Study On Preparation And Energy Storage Mechanism Of Manganese Oxide Electrode Materials

Manganese oxide has been considered as one of the most promising materials owing to its advantages such as high theoretical capacitance,natural abundance,and low price.However,manganese oxide electrode materials often suffer from poor rate capability and cycling stability because of its poor electronic/ionic transport.In this paper,the electrochemical performance of Mn-based oxides electrode materials was improved by the fabrication of an unique core-shell structure.At the same time,the evolution of crystal microstructure was studied under in-suit Raman and XRD tests to explore the energy storage mechanism in alkaline electrolyte system.A stable MnO_x@MnO_x core-shell heterostructure electrode materials consisting of MnO_x nanosheets grown uniformly on the surface of MnO_x nanowires has been prepared by a simple liquid phase method followed by thermal treatment.The electrode displays a specific capacity of 336 F g^-1 at 1 A g^-1 and good cycling life.This is mainly due to the synergy effect between the one-dimensional Mn O_x nanowires as the backbone structure and the two-dimensional MnO_x nanosheets with large surface area provide more active sites and the rapid transmission of electrons.To improve the electronic conductivity of MnO_x@MnO_x core-shell heterostructure,a stable MnO_x@C@MnO_x heterostructure are obtained by simple liquid phase method combined with thermal treatment.The hierarchical MnO_x@C@MnO_x heterostructure electrode possesses a high specific capacitance of 350 F g^-1 and an excellent cycle performance.Particularly,according to the in situ Raman spectra analysis,no characteristic peaks corresponding to MnOOH are found during charging/discharging,indicating that pseudocapacitive behavior of the MnO_x electrode have no relevance to the intercalation/deintercalation of protons?H⁺?in the electrolyte.Further combining in situ X-ray powder diffraction analysis,the diffraction peak of?-Mn O₂ can be detected in the process of charging,while Mn₃O₄ phase is found in discharge products.Therefore,these results demonstrate that the MnO_x undergoes a reversible phase transformation reaction of Mn₃O₄?MnO₂.In order to explore the energy storage mechanism of Mn-based oxides,Mn₃O₄@rGO core-shell nanostructure as a typical measured electrode material was synthesized by a hydrothermal and electrostatic self-assembly method combined with thermal treatment.According to in-situ Raman spectroscopy and electrochemical quartz crystal microbalance measurements,it found that the Mn₃O₄ in core-shell nanostructure undergoes a reversible phase transformation reaction of Mn₃O₄?K-birnessite manganese oxides during charging/discharging.Furthermore,based on the First principle calculation,it indicated that K-birnessite manganese oxides obtained by electrochemical oxidation in the process of charging mainly reveals the K⁺intercalation pseudocapacitance behavior during discharging.With the increasing of K⁺intercalation,the distortion of MnO₆ octahedron in the structure of K-birnessite manganese oxides is becoming more and more serious due to the Mn-O bond length variable longer.Therefore,the lattice oxygen is released to produce oxygen vacancies,which are further transformed into a stable spinel-type Mn₃O₄ phase.