Identification and Catalysis of the Potential-Limiting Step in Lithium-Sulfur Batteries

Abstract

The Li-S chemistry is thermodynamically promising for high-density energy storage but kinetically challenging. Over the past few years, many catalyst materials have been developed to improve the performance of Li-S batteries and their catalytic role has been increasingly accepted. However, the classic catalytic behavior, i.e., reduction of reaction barrier, has not been clearly observed. Crucial mechanistic questions, including what specific step is limiting the reaction rate, whether/how it can be catalyzed, and how the catalysis is sustained after the catalyst surface is covered by solid products, remain unanswered. Herein, we report the first identification of the potential-limiting step of Li-S batteries operating under lean electrolyte conditions and its catalysis that conforms to classic catalysis principles, where the catalyst lowers the kinetic barrier of the potential-limiting step and accelerates the reaction without affecting the product composition. After carefully examining the electrochemistry under lean electrolyte conditions, we update the pathway of the Li-S battery reaction: S8 solid is first reduced to Li2S8 and Li2S4 molecular species sequentially; the following reduction of Li2S4 to a Li2S2-Li2S solid with an almost constant ratio of 1:4 is the potential-limiting step; the previously believed Li2S2-to-Li2S solid-solid conversion does not occur; and the recharging reaction is relatively fast. We further demonstrate that supported cobalt phthalocyanine molecules can effectively catalyze the potential-limiting step. After Li2S2/Li2S buries the active sites, it can self-catalyze the reaction and continue driving the discharging process.