| Lithium-sulfur (Li-S) and lithium-selenium (Li-Se) batteries have higher energy density because of high reduction potentials of S and Se and two-electron reduction (per atom) process compared to the traditional lithium-ion battery. They have attracted a lot of attention as prospective rechargeable batteries which are critical power sources widely used in portable electronic devices as well as large-scale stationary energy storage. However, some inherent problems hinder the commerlization of Li-S battery and Li-Se battery, majorly including serious capacity fading and low coulombic efficiency. To overcome these limitations, the key lies on improving the electronic conductivity of active materials and suppressing the shuttling effect of soluble intermediates. The combination of porous carbon materials, especially mesoporous carbon, and chalcogens have been demonstrated as one of most effective approaches. Although great progresses have been achieved, the systematic research of carbon properties, such as pore size, porosity, and surface chemistry have been rarely reported yet. Therefore, this work mainly focus on the control of structures of carbon hosts and studying the effect of surface chemistry on the performance of Li-S and Li-Se batteries. What’s more, a new SexSy composite system was developed as cathodes for the first time, which could combine the synergetic effect of selenium and sulfur, thus demonstrating excellent electrochemical performance. Such useful information provides valuable hints for a new perspective of high performance cathode materials in energy storage technology. The main results of this thesis are summarized as follow:(1) Mesoporous carbon microsphere/sulfur composites as cathodes for Li-S batteries. Mesporous carbon microspheres (MCMs) with tunable pore sizes (3.8 nm,5 nm,6.5 nm,9.5 nmn) were prepared by a simple and scalable spray drying method, using resorcinol-formaldehyde polymer as carbon precursor and home-made silica sol as hard templates. The pore structure of MCMs can be precisely turned by adjusting the hydrolysis conditions of water glass. The as-prepared carbon microspheres were employed as the conducting host to incorporate sulfur for lithium sulfur batteries. It was found that the smaller pores could effectively constrain the diffusion of polysulfide species via stronger confinement effect, thus improving the cycling performance. A reversible discharge capacity of 700 mAh g-1 was obtained at 0.5 C.(2) Mesoporous carbon microsphere/selenium as cathodes in lithium-selenium batteries. Mesoporous carbon microsphere and nitrogen-doping mesoporous carbon microsphere were prepared by spray drying method and used as matrix to load selenium for Li-Se batteries. The results demonstrated the positive effects of smaller pore size in electrochemical performances. Moreover, the rich nitrogen functionalities can effectively enhance surface interaction between carbon matrix and Se and improve the electronic conductivity of carbon host, thus retarding polyselenide shuttling and promoting the performance of lithium selenium batteries. An initial discharge capacity of the N-doping composites was as high as 587 mAh g-1 at 0.5 C. After 100 cycles, it was still retained 260 mAh g-1.(3) A new SexSy composite system was firstly developed as the cathode by physically combination of S and Se. Mesporous carbon microspheres prepared by the spray drying method were used as carbon matrix to load SexSy with different Se/S ratios as cathodes. The results showed that the SexSy composite cathodes could combine the advantages of good conductivity of Se and high capacity of S, and effectively alleviate the shuttle effect in bulk electrolyte, thus achieving high reversible capacity as well as good cycling performance. The optimal MCM-SeS exhibit a satisfactory cycling stability with a good reversible capacity of 626 mAh g-1 after 100 cycles at 5 C. These results provide valuable hints for a new perspective of high performance cathode materials in energy storage technology. |
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