Online electrochemical mass spectrometry on large-format Li-ion cells

Advances in methodologies for real-time analysis of batteries have come a long way, especially with the development of Operando Electrochemical Mass Spectrometry (OEMS). These approaches allow for the deter-mination of side reactions during battery cycling with unprecedented selectivity and sensitivity, providing vital information necessary for determination of lifetime-limiting processes. However, the work thus far has primarily been carried out on model battery systems, where cell atmospheres are largely altered (through open flow, closed cell, and intermittent sampling approaches) and operation conditions are therefore not comparable with real-life situations. Herein, the development and validation of an intermittently closed OEMS system adapted for readily available commercial batteries is showcased. We provide a detailed description of a unique analysis design for large-format PHEV2 cells, with subsequent pressure and gassing data. A qualitative analysis of the results shows that side reactions brought on by structural transitions within both electrodes can be clearly observed. Transi-tions causing large volume changes in graphite induce H2 and C2H4 as SEI reformation products while the c lattice collapse in NMC induces CO2 evolution (through O2 release). OEMS can therefore be used for the quick and effective study of commercially available rechargeable batteries without influencing the internal battery chemistry.