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 Na_2/3[Fe_1/2Mn_1/2]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 Na_2/3Ni_1/3Mn_2/3O₂ (P2-NiMn) positive electrodes. We demonstrate the synergistic effect that promotes interactions among CMC, H₂O molecules, Na⁺, and CO₃^2- 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.