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.