Upon charge, the mixed valence state represented by F2F3 increases at the expense of F2F2, implying a continuous shift from Fe 2+ to Fe 3+ within the structure. F2F3 sharply increases to a maximum at ~3.10 V where the HS-FeN (high-spin N bonded Fe) undergoes oxidation. At roughly the same point, F3F3 starts to increase, implying the appearance of the nearly fully oxidised Fe 3+ -CN-Fe 3+ . The other components disappear completely at >3.70 V indicating all the Fe present in the electrode has been oxidised. No further changes in the Fe valence are observed when the voltage is increased past this point, here suggesting the absence of any overcharge processes, at least as detectible via FTIR. Upon discharge, the reverse processes are observed as expected, with F3F3 dropping, F2F3 increasing followed by the return of F2F2 close to 0 V. Our data explains why commercial PW|HC cells are typically specified with an upper and lower cut-off of 3.6 and 1.3 V (i.e. 3.70 V and 1.40 V vs Na + /Na for PW), respectively, to harbour the complete Fe 2+ /Fe 3+ redox of PW. Battery cycling beyond this voltage window would not increase cell capacity, but only induce detrimental oxidative and reductive processes (in current state-of-the-art organic carbonate-based electrolytes), which we will show is partly related to Na plating and PW over-oxidation along with the formation of (CN)2. It is notable that F2F3 does not decrease to 0 concentration as would be expected during the second cycle, suggesting the PW is never fully reduced back to the pristine state, predominantly as a result of irreversible Na loss at the negative electrode. Processes, such as SEI formation, Na trapping, and other side-reactions at HC further contributes to cell capacity loss, but are scrutinized in more detail below. The negative HC electrode has previously been much more extensively investigated elsewhere and is known to undergo a three-stage sodium storage process . First, during the sloping region, Na + ions are inserted into graphitic domains of HC, causing the electrode to swell. Second, along a plateau (usually when HC reaches ~100 mV vs Na + /Na), Na + ions fill HC pores, leading to slow, constant expansion. Third, near the cut-off potential, Na-like plating on pore surfaces takes over, expanding the electrode similar to the insertion step. Important to note, however, is that no degradation products were observed in the IR spectra even though the cell was polarized up to 4.5 V. With our FTIR configuration, degradation products must reach a few percent of the total cell composition in order to be detected and focus was therefore shifted to more sensitive analytical techniques.