δ-MnO2 Nanofibers: A Promising Cathode Material for New Aluminum-Ion Batteries

delta-MnO2 nanofibers, synthesized by using a simple, low-cost sol-gel method, showed high electrochemical performance as a cathode for rechargeable Al-ion batteries (AIBs). delta-MnO2 presented an initial discharge capacity of 59 mA h g(-1) and stabilized at 37 mA h g(-1) at a current rate of 100 mA g(-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 delta-MnO2 in AIBs. The obtained charge/discharge capacities are ten times higher than previous studies performed with this material. Ex situ Raman and high-resolution TEM measurements in different charge/discharge states revealed structural information of delta-MnO2 upon Al-ion intercalation/deintercalation.