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.
A set of multiphase manganese‐oxide composite materials (Mn2O3@Mn3O4 and Mn3O4@Mn5O8), and a birnessite‐type KxMnO2 oxide are prepared and evaluated as cathodes for Zn‐ion batteries. The species formed when the electrodes are subjected to 2 V in aqueous solutions of MnSO4 and ZnSO4 are analyzed, suggesting an interphase activation leading to enhancement of electrochemical response. For the first time, it is shown that a Zn4(SO4)(OH)6.xH2O phase coats the composite‐type electrodes in the charging stage, contributing to extending the lifetime of the batteries. KxMnO2 electrode with layered birnessite structure shows long cycling life at low current densities (122 mAh g−1 at 30 mA g−1 after 50 cycles) and good efficiencies (ca. 99%) in the 0.1 Mn2+ electrolyte. In contrast, in the 0.5 m Mn2+ electrolyte, high values of specific capacity are delivered by the cell at higher rates, that is, 150 mAh g−1 at 600 mA g−1. In Mn5O8@Mn3O4 the good performance is due to the synergistic effect of the two compounds forming the composite. Thus, after more than 100 cycles this composite displays specific capacity values of 175 mAh g−1 at 2150 mA g−1 in the 0.1 m Mn2+/1 m Zn2+ electrolyte.
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