Relationship between electrode performance and chemical bonding nature in mesoporous metal oxide-layered titanate nanohybrids

We investigated the applicability of mesoporous MO_x-layered titanate (M = Fe or Ni) nanohybrids as a lithium intercalation electrode to elucidate the relationship between electrode activity and chemical bonding nature in these materials. Electrochemical measurements clearly demonstrate that the mesoporous nanohybrids show better performance of lithium cation intercalation electrodes, compared with the pristine titanate. This finding underscores that hybridization with metal oxide nanocrystals is quite effective in improving the electrochemical property of titanium oxide. The anode performance of the NiO_x-layered titanate nanohybrid can be further improved by a postcalcination at 300 °C whereas there is less significant increase in the discharge capacity of the iron oxide homologue after the heat-treatment at the same temperature. Such different effects of the postcalcination can be understood in terms of local structural evolution of guest species; the nickel species in the as-prepared NiO_x-layered titanate nanohybrid exist in the form of nickel hydroxide and the postcalcination at ≥300 °C induces a structural transformation to rocksalt-structured nickel oxide. On the contrary, the local structure around iron ions in the FeO_x-layered titanate nanohybrid does not experience any notable modifications upon the postcalcination. According to X-ray absorption spectroscopic analyses for the chemically lithiated nanohybrids, the lithium insertion via n-BuLi treatment decreases the oxidation states of iron and nickel ions with negligible change in Ti valency. This result strongly suggests that the insertion of Li⁺ ions into the present nanohybrids can be achieved by a redox process of hybridized iron oxide or nickel oxide, and hence this functionality is strongly dependent on the chemical bonding nature of hybridized guest species.