Inexpensive and eco-friendly nanostructured birnessite-type δ-MnO2: A design strategy from oxygen defect engineering and K+ pre-intercalation

Nanostructured birnessite-type δ-MnO2 (KMO-V) with intercalated K+ and oxygen vacancies is synthesized by a hydrothermal method at 160 °C followed by annealing under 300 °C in Ar atmosphere with non-toxic and inexpensive γ-MnO2 (Unit price: 0.96–1.6 US$ Kg−1) as raw material. The interplanar spacing in (001) diffraction direction of KMO-V is near 0.719 nm. The considerable initial discharge potential of about 1.8 V, specific energy of 389.88 Wh kg−1, and cyclic stability of 91.9% after 1500 cycles at 1.0 A g−1 of KMO-V are obtained. A hybrid intercalation mechanism of H+ and Zn2+ is revealed, KMO-V has the best diffusion kinetics performance of Zn2+ among the samples according to the calculation based on density functional theory (DFT). The by-product Zn4(OH)6SO4·4H2O and Mn2+ are thought to be the primary factors of capacity decay, the synergy of K+ pre-intercalation and oxygen vacancies plays a critical role of enhancing structural stability and reaction kinetics in KMO-V. Thus, this work provides an eco-friendly research method of inexpensive electrode material in aqueous Zn-ion batteries (AZIBs) for large-scale energy storage.