Gas evolution in sodium ion batteries – DEMS setup, data evaluation and application to a Mn-rich layered cathode material

Exceeding the electrochemical stability window of battery cells causes side reactions that are often accompanied by the release of gas molecules. A powerful operando method to analyze such side reactions and their onset potentials is differential electrochemical mass spectrometry (DEMS). While the method provides valuable information, the correct assignment of the DEMS signals and deriving quantitative information on the amounts and types of gases released can be challenging. A frequent limitation is that gas concentrations are often calculated from single m/z ratios only. This has the drawback of overlooking unexpected gases which can moreover cause misinterpretation of the signal intensities, or even the attribution to gases which are not actually formed. Here, we describe a multiple concentration determination (MCD) algorithm, that uses the full MS spectra as a basis. The approach allows a more reliable determination of the gas release and is, to our knowledge, for the first time applied to DEMS for batteries. As study case, Na-ion half cells with P2-Na0.67Mn3/4Ni1/4O2 (NaMNO) as cathode active material (CAM) are chosen. The gassing behavior for two electrolyte formulations (1M NaPF6 in propylene carbonate (PC) and 1M NaPF6 in diglyme (2G)) and for two different upper cut-off potentials (4.25 and 3.80 V) is determined. Against the general belief that glymes lead to more gassing at high potentials, we find that gas evolution for PC electrolytes is generally larger compared to 2G electrolytes. In case of 2G, dimethyl ether is found as decomposition product. Pressure change measurements in a closed cell are used as a second, independent method to validate the gas quantification of the MCD algorithm. The study also highlights the relevance for implementing a reference electrode into DEMS cell setups.