The amount of (CN)2, however, decreases significantly after the first cycle, with the onset of evolution pushed up to 1.3 V (~4.4 V vs Na + /Na) suggesting that the loss of CN generates a partly passivated surface. When the (CN)2 evolution across the three sweeps is integrated, a total of ~1023 nmol is calculated, equivalent to roughly 4.1% of the total molar amount of PW in the cell. Assuming an even loss of CN across the whole PW surface (with BET surface area of 5.49 m 2 g -1 ) and density of the PW (calculated via XRD refinement, 1.81 g cm 3 ), about ~4.2 nm deep surface layer has been reconstructed as a result of CN ligand loss (calculation values shown in Table ). Such a layer thickness value aligns well with observations associated with Ni-rich Li-ion cathodes where a ~4 nm rock salt phase is typically found after O2 release .