Huilin Pan1 Junzheng Chen1 Lili Shi1 Yuyan Shao1 Jun Liu1

1, Pacific Northwest National Laboratory, Richland, Washington, United States

Currently, sulfur encapsulation in high surface area, nanoporous conducting carbon is the most widely studied approach to improve the cycling stability of Li-S batteries. However, the relatively large amount of high surface area carbon results in two fundamental problems with this approach. First, a large amount of electrolyte volume to sulfur (E/S) ratio (typically > 20 mlE/gs) is needed to fully wet the porous sulfur cathode. Second, the large amount use of high surface area carbon greatly decreases the overall energy density in the system, especially for volumetric energy density, and makes it difficult to compete with other battery technologies.
Here, we report a soft gel encapsulation approach for rechargeable Li−S cell under lean electrolyte conditions. The polymer gel immobilizes the electrolyte and confines polysulfide within the ion conducting phase, enabling smooth charge transfer on the interface under a very lean electrolyte condition of E/S of 3.3 mLE/gS and good cycle life. The cell failure mechanism study using solid NMR indicates that the passivation of the cathode which needs further investigation (Nano Lett. 17, 3061, 2017). To solve the critical passivation issue of cathode especially under lean electrolyte condition, we proposed a new approach that does not depend on the conventional sulfur encapsulation with high surface area carbon was proposed to reduce electrolyte absorption. The new approach generates a large spherical porous agglomerated particles with self-sustaining structures to avoid cathode passivation, leading ~100% sulfur utilization with good cycling (Nature Energy 2, 813, 2017).
In addition, the dissolution of polysulfide would cause the sulfur redistribution more or less during cycling in the conventional solvating electrolyte system, which is considered to harmful for the long-term cycling of Li-S batteries. Our recent research on non-solvating electrolyte suggests that it would be alternative new strategy for sulfur batteries to avoid active material redistribution enabling stable cycling.