Credit: ACS, Li et al. Click to enlarge.

The lithium sulfur (Li-S) rechargeable battery has a theoretical energy density as high as 2600 Wh kg^-1, promising a revolutionary advantage as the next-generation energy storage system. However, the realization of practical application is still hindered by the low capacity release of S and the severe capacity fading over cycling. The low sulfur utilization is generally caused by the insulating nature of S (5 10^-28 S m^-1), while the poor cycle stability is mainly due to the high dissolubility of the reaction intermediates of lithium polysulfides (Li₂S_x, 4 < x ≤ 8). In order to overcome the hurdles in Li-S battery technology, various approaches have been proposed to enhance the actual capacity and the cycle stability of the sulfur cathode.

…In general, to achieve an ideal electrochemical performance, the carbon host for the sulfur composite should meet the following requirements: (1) high electrical conductivity to ensure an efficient conductive network in the sulfur cathode; (2) large pore volume to obtain a high maximum sulfur loading; (3) hierarchical interconnected micro/mesopores to accommodate the sulfur species and suppress the diffusion of the dissolved polysulfides to the bulk electrolyte, and meanwhile to favor the infiltration of the electrolyte to ensure fast transport of Li ions during the redox process; (4) robust mechanical properties to endure the volumetric change of the electrode during the lithiation/delithiation reaction. Although many kinds of carbonaceous materials have been designed to confine sulfur species, no one can completely meet all the above requirements. Some contradictions always exist between the energy density and the electrochemical performance.

…Here, we demonstrate a hybrid nanoarchitecture with highly ordered meso-microporous core
shell carbon (MMCS) as sulfur container. Such a core
shell-structured carbon inherits the natures of both mesoporous carbon (mesoC) and microporous carbon (microC), showing a large pore volume and highly ordered porous structure.

—Li et al.

With large pore volume and highly ordered porous structure, the “core” promises a sufficient sulfur loading and a high utilization of the active material, while the “shell” containing microporous carbon and smaller sulfur acts as a physical barrier and stabilizes the cycle capability of the entire S/C composite.

The team reported that such a S/MMCS composite exhibited a capacity as high as 837 mAh g^–1 at 0.5 C after 200 cycles with a capacity retention of 80% vs the second cycle (a decay of only 0.1% per cycle), thereby demonstrating that the diffusion of the polysulfides into the bulk electrolyte can be greatly reduced.

We believe that the tailored highly ordered meso-microporous core–shell structured carbon can also be applicable for designing some other electrode materials for energy storage.

—Li et al.

Resources

• Zhen Li, Yan Jiang, Lixia Yuan, Ziqi Yi, Chao Wu, Yang Liu, Peter Strasser, and Yunhui Huang (2014) “A Highly Ordered Meso@Microporous Carbon-Supported Sulfur@Smaller Sulfur Core–Shell Structured Cathode for Li–S Batteries,” ACS Nano doi: 10.1021/nn503220h