. The p rogress of graphitization densifies the carbon and reduces the free su rface of the hexagonal sheet and the charging to such sites. The perfo rmance of Type III ions reflects the characteristic of anisotropic car bon in which the clusters are aligned to have more faced edges than th ose in isotropic carbon. The heat-treatment combines the edges to enla rge considerably the hexagonal plane in this temperature range. (C) 19 98 Elsevier Science S.A. All rights reserved.