| With the progress of science and technology,new energy-storage devices are continuously developing,too.The electrode material in an electrochemical cell is the place where the energy-conversion reaction takes place,which determines the device’s energy-storage performance to a great extent.Therefore,the research on new electrode materials has been an important topic in the electrochemical energy-storage field.Manganese is abundant in the earth’s crust.With high safety and low environmental pollution,its oxides are widely applied in the energy-storage field.However,their low intrinsic conductivity,the multiple valence changes of manganese,and the possibly large volume changes during charging and discharging,would greatly restrict the stable play of their capacities.Hybridizing them with carbon or adjusting the morphology and structure can effectively alleviate or solve the above problems.In this thesis,to improve their energy-storage performances in lithium-ion batteries（LIBs）and supercapacitors（SCs）,manganese oxide/carbon composites with different structure and morphology have been developed by using different carbon sources,and their electrochemical reaction mechanisms and structure-activity relationships have been revealed,as follows:Firstly,considering the molecular-level interactions between organic functional groups and metal ions,a kind of small water-soluble polycyclic aromatic hydrocarbon molecules,composed by amphiphilic carbonaceous material（ACM）and containing some nitrogen and sulfur elements,was used to prepare a homogeneous manganese oxide/carbon composite as anodes of LIBs.Benefited by the nanoscale reactants,continuous and highly conductive carbon network,and developed nano-porous structure,the composite material demonstrated increased reactive sites and structural stability but reduced resistance of charge transport.As a result,it showed much enhanced electrochemical energy-storage performance in LIBs.When tested under the current density of 0.3 A g-1,its reversible capacity was maintained at 699.7 m Ah g-1 after500 cycles,implying a capacity retention of 87.8%.Further study showed that,improving the carbonization process of ACM significantly improved the micro-crystalline structure of the carbon component in the composite and reduced its defects and surface oxygen content,so that the initial coulomb efficiency of the target material was also improved.Secondly,two core-shell structured manganese oxide/carbon composites,namely the mulberry-like C@Mn O2 nano material and sphere-like Mn O micron material,were developed and comparatively studied to reveal the structure-activity relationship between the material’s morphology and its electrochemical lithium-storage performance.Thereinto,the former one had a continuous and highly conductive carbon framework and a nanoporous Mn O2 shell,which would enable the composite to possess both high electronic conduction and ionic diffusion.Whereas,for the latter,its above properties should be greatly restricted due to the coating of the insulated Mn O shell on its conductive carbon core.The electrochemical performance test also proved that,the mulberry-like C@Mn O2 material exhibited larger capacity,better rate performance,and improved electrochemical cycling capability.Furthermore,a novel micro-nanoporous Mn O material with unique hollow six-branched star-like morphology was fabricated through a facile annealing-assisted hydrothermal method.Such a design would enable the target material to possess several advanced electrochemical energy-storage architectures,including large surface area and short charge-diffusion distance.As a result,it in LIBs exhibited substantially higher electrochemical energy-storage performance than the nonporous Mn O2 contrasting material with the same hollow six-branched star-like morphology.After 200 cycles at0.5 A g-1,the former’s reversible capacities maintained at 834 m Ah g-1,but it was only338.4 m Ah g-1 for the above counterpart.Besides,at the same current densities of 0.1,0.3,0.5,1.0,and 1.5 A g-1,their reversible capacities were 769.7,741.7,718.9,713.2,and 704.4 m Ah g-1,and 476.7,392.4,357,303.4,and 269.9 m Ah g-1,respectively.In addition,a series of C@Mn O2 core-shell materials were prepared through oxidizing acetylene black with potassium permanganate,and the energy-storage structure-function relationship of the materials in SCs was determined based on their pseudocapacitive performances.The results showed that,by varying the mass ratio of the two precursors,it was possible to precisely control the composition,morphology and structure of the target material,so that the maximum load of active material and the further optimization of its charge-transmission dynamic characteristics could be achieved.It was also found that,when the mass ratio of potassium permanganate to acetylene black was 3:1,the as-prepared material showed the optimal core-shell structure in terms of electron and ion transport,when it used chain-like acetylene black as the conductive skeleton and porous Mn O2 nanoflower as the shell.Accordingly,it exhibited the best electrochemical energy-storage performance in a pseudocapacitor using KOH aqueous of 6 mol L-1 as electrolyte.For example,at 1,1.5,2,5,8,and 10A g-1,its specific capacitance archived 274.4,258.3,244.4,198.8,174.2,and 162.2 F g-1,respectively.Even after 5000 cycles at 1 A g-1,the capacity retention still reached90.2%,implying a good application prospect for this material.In summary,this thesis has developed multiple manganese oxide-based electrode materials with different components and morphology,and systematically studied their electrochemical energy-storage performances in LIBs and SCs.Based on these results,this thesis has successfully revealed the important role of carbon composite,core-shell structure,and nanoporous morphology in improving manganese oxide electrode materials in the energy storage-related properties like electron-ion transport,structure stability,and so on.The whole study provides important theoretical basis for the preparation,modification,and energy-storage application of high-performance manganese oxide-based electrode materials. |
PDF Full Text Request