Unsaturated coordination modes of Mn/V in manganese vanadate: Inner capture and surface migration of zinc ions for high performance zinc-ion battery

Abstract

It was synthesized Mn0·2V2O5(H2O)·H2O (MnVO) porous microflowers and Ti0·055V2O5(H2O)2·H2O (TiVO) nanosheets. Rietveld refinements and high-angle annular dark-field (HAADF)-scanning transmission election microscope (STEM) reveal they are both constructed by two-dimensional (2D) V-O-V monolayers, lamellar hydrated metal ions and uncoordinated water molecules, in which all the V centers exhibit unsaturated coordination modes. Density functional theory (DFT) calculation proves that the exposed unsaturated V centers on the MnVO (001) surface can boost Zn2+ migration along the surface with a low barrier of 0.61 eV. Therefore, MnVO shows a dominant surface-controlled capacity. MnVO displays excellent Zn2+ storage behavior (435 mAh g-1 at 0.1 A g-1) with a capacity retention of 96.4 % after 1000 discharge/charge cycles at 5 A g-1, which is superior to TiVO. And MnVO and TiVO both show higher capacities than the commercial V2O5. Ex situ characterizations of MnVO discover a conversion of MnVO → Znm(MnVO) and a partial phase transformation mechanism of MnVO → Zn3(OH)2V2O7·2H2O during discharge/charge process. And DFT calculations reveal the intercalation of Zn2+ into some sites in the inner channel of MnVO is irreversible, leading to the accumulation of Zn2+ with almost unchanged interlayer spacing.