Graphene-wrapped mesoporous cobalt oxide hollow spheres anode for high-rate and long-life lithium ion batteries

Transition metal oxides, used as LIB anodes, typically experience significant capacity fading at high rates and long cycles due to chemical and mechanical degradations upon cycling. In this work, an effective strategy is implemented to mitigate capacity fading of Co₃O₄ at high rates by use of hollow and mesoporous Co₃O₄ spheres and graphene sheets in a core-shell geometry. The core-shell structure exhibits a high reversible capacity of 1076 mAh g^-1 at a current density of 0.1 A g^-1, and excellent rate performance from 0.1 to 5.0 A g ^-1. The graphene/Co₃O₄ nanosphere composite electrode also displays an exceptional cyclic stability with an extraordinarily high reversible capacity over 600 mAh g^-1 after 500 cycles at a high current density of 1.0 A g^-1 without signs of further degradation. The highly conductive graphene nanosheets wrapping up on surfaces and interfaces of metal oxide nanospheres provide conductive pathways for effective charge transfer. The mesoporous features of graphene and hollow metal oxide nanosphere also enable fast diffusion of lithium ions for the charge/discharge process. The highly flexible and mechanically robust graphene nanosheets prevent particle agglomeration and buffer volume expansion of Co₃O₄ upon cycling. The unique nanostructure of Co₃O₄ wrapped up with highly flexible and conductive graphene nanosheets represents an effective strategy that may be applied for various metal oxide electrodes to mitigate the mechanical degradation and capacity fading, critical for developing advanced electrochemical energy storage systems with long cycle life and high rate performance.