Controllable synthesis of core–shell Co@CoO nanocomposites with a superior performance as an anode material for lithium-ion batteries

The present study reports a straightforward template-free route for the synthesis of core–shell Co@CoO nanocomposites by the controlled reduction of Co3O4 nanospheres. The target Co@CoO nanoparticles consist of an unsealed hollow porous CoO shell with a metal Co core, in which the outer porous CoO shell as the active anode material can be fully in contact with the electrolyte. The void within the particles provides a remarkable buffer to tolerate volume changes of the electrode materials during the insertion and extraction of lithium. Most importantly, the inner nanosized metal Co core gives a new impetus to the reversible decomposition of Li2O due to its catalytic activity. Furthermore, the exposed metal Co portion outside the nanoshells provides a favorable electrical contact between adjacent particles and greatly improves the efficiency of the electronic connection between the active material and the current collector. The Co@CoO nanocomposite maintains an excellent reversible capacity over 800 mA h g−1 after 50 cycles with an initial coulombic efficiency of 74.2%, which is much higher than that of pure CoO (67.8%). This superior electrochemical performance is closely related to the unique composition and nanostructure of the electrode material. Notably, it is the first case of a hybrid-structured Co@CoO anode material derived from the reduction process from oxide precursors. Such a conclusion may be advantageously used to guide the design of a wide range of nanostructured metal oxides.