The Effect of Binder on the Structure and Performance of Sulfur Cathodes in Lithium-Sulfur Batteries

Lithium-ion batteries (LIBs) are a reliable energy storage technology that have been used in various applications such as portable devices and power tools. However, the specific capacities of the electrode materials in the current LIB technology are approaching their theoretical limits which impedes their utilization in a variety of emerging applications such as long-range electric vehicles, next-generation mobile devices, and grid level energy storage and delivery. Therefore, alternative electrode materials with high specific capacity beyond the conventional LIB electrode materials are needed 1. Sulfur has been touted as a promising alternative cathode material in recent years. Sulfur offers superior theoretical capacity, and a high practical energy density when it is paired with a Li metal anode in so called Li-S batteries2. Non-toxicity, low cost and high natural abundance also make Sulfur environmentally and economically appealing. However, achieving the desired high energy density and long cycle life in Li-S batteries have been proven difficult because of the: (1) insulating nature of the two end products of charge and discharge; S8 and Li2S, (2) electrode degradation due to the volumetric change during cycling, and (3) dissolution of the Sulfur discharge products, Li-polysulfides (LiPSs), in the ether-based electrolyte, resulting in the “shuttling effect” that leads to capacity decay over extended cycling 3,4. In this work, new insights are presented on how the binder, its solvent, and dissolution process affect the electrode microstructure and performance. The Sulfur cathodes were prepared using commercially available Sulfur powder, carbon black and various binders and solvents. The cathode structures prepared using different binder and solvent combinations were characterized using scanning electron microscopy (SEM). The cycling performance of the Sulfur cathodes were tested in coin cells. The results showed considerable structural and performance variations between cathodes with similar binders but different solvents, or different treatment conditions with the same solvent. In particular, when binders were minimally dissolved in N-Methylpyrrolidone a porous shell-like structure was observed around the sulfur particles that evolved to a denser sponge-shape structure upon excessive dissolution. The porous shell structure resulted in enhanced performance and cycle life. Using spectroscopic data, it is possible that enhanced cycle life might be attributable to physical trapping of the LiPSs and providing a buffer for the volumetric change during discharge. Thus, a new perspective will be presented that the binder/solvent interaction can impact the performance of sulfur cathodes by manipulating both its structural and chemical behavior. These results are expected to provide a new understanding regarding the effect of binder and its processing on the performance of Li-S batteries and help to write a new narrative regarding electrode chemistry and preparation techniques for future applications. References M. Zhao et al., ACS Cent. Sci., 6, 1095–1104 (2020). A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, and Y.-S. Su, Chem. Rev., 114, 11751–11787 (2014). A. Manthiram, Y. Fu, and Y.-S. Su, Acc. Chem. Res., 46, 1125–1134 (2013). W. Ren, W. Ma, S. Zhang, and B. Tang, Energy Storage Mater., 23, 707–732 (2019).