Construction And Energy Storage Of Self-Supported Manganese Cobaltate Composite Materials

As a new type of energy storage device,supercapacitors have attracted extensive attention due to their advantages such as fast charge and discharge,high power density,and long cycle life.Among them,the energy storage electrode materials with excellent electrochemical performance are believed to be a key driving factor for meeting the demands for the applications of the supercapacitors.However,the primary problems for current electrode materials lie in the factors including the low practical specific capacitance,especially poor rate capability and cycle stability.Therefore,exploring and investigating the effeicent supercapacitor electrode materials with stable electrochemical performance are still a hot and frontier topic in this research field.The focus of this thesis is to investigate the structure design,controllable synthesis,and the correlation between the components,hierarchical structure and electrochemical performance of the MnCo₂O₄-based composite materials.The intrinsic reasons for the improvement of electrochemical performance of composite materials are explored to clarify the energy storage mechanism of the composite materials.As a result,a valuable reference and basis can be provided to improve the electrochemical performance of MnCo₂O₄-based composite materials.The main topics of this thesis are described as follows:?1?Preparation and electrochemical performance of MnCo₂O₄@Ni₃S₂ core-shell heterostructure composite electrode materialsThe hexagonal prism-like MnCo₂O₄@Ni₃S₂ composite materials were prepared by a two-step hydrothermal method.The morphology of MnCo₂O₄ could be varied by changing the molar ratio of dicyandiamide to Co+Mn,which in turn changed the electrochemical performance of the composite materials.The electrochemical measurements results indicated that the specific capacitance of the MnCo₂O₄@Ni₃S₂composites was as high as 2807 F g^-11 at a current density of 3 Ag^-1,which was much larger than that of the pristine MnCo₂O₄ or Ni₃S₂ electrodes materials.And 69%capactiance retention was obtained?the capacitance retention of the pristine MnCo₂O₄and Ni₃S₂ electrodes was 53%and 63%,respectively?when the current density was increased to 30 Ag^-1.In addition,the pristine MnCo₂O₄ and Ni₃S₂ electrodes exhibited the capacitance retention of 78%and 83%,respectively,after 5000 cycles at 10 Ag^-1,whereas the MnCo₂O₄@Ni₃S₂ composite electrode demonstrated a capacitance retention of up to 92%.These results indicate that the electrochemical performance of the MnCo₂O₄@Ni₃S₂ composite electrode is significantly higher than that of the pristine MnCo₂O₄ and Ni₃S₂ electrodes,which is attributed to the unique hierarchical structure of the composite electrode materials and the synergistic effect of the MnCo₂O₄ and Ni₃S₂components.?2?Preparationandelectrochemicalperformanceofthree-dimensional MnCo₂O₄@MnMoO₄ electrode materialsRational design and preparation of core-shell nanoarrays?CSNs?with distinctive architectures and desirable capacitive performance are believed to be a promising and challenging strategy toward advanced electrode materials for supercapacitors.In this work,we report a two-step synthesis of three-dimensional MnCo₂O₄nanosheets@MnMoO₄ nanosheets CSNs on Ni foam,which are built by vertically growing MnMoO₄ nanosheets onto the interconnected MnCo₂O₄ nanosheets arrays.Such MnCo₂O₄@MnMoO₄ CSNs can fully utilize the unique sheet-by-sheet microstructure and the synergistic effect between MnCo₂O₄ and MnMoO₄ to improve the electrochemical performance.The results show that the composite materials demonstrated a significantly enhanced specific capacitance(2212 F g^-1 at 3 Ag^-1),higher rate capability(1573F g^-1 at 30 Ag^-1),and better cycling stability(95%capacitance retention after 5000 cycles at 10 Ag^-1)compared to the pristine MnCo₂O₄ or MnMoO₄nanoarray electrodes.Moreover,the assembled MnCo₂O₄@MnMoO₄//activated carbon asymmetric supercapacitor cell also exhibited a certain specific capacitance(138.8 F g^-1at 1 Ag^-1)and an energy density(49.4 Wh kg^-1 at 815 Wkg^-1),indicating a promising practical application in high-performance supercapacitors.?3?Preparation and energy storage performance of MnCo₂O₄/NiMn layered double hydroxide?LDH?composite nanosheet arrays electrode materialsThis work is related to the construction of hierarchical MnCo₂O₄/NiMn LDH composite nanosheet arrays by in situ growing the NiMn LDH nanosheet arrays onto the surface of the MnCo₂O₄ nanosheet arrays?NAs?and their electrochemical performance.The layer-by-layer structure and the synergistic effect between NiMn LDH and MnCo₂O₄components greatly improved the energy storage performance of the prepared composite materials.The results show that the composite electrodes exhibited a specific capacitance of 3063 F g^-1 at 3 Ag^-1?four times with respect to the individual MnCo₂O₄ electrode?and2315 F g^-1 at 30 Ag^-1.And 94.7%capacitance retention was achieved after 5000 cycles at 20 Ag^-1.Furthermore,an asymmetric solid-state supercapacitor device was fabricated using MnCo₂O₄/NiMn LDH and active carbon as the positive and negative electrode,respectively,which delivered a high energy density of 51.9 W h kg^-1 at a power density of 806 W kg^-1.These findings might offer a feasible strategy to synthesize the high-performance composite electrodes for supercapacitors...