Degradation reactions in Li-ion and Na-ion batteries are undesirable chemical processes that occur alongside the intended electrochemical reactions, often leading to performance loss and/or safety issues. Determining exactly what these processes are and what is causing them is one of the biggest challenges in the battery field, in particular since they account for only a very minor fraction (ppm level) of all reactions taking place inside the battery cell. Limiting the voltage and/or temperature window of a SIB often results in longer cycle life since side reactions mostly occur at the extremes. However, side reactions still occurs under normal operating conditions, the most common of these is continuous SEI formation (as a result of an unstable SEI), which simultaneously drains the Na-ion reserve and increases impedance at the anode surface (through reactions between Na and electrolyte components) . By understanding the side reactions occurring throughout a cell's life, steps could be taken to prevent them, reducing cell aging significantly. So far, substantial progress has been made in understanding the negative role of water in PBAs (more notably PW) and Na-ion trapping/plating on the hard carbon (HC) counter electrode . Furthermore, in recent years, the focus has shifted towards monitoring the volatile products evolving in SIBs; especially associated with electrolyte degradation, but also gases forming from the CN bridging ligands in PBAs .