The sodium deficiency issue of Na0.67[Fe0.5Mn0.5]O2, P2-type layered oxide, is compensated with the addition of Na2C4O4 as sacrificial salt. Na2C4O4 is easy to synthesize and stable, which makes it easy to handle and scale-up. Sodium-free hard carbon anode and sodium-deficient layered oxide cathode, mixed with different amounts of Na2C4O4, are used to assemble Na-ion full-cells and evaluate the effect of the sacrificial salt content in the electrochemical performance of the cells. Ex-situ SEM and XRD analyses confirmed that the Na+ ions formed as a result of the electrochemical decomposition of the salt, are reinserted back into the cathode structure. Thus, Na0.67[Fe0.5Mn0.5]O2 undergoes same phase transitions when tested as full-cell using Na-free hard carbon or with a Naº anode that compensates for the Na deficiency. In comparison with NaN3 sacrificial salt, Na2C4O4 seems to be a better candidate as sodiation agent in terms of safety and handling. The electrochemical performance of the P2-type layered oxide with 31% of Na2C4O4 sacrificial salt outperforms the cathode with NaN3 with improved cycling stability, delivering a reversible capacity of 155 mAh g−1 and an energy density of 165 Wh kg−1.