General Synthesis of Transition-Metal Oxide Hollow Nanospheres/Nitrogen-Doped Graphene Hybrids by Metal–Ammine Complex Chemistry for High-Performance Lithium-Ion Batteries

We present a general and facile synthesis strategy, on the basis of metal–ammine complex chemistry, for synthesizing hollow transition-metal oxides (Co₃O₄, NiO, CuO-Cu₂O, and ZnO)/nitrogen-doped graphene hybrids, potentially applied in high-performance lithium-ion batteries. The oxygen-containing functional groups of graphene oxide play a prerequisite role in the formation of hollow transition-metal oxides on graphene nanosheets, and a significant hollowing process occurs only when forming metal (Co²⁺, Ni²⁺, Cu²⁺, or Zn²⁺)–ammine complex ions. Moreover, the hollowing process is well correlated with the complexing capacity between metal ions and NH₃ molecules. The significant hollowing process occurs for strong metal–ammine complex ions including Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ ions, and no hollow structures formed for weak and/or noncomplex Mn²⁺ and Fe³⁺ ions. Simultaneously, this novel strategy can also achieve the direct doping of nitrogen atoms into the graphene framework. The electrochemical performance of two typical hollow Co₃O₄ or NiO/nitrogen-doped graphene hybrids was evaluated by their use as anodic materials. It was demonstrated that these unique nanostructured hybrids, in contrast with the bare counterparts, solid transition-metal oxides/nitrogen-doped graphene hybrids, perform with significantly improved specific capacity, superior rate capability, and excellent capacity retention.