Graphene-encapsulated mesoporous metal oxides (e.g. Co₃O₄, Cr₂O₃ and NiO) as novel anode materials for lithium-ion batteries were simply synthesized by adjusting the pH of mesoporous metal oxides and graphene oxide suspensions to set values (e.g. 5–6 for metal oxides, 7–8 for graphene oxide) and mixing the two suspensions in the presence of reducing agents. These composites synthesized by this facile method exhibited superior electrochemical performance, including remarkably high capacity, high rate capability and excellent cycle performance. The improved performance may be attributed to the high dispersion of mesoporous particles separated by graphene nanosheets and the high electrical conductivity of graphene.

Graphene-based metal oxides commonly show outstanding electrochemical performance due to the superior properties of graphene. However, the as-formed metal oxides decorated on graphene prefer to disintegrate or aggregate together, and the graphene–metal oxide hybrids also randomly aggregate themselves, resulting in capacity fading and poor cycling stability. Herein, the graphene-based metal oxides are further protected by graphene nanosheets through a stepwise heterocoagulation method, producing a layered sandwich structure. Compared to the normal graphene-based metal oxides, the sandwich-like graphene-based composites exhibit higher reversible capacities, better cycle performances, and higher rate capabilities. Such sandwich structure can avoid the aggregation of the composites and also act as an ideal strain buffer to alleviate the volume change of metal oxides during cycles. Moreover, the electronic conductivity of the electrode can be further enhanced by the introduction of additional graphene nanosheets. This double layer protection strategy is very effective and may be extended to prepare other high-capacity electrode materials for lithium-ion batteries.