Synthesis, Structural Characterization Of Manganese Dioxide And Their Electrochemical Performances

Manganese oxide (MnO₂) has been investigated widely because of its application in batteries, separation and catalysis. However, a complete understanding of its structure is still out of the knowledge because of its many various polymorphs and also owing to the intercystalline water and other cations settled in structure. MnO₂ has been investigated as cathodes for batteries for long time because of its good economy and low toxicity. However, the specific capacity of MnO₂ in practical Li/MnO₂ batteries is low compared to its theoretical one, and its cyclic performance is not good for application in rechargeable batteries. Concerning these issues, intensive and systemetical investigations were carried out in this dissertation and main conclusions were summarized as follows:Interlayer water of bimessite (IWB) was investigated by thermalgravimetric /differential thermal analysis - mass spectrum (TG/DTA-MS) and temperature -programmed X-ray diffraction (TP-XRD) techniques. Our results showed that the nearly complete removal of IWB did not destroy its structure but only shorten its interlayer distance. Once the dehydrated birnessite exposed to atmospheric air, it will rehydrate very fast, i.e., about 45% of IWB recovered in 20 min, while 73% in 1 h. However, a prolonged time exposure in atmospheric air did not alter significantly the quantity of IWB. For interlayer potassium of birnessite (IPB), its removal assisted by soaking in acidic solution at different concentrations and temperatures, collapsed its layered structure to some extent, sometimes even collapsed completely. Furthermore, the effects of IPB on the layered structure were discussed by comparing the previous reports and concluded to be crucial for the layered structure of birnessite. In addition, a well-crystallized birnessite was believed not to be obtained directly in an acidic solution.CoOOH coated layered MnO₂ was prepared by creative deposition of Co²⁺ on the surface of MnO₂ with OH" left on surface during washing. The point is washing times, controlled according experiments. Co₃O₄ coated layered MnO₂ (Co-MnO₂) was subsequently obtained after heat-treatment at 300℃, which was determined based on the analysis of TG/DTA-MS and XRD results. Compared with MnO₂ samples; Co-MnO₂ delieved a greater specific capacity and showed a better cyclic performance.Amorphous MnO₂/C composite was synthesized by acetylene black which was able to reduce permanganate directly. The reaction was described by the following equation:4KMnO₄ + 3C + 2H₂O→4MnO₂ + 3CO₂ +4KOH,The proposed reaction confirmed by analyzing products, such CO₂ gas and CO₃^2-/7HCO₃^- left in the mother liquor. The composite has a structure of tens of nanometers of MnO₂ covering on the remaining C as evidenced by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) line scan map. To improve Mn content in composites, the as-prepared products were treated deliberately by acid or by calcinations to remove some of interlayer potassium and water, respectively. In both cases, the specific capacities were improved while the polarizations shifted greater and smaller for acid and heat treatment, respectively. The heated composite has a good discharge and rate performances and is anticipated as a promising electrode material for heavy drain.The successful controllable synthesis ofα-,β- andδ-MnO₂ were based on our previous experiments and well understanding on the effect of potassium on the structure of MnO₂. The mechanism controlling this process is a condensation reaction, where Mn²⁺ reacted rapidly with MnO₄ forming amorphous MnO₂. MnO₂ subsequently dissolved in acidic or basic solution and then condense to form crystalline MnO₂. The formation of MnO₂ polymorphs depends on the K⁺, H⁺ and/or OH^- concentration. As a result, we propose that K⁺ and H⁺ were in competition for growth of crystallineα-MnO₂ andβ-MnO₂, whereas K⁺ and OH^- are both necessary for formation ofδ-MnO₂. Such conclusions would benefit understanding crystallzaiton of MnO₂, and also help analyze thier transformation between these polymorphs and expire mechanisms in the application in batteries, separation and catalysis, etc.