SPAN and Beyond: Sulfur-Based Cathodes Free of Polysulfides

Li-S battery is a highly desirable technology featuring high energy density and low-cost materials. However, the challenges with the technology are also well documented. In ether-based electrolytes, the redox of sulfur goes through a soluble polysulfide mechanism, which facilitates the reaction kinetics but greatly impacts the cycle life and practical energy density due to the need for elevated amount of electrolytes. Sulfurized polyacrylonitrile, SPAN, in contrast, does not involve the polysulfide mechanism. Instead, the material goes through a solid-solid conversion process. Coupled with the presence of a robust cathode electrolyte interface (CEI), the material has demonstrated exceptionally long life, well over 1000 cycles.<br/><br/>SPAN’s limitation lies in its limited sulfur content (~ 43 wt%) and specific capacity (&lt; 700 mAh/g). These values put a practical limit for the energy density of Li-SPAN battery at around 300 Wh/kg. In order to increase the capacity, we have embarked on a study to understand the structure and the capacity-limiting mechanism. During the first cycle, lithiation leads to a loss of H₂S which in turn improves the degree of conjugation and electronic conductivity. Both sulfur and the nitrogen on the pyridine ring are involved in the charge storage during the subsequent cycles. Recently, we have focused on further raising sulfur contents in SPAN by introducing sulfur species that resist the formation of long-chain polysulfide in the solid state. Structural analysis reveals the critical roles played by the nitrogen in facilitating the redox reaction of the additional sulfur. More than 20% improvement in capacity is obtained without the introduction of soluble polysulfide process. We will discuss in detail the reaction mechanisms of these high-sulfur SPAN materials and the pathway towards their implement in high energy density batteries.