Schematic of electron pathway of (a) conventional sulfur cathode, (b) ACFC–S cathode with 3D conductive framework. The aluminum foil of the conventional Al–S cathode has a limited contact area with active material, so it only acts as current collector and leads it to external circuit. The electrical conductivity of the cathode is therefore not satisfied. For the composite cathodes with 3D conductive framework, however, the continuous carbon fibers act as high-speed electron pathway. Active material can contact firmly each other, in which continuous carbon fiber and a small amount of spherical carbon black built up a hierarchical “dot-line” conductive framework. He et al. Click to enlarge.

Well-known obstacles hinder the practical application of Li-sulfur batteries, including the poor electrical conductivity of elemental sulfur and its discharge product Li₂S, which results in limited active material utilization and low rate capability; and the volumetric expansion of sulfur upon lithiation and the high solubility of the intermediate products of lithium polysulfides (Li₂S_x, 4 ≤ x ≤ 8) in the organic electrolyte solution, resulting in irreversible active sulfur loss, rapid capacity decay and low Coulombic efficiency during cycles.

To overcome these drawbacks of the sulfur cathode, many approaches have been explored to improve the electronic conductivity of sulfur and prevent the dissolution of polysulfides into the electrolyte, including preparation of hierarchical porous carbon–sulfur composites as sulfur host. And it is believed that encapsulating the sulfur in carbon improves its utility as an active mass and avoids the diffusion of the Li₂S_n species to the electrolyte solution, thus it reduces the shuttle phenomena that limit the capacity of sulfur cathodes.

… In this work, we report here with a novel, simple, and very promising method for a foldable and highly sulfur loading electrode for Li-S batteries. A commercial foldable activated carbon fiber cloth (ACFC) was used as cathode matrix. ACFC–S cathodes were simply prepared by blade coating method. ACFC has high electric conductivity with continuous long-distance conductive structures and plentiful internal space to accommodate a large amount of active sulfur. What’s more, ACFC has a surface area of 471 m² g⁻¹ and plenty of micropores, which is expected to keep the polysulfide within the cathode.

—He et al.

To prepare the active material slurry, the team mixed 70 wt% sublimed sulfur, 20 wt% super P as a conducting agent, and 10 wt% PVDF as a binder. A commercial activated carbon fiber cloth was treated and modified, and then used as cathode matrix (ACFC). Composite cathodes were prepared by a simple blade coating method to permeate the slurry into the matrix.

Cycling performance at current density: 0.05 C of the first two cycles and 0.2 C of the follow cycles. He et al. Click to enlarge.

The ACFC–S cathode can be folded freely without any brittle fracture, which meets the requirement of both pouch battery and cylindrical batteries, leading to potential for commercial applications, the team noted.

The researchers also suggested that the electrochemical performance of the novel 3D cathode could be further enhanced with further increasing sulfur loading.

Resources

• Na He, Lei Zhong, Min Xiao, Shuanjin Wang, Dongmei Han & Yuezhong Meng (2016) “Foldable and High Sulfur Loading 3D Carbon Electrode for High-performance Li-S Battery Application” Scientific Reports 6, Article number: 33871 doi: 10.1038/srep33871