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

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_1/3 Fe_1/3 Mn_1/3 O₂ (NMFO)||HC full cell and achieve a compensated capacity ratio of 18.2%. In the Na₃ V₂ (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.