Fabrication Of Composites Of Manganese Compounds And Their Applications In Supercapacitors

Compared with conventional electrochemical energy storage devices,supercapacitors（SCs）possess longer cycle life,higher power density,faster charge and discharge speed,higher safety and lower maintenance cost,etc.,which have attracted increasing interests in recent years.Notably,as a crucial segment,the performance of SCs strongly depends on the electrode materials.Manganese compounds,such as MnO₂and MnCO₃,exhibit the advantages of natural abundance,eco-friendly,and excellent theoretical specific capacitance,are considered to be promising electrode materials for SCs.Nevertheless,studies found that the actual specific capacitance of these materials is far lower than the theoretical value,and the cycle stability is unsatisfactory.These shortcomings greatly hinder their practical applications.In this paper,two kinds of manganese compounds--plasma-h-MnO₂ and La₂O₃@MnCO₃ with high specific capacitance and cycle stability,were fabricated directly on the nickel foam collector by hydrothermal-plasma treatment and one-step hydrothermal method,respectively.The morphology,structure and composition of the materials were characterized by field emission scanning electron microscope（FESEM）,X-ray diffraction（XRD）,transmission electron microscope（TEM）,Raman spectra（Raman）and X-ray photoelectron spectroscopy（XPS）.Electrochemical test methods such as cyclic voltammetry（C V）,galvanostatic charge-discharge（GCD）and electrochemical impedance spectroscopy（EIS）were employed to assess the performance.To investigate the practicability,asymmetric SCs were further assembled using these electrode materials.Afterwards,the reasons for the excellent performance of the electrode materials,charge and discharge mechanisms were also discussed.The main content is listed as follows:（1）Plasma-h-MnO₂ ultra-thin nanoflakes were prepared directly on the nickel foam via hydrothermal method followed by plasma treatment.These nanoflakes are approximately 20 nm thick and closely packed on the three-dimensional nickel foam framework.The majority of the synthesized material isδ-MnO₂ crystal structure,while plasma treatment introduces a small amount ofα-MnO₂ phase on the surface.Electrochemical results suggest that plasma-h-MnO₂ achieves a high specific capacity of 736.0 F/g（at the current density of 1 A/g）and desirable cycle stability（nearly 92%capacity retention after 1000 cycles）,superior to the MnO₂ electrode without plasma treatment.After assembled into an asymmetric supercapacitor device with the plasma-h-MnO₂ positive electrode and the activated carbon negative electrode.The device exhibits favorable specific capacity of 99.1 F/g at the current density of 1 A/g,and its capacitance retention maintains at 92%within 4000 cycles,indicating excellent cycle stability of the device.Besides,the energy density of the device reaches 44.6Wh/kg at a power density of 899.9 W/kg and can light on a red LED.Based on the characterization results of Raman,XPS and TEM,it is speculated that the mechanism of plasma treatment to improve the stability ofδ-MnO₂ is:after plasma treatment,theδphase of the co-edge structure on the MnO₂ surface partly turns into theα-phase of the co-point structure.Theoretically,the co-point structure shows higher stability than that of the co-edge structure,hence the plasma treatment significantly improves the stability.This work is helpful for revealing the tailoring mechanism of plasma treatment towards surface structure and properties of the electrode materials,which can provide a reference on simply and quickly strengthening the stability of materials.（2）A simple one-step hydrothermal method was carried out for in-situ fabricating La₂O₃@MnCO₃ compound on nickel foam collector.The material presents hydrangea-shaped,with a specific surface area of～16.4 m²/g and a pore size of～3.7 nm.In addition,it consists of rhodochrosite phase MnCO₃ and hexagonal phase La₂O₃ in a mass ratio of 5:1.Electrochemical assessments suggest that the specific capacity of La₂O₃@MnCO₃material attains 955.0 F/g at a current density of 0.5 A/g,and the capacitance retention keeps at 93%even after 5000 cycles,which is superior to the most MnCO₃ materials reported in the current literature.Besides,an asymmetric SCs device was assembled using La₂O₃@MnCO₃ as the positive electrode and activated carbon as the negative electrode.The energy density of the device at power density of 849.6 W/kg is as high as 71.6 Wh/kg,and its energy is sufficient to light on small-power LED lamps.More importantly,the specific capacity retention maintains at 96%even after 5000cycles with a current density of 5 A/g,showing excellent supercapacitor performances.In conclusion,the combination of rare earth oxide La₂O₃ can effectively improve the properties of MnCO₃,thereby significantly improving the electrochemical performance of the material.The facile preparation method in this st udy fabricated La₂O₃@MnCO₃shows excellent specific capacitance and cycle stability,which exhibits promising application prospects.