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Carbon-coated magnetite embedded on carbon nanotubes for rechargeable lithium and sodium batteries.

Fe3O4, carbon-coated Fe3O4, and carbon-coated Fe3O4 embedded on carbon nanotubes are synthesized via hydrothermal reaction. Scanning electron microscopic analysis reveals that particle size of the as-synthesized Fe3O4 ranges 100-250 nm, whereas carbon-coated Fe3O4 ranges 10-15 nm in diameter and is surrounded by a thin carbon layer derived from sucrose. The surface modification by carbon is effective in prohibiting crystal growth during hydrothermal reaction. The carbon-coated Fe3O4 is loaded on conductive carbon nanotubes during a hydrothermal reaction where the carbon nanotubes are added prior to the reaction. Li and Na cell tests indicate that the carbon-coated Fe3O4 embedded on carbon nanotubes exhibits excellent capacity retention and a good rate capability compared to those of Fe3O4 and carbon-coated Fe3O4. For both cases, the presence of conductive carbon nanotubes provides a conduction path of electrons and is thereby responsible for good capacity retention. These results demonstrate the feasibility of dual alkali ions (Li(+) and Na(+)) storage in inexpensive magnetite.