关键词: 锂硫电池, 多孔碳, N、S共掺杂

Abstract:

In recent years, lithium-sulfur (Li-S) batteries have been considered as a promising candidate for the next generation of energy storage system due to their ultrahigh theoretical capacity (1675 mAh·g^-1) and energy density (2600 Wh·kg^-1). However, the practical application of Li-S batteries is seriously limited by their insulating nature of sulfur, the shuttle effect of polysulfides (LiPSs), and volume expansion during charging and discharging. To overcome those disadvantages, one of the commonly methods is to infiltrate sulfur into porous conductive carbon framework, such as porous carbon, hollow carbon spheres, graphene, carbon nanotubes and some composites of the above structures to achieve the purpose of physically limiting the shuttle effect of polysulfides, thereby improving the performance of Li-S batteries. However, due to the nonpolarity of traditional carbon materials, the interaction with polar polysulfides is very weak, which cannot effectively inhibit the shuttle effect of polysulfides. Previous studies have shown that introducing heteroatom (N, S, P, B, etc.) doping into carbon matrix is a feasible method to adjust the nonpolarity of carbon materials. It is reported that the introduction of N atoms is conducive to improving the electrochemical activity. The Li-N bond formed by the interaction between N and Li⁺ can anchor polysulfides, effectively inhibit the dissolution of polysulfides and improve the utilization rate of sulfur. The introduction of nitrogen and sulfur heteroatoms can increase polar sites and active centers, thus, enhancing the adsorption capacity of carbon materials for polysulfides and capturing polysulfides. Therefore, ionic liquids are selected as nitrogen and sulfur sources to improve the polarity of carbon materials. In this paper, nitrogen and sulfur co-doped porous carbon (NSPC) was synthesized by using glucose as carbon source, KCl and ZnCl₂ as templates, KOH as activator and ionic liquid as heteroatom source. XPS and adsorption experiments show that nitrogen and sulfur heteroatoms had been successfully introduced into NSPC, which improved the adsorption capacity of carbon materials for polysulfides, effectively alleviated the shuttle effect of polysulfides. The higher specific surface area (1290.67 m²·g^-1) could help to improve the sulfur loading. After loading 70.1wt.% sulfur into NSPC (S@NSPC) and tested as a cathode material of Li-S battery, the initial discharge capacity was 1229.2 mAh·g^-1 at 167.5 mA·g^-1, higher than the 861.6 mAh·g^-1 of S@PC, and the capacity remained at 328.1 mAh·g^-1 after 500 cycles. When the current density returned to 167.5 mA·g^-1, the reversible capacity almost went back to its initial value, which was 80% of its initial value. The good performance was mainly ascribed to both the porous structure and N, S co-dopants, which provided physical blocks and chemical affinity, respectively, for the efficient immobilization of intermediate lithium polysulfides. The results would provide an effective example in the surface chemistry and sulfur host materials design for high performance Li-S batteries.

Key words: lithium-sulfur batteries, porous carbon, heteroatom doping