Accessing the Primary Solid–Electrolyte Interphase on Lithium Metal: A Method for Low‐Concentration Compound Analysis

Despite large research efforts in the fields of lithium ion and lithium metal batteries, there are still unanswered questions. One of them is the formation of the solid−electrolyte interphase (SEI) in lithium‐metal‐anode‐based battery systems. Until now, a compound profile analysis of the SEI on lithium metal was challenging as the amounts of many compounds after simple contact of lithium metal and the electrolyte were too low for detection with analytical methods. This study presents a novel approach on unravelling the SEI compound profile through accumulation in the gas, liquid electrolyte, and solid phase. The method uses the intrinsic behavior of lithium metal to spontaneously react with the liquid electrolyte. In combination with complementary, state‐of‐the‐art analytical instrumentation and methods, this approach provides qualitative and quantitative results on all three phases revealing the vast variety of compounds formed in carbonate‐based electrolytes. SEI accumulation: A study on method development and application of solid−electrolyte interphase (SEI) molecular structure elucidation is presented, giving insights into the compound profile of the primary SEI on lithium metal in contact with an electrolyte based on lithium hexafluorophosphate in ethylene and ethyl methyl carbonate, with qualitative and quantitative results through the analysis of gas, liquid and solid phase using complementary analytical methods.