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.