Anodic performance and insertion mechanism of hard carbons prepared from synthetic isotropic pitches

Anodic performances and structures of hard carbons prepared from synthetic isotropic pitches through solid phase oxidation were investigated to discuss their insertion sites for lithium ion and correlation between the capacity and structure. The derived hard carbons showed complicated voltage profiles of charge and discharge, indicating five kinds of insertion sites; partially charge transferring surface site (Type I), intercalation site like graphite (Type II), cluster gap between edges of carbon hexagon clusters (Type III), microvoid surrounded by hexagonal planes (Type IV) and atomic defect created by evolution of heteroatom (Type V). Types I–III are common to those found in the soft carbon. Very unique capacities of the present hard carbons calcined at 1000°C were found at the potentials of 0–0.13 V (Type IV) and 0.5–2.0 V (Type V) and varied markedly with oxidization extent and heteroatom contents of precursor pitch, respectively. The microvoid among the hexagonal planes and atomic defects created by evolution of heteroatom are suggested to serve such insertion sites of Types IV and V, respectively. The microvoid holds reduced lithium ions with different extents of charge transfer to the carbon, which exist independently at 140 K but exchange rapidly at room temperature to their averaged chemical shift in 7Li-NMR. Type V showed very similar charge–discharge behaviors to that of Type III, which was found more in the soft carbon carbonized at around 700°C and decreased sharply by the heat-treatment at 1000°C due to the coupling of hexagon clusters. The amount of microvoid or capacity at 0–0.13 V is correlated to XRD parameters and oxygen content of the precursors. Smaller crystal, lower stacking or more oxygen content in the precursor allows apparently larger capacity. Random assemble of smaller graphitic units may induce the microvoid among the cluster units, although the microvoid has not been specifically defined yet.