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 discus s their insertion sites for lithium ion and correlation between the capacit y and structure. The derived hard carbons showed complicated voltage profil es of charge and discharge, indicating five kinds of insertion sites: parti ally charge transferring surface site (Type I), intercalation site like gra phite (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 tho se found in the soft carbon. Very unique capacities of the present hard car bons calcined at 1000 degreesC were found at the potentials of 0-0.13 V (Ty pe IV) and 0.5-2.0 V (Type V) and varied markedly with oxidization extent a nd heteroatom contents: of precursor pitch, respectively. The microvoid amo ng the hexagonal planes and atomic defects created by evolution of heteroat om are suggested to serve such insertion sites of Types IV and V, respectiv ely. The microvoid holds reduced lithium ions with different extents of cha rge transfer to the carbon, which exist independently at 140 K but exchange rapidly at room temperature to their averaged chemical shift in Li-7-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 degreesC a nd decreased sharply by the heat-treatment at 1000 degreesC due Lo the coup ling of hexagon clusters. The amount of microvoid or capacity at 0-0.13 V i s correlated to XRD parameters and oxygen content of the precursors. Smalle r crystal, lower stacking or mon oxygen content in the precursor allows app arently larger capacity. Random assemble of smaller graphitic units may ind uce the microvoid among the cluster units, although the microvoid has not b een specifically defined yet. (C) 2001 Elsevier Science Ltd. All rights res erved.