Suppressed Layered-to-Spinel Phase Transition in δ-MnO 2 via van der Waals Interaction for Highly Stable Zn/MnO 2 Batteries.

Although birnessite-type manganese dioxide (δ-MnO 2 ) with a large interlayer spacing (≈7 Å) is a promising cathode candidate for aqueous Zn/MnO 2 batteries, the poor structural stability associated with Zn 2+ intercalation/deintercalation limits its further practical application. Herein, δ-MnO 2 ultrathin nanosheets are coupled with reduced graphene oxide (rGO) via van der Waals (vdW) self-assembly in a vacuum freeze-drying process. It is interesting to find that the presence of vdW interaction between δ-MnO 2 and rGO can effectively suppress the layered-to-spinel phase transition in δ-MnO 2 during cycling. As a result, the coupled δ-MnO 2 /rGO hybrid cathode with a sandwich-like heterostructure exhibits remarkable cycle performance with 80.1% capacity retained after 3000 cycles at 2.0 A g -1 . The first principle calculations demonstrate that the strong interfacial interaction between δ-MnO 2 and rGO results in improved electron transfer and strengthened layered structure for δ-MnO 2 . This work establishes a viable strategy to mitigate the adverse layered-to-spinel phase transition in layered manganese oxide in aqueous energy storage systems.