Study On Molten Salt Synthesis Of Transition Metal Mn/Co Compounds And Their Supercapacitive Properties

Global warming and the ever worsening energy loss,have greatly affected the economy,ecology and human life.Hence,the developments of environmentally friendly,more advanced energy storage and easy operation devices are vitally important.Research on supercapacitors have intensively attracted considerable attention over the years because of their long cycle life(10⁵ cycles),quick charge-discharge,excellent high power density,low toxicity and simple principle.In this dissertation,Mn₂O₃,LiMn₂O₄,Co₃O_4,MnCo₂O₄ and NiCo2O4 with different microstructure and morphology have been synthesized by molten salt synthesis.This dissertation focuses on supercapacitive properties of Mn2O3 and LiMn2O4materials in molten nitrate electrolyte.And emphasizes on studying supercapacitive properties of Co3O4,MnCo2O4 and NiCo2O4 materials in aqueous electrolyte.X-ray Drffraction（XRD）,scanning electron micrograph（SEM）,transmission electron microscopy（TEM）and other characterization methods were employed to study the microstructure and morphology of active materials.And the electrochemical behavior of electrode materials were investigated by cyclic voltammetry,galvanostatic charge discharge and electrochemical impedance spectroscopy in molten salt electrolyte or aqueous electrolyte.The main works were as follows:（1）Firstly,the potential window and thermal stability of KNO₃-NaNO₃-NaNO₂（53:7:40wt.%）molten salt were studied by steady-state polarization,thermogravimetric analysis and ICP.The results show that KNO3-NaNO3-NaNO2（53:7:40 wt.%）molten salt has a large potential window and good thermal stability in the temperature range from 50 to 380°C.The Mn2O3 nanoparticles with good crystallinity,high purity and excellent thermal stability were synthesized by molten salt synthesis,which is alkali metal nitrate as reaction medium and（CH3COO）2Mn as sources of manganese element.Electrochemical properties of Mn2O3electrodes were evaluated by electrochemical measurements.The results indicate that the specific capacitance of Mn2O3 electrodes is as high as 558.0 F/g at a current density of 5mA/cm² in molten salt electrolyte,and the coulombic efficiency is greater than 95.0%.The Mn2O3 electrodes also reveal excellent cycle stability in 3 M Na2SO4 aqueous electrolyte,and the capacitance retention rate is as high as 95.9%after 3000 cycles.（2）LiMn₂O₄ nanorods were synthesized by molten salt synthesis,which was KMnO₄and MnCl2 as sources of manganese element,LiCl and KCl mixed molten salts as reaction media.Electrochemical measurements show that the LiMn2O4 electrodes have excellent supercapacitive properties.The specific capacitance of LiMn2O4 electrodes are 420.0 F/g in KNO3-NaNO3-NaNO2（53:7:40 wt.%）molten salt electrolyte at a current density of 20mA/cm².And the capacitance retention rate of the LiMn₂O₄/LiMn₂O₄ symmetric supercapacitors is as high as 90.5%after 3000 cycles.Thus the LiMn2O4/LiMn2O4 symmetric supercapacitors has a certain application prospect and research value.（3）In this section,Co3O4 and MnCo2O4,NiCo2O4 electrode materials with polyhedron structure were successfully synthesized by using nitrate molten salt as reaction medium at250°C.By electrochemical analysis,finding out that the NiCo2O4 electrodes exhibit excellent supercapacitive properties,the specific capacitance of NiCo2O4 electrodes is 373.2 F/g in 3 M KOH aqueous electrolyte at a current density of 10 mA/cm².However,Co₃O₄ and MnCo₂O₄electrodes can only reach 172.1 and 155.3 F/g.And the NiCo2O4/AC asymmetric supercapacitors have excellent cycle stability and high power density.The specific capacitance could reach 69.8 F/g after galvanostatic charge-discharge 5000 cycles,coulombic efficiency and capacitance retention are up to 97.0%and 94.0%,respectively.