Revealing the lithium solid electrolyte interphase in liquid electrolytes via in situ Fourier transform infrared spectroscopy

Unlocking lithium metal batteries requires a robust solid electrolyte interphase (SEI) capable of sustaining high Coulombic efficiency (CE). Here, we develop in situ Fourier transform infrared (FTIR) spectroscopy to directly probe potential- and cycling-dependent formation of an organic SEI in carbonate electrolytes containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) and systematically correlate interfacial chemistry with CE. In 1.2 M LiPF₆ EC, an organic-dominated SEI comprising lithium ethylene dicarbonate (LEDC) forms starting at pre-plating potentials, yielding the highest CE (∼90%). In contrast, 1.0 M LiPF₆ EMC produces primarily soluble lithium ethyl carbonate (LEC) and develops a thick, LiF- and Li₂O-rich, spatially heterogeneous SEI with poor reversibility (<15% CE). LP57 exhibits intermediate behavior, where the emergence of alternative semi-carbonates due to trace water destabilizes the organic SEI. Such observations demonstrate that an inorganic-dominated SEI does not intrinsically ensure high CE and instead highlight organic, LEDC-based interphases as critical to stable cycling.