Nonuniform Redistribution of Sulfur and Lithium upon Cycling: Probing the Origin of Capacity Fading in Lithium-Sulfur Pouch Cells

Lithium-sulfur (Li-S) batteries have emerged as a promising candidate for the next-generation high-energy-density system for energy-demanding applications. Despite innovations in concepts and materials that significantly improve the electrochemical performance of coin cells, Li-S pouch cells have the disadvantages of short cycle life and inferior rate capability in comparison with coin cells. Bridging the fundamentals of Li-S chemistry to the hindrance on its practical application is of great importance for the development of Li-S batteries. Herein, the nonuniformity of the distribution of sulfur and lithium upon cycling is probed as one of the origins for the rapid capacity fading in a Li-S pouch cell. In particular, the nonuniform evolution of sulfur/lithium distribution impairs the discharge capacity of a low-voltage plateau. Lithium polysulfide intermediates produced on discharge tend to diffuse toward the bottom of a pouch cell, leading to agglomeration of sulfur and thus passivating the cathode. The migration of polysulfides also etches lithium away from the central region of the anode and induces nonuniform anode pulverization. Herein, the importance of a rational design of a pouch cell to mitigate the nonuniform redistribution of the active material toward stable Li-S pouch cells is highlighted.