Synergistic Effects of Na₂CO₃ as Sacrificial Salt and Water-Soluble Binder for Na-Ion Battery with Na-Deficient P2–Na_2/3Ni_1/3Mn_2/3O₂

P2-type transition metal layered oxides have attracted attention as high-capacity positive electrode materials for sodium-ion batteries (NIBs). However, due to their Na-deficient compositions, an additional Na+ supplement is necessary for their practical use in NIBs. As we reported recently, the addition of Na2CO3 powder into the P2-type Na2/3[Fe1/2Mn1/2]O2 electrode has proven to be effective in addressing this challenge, as its electrochemical oxidative decomposition compensates for the Na+ deficient capacity. In this study, a combination of water-soluble Na2CO3 sacrificial salt and an aqueous slurry process using carboxymethyl cellulose (CMC) binder is successfully applied to sodium-deficient P2-type Na2/3Ni1/3Mn2/3O2 (P2-NiMn) positive electrodes. We demonstrate the synergistic effect that promotes interactions among CMC, H2O molecules, Na+, and CO32– in the aqueous slurry solution and the slow precipitation of low-crystalline Na2CO3 during the electrode drying process. This mechanism eventually enhances the homogeneous dispersion with nanoparticulation of Na2CO3, thus synergistically improving the decomposition efficiency of the sacrificial salts, enabling the achievement of superior reversible capacity in the P2-type electrode from the first cycle of the Na-ion full cell. Based on these findings, the combination of water-soluble sacrificial salt and aquosity slurry process totally offers a promising approach for developing NIBs with Na-deficient positive electrodes.