All OEMS work was conducted using the setup and dedicated cell previously described by Lundström et al. which samples gas from the headspace every 10 minutes. After each measurement point, the cell is refilled with an Ar carrier gas (purity 99.9999%, Air Liquid) allowing for extended cycling without drying out the cell. The mass spectrometer was calibrated for H2, CO2, CH4, C2H4 and O2 in the same was as described in our previous works with the exception of (CN)2 which was calibrated as described in section 3.3 using the pressure sensor connected to the dedicated cell housing . All experiments started with a 4-hour rest at open circuit voltage (OCV) while connected to the OEMS allowing for the background to stabilise before any cycling commenced. The cells were either cycled galvanostatically (0.33-0.1C) or potentiodynamically (0.1 mV s -1 ) with the negative terminal always connected to the base of the cell housing. All experiments were carried out in a climate chamber set to 30°C and assembled in an argon glovebox (O2 & H20: <0.1 ppm, 1 ppm). In order to understand the electrochemical behaviour of the PW synthesized for this study and possibly detect the formation of side-products, operando FTIR was applied and plotted in Figure . Most of the observed spectral changes can be associated with the well-known evolution of in Na/PF6 solvation and coordination structure upon de-/sodiation of the active materials (as shown in Fig. and S4) . However, while monitoring the frequency shifts of PW as a function of time, three unique components of interest in the range 1900-2300 cm -1 are identified with multivariate curve resolution-alternating least squares (MCR-ALS) and labelled as F2F2, F3F3 and F2F3, as they represent Fe 2+ -CN-Fe 2+ (with a peak at 2049 cm -1 ), Fe 3+ -CN-Fe 3+ (with a peak at 2167 cm -1 ), and Fe 2+ -CN-Fe 3+ (with a peak at 2071 cm -1 ), respectively (in accordance with previous studies ).