Layered VSe₂ as an Al-Ion Storage Cathode for Aqueous Aluminum-Ion Batteries with Fast Diffusion and Stability and Its Charge Storage Mechanism

Aluminum (Al)-ion batteries have gained popularity because of their improved energy density, increased safety, eco-friendliness, abundant Al resources, and extremely attractive three-electron redox, making Al-ion batteries an appealing candidate. However, the progress in Al-ion batteries has been hindered by the unavailability of potential cathode materials that could reversibly host Al³⁺ ions. In this work, we investigated VSe₂, a 2D material with a graphene-like layered structure, as a potential cathode for aqueous aluminum-ion batteries. The VSe₂ cathode effectively accommodated Al³⁺ ions, delivering a high discharge capacity of 163 mAh g^-1 at 1 A g^-1, and maintained a capacity of 63 mAh g^-1 even at a high current density of 10 A g^-1, with excellent capacity retention over 2500 cycles. The Al-ion storage mechanism in the VSe₂ cathode is examined using ex situ X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and gravimetric intermittent titration technique studies, and it is revealed that the large interlayer spacing enabled by Se defects in VSe₂ can reversibly host Al ion through a combination of diffusion- and capacitive-controlled intercalation-type storage. The strong metallic characteristic, along with fast Al-ion diffusion and rigid structure, enabled VSe₂ to attain high capacity, rate, and stability. These findings have implications for the study of various 2D layered metal selenides as potential cathodes for Al-ion batteries.