Researchers at South China Normal University in Guangzhou have developed a novel composite of sulfur loaded in micropore-rich carbon aerogel (CA-S) for use as a cathode in Li-sulfur batteries.
Compared to sulfur loaded in a common carbon material, acetylene black (AB-S), the CA-S exhibited significantly improved cyclic stability and rate capability. The CA is micropore-rich with micropore volume over 66% of total pore volume. In a paper in the Journal of Power Sources, and team attributed the improved performance of CA-S to the confinement of the micropores in CA to small sulfur allotropes and corresponding lithium sulfides.
Because of the abundance of sulfur on the Earth, the high theoretical specific capacity (1675 mAh g-1) of sulfur cathode, and the high theoretical gravimetric as well as volumetric energy density (2567 Wh kg-1 and 2800 Wh L-1, respectively) of the battery, lithium-sulfur battery is attracting more and more attention. However, lithium-sulfur battery exhibits poor rate capability, inadequate cyclic stability and short calendar life, due to the insulating nature of sulfur and lithium sulfide, the high solubility of the intermediate lithium polysulfides (Li2S5-8) in organic electrolyte, and the significant volume change during charge/ discharge process. These issues limit the practical application of lithium-sulfur battery.
Many efforts have been made to improve the performances of lithium-sulfur batteries. Among these efforts, introducing porous carbon materials into sulfur cathode has been found to be an effective strategy. … It has been found that the size of pores in carbon materials plays an important role in the cyclic stability improvement of sulfur. … Compared with other carbon materials, carbon aerogel synthesized by using special method possesses large specific surface area as well as abundant pores, and has been applied as support of sulfur cathode of lithium-sulfur battery However, the resulting carbon aerogels, which were obtained from the precursors synthesized with high catalyst concentrations and through carbonization under lower temperature, were not characteristic of microporous structure.
In this work, we proposed a novel composite, sulfur loaded in micropore-rich carbon aerogel, as the cathode of lithium-sulfur battery. Carbon aerogel was synthesized through phenol-formaldehyde reaction with a low catalyst concentration and carbonization under high temperature, which favors the formation of micropores. The resulting carbon aerogel possesses microporous structure, and the corresponding sulfur composite exhibits improved cyclic stability and rate capability.—Li et al.
|Cyclic stability of Li/CA-S and Li/AB-S cells at 0.5 C. Li et al. Click to enlarge.|
To create the carbon and sulfur composite (CA-S), the team loaded sulfur in the resulting CA via chemical deposition and heat treatment. During the formation of CA-S, the heat treatment allows S8 to decompose to S2-7 allotropes, among which smaller chain-like S2-4 migrate into the micropores in the aerogel. These micropores are capable of immobilizing small S2-4 allotropes and prevent them from transforming back to electrolyte-soluble Li2S4-8, leading to the improved cyclic stability, the team concluded.
Due to the highly electronic conductivity of CA, the resulting CA-S also exhibits excellent rate capability.
Zihao Li, Xiaogang Li, Youhao Liao, Xiaoping Li, Weishan Li (2016) “Sulfur loaded in micropore-rich carbon aerogel as cathode of lithium-sulfur battery with improved cyclic stability,” Journal of Power Sources, Volume 334, Pages 23-30, doi: 10.1016/j.jpowsour.2016.10.003