In Situ Gas Analysis - Method Development and Application to Study Cathode Performance in Li-O2 Cells

New insights into the electrochemical formation of volatile species in Li-O₂ cells are provided by combining differential electrochemical mass spectrometry with online pressure analysis. In situ gas analysis during Li-O₂ battery cycling is crucial to identify parasitic side-reactions and determine reversibility of the cell. The figure shows the potential and gas evolution profiles of O₂, CO₂, H₂, H₂O, CO, DME during the 1^st and 3^rd galvanostatic charge (0.1 mA/cm²) of a Li-O₂ cell  (20:80 PTFE:BP2000 carbon, 0.2 M LiTFSI in Diglyme) after a shallow discharge (~5% of total capacity). On the 1^st and 3^rd discharge, oxygen is consumed at a lower rate (~2.12 and ~2.28e^-/O₂, respectively, as probed by ΔP(O₂)) than expected for Li₂O₂ formation (2e^-/O₂), clearly indicating  influence of parasitic side-reactions during cycling. On charge, O₂ evolves slightly above 3 V initially with a rate expected for Li₂O₂ oxidation. At 3.2 V, O₂ evolution rate gradually drops and CO, H₂, H₂O appear. At 3.8 V, CO₂ evolves possibly as results of oxidation of carbonates, which formed during discharge. Above 4.5 V, DME is observed, which is related to direct electrolyte oxidation. In subsequent cycles, the cell over-potentials increase, the maxima of O₂ evolution rate gradually shift to higher potentials, while the gas evolution associated with parasitic side-reactions becomes more pronounced. In our contribution we demonstrate how development and application of in situ gas analysis with high sensitivity provide detailed insights into Li-O₂ cell chemistry.

BASF SE is greatfully acknowledged for financial support.