| The electrical energy storage technology is attracting extensive attention nowadays, considering the expanding market for battery-powered vehicles and electric grid. Lithium-Oxygen (Li-O2) batteries are considered as the next generation of energy storage system due to their low cost, environmental friendliness and extremely high energy density, which is several times higher than that of Li-ion batteries. Similar to research mode of Li-ion batteries, specific capacity based on the mass of cathode material is widely adopted to evaluate the electrochemical performance of Li-02 batteries. It is essential to investigate the prerequisite for adopting this evaluated method by examining the linear correlation between the delivered capacity of Li-02 batteries and cathode mass. Besides, to date, the Li-02 batteries suffer from low practical specific capacity, large charge overpotential and poor cycling life, which limits the practical application of this smart system. To our knowledge, the electrochemical performance of Li-O2 batteries is mainly dependent on the structure and inherently chemical property of oxygen electrode. Therefore, in this thesis we conduct a series of studies in optimizing the porous structure and improving the stability of oxygen electrode. The reaction mechanisms during charge in the Li-02 batteries are further investigated via X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and differential electrochemical mass spectrometry (DEMS). The major research works are presented as follows:First of all, we explore the factors influencing the discharge capacity of Li-O2 batteries. We demonstrate the rationality of specific capacity adopted in Li-ion batteries with classic LiCoO2 cathode by confirming the linear correlation between battery capacity and LiCoO2 mass. Delivered capacities of Li-O2 batteries with different cathode mass are simultaneously measured and nonlinear correlation is obtained. The discharge product of Li2O2 is identified by XRD analysis to ensure reaction mechanism. Considering the reactant of oxygen during discharge, delivered capacities of Li-O2 batteries with various areas of oxygen window are further studied, which exhibits that battery capacity increased linearly with the area of oxygen window. SEM is employed to observe the discharged electrode and presents that Li2O2 deposition during discharge mainly occurs in the electrode area exposure to the oxygen, which is consequently defined as effective area for accommodating Li2O2. Moreover, a plausible route for formation of effective area in the oxygen electrode is proposed properly.On the basis of above results, freestanding and hierarchically porous graphene aerogels (GA) are successfully prepared by a one-pot reaction and directly used as binder-free cathode in the Li-O2 batteries. Hierarchical and three-dimensional porosity in GA cathode facilitated electrolyte permeation and oxygen diffusion, provided facile electron transfer, offer abundant active sites and accommodate plenty of discharge products. As a result, a large specific capacity of above 10000 mAh g-1 can be achieved. Ru nanoparticles are further decorated on graphene sheets of GA and demonstrate superior catalytic activity towards oxygen evolution reaction (OER). Ru-GA cathode can efficiently enhance discharge specific capacity (12000 mAh g-1), reduce charge overpotential, and improve cycling stability up to 50 cycles at a curtailing capacity of 500 mAh g-1. More importantly, according to the results of in situ DEMS analysis, the reaction mechanism during charge is proposed by the theory of three oxidation stages.In addition, to circumvent the corrosion of carbon materials during cycling, ordered porous RuO2 materials with various pore structure parameters are prepared via the hard-template method and used as the carbon-free cathodes for Li-O2 batteries. Meanwhile, the influence of the pore structure parameters of porous RuO2 cathode on the electrochemical performance of Li-02 batteries is systematically studied. The results of electrochemical tests and analysis indicate that the Li-O2 battery based on the RuO2 cathode with a pore size of 16 nm (RuO2-16) exhibits large specific capacity, high round-trip efficiency and excellent cycling stability to 70 cycles at a current density of 100 mA g-1 with a voltage window form 2.5-4.0 V. The superior performance can be attributed to the good conductivity, large BET specific surface area, appropriate pore size and good catalytic activity of RuO2-16 towards OER. Furthermore, the result of in situ DEMS confirms that RuO2-16 cathode can effectively reduce parasitic reactions of Li-02 batteries compared with carbon materials.Finally, we probe deeply into the effect of moisture adsorbed by activated graphene with high specific surface area on the electrochemical performance and reaction mechanism of Li-02 batteries. Two voltage plateaus are appeared during charge when activated graphene is used as the cathode materials of Li-O2 batteries. XRD and SEM are employed to identify the product during discharge and recharge, which demonstrate that Li2O2 accompanied with LiOH is formed in the discharging process and LiOH will be completely decomposed when charging to 3.5 V. This result is also confirmed by in situ DEMS. However, LiOH can’t be observed when using a cathode based on SP carbon black, which indicates that the formation of LiOH is related to the activated graphene cathode. By means of Karl-Fisher Titration, we detect the moisture adsorbed by activated graphene due to its high specific surface area. As a result, the discharge product Li2O2 easily reacts with moisture to form LiOH, whose decomposition leads to the low charge voltage plateau. |
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