Studies On Preparation Of Partly Reduced MnO₂ And Modified MnO And Their Electrochemical Performance

Alkaline manganese rechargeable batteries are. attractive because of their low cost, high capacity and long shelf life. However, they have the disadvantage of short cycle-life. On the other hand, there is abundant of rare earth in our country that can be used as storing hydrogen materials. Therefore, it is considered that Zn anode is replaced by MH electrode in alkaline manganese battery to improve its performance. Some work have been done and testified its feasibility. However, the main obstacle of investigation of MH/MnO2 batteries is the mismatched states of discharge and charge of cathode and anode, i.e. MnO2 is in the state of charge, while MH is in the state of discharge. Therefore, some manganese oxides were prepared that the valence of manganese is lower than four. It is expected that the obstacle of mismatched state could be overcome by assembling these manganese oxides used as cathode with MH electrode.The following items have been done:1) I.C.No 1 HMD were partly reduced under different conditions, such as stirring in aqueous and organic environment and ultrasound in aqueous and organic environment respectively. The reduced samples were characterized by oxidation degree analysis, X-ray diffraction, IR spectrum and TEM technique. Some conclusions can be drawn that in aqueous reduction, ultrasound can accelerate the reduction and plays a role as catalyst to change the crystallgraphic form of products.2) The electrochemical performance of reduced samples was studied by constant-current charge-discharge, cyclic-voltammetry and potentiastaticpulse technique. The accumulative discharge capacity of partly reduced samples and these samples added proper amount of nickel hydroxide are lager than the capacity of I.C.NolEMD added nickel hydroxide. The results of measuring H+ diffusion coefficient of each charge-discharge cycle show that the coefficient of the sample electrode and the sample electrode added Ni(OH)2 do not decrease with the cycles. Therefore, the reversibility of electrode is improved. Meanwhile, it is confirmed that Ni(OH)2 can reduce the formation of inactive Mn3O4.3) MnO and modified MnO were prepared under nitrogen atmosphere by thermally decomposition of MnC2O4 and doped MnC2O4 and MnCO3 synthesized by solid-state reaction respectively. Samples were characterized by TG/DTA, X-ray diffraction, TEM and measurement of solubility. The particle size of sample is well distributed and the cluster decreases. The solubility of MnO in KOH increases with the concentration of KOH.4) The samples were studied by galvanistatic charge-discharge, cyclic-voltammetry, constant current polarization. The results show that MnO which not be modified can not be use as electrode active materials because of its poor electrochemical activity. Meanwhile, the rechargeabilify of MnO modified by Bi and Pb is improved. Bismuth could prolong the second electron equivalent discharge and Lead could improve the first electron equivalent discharge performance. It is concluded by kinetic study that the electrochemical polarization is the controlled step in the charge process of MnO doped with Bismuth.