Side-reaction pathways in Na-ion PW|HC full-cells were studied primarily through the analysis of the evolving volatile species during cycling. Cells were operated under both "normal" or "extreme" voltage conditions (1.3-3.6 V and 0.0-4.0 V, respectively). All identified reaction pathways (along with those previously reported) are summarised in . Y represents a negatively charged ligand originating from the electrolyte (gas or other) that stabilises Fe[Fe(CN)4] after it has been oxidised and undergone reductive coupling. These findings imply that if an overcharge event occurs on a large scale, significant amounts of (CN)2 (4% of all PW present) could be released. However, assuming this new surface-reconstructed phase does not impart significant impedance on the cell, PW could theoretically be pre-oxidized such that all possible (CN)2 is released and the surface passivated. This would add to SIBs already positive status as an environmentally friendly and safe cathode.