Exemption of Lattice Collapse in Ni-MnO2 Birnessite Regulated by the Structural Water Mobility

Lattice collapse and associated mechanical fracture frequently occur in Li-intercalated metal oxide cathodes at the deep charge upon Li-ion removal, governed by chemical compositions and the resulting electron density of the oxygen atoms. However, similar lattice collapse for metal oxide electrodes in aqueous storage and its mitigation have not been well studied. Herein, we reported the lattice collapse of MnO2 layered birnessite during the aqueous de-sodiation process at high voltage due to the structural water motion, evidenced by in situ XRD. Unlike non-aqueous Li-intercalated electrodes, Ni-dopants mitigated the lattice collapse of birnessite at deep charge states. Moreover, density functional theory (DFT) calculations showed that Ni doping induces charge depletion of lattice oxygen due to its higher electronegativity than Mn. This charge reduction, in turn, yields a significant decrease in electrostatic repulsion between the oxygens belonging to lattice and structural water. Classical molecular dynamics simulations based on atomic charges obtained from DFT elucidate that Ni-doping facilitates immobilization of structural water in (Ni)MnO2 and prevents lattice collapse upon Na-ion removal. (Ni)MnO2 exempted from lattice collapse shows an improved storage capacity relative to MnO2 while maintaining similar cycling stability.