Revealing the Sulfur Redox Paths in a Li-S Battery by an In Situ Hyphenated Technique of Electrochemistry and Mass Spectrometry

The lithium-sulfur (Li-S) battery is one of the most promising next generation energy storage systems due to its high theoretical specific energy. However, the shuttle effect of soluble lithium polysulfides formed during cell operation is a crucial reason for the low cyclability suffered by current Li-S batteries. As a result, an in-depth mechanistic understanding of the sulfur cathode redox reactions is urgently required for further advancement of Li-S batteries. Herein, the direct observation of polysulfides in a Li-S battery is reported by an in situ hyphenated technique of electrochemistry and mass spectrometry. Several short-lived lithium polysulfide intermediates during sulfur redox have been identified. Furthermore, this method is applied to a mechanistic study of an electrocatalyst that has been observed to promote the polysulfides conversion in a Li-S cell. Through the abundance distributions of various polysulfides before and after adding the electrocatalyst, compelling experimental evidences of catalytic selectivity of cobalt phthalocyanine to those long-chain polysulfide intermediates are obtained. This work can provide guidance for the design of novel cathode to overcome the shuttle effect and facilitate the sulfur redox kinetics.