Chemical Synthesis of K<sub>2</sub>S<sub>2</sub> and K<sub>2</sub>S<sub>3</sub> 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 K<sub>2</sub>S<sub><i>n</i></sub> (<i>n</i> = 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, K<sub>2</sub>S<sub>2</sub> and K<sub>2</sub>S<sub>3</sub>, and probed their electrochemical pathways. When K<sub>2</sub>S<sub>2</sub> and K<sub>2</sub>S<sub>3</sub> are coated directly on a current collector, both species can be further reduced. However, when K<sub>2</sub>S<sub>2</sub> and K<sub>2</sub>S<sub>3</sub> are electrically isolated from the current collector, no further reduction is observed, showing the reduction occurs through a solid-state conversion pathway. Interestingly, K<sub>2</sub>S<sub>2</sub> and K<sub>2</sub>S<sub>3</sub> can be charged even when they are electrically isolated from the current collector, proving solution-mediated charging pathways. Only K<sub>2</sub>S is the “dead” sulfur species that cannot be charged.