Chemical Synthesis of K2S2 and K2S3 for Probing Electrochemical Mechanisms in K–S Batteries

Understanding the properties of polysulfide intermediates is crucial for explaining and optimizing metal–sulfur batteries. Unlike the unstable or inseparable low-order polysulfide intermediates in Li–S systems, the K–S phase diagram displays a series of stable phases of K2Sn (n = 1, 2, 3, 4, 5, 6), which provides the accessibility of an individual polysulfide to investigate the mechanism using pure-phase polysulfides. Herein, we synthesized two key intermediate polysulfides, K2S2 and K2S3, and probed their electrochemical pathways. When K2S2 and K2S3 are coated directly on a current collector, both species can be further reduced. However, when K2S2 and K2S3 are electrically isolated from the current collector, no further reduction is observed, showing the reduction occurs through a solid-state conversion pathway. Interestingly, K2S2 and K2S3 can be charged even when they are electrically isolated from the current collector, proving solution-mediated charging pathways. Only K2S is the "dead" sulfur species that cannot be charged.