摘要/Abstract

多硫化物(LiPSs)的穿梭效应和低硫利用率会导致电池容量的快速衰减, 这严重阻碍了锂硫电池的商业化进程. 为了抑制LiPSs的穿梭效应和提高硫的利用率, 本工作采用一步高温还原法合成了具有分级多孔结构的N, P共掺杂还原氧化石墨烯(NPG), 并将其用于锂硫电池的隔膜改性. 高导电性NPG具有丰富的分级多孔结构, 提供了大量的LiPSs锚定位点和丰富的离子/电子传输通道, 实现了可溶性中间体的快速转化, 高效抑制了LiPSs的穿梭效应. 归因于以上优点, NPG改性聚丙烯隔膜(NPG/PP)能够有效抑制LiPSs的穿梭并提高硫的利用率. 结果表明, NPG/PP改性隔膜的电池展现出优异的循环性能(在1 C的电流密度下, 循环500圈以后容量仍保持在612.5 mAh•g^-1, 每圈的衰减仅为0.052%)和出色的倍率性能(在2 C的电流密度下容量仍保持在617.9 mAh•g^-1). 这种构建分级多孔N, P共掺杂rGO改性隔膜的思路为锂硫电池的研究提供了新的方向.
关键词: 锂硫电池, 多硫化物, N,P共掺杂rGO, 隔膜改性, 穿梭效应
Lithium-sulfur batteries with high energy density (2600 Wh•kg^-1) and theoretical capacity (1675 mAh•g^-1) have attracted much attention. Furthermore, as a cathode active material, sulfur has prominent advantages such as rich in natural resources, low cost and environmental friendliness. Attributed to the above merits, lithium-sulfur batteries deemed to be one of the most promising energy storage devices. However, poor utilization of sulfur and the shuttle effect of lithium polysulfides (LiPSs) causes dramatic capacity degradation, which severely restricts the commercial application of lithium-sulfur batteries. These problems are mainly attributed to the insulating nature of sulfur and its final discharge products (Li₂S₂/Li₂S), which reduces the sulfur utilization, as well as the poor adsorption capability and slow reaction kinetics, which give rise to the shuttle effect of soluble LiPSs. To solve the above problems, carbon materials are regarded as the most suitable cathode materials for lithium-sulfur batteries, because its superior electrical conductivity and rich porous structure can effectively improve the sulfur utilization and mitigate the shuttle effect of LiPSs. However, the shuttling of LiPSs is difficult to suppressed completely due to the weak adsorption interaction between nonpolar carbon materials and polar LiPSs. Based on this, heteroatom doping is beneficial to enrich the chemical adsorption sites of LiPSs in carbon materials, enhancing the interaction between carbon materials and LiPSs. Thus, the shuttle effect of LiPSs is efficiently suppressed and the cycle stability of lithium-sulfur batteries is improved. Hence, N, P co-doped reduced graphene oxide (NPG) with hierarchical porous structure was prepared by one-step high-temperature reduction method and used for the polypropylene (PP) separator modification of lithium-sulfur batteries. The highly conductive NPG with abundant hierarchical porous structure provides a large number of anchor sites for LiPSs and sufficient ion/electron transport channels, facilitating the conversion of the soluble intermediates and efficiently suppressing the shuttle effect of LiPSs. In consequence, the NPG/PP modified separator can effectively inhibit the shuttle of LiPSs and improve the sulfur utilization. The results show that the cells with NPG/PP modified separator exhibit excellent cycling performance (the degradation per cycle is only 0.052% and the capacity remains at 612.5 mAh•g^-1 after 500 cycles at 1 C) and excellent rate performance (high specific capacity of 617.9 mAh•g^-1 at 2 C). This idea of constructing hierarchical porous N, P co-doped rGO modified separators provides a new strategy for the study of lithium-sulfur battery.
Key words: lithium-sulfur battery, lithium polysulfide, NPG, modified separator, shuttle effect


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