Template-free hydrothermal synthesis of δ-MnO2 hierarchical nanoflowers with potassium ions intercalation as cathodes for high-performing aqueous zinc ion batteries

•Self-assembly of δ-MnO2 hierarchical nanoflowers with (001) nanosheets.•Larger interlayer spacings of 0.706 nm, making fast insertion of Zn2+ and H+ ions.•Mesostructures endow well contacts with electrolytes and narrow insertion pathway.•293.6 mAh/g close to theoretic value is available to discharge capacity at 0.3 A/g.•Excellent long cycle discharge capacity, 70.3 mAh/g at 3 A/g after 1000 cycles. δ-MnO2 (K0.44Mn2O4·1.5H2O) hierarchical nanoflowers self-assembled with nanosheets were synthesized via a template-free hydrothermal route by using a mixture suspension of KMnO4 and MnSO4·H2O as precursors. The primary δ-MnO2 nanosheets spread along (001) plane with a thickness of ca. 2–5 nm. It is argued that the intercalations of K+ ions and water molecules among the interlayers of MnO6 octahedrons make the layered lattice structure of δ-MnO2 much stable, resulting in the δ-MnO2 hierarchical nanoflowers. The enlarged interlayer spacings effectively activate the storage of Zn2+ ions in the δ-MnO2 hierarchical nanoflowers. As cathodes for aqueous zinc ion batteries, thus, the δ-MnO2 hierarchical nanoflowers exhibit excellent electrochemical performances. The Zn//δ-MnO2 battery can deliver a high specific capacity of 293.6 mAh/g at a current density of 0.3 A/g and still maintains 217.8 mAh/g after 100 cycles. Besides, a high rate capacity of 84.6 mAh/g can be achieved at 3 A/g. It is believed that but for the promotion of Zn2+ ions diffusion by expanding interlayer spacings, the high specific area and mesoporous structure of the hierarchical architecture of the δ-MnO2 nanoflowers strongly improve the electrochemical performances through fully contacting with the electrolytes.