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

For the first time, we suggest VO1.52(OH)(0.77) and Al-doped VO1.52(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 VO1.52(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 V1-xAlxO1.52(OH)(0.77). Electrochemical test results indicate significant improvement in capacity and retention in the Al-doped V1-xAlxO1.52(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 V1-xAlxO1.52(OH)(0.77) is activated by the V4+/3+ redox reaction, accompanied by Zn2+ insertion into the tunnel structure upon reduction and Zn2+ extraction upon oxidation.