δ-MnO 2 nanofibers, synthesized by using a simple, low-cost solgel method, showed high electrochemical performance as a cathode for rechargeable Al-ion batteries (AIBs). δ-MnO 2 presented an initial discharge capacity of 59 mA h g À 1 and stabilized at 37 mA h g À 1 at a current rate of 100 mA g À 1 after 15 cycles and for more than 100 cycles with almost a 99 % coulombic efficiency. Different plateaus in charge/discharge curves, consistent with CV peaks, revealed the Al-ion insertion/ deinsertion and the electrochemical stability of the battery. Moreover, different rate CV measurements revealed the pseudocapacitive behavior of δ-MnO 2 in AIBs. The obtained charge/discharge capacities are ten times higher than previous studies performed with this material. Ex situ Raman and highresolution TEM measurements in different charge/discharge states revealed structural information of δ-MnO 2 upon Al-ion intercalation/deintercalation.