In Situ, Time-Resolved Measurements of Lithium-Ion Battery Gases During Cell Formation Using Mid-IR Laser Absorption Spectroscopy

Real-time non-extractive measurements of gaseous byproducts are needed to understand the buried interfacial formation chemistry of lithium-ion batteries at various scales; however, conventional gas sensors (e.g., those based on mass spectrometers) do not fully meet this need. This manuscript presents the design and application of a mid-infrared laser absorption spectroscopy (LAS) diagnostic for in situ measurements of CO, CO₂, CH₄, C₂H₄, C₂H₆, and OCS near-simultaneously during the formation cycle of pouch-style lithium-ion batteries. Scanned-wavelength direct-absorption measurements of these species were made near 4.85, 4.19, 3.27, and 3.35 μm, respectively, at 100 Hz using four distributed-feedback semiconductor lasers. Tests were conducted with NMC622-Gr pouch batteries containing 30:70 (v/v) ethylene carbonate (EC)/dimethyl carbonate (DMC) and 1.2 M lithium hexafluorophosphate charging at a rate of C/8. Between 40%–60% by mole of the total gas produced was measured consistently, with C₂H₄ and CO representing the largest fractions. Trace amounts (100–300 ppm) of carbonyl sulfide (OCS) were also measured. The evolution of gas mole fractions, moles, and their production rates with respect to time and voltage are discussed in the context of production mechanisms and electrochemical events. The presented results demonstrate the ability of LAS to provide quantitative non-extractive and calibration-free speciation measurements with high temporal resolution in Li-ion battery cells.