Study On Electrochemical Cycling Stability Of Manganese Oxides

Supercapacitors are one of the most promising new energy storage devices because they can instantaneously provide higher power density than batteries,and higher energy density than conventional dielectric capacitors,and are environmentally friendly.Manganese oxide（Mn O_x）,with so many outstanding properties such as low cost,high theoretical specific capacitance and various crystal forms,is one kind of competitive electrode among the many electrode materials of supercapacitors.However,poor electrical conductivity and cycle stability limit its further practical application.On the other hand,stable output and long cyclic life are very important to the supercapacitors,which can reduce the cost of maintenance and replacement.Therefore,a systematic study has been carried out to reveal what key factors and how they affecting the cycling behaviors of manganese oxides.And carbon-coated Mn O2hierarchical spheres（MHSs）were prepared to improve their electrochemical performance.The main research results are as follows:（1）The specific capacitance degradation of Mn O_xis usually attributed to the so-called“dissolution”issue.Our results indicate that“dissoluted Mn O_x”is in the form of Mn O_x:Na tiny nanostructures（or flotsam）derived from the detached active materials instead of Mn²⁺in the solution,which causes color change of electrolyte and the loss of specific capacitance.During the cycling,the morphology of manganese oxides transformed to flowerlike flakes regardless of the starting structures.After that,it tends to form nanowires especially at elevated temperatures.According to the relative low electrochemical utility of nanowires,specific capacitance might decrease at this stage.Besides,electrochemical oxidation of Mn O_xleads to an increase in specific capacitance.The cycling behavior of Mn O_xis mainly determined by these three factors.These results provide a deeper understanding of the fundamental cycling behavior of manganese oxide-based electrode materials.（2）By coating PDA on MHSs followed by carbonization,sphere like carbon coated Mn O_xnanostructures have been synthesized.The major part is Mn₃O₄in the Mn O_x.A specific capacitance of 224.8 F g^-1has been achieved at 1 A g^-1.During cycling test,the specific capacitance kept increasing.After 12000 cycles,the specific capacitance increased to 150%of its initial value,indicating a good stability.Although there is carbon coating as protecting layer,the morphology of the sample still transformed to flower-like sheet structure.It suggests that the electrode materials may endure a complex structure change and reorganization process during the storage of sodium ions.It also puts forward new questions on charge storage mechanism,which have important guiding significance for the development of high-stability supercapacitors.