关键词: 锂硫电池, 正极材料, 金属有机框架衍生碳纳米笼, 电催化, 限域效应

Lithium-sulfur battery has attracted extensive research interest because of its high theoretical specific capacity, low cost and environmental friendliness. However, its practical application is still limited by the low utilization of sulfur, serious shuttle and polarization effects. To solve these problems, confining sulfur in a highly conductive host, with the assistance of chemical adsorption of intermediate lithium polysulfides (LiPS) by polar sites and catalytic promotion of LiPS conversion by catalytic sites, can effectively boost the charge transfer and suppress the shuttle and polarization effects, hence leading to the high utilization of sulfur and the improved performances of lithium-sulfur batteries. Hollow carbon nanocages have become an ideal host for sulfur encapsulation because of the large inner cavity, high conductivity, and tunable surface and electronic structures. In this study, three kinds of hollow carbon nanocages with similar size but different composition and structure of shells were prepared by etching the ZIF8, ZIF67 and ZIF8@ZIF67@ZIF8 precursors with tannic acid solution, followed by the carbonization. Used as the cathodes for lithium-sulfur batteries after sulfur filling, the sample derived from ZIF8@ZIF67@ZIF8 shows the best electrochemical performance. Specifically, the specific capacity reaches 1010 mAh•g^-1 at 0.1 C (1 C=1675 mA•g^-1), and remains 664 mAh•g^-1 at 1 C; after 300 cycles tests at 0.5 C, the capacity is retained at 492 mAh•g^-1, significantly better than the control samples derived from ZIF8 and ZIF67. The excellent performance of the former is closely related to its unique structure and composition: (ⅰ) the Co species presented in the precursor can improve the conductivity of derived carbon nanocages; (ⅱ) the evaporation of Zn species brings about large specific surface area and rich micro/mesopores, which is conducive to the sulfur filling and the catalytic conversion of S species by the active Co species. Therefore, the shuttle and polarization effects are efficiently suppressed and the utilization of sulfur is improved accordingly, leading to the better performances of lithium-sulfur batteries. This study opens a new avenue to regulate the performance of lithium-sulfur batteries by constructing novel carbon nanocages hosts based on the metal organic framework (MOF) precursors.

Key words: lithium-sulfur battery, cathode material, metal organic framework (MOF)-derived carbon nanocages, electrocatalysis, confinement effect