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·H2O to boost Al3+ migration via the charge shielding effect of water is reported. It is revealed that VOPO4·H2O 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·H2O@MXene nanosheets are fabricated via the formed TiOV bond between VOPO4·H2O and MXene. The MXene aided exfoliation results in enhanced VOwater bond strength between H2O and VOPO4 that endows the obtained composite with strong water holding ability, contributing to the extraordinary cycling stability. Consequently, the VOPO4·H2O@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. The water lubrication effect for Al3+ diffusion is uncovered using VOPO4·nH2O (n = 0, 1, 2) as model. The water stability of VOPO4·H2O is enhanced via hybridization with MXene, which can strengthen the VOwater bond via introduction of the abundant oxygen vacancies. The resulted VOPO4·H2O@MXene owns high capacity and excellent cycling stability.