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 GuSichen XiaoNeng WuFeng BaiYing WuChuan WuYiying 2018 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.