VOPO4·H2O: A Stacking Faults Structure Studied by X-ray Powder Diffraction and DFT-D Calculations

Water-Stabilized Vanadyl Phosphate Monohydrate Ultrathin Nanosheets toward High Voltage Al-Ion Batteries

Al ion batteries (AIBs) are attracting considerable attention owing to high volumetric capacity, low cost, and high safety. However, the strong electrostatic interaction between Al³⁺ and host lattice leads to discontented cycling life and inferior rate capability. Herein, a new strategy of employing water molecules contained VOPO₄ ·H₂ O to boost Al³⁺ migration via the charge shielding effect of water is reported. It is revealed that VOPO₄ ·H₂ O with water lubrication effect and smaller steric hindrance owns high capacity and fast Al³⁺ diffusion, while the loss of unstable water upon cycling leads to a rapid performance degradation. To address this problem, ultrathin VOPO₄ ·H₂ O@MXene nanosheets are fabricated via the formed TiOV bond between VOPO₄ ·H₂ O and MXene. The MXene aided exfoliation results in enhanced VO_water bond strength between H₂ O and VOPO₄ that endows the obtained composite with strong water holding ability, contributing to the extraordinary cycling stability. Consequently, the VOPO₄ ·H₂ O@MXene delivers a high discharge potential of 1.8 V and maintains discharge capacities of 410 and 374.8 mAh g^-1 after 420 and 2000 cycles at the current densities of 0.5 and 1.0 A g^-1 , respectively. This work provides a new understanding of water-contained AIBs cathodes and vital guidance for developing high-performance AIBs.

Combined NMR and X-ray diffraction study of structural aspects, dynamics and charge ordering mechanism in LixVOPO4.2H2O intercalation compounds

We carried out a detailed investigation of the local ordering and dynamics of the lithium intercalation in paramagnetic Li_xVOPO₄.2H₂O (with 0 < x ≤ 1) materials. This question was addressed using a combination of X-ray diffraction, ³¹P and ⁷Li MAS NMR experiments. We first studied the structure of the fully ordered end-member of the series, Li₁VOPO₄.2H₂O, revisiting the X-ray single crystal diffraction data on the basis of the information provided by ³¹P MAS NMR. We then carried out ⁷Li MAS and exchange NMR experiments and ³¹P MAS experiments on the polycrystalline powders obtained after partial lithium insertion in VOPO₄.2H₂O phases. These experiments evidenced an unexpected ageing of the material related with lithium dynamics between the VOPO₄ layers and a V⁴⁺/V⁵⁺ charge ordering mechanism within the layers.