Hollandite-type Al-doped VO1.52(OH)0.77 as a zinc ion insertion host material

Abstract

For the first time, we suggest VO_1.52(OH)_0.77 and Al-doped VO_1.52(OH)_0.77 as possible candidate electrode materials for aqueous zinc-ion batteries (ZIBs). Structural analysis data indicate successful doping of Al into the hollandite VO_1.52(OH)_0.77 tunnel structure, as confirmed by Rietveld refinement of the X-ray diffraction data (XRD) and X-ray absorption near edge structure (XANES) spectroscopy data. Al is preferred to replace V³⁺, such that the resulting average oxidation state of V increases with increasing Al content in V_1−xAl_xO_1.52(OH)_0.77. Electrochemical test results indicate significant improvement in capacity and retention in the Al-doped V_1−xAl_xO_1.52(OH)_0.77 due to the presence of strong Al–O bonds, which stabilizes the crystal structure. This is confirmed from the post-cycled data that show less variation in the lattice parameters. Combining the XRD and XANES data, we suggest that the present V_1−xAl_xO_1.52(OH)_0.77 is activated by the V^4+/3+ redox reaction, accompanied by Zn²⁺ insertion into the tunnel structure upon reduction and Zn²⁺ extraction upon oxidation.