Only H2 and CO2 are observed to evolve at ~2.3 V during the second charge, although to a lesser extent, indicating the continued presence of H2O/OH -species in solution/surface of the cell housing. EC is well-known to form a SEI largely permeable to H2O . In fact, CO2 is seen to evolve throughout the charge step in all cycles, which is likely also a result of reactions between the cathode surface and the electrolyte. Brant et al. have shown that the longer PW powder/electrodes sit outside of a cell, the more OH -groups attach to its surface . In addition, NaPF6 salt hydrolysis is expected alongside those previously mentioned reactions, forming reactive HF (Eq. 5) , which can be reduced forming NaF and H2 gas (Eq. 6) as well as degrade inorganic HC surface species such as Na2CO3 forming more CO2 (Eq. 7) . However, throughout the cycling in the normal voltage range, POF3 evolution was rarely detected with none observed at all in Figure . This suggest that unlike with LiPF6, the Na + /PF6 -interaction is stronger and less prevalent to degradation (likely due to a reduced disparity in ion size).