Hierarchical Porous N, P co-doped rGO Modified Separator to Enhance the Cycling Stability of Lithium-sulfur Batteries

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 degrees 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.

Automated summary

Lithium-sulfur batteries with high energy density have garnered attention, but face challenges such as poor sulfur utilization and the shuttle effect of LiPSs. Carbon materials are considered ideal cathode materials to enhance sulfur utilization and mitigate the shuttle effect. Heteroatom doping enriches the chemical adsorption sites in carbon materials, effectively suppressing the shuttle effect of LiPSs.