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.