Cobalt oxide nanowall arrays on reduced graphene oxide sheets with controlled phase, grain size, and porosity for Li-ion battery electrodes

A facile chemical approach has been developed to produce nanohybrids with ultrathin Co oxides nanowall arrays on reduced graphene oxide (rGO) sheets. The Co oxides exhibited porous structure. The porosity of the Co oxide/rGO nanohybrids and the grain size of the Co oxides could be tailored by varying the annealing temperature, which directly affected their performance as Li-ion battery electrodes. When tested as anode materials for Li-ion batteries, these Co oxide/rGO nanohybrids showed structural-process-dependent performances. For example, Co₃O₄/rGO hybrids obtained by annealing α-Co(OH)₂/rGO at 350 °C showed a high specific capacity of 673 mAh g^-1 after 100 cycles at a discharge current density of 180 mA g^-1 (0.2 C), which was better than Co₃O₄/rGO samples obtained at other annealing temperatures. Similarly, CoO/rGO hybrids obtained by pyrolysis of α-Co(OH)₂/rGO at 350 °C showed optimum performance, as compared to that of CoO/rGO samples obtained at other annealing temperatures, with a capacity of 732 mAh g^-1 after 100 cycles at a discharge current density of 150 mA g^-1 (0.2 C). Although many metal oxide/rGO hybrid systems have been investigated as electrode materials for Li-ion batteries, this study indicates that optimization of such nanohybrids by adjusting the phases, grain sizes, and porosities is necessary to achieve ideal Li storage performances.