李 墨宸 理工総研が募集する次席研究員
LI Mochen Researcher
理工学術院総合研究所 野田 優研究室
Waseda Research Institute for Science and Engineering
Lithium-sulfur (Li-S) batteries are one of the most promising candidates for next-generation energy-storage systems beyond routine lithium-ion batteries due to their high energy densities, low cost and natural abundance of sulfur. Their extremely high gravimetric energy density owing to the huge theoretical specific capacities of cathode and anode and cost-effectiveness of sulfur provide them huge potentials to satisfy the increasing demands of energy storage technologies, especially for electric vehicles (EVs), hybrid EVs and stationary energy storage systems. In spite of these advantages, the practical applications of Li-S batteries are impeded by several challenges, such as low sulfur utilization, fast capacity fade and capacity loss, which mainly originate from the low conductivity of sulfur and its end-discharge product Li2S, shuttling effect of the soluble polysulfide intermediates and large volume changes.
Among numerous efforts have been devoted to address these unsolved issues, carbon nanotubes (CNTs) have shown promising results when hybridized with sulfur to fabricate composite cathodes. Peng et al. improved the cycling stability of high-sulfur-loading Li-S batteries by applying 3D CNT current collectors. Sulfur-carbon composite cathode with high areal capacities was also produced by vapor-phase filtration of sulfur onto CNT foams. These works show the effectiveness of CNTs, however, they overlook the critical parameters, including low sulfur loading, small sulfur content, and impractically high electrolyte/sulfur ratio. With these conditions, it is easy to achieve good electrode-based performance, while it results in impractically small cell-based performance due to the small fraction of S in the full cell.
In this research, the main purpose is to synthesis long CNTs with controlled wall number and use them to fabricate hybridized sulfur and CNT (S-CNT) composite cathodes, which minimize the content of inactive materials without using any current collector and polymeric binder and maximize S content (Fig 1).
The systhesis of CNTs will be carried out in fluidized-bed CVD method that developed by our group previously. S-CNT cathodes will be fabricated by facile filtration, melt-diffusion and evaporation process. Their electrochemical performance will be enhanced by examining a series of CNTs of different types for the 3D current collector which captures and activates sulfur inside the sponge-like, free-standing CNT papers. By analyzing the structure and electrochemical performance of the cathodes, the mechanism of improved electrochemical performance of Li-S batteries will be clarified.
Fig 1. Different structures between traditional Li-S batteries cathodes and S-CNT cathodes. Advantages of S-CNT cathodes are: (1). Elimination of heavy metal current collectors and replace them with CNTs; (2). Without using polymeric binders and conductive materials; (3) Inhibit the shuttle effect of polysulfide and activates sulfur.
 H. J. Peng, W. T. Xu, L. Zhu, et al., Adv. Funct. Mater. 26, 35(2016).
 M. Li, R. Carter, A. Douglas, et al., ACS Nano 11, 5(2017).