Chromium vanadate with unsaturated coordination sites for high-performance zinc-ion battery

Cr0.09V2O5(H2O)2·H2O (CrVO) curly nanosheets and CoVO porous microflowers were synthesized. Rietveld refinement unveils CrVO consists of V-O-V layers, lamellar {Cr(H2O)6}3+ cations and dissociated water, in which all the V centers are unsaturated coordinated. As a result, the V-O-V bilayer in V2O5 is split into two discrete V-O-V monolayers in CrVO with the (0 0 1) facet as the division plane and the interlayer spacing is large up to 14.5 Å. The exposed V centers on the (0 0 1) surface are unsaturated with oxygen vacancies, which can facilitate the migration of Zn2+ along the surface with a lower energy barrier of 0.68 eV, as evidenced by DFT calculations. CrVO shows a dominant surface-controlled capacity, which can deliver a large capacity of 497 mAh g−1 at 0.1 A g−1 with excellent long-term durability. The capacity retention is 96 % after 1000 discharge/charge cycles at 5 A g−1, which is superior to the isostructural CoVO. Ex situ characterizations of CrVO indicate a co–(de)intercalation mechanism of H+/Zn2+, and DFT calculations reveal that the intercalation of Zn2+ on some sites in the inner channel of CrVO is partial irreversible with very negative binding energies, leading to the accumulation of Zn2+ in CrVO. However, the transformation of CrVO → Znx(CrVO) doesn’t change the lattice spacing of (0 0 1) facet obviously, and Znx(CrVO) can be maintained after cycling test.