Ammonia-Treated Ordered Mesoporous Carbons with Hierarchical Porosity and Nitrogen-Doping for Lithium-Sulfur Batteries
Authors: Li Yabo , Qu Deyu , Liu Dan 
Published: February 2020

The theoretical specific capacity and energy density of lithium sulfur battery are as high as 1675 mAh/g and 2600 Wh/kg，which is an order of magnitude higher than that of the commercial insertion cathodes. In combination with the natural abundance, low cost and environmental friendliness of sulfur, the Li-S battery becomes a promising candidate for the next generation power source. However，the commercialization of Li-S battery is inhibited by the insulating nature of sulfur and Li2S, the volume expansion, “Shuttle effect” caused by the high solubility of lithium polysulfides in the electrolyte, the severe corrosion of lithium anode and irreversible reaction during the charge/discharge process, which lead to a low utilization rate of sulfur, serious capacity fading, and low Coulombic efficiency of Li-S battery.

One of the most effective ways to circumvent these three limitations is to fabricate sulfur/carbon composite cathodes by incorporating sulfur within a variety of carbon materials including porous carbons, carbon nanotubes, graphene nanosheets and many other forms. These carbon hosts can enhance the conductivity of sulfur electrodes and accommodate their volume changes. Furthermore, the weak interactions between carbon and polysulfides also alleviate the dissoluble loss of polysulfide intermediates. In particular, hierarchically porous carbon materials are recently receiving discernable attention. It is believed that the micropores enable better cycle stability due to solvent-restricted lithiation/delithiation of sulfur, while the larger pores (mesopores and macropores) promise high sulfur loading and rapid ion transport. Therefore, both of large capacity and high rate performance may be achieved. Further, the nitrogen doping of carbon materials has been recently demonstrated to be a powerful method for enhancing the overall electrochemical performance of sulfur cathodes. The nitrogen heteroatom with high electronegative improves the carbon conductivity and promotes chemical adsorption of polysulfides, resulting in enhanced sulfur utilization, rate capability, and cyclic stability.

For these reasons, it is essential to develop facile and scalable methods to prepare nitrogen-doped carbon materials with hierarchical porosity. However, there is a trade-off between well-developed porosity and high nitrogen-doping level. Generally, current methods that favor high nitrogen contents tend to result in low specific surface area or small pore volume, or vice versa. To produce hierarchical porous carbon materials with both developed porosity and high nitrogen content, we herein introduce a simple and cost-efficient approach using two ease-to-scale-up steps including 1) one-pot aqueous self-assembly route to firstly get nitrogen-doped ordered mesoporous carbons (NOMCs) and then 2) ammonia activation treatment to achieve desirable porosity and nitrogen-doping. For comparison, another important and industrial method, KOH activation, was also used to obtain hierarchically porous carbon. Those gained carbons were then used as the sulfur hosts in the Li-S batteries. The effect of activation conditions on the nitrogen content, specific surface area, and pore size distribution of the hierarchically porous carbons as well as their electrochemical performance were investigated in detail. It clearly exhibits that ammonia activation enables the hierarchically porous carbon with relatively high porosity (specific surface area up to 1565 m2 g−1) and nitrogen content (up to 4.4 wt%). These combined superior properties make them good candidates as the host materials of sulfur cathodes in Li-S batteries with high sulfur loading.