Interface Regulation in Sodium-Ion Batteries Using a Magnesium Fluoride-Modified Polyimide Composite Separator

As a pivotal component in sodium-ion batteries (SIBs), the separator critically governs interfacial stability and ion transport efficiency. Herein, an interface engineering strategy is proposed for constructing a high-performance polyimide-based separator modified with a nano-MgF₂ coating via a facile alkaline activation and in situ precipitation approach. The introduced MgF₂ layer, characterized by strong polarity and negative surface charge, effectively enhances electrolyte affinity and regulates sodium-ion flux. This modification not only facilitates rapid Na⁺ migration but also promotes the formation of a robust solid electrolyte interphase (SEI), leading to substantially improved interfacial kinetics and deposition homogeneity. As a result, the Na||hard carbon (HC) half-cell equipped with the modified separator delivers a high discharge capacity of 160 mAh g^-1 at 0.5C, a significant increase from 82.1 mAh g^-1 achieved with the unmodified separator. Furthermore, exceptional cycling stability is demonstrated in a Na||NFPP half-cell, maintaining a capacity of 90.9 mAh g^-1 after 500 cycles. This work underscores the potential of interface-engineered separators for advancing high-performance SIBs.