Enhanced reversible divalent zinc storage in a structurally stable α-MnO2 nanorod electrode

In the present study, a nanorod-type α-MnO2 cathode is prepared by a facile hydrothermal method for rechargeable aqueous zinc-ion battery (ZIB) applications. Electron microscopy studies reveal rod shaped particles with approximately 20 nm of width and 200 nm of length. When tested for aqueous ZIBs, the α-MnO2 nanorod cathode exhibits an initial discharge capacity of 233 mA h/g at a current density of 83 mA/g with nearly 100% Coulombic efficiencies under prolonged cycling. Besides, the prepared cathode demonstrates decent rate capabilities at higher current densities (43.33 and 31.48 mA h/g at 1333 and 1666 mA/g, respectively). Ex-situ synchrotron XAS investigations clearly establish the reversibility of electrochemical Zn-insertion into the α-MnO2 nanorod cathode. The analyses also reveal that the host α-MnO2 structure demonstrates considerable structural stability during Zn-insertion/extraction. Further, a combination of ex-situ synchrotron XRD studies, visualization and pattern-fitting software programs not only confirm electrochemical Zn-insertion into the host α-MnO2 structure but also suggest that the unit cell volume of the [2×2] tunnels in the α-MnO2 host expands by approximately 3.12% during Zn-insertion. The present study thus paves the way for further development of eco-friendly ZIB as an ideal energy storage system due to its excellent safety and reliability.