Nano‐Treated Na ₂ C ₂ O ₄ With Reduced Oxidation Potential for Enhancing Sodium Compensation Performance in High‐Energy Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) have emerged as a promising solution for large‐scale energy storage due to their cost‐effectiveness and the abundant availability of sodium. However, the formation of a solid electrolyte interphase film on the anode surface during initial cycling results in irreversible Na + loss, negatively impacting the initial coulombic efficiency of SIBs. To address this challenge, a cathode‐oriented sodium compensation additive, Na 2 C 2 O 4 @5%Ketjenblack (NCO@5%KB), was developed through a simple sand‐milling process. Na 2 C 2 O 4 is particularly noteworthy because of its high theoretical capacity (400 mA h g –1 ) and its complete decomposition into Na + and CO 2 . In this study, sand milling was employed to reduce the particle size of NCO, while Ketjenblack (KB) was incorporated to enhance electrical conductivity, leading to a synergistic effect that lowered the oxidation decomposition potential. Electrochemical characterization demonstrated that the combined effects of KB coating and nanosizing significantly improved the electrochemical kinetics of NCO, reducing its oxidation decomposition potential from 4.43 to 4.18 V. When used as a sodiation reagent in Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 ‐based full cells, the modified NCO@5%KB substantially enhanced the initial reversible capacity, cycling stability, and energy density. This work presents an effective strategy for sodium compensation, paving the way to improve the electrochemical performance of SIBs.