Synthesis And Electrochemical Performances Of Several Manganese Oxides

Three electrolytic manganese dioxide (EMD) samples were characterized by XRD and chemical analyses. Their electrochemical performances were studied by cyclic voltametry, constant-current discharge, and choronopotentiometry. It is found that three samples have the same structure, γ-MnO₂, but their combined water contents are different. The chemical compositions of three samples can be formulated as A: Mn_0.881⁴⁺Mn_0.044³⁺O_1.655^2-(OH)₍0.345_<sup>-, B: Mn_0.880⁴⁺ Mn_0.048³⁺O_1.666^2-(OH)^0.334- and C: Mn_0.879⁴⁺Mn_0.054³⁺O_1.677^2-(OH)_0.322^-. The electrochemical performances are influenced by the combined water contents, which are related to the number of cation-vacancy in the samples. The open circuit potential, discharge capacity and peak potential of one electron reduction increase with the increase of the combined water contents in the samples.EMD were inserted with H by chemical methods in a nonaqueous environment. Compounds with compositions varying from the staring material to fully H-inserted material were prepared and investigated by FTIR spectroscopy. Slow-scan cyclic voltammery was used to investigate the voltammetric behavior of EMD. The concentrations of soluble Mn(III) species, during the reduction of electrolytic manganese dioxide in alkaline solution, have been monitored in situ by UV-visible spectroscopy. The concentration of Mn(III) ion in solution changes with discharge time. The first electron discharge process of EMD can be described by three different steps: (1) the reduction of Mn⁴⁺ ions on the surface of EMD particles and the structural defect regions within the EMD, (2) the reduction of Mn⁴⁺ ions in ramsdellite, (3) the reduction of Mn⁴⁺ ions in pyrolusite.Microporous todorokite-type manganese oxides had been synthesized by a route inwhich the key Na-birnessite precursor was prepared by oxidation of Mn(0H)2 with K2S2O8 in aqueous NaOH. The foreign metal cations, such as Mg2+ and Ni2+, were used in a subsequent ion-exchange reaction that converted Na-birnessite into a related layered material, buserite. Hydrothermal treatment of the buserite ultimately yielded Mg-todorokite or Ni-todorokite. In lmol-L"1 LiPF6 solution, it was found that the initial capacity of Mg-todorokite was 286 mAh-g"1 and the second discharge only had a capacity of 103 mAhg1. On the other hand, the initial capacity of Ni-todorokite was 124 mAh-g"1, and a capacity of 104 mAh g'1 was still found in the second cycle. Therefore, Ni-todorokite was better than Mg-todorokite in the performance of reversiblility.Cryptomelane and amorphous manganese dioxide were prepared by using reflux methods after the oxidation of Mn2+ by KMnO4. Cryptomelane was attained by using water as solvent, while yielded amorphous manganese dioxide was yielded by using ethanol as solvent. The initial capacity of Cryptomelane was 196 mAhg"1, but it had a poor rechargeability. The amorphous manganese dioxide yielded superior electrochemical cycling stability. It was found that the initial capacity of amorphous manganese dioxide was 208 mAh-g"1, and it still had a capacity of 167.6 mAh-g"1 after four charge/discharge cycles.