Impact of separator design on performance and energy density in sodium-ion batteries

The performance of sodium-ion batteries (SIBs) is critically influenced by separator design, which affects separator–electrolyte interactions, ion transport, cycling stability and cell-level energy density. This study systematically evaluates the impact of polyolefin-based, cellulose-based, and ceramic-coated separators in O3-NaNi 1/3 Mn 1/3 Fe 1/3 O 2 //hard carbon full cells. In parallel, a theoretical 15 Ah cell is used to assess how separator thickness, density, porosity, and electrolyte inventory contribute to gravimetric and volumetric energy density under a fixed-capacity assumption. The model indicates that gravimetric energy density varies by ∼3%, whereas volumetric energy density varies by ∼6% across the tested separators, primarily due to separator thickness and porosity-dependent electrolyte requirements. Experimentally, the separators with enhanced wettability, particularly boehmite-coated polyethylene monolayer (SP3) and cellulose-based separator (SP5), deliver superior capacity retention (∼90%). In contrast, conventional polyolefin/trilayer separators with limited wettability or high internal resistance show diminished performance. A qualitative Pearson trend analysis suggests that lower contact angles, corresponding to higher wettability, may be associated with improved performance. Overall, these findings underscore the importance of optimizing separator properties (i.e. thickness, porosity, and surface morphology/chemistry) to enhance the performance, efficiency, and scalability of next-generation SIBs.