Aluminium vanadate with unsaturated coordinated V centers and oxygen vacancies: surface migration and partial phase transformation mechanism in high performance zinc-ion batteries

Abstract

Al_0.17V₂O₅(H₂O)₂·H₂O (AlVO) and Ca_0.23V₂O₅(H₂O)·H₂O (CaVO) are synthesized herein, and their Rietveld refinements reveal that they are composed of V–O–V layers, lamellar hydrated Al³⁺ or Ca²⁺ and dissociated water molecules. The preintercalation of metal ions into V₂O₅ leads to the elongation of V–O bonds, and all the V centers are unsaturated coordinated. The V–O–V bilayer in V₂O₅ is split into two discrete V–O–V monolayers with the (001) facet as the division plane. Meanwhile, the exposed V centers on the (001) surface are unsaturated with oxygen vacancies, which can facilitate the migration of Zn²⁺ along the (001) surface of AlVO with a lower energy barrier of 0.88 eV, as evidenced by density functional theory (DFT) simulations. AlVO shows a dominant pseudocapacitive behavior with excellent rate capability and outstanding cycling performance. It can deliver a high capacity of 441 mA h g⁻¹ at 0.1 A g⁻¹ with capacity retentions of 96.7% and 90.5% after 1000 and 2000 discharge/charge cycles at 5 A g⁻¹, respectively, which are superior to that of CaVO. In addition, DFT calculations also reveal that the intercalation of Zn²⁺ on some sites in the interlayer channels of AlVO and CaVO is irreversible with very negative binding energies, leading to the accumulation of Zn²⁺ in AlVO and CaVO. Furthermore, ex situ characterization of AlVO and quasi-reversible change of the pH value during the 1^st discharge/charge process indicate a co-(de)intercalation mechanism of H⁺ and Zn²⁺. The pH-induced V species and the accumulation of Zn²⁺ give rise to partial transformation of AlVO → Al³⁺-doped Zn₃(OH)₂V₂O₇·2H₂O, which is also observed for CaVO.