Fabrication of LiCoO2/helical nanocarbon composites and their effect on lithium cell performance

We fabricate LiCoO₂/helical nanocarbon (HCN) composites by forming HCNs on LiCoO₂ on which iron oxides (Fe₂O₃ or Fe₃O₄) are dispersed (LiCoO₂(Fe₂O₃) or LiCoO₂(Fe₃O₄)) as catalysts for HCN formation, and estimate their electrochemical properties. Granular nanocarbons form on LiCoO₂(Fe₂O₃) and LiCoO₂(Fe₃O₄) at 350 °C although HCNs of about 100 nm in diameter form on LiCoO₂(Fe₂O₃) at 450 °C. Transmission electron microscopy and energy dispersive X-ray spectroscopy measurements show that HCNs consist of stacked graphene layers for LiCoO₂(Fe₂O₃)/HCN composites fabricated at 450 °C. On the other hand, several-nm-thick tetragonal layer exists on the LiCoO₂ substrate and amorphous nanocarbons form on the tetragonal layer for LiCoO₂(Fe₂O₃)/HCN and LiCoO₂(Fe₃O₄)/HCN composites fabricated at 350 °C. X-ray diffraction measurements suggest that Fe₂O₃ and Fe₃O₄ do not completely inhibit LiCoO₂ decomposition. Cathodes containing LiCoO₂(Fe₂O₃)/HCN or LiCoO₂(Fe₃O₄)/HCN fabricated at 350 °C improve rate capability of lithium cells. However, this rate capability is not better than that of cathodes containing a mixture of LiCoO₂ and acetylene black.