Dong-Liang Peng’s group from Xiamen University in China reported a novel metal-based carbon composite material as a host material applied in Lithium-sulfur (Li-S) batteries, which could accelerate the redox reaction kinetics and minimize the polysulfides loss by means of the strong chemisorption and electrocatalytic conversion activity to lithium polysulfides (LiPSs), and then effectively enhance the cycling stability and rate capability. [Qiao et al., Energy Storage Materials (2019), doi: 10.1016/j.ensm.2019.05.032].

Li-S batteries have attracted considerable attentions as a promising candidate for next-generation energy storage systems due to their high theoretical specific capacity and specific energy. However, the polysulfide shuttle, huge volume change of S cathode, and kinetics sluggishness during cycling limit their practical applications. To address such issues, the most popular approach is to combine sulfur with various host materials, especially carbon-based materials. However, the physical interactions between nonpolar carbon and polar polysulfide species are so poor that it is difficult to settle the “shuttle effect” and increase the sulfur utilization. The metallic-based carbon composites (such as Sn-carbon composite) have merits of high surface area, porous configuration, excellent conductivity of carbon materials and strong chemisorption effects to restrain the polysulfide shuttle. In addition, fast conversion kinetics from long-chain polysulfides to short-chain polysulfides and final to Li2S is another important factor to strengthen the electrochemical performances of Li-S batteries.

An ideal host material in Li-S batteries should simultaneously have strong chemisorption and electrocatalysis activity for lithium polysulfides (LiPSs). It can not only effectively anchor LiPSs and then minimize sulfur loss, but also accelerate the redox reaction kinetics from S to polysulfides and to Li2S. “Fortunately, the hollow CoxSny modified N-doped carbon (E-CoxSny/NC) composite prepared in our work possesses the two properties” says Dong-Liang Peng, corresponding author of the study.
The researchers prepared the hollow E-CoxSny/NC host materials for Li-S batteries using a stepwise coating-etching approach. “The synthetic process is interesting, SiO2 from tetraethyl orthosilicate (TEOS) as an additive and the pyrogenation of PDA can induce the directly-alloy of Co and Sn,” Peng says, “Hollow CoSnO3 nanoboxes, the obtained SiO2, and PDA would interact during the preparation process. Then, CoSnO3 would be reduced to CoxSny alloy under pyrogenation condition using carbon from the carbonization of PDA as a reductant and SiO2 as a crystal growth inhibitor. In addition, EDA (ethanediamine) is employed to effectively increase N content in carbon host material.”

Chemisorption and electrocatalysis from CoxSny alloy for high-performance Li-S batteries

The designed CoxSny alloy can not only provide the chemisorption to enhance the anchor effect to LiPSs but also accelerate the redox conversion to minimize the polysulfides loss and achieve homogeneous deposition of solid products. Therefore, the prepared E-CoxSny/NC/S cathode shows high specific capacity of 1006 mAh g-1 after 100 cycles at 0.2 C, good long-term cycling stability with a capacity retention of 81.2 % after 500 cycles at 1.0 C, and superior rate capacity (~778 mA h g-1 at 2.0 C). “In addition, we find that the potential gaps of the E-CoxSny/NC/S cathode are lower than other two cathodes without the modification of CoxSny alloy. The higher content of CoxSny, the lower potential gaps. The results are mainly attributed to the high electrocatalytic activity of CoxSny alloy to promote conversion between S, polysulfides, and Li2S, accelerating their reaction kinetics.” explains Peng.

The nanoscale CoxSny alloy particles are embedded in the carbon framework, which would cause more disorder degree and structural defects of carbon in E-CoxSny/NC composites. “The induced defects in carbon provide more electrochemical active sites, increase the overall electrical conductivity of electrode materials and also enhance the electrochemical properties of Li-S batteries.” says Peng.
“The study demonstrates that CoxSny alloy in hollow carbon is a good host material for sulfur to enhance the electrochemical performance of Li-S batteries, however, the more defect mechanism in carbon layer needs a more in-depth investigation. In addition, the higher area sulfur loading and higher areal capacity in the cathode are also necessary to realize for practical industrial applications,” says Prof. Liqiang Mai, at Wuhan University of Technology, China. “The work is meaningful, which may inspire researchers to design more effective and cost-effective cathode materials for high-performance Li-S batteries.”