Exploring Aluminum-Ion Insertion into Magnesium-Doped Manjiroite (MnO2) Nanorods in Aqueous Solution

Aqueous aluminum-ion batteries are promising for sustainable energy storage given the abundance of aluminum and the ease of recycling. However, finding suitable electrode materials that can intercalate Al ions remains challenging. Here, we investigate Mg-ion-doped MnO₂ nanorods in the manjiroite structure that allow Al-ion insertion into the open tunnels of this material. The electrode material is characterized before and after Al-ion insertion with X-ray diffraction and X-ray photoelectron spectroscopy, and the insertion process is studied via in situ transmission electron microscopy, which highlights a change in the length of the MnO₂ nanorod. The results indicate that Al ions can be successfully inserted into the MnO₂ nanorods, which has also been determined by density functional theory calculations to be an energetically favorable process. The electrochemical data indicate that diffusion-controlled Al insertion as well as a surface pseudocapacitive effect occur. This indicates that MnO₂ nanomaterials synthesized in the presence of metal cations is potentially a viable route for facilitating their use as electrode materials in aqueous Al-ion batteries or supercapatteries.