| Nonaqueous Li-air batteries(LABs) are of great potential in the fields of electrification regarding vehicles and military due to their ultrahigh theoretical energy density. However, it is still the initial stage where the LABs need further investigations, in order to address many fundamental and technological issues. In this thesis, the surface chemistry of carbon materials, the structures and composites of cobalt based oxide catalysts, and their effects on the LABs performance were studied, while the relationship among which was also disscussed.Initially, carbon fiber papers(CP) were chosen as the air cathodes to investigate the effects of oxygen functional groups towards ORR and OER processes. XPS analysis showed that oxygen functional groups and defects could be effectively introduced through heat and acid treatment, which could significantly promote the ORR and OER process as well as the charge-discharge efficiency. The samples after heat and acid treatment(denoted as Heat-CP and Acid-CP, respectively) exhibited the increases of 38.2 m V and 0.11 V in the discharge plateau, while the decreases of 24.5 m V and 0.837 V in charge overpotential, respectively. The pristine, Heat-CP and Acid-CP could maintain 54, 41 and 82 cycles till the discharge voltage down to 2.5 V at a current density of 0.1 m A/cm2 with a limiting capacity of 0.4 m Ah/cm2 under the high purity oxygen atmosphere(1 atm). In addition, Li2O2 formed on Acid-CP cathodes possessed poor crystallinity and was more of amorphous structure, which was more easier to be oxidized. And this was due to the strong adsorption of oxygen functional groups and defects towards the ORR intermediates, such as Li O2, thus inhibiting the subsequent disproportionation reactions.In addition, Co3O4 was employed as the electrocatalysts in Li-air batteries in this thesis. The researches showed that the ordered mesoporous Co3O4 prepared by nano casting method using KIT-6 as a template exhibited better performance in comparison to Co3O4 nanoparticles. Thereinto, the ordered mesoporous Co3O4 could lower the charge overpotential effectively, improving the discharge capacity and cycle performance even more. However, neither of them showed remarkable promoting effects towards the ORR process. The morphologies of Li2O2 were of great differences on nanoparticle-like and mesoporous Co3O4 catalytic electrodes, on which Li2O2 tended to be schistose and petaloid, respectively.Furtherly, Co3O4 was pretreated by using concentrated HNO3, the performance of which was then studied. The results suggested that the oxygen vacancies and defects were introduced on the surfaces of Co3O4 after the pretreatment. The charge-discharge measurements revealed that the pretreated sample resulted in lower ORR overpotential but inversely higher OER overpotential due to the compact Li2O2 products.Finally, the cobalt oxide/carbon nanocomposites were investigated in LABs. The researches showed that the different pyrolysis temperature led to different composition of products during the synthesis of nitrogen-doped Ketjen black supported Cox Oy/NC catalysts. The products were proved to be Co3O4 at 700 °C, coexisted Co3O4, Co O and Co at 800 °C, and only Co3O4 and Co O at 900 °C, of which the sample porolysis at 900 °C showed the best electrochemical performance. For examples, the initial discharge capacity could reach 6 543 m Ah/g and the discharge plateau was about 2.82 V at the current density of 200 m A/g. Meanwhile, the charge potential decreased for 0.14 V and 0.26 V compared to the samples porolysis at 700 °C and 800 °C, respectively. On the other hand, the composition of the ordered mesoporous Cox Oy/NC products was also highly dependence on the pyrolysis temperature. The sample pyrolysis at 900 °C exhibited the best catalytic activity towards ORR and OER processes, such as the initial discharge plateau of 2.77 V and the corresponding charge potential of 4.37 V at 200 m A/g with a limiting capacity of 600 m Ah/g. |
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