Qualitative Determination of Polysulfide Species in a Lithium–Sulfur Battery by HR-LC-APCI-MSn with One-Step Derivatization

Lithium-Sulfur (Li-S) batteries are one of the most promising next-generation batteries due to their ultrahigh energy density up to 500 W h kg^-1. However, despite the steady progress during the last several decades, there have been significant challenges for practical applications and commercialization. One of the major issues is controlling the lithium polysulfide (LiPS) shuttling process, which causes premature cell failure. To better understand the mechanism of the LiPS shuttling chemistry, a qualitative and quantitative analysis on polysulfide species in Li-S cell has profound significance for realizing highly efficient sulfur electrochemistry. Here we report a qualitative determination of the derivatized polysulfides in the electrolyte of a custom-made Li-S pouch cell with a high-resolution liquid chromatography-atmospheric pressure chemical ionization tandem mass spectrometry method for the first time. The ionization efficiency of the methylated polysulfides was affected by the tune parameters such as the corona discharge current, the vaporizer temperature, and the source capillary temperature. It was found that the source capillary temperature was the dominant parameter to increase the peak intensity of CH₃S₇^- ion, which was the smallest peak in the spectrum. An unusual and unique ionization pattern for methylated polysulfides detected in atmospheric pressure chemical ionization negative mode was elucidated by using first-principles calculations.