High‐Efficiency Separator Capacity‐Compensation Strategy Applied to Sodium‐Ion Batteries

Abstract Sodium‐ion batteries (SIBs) are expected to replace partial reliance on lithium‐ion batteries (LIBs) in the field of large‐scale energy storage as well as low‐speed electric vehicles due to the abundance, wide distribution, and easy availability of sodium metal. Unfortunately, a certain amount of sodium ions are irreversibly trapped in the solid electrolyte interface (SEI) layer during the initial charging process, causing the initial capacity loss (ICL) of the SIBs. A separator capacity‐compensation strategy is proposed, where the capacity compensator on the separator oxidizes below the high cut‐off voltage of the cathode to provide additional sodium ions. This strategy shows attractive advantages, including adaptability to current production processes, no impairment of cell long‐cycle life, controlled pre‐sodiation degree, and strategy universality. The separator capacity‐compensation strategy is applied in the NaNi_1/3Fe_1/3Mn_1/3O₂ (NMFO)||HC full cell and achieve a compensated capacity ratio of 18.2%. In the Na₃V₂(PO₄)₃ (NVP)||HC full cell, the initial reversible specific capacity is increased from 61.0 mAh g⁻¹ to 83.1 mAh g⁻¹. The separator capacity‐compensation strategy is proven to be universal and provides a new perspective to enhance the energy density of SIBs.