Interfacial Reconstruction and Spatial Decoupling Enable High‐Efficiency Sodium Compensation

ABSTRACT Irreversible loss of active sodium severely limits the attainable energy density and cycling stability of sodium‐ion batteries. Sodium oxalate is a promising sacrificial sodium source yet suffers from sluggish oxidation kinetics. Moreover, because hard carbon anodes exhibit low initial Coulombic efficiency, practical sodium compensation often requires high loadings of sodium oxalate, making gas evolution during activation a critical challenge. Under practical compensation levels, sodium oxalate can generate ∼0.18 L Ah − 1 of CO 2 (≈0.3 mL cm − 2 for a 2 mAh cm − 2 cathode), imposing a substantial structural and interfacial burden on closed battery systems. Here, we show that efficient sodium compensation is governed not only by the intrinsic activation kinetics of sodium sources, but also by their spatial configuration within the cell. Conductive polymer‐induced interfacial electronic reconstruction enables sodium oxalate activation at a reduced decomposition voltage of ∼4.08 V. More importantly, a sodium compensation separator spatially decouples sodium release from cathode reactions and facilitates CO 2 escape from the electrode framework. This configuration mitigates gas‐induced cathode degradation and parasitic sodium consumption, improving sodium utilization efficiency and electrode stability. Consequently, reversible capacity is enhanced by 78% in polyanionic full cells and 63% in layered oxide full cells after extended cycling.