Differential electrochemical mass spectrometry DEMS is a powerful operando method for analyzing side reactions in batteries. We describe our DEMS setup highlighting the relevance for implementing a reference electrode. Although the method provides valuable information, the correct assignment of the DEMS signals to types of gases and quantifying the amounts released can be challenging. A frequent limitation is that gas concentrations are calculated from single m z ratios. This has the drawback of overlooking unexpected gases which can cause misinterpretation of the signal intensities, or even attributing to gases which are not actually formed. We present a multiple concentration determination MCD algorithm that uses the full MS spectra, which allows a more reliable determi nation of the gas release. We demonstrate this approach for Na ion half cells with P2 Na0.67Mn3 4Ni1 4O2 NaMNO as cathode active material CAM . Studying the gassing behavior for two electrolyte formulations 1 M NaPF6 in propylene carbonate PC and in diglyme 2G . Against the general belief that glymes lead to more gassing at high potentials, we find that gas evolution for PC electrolytes is larger compared to 2G electro lytes. Dimethyl ether is found to be a decomposition product of 2G. Pressure change measurements are used to independently validate the gas quantificatio
