Probing Inorganic Clusters in the Solid Electrolyte Interphase by MALDI-TOF Mass Spectrometry

Abstract The cycle life of graphite anodes is governed by the chemistry of the solid electrolyte interphase (SEI), yet the molecular identity of its inorganic constituents has remained elusive despite extensive characterizations. Here, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) is employed to resolve the inorganic SEI as discrete molecular formulas, tracking LiFePO4/graphite cells from initial formation to 80% state of health (SOH). In a baseline LiPF6 carbonate electrolyte, the SEI is dominated by H-rich (LiF)m·(LiOH)n composite clusters together with ester-terminated poly(ethylene oxide) chains that degrade into methoxy-terminated polyethers, and the cell reaches 80% SOH within 25 cycles at 3 C and 45 °C. The addition of fluoroethylene carbonate (FEC) reverses the compositional balance: dense (LiF)m clusters coexist with F-rich (LiF)m·(LiOH)n composites, and the cell sustains 330 cycles under identical conditions. The F-rich versus H-rich distinction thus emerges as a molecular descriptor of SEI longevity, directly resolved by MALDI-TOF-MS.