An attempt was made to clarify the transport mechanism of a promising energy storage system of Al-I-2 batteries in both aqueous and non-aqueous solution for the first time. The different discharge features of the Al-I-2 cells in the water and the acetonitrile solution were revealed. Ac impedance spectra indicated that the resistance of the Al/I-2 cell in water solution reduced during discharging process gradually, while the resistance of the Al/I-2 cell in the acetonitrile solution increased during discharging process. This could be due to the formation of a layer of amorphous AII(3) on the surface of aluminum anode, which was confirmed by the scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and selected area electron diffraction measurements. An I- ion conductor, Lil-(3-hydroxypropionitrile)(4) (Lil(HPN)(4)) with 15 Wt% SiO2/l(2) cell was used as a solid electrolyte sandwiched between Al and I-2 electrode to assemble the solid-state Al-I-2 cell for further clarifying the transport mechanism. Our results demonstrated that the transport mechanism of Al-I-2 cell in the water may be involved in both cation and anion transport mechanism, but in non-aqueous solution, it may be involved in cation transport mechanism. (C) 2008 Elsevier Ltd. All rights reserved.