Electrochemical Performance Optimization Of Manganese Oxide-based Composites And Their Energy Storage Devices

As a transition metal oxide,manganese oxide has been widely used in the field of electrical energy storage due to its high theoretical specific capacitance.However,manganese oxide is very easy to agglomerate during the preparation process,and it can not fully contact with the electrolyte.Only the electrode surface can participate in the reaction,resulting in the actual specific capacitance to be far from the theoretical value,which is often an order of magnitude smaller.In addition,its poor electrical conductivity further limits its application in the energy storage field.Nowadays,there are mainly two aspects to improve the energy storage performance of manganese oxide:one is compositing with materials with good conductivity and comprehensively improving the performance of the composites by using the synergistic effect of the two materials;the other is constructing special structures.The structure optimization can not only slow down the agglomeration of manganese oxide,but also improve the overall stability of the materials.Two manganese oxides,Mn₃O₄and MnO₂,were studied in this paper,and three composites were prepared.Based on the analysis of composition,structure and performance,these composites are applied to supercapacitor devices.The results not only provide an effective synthesis path for improving the electrochemical performance of manganese oxide,but also provide an important reference for broadening the application of the composites in supercapacitor devices.Mn₃O₄/NC composite was prepared by a facile liquid-phase precipitation method.The 0D/2D structure was formed by nano-sized Mn₃O₄ particles and micron-sized activated carbonitride sheets（NC）.NC with large surface area and abundant pore structure can increase the contact area between electrode and electrolyte,and N-atom doping can provide additional pseudocapacitance.When the percentage of NC is15.8%,the Mn₃O₄/NC composite has the largest surface area of 77.42 m² g^-11 and the highest specific capacitance of 180 F g^-1.At a current density of 1 A g^-1,the assembled symmetrical coin cell supercapacitor achieves a specific capacitance of 44.4 F g^-11 and exhibits good stability.MnO₂/CC composite was prepared by a one-step hydrothermal method.MnO₂fibers with a diameter of 14-15 nm formed a special"marshmallow-like"structure on the carbon cloth substrate（CC）.The fibers are crossed with each other and formed a three-dimensional network,which can expose more active sites.In the neutral electrolyte,MnO₂/CC composite exhibits a higher specific capacitance of 249.6 F g^-1and a better cycle stability of 97.7%.With MnO₂/CC as the positive electrode,activated carbon as the negative electrode,and 1 M Na₂SO₄ as the electrolyte,the asymmetrical coin cell supercapacitor has a specific capacitance of up to 67.8 F g^-1,and its maximum energy density can reach 18.46 Wh kg^-1.Mn₃O₄/CC composite was prepared by a two-step hydrothermal method.Mn₃O₄particles uniformly covered the entire carbon fiber to form an 0D/1D structure.As a good conductive matrix,carbon cloth can effectively slow down the agglomeration of Mn₃O₄,and ensure the stability of the composite structure.In the neutral electrolyte,Mn₃O₄/CC composite has excellent stability,and the specific capacitance remains99%after 2000 cycles.In the alkaline electrolyte,the specific capacitance of the composite can reach 555 mF cm^-2,and its cycle stability is improved,compared with MnO₂/CC composite.After 2000 cycles,it can maintain 63.6%of the initial specific capacitance.Asymmetrical flexible supercapacitor is assembled with Mn₃O₄/CC as the positive electrode,activated carbon as the negative electrode,and PVA/LiCl as the gel electrolyte.The device has good flexibility and mechanical stability.When the current density is 1 mA cm^-2,the specific capacitance of the device can reach 108 mF cm^-2.At the same current density,the energy density is 25μWh cm^-2 and the power density is 0.64 mW cm^-2.