Anodic performance and mechanism of mesophase-pitch-derived carbons in lithium ion batteries

Abstract

The anodic performance of soft carbons prepared from synthetic mesophase pitches by heat-treatment at 500 to 1200°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-treated at low temperatures is divided into the following three cases: (i) lithium ions partially charge transferred on the surface of hexagonal planes or in the unstacked carbon layers to be charged and discharged at 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 clusters to be charged at 0.0 to 0.1 V and discharged at 0.8 to 2.0 V (Type III). Lithium ions of Types I and II are charged and discharged reversibly, 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 ions suggest some chemical reactions during charge–discharge that increase the discharge potential and modify the carbon structure. Bonding of carbon planes at facing edges in the anisotropic carbon may be responsible for the poor cycle stability. The capacity of Type II ions increases gradually with heat-treatment which graphitizes carbon to allow intercalation. By contrast, the capacities of Types I and III ions are decreased gradually and sharply, respectively, by heat-treatment. The progress of graphitization densifies the carbon and reduces the free surface of the hexagonal sheet and the charging to such sites. The performance of Type III ions reflects the characteristic of anisotropic carbon in which the clusters are aligned to have more faced edges than those in isotropic carbon. The heat-treatment combines the edges to enlarge considerably the hexagonal plane in this temperature range.