A solid dual-salt plastic crystal electrolyte enabling rapid ion transfer and stable interphases for high-performance solid-state sodium ion batteries

As promising next-generation energy storage systems, solid-state sodium ion batteries (SIBs) are hindered by the low ionic conductivity of their solid electrolytes and poor interfacial compatibility. Here, we developed a solid dual-salt plastic crystal electrolyte (PCE) composed of succinonitrile (SN), fluoroethylene carbonate (FEC), and dual salts (NaClO₄ and NaBF₄). The synergistic redox reactions of FEC and BF₄ ^- form a robust, F- and B-rich interphase at electrode/electrolyte interfaces, which significantly enhances interfacial stability and minimizes side reactions that cause cell degradation. Meanwhile, the derived interphase combined with SN's intrinsic oxidation stability endows the PCE with high-voltage tolerance (≥4.75 V), enabling stable operation with high-voltage cathodes. Furthermore, SN's high polarity and plastic character facilitate Na-salt dissociation and optimize interfacial contact, while the competitive coordination of BF₄ ^- with Na⁺ over ClO₄ ^- increases the free Na⁺ concentration, synergistically enhancing ionic conductivity to 3.79 mS cm^-1. Consequently, the dual-salt PCE enables high-performance SIBs when paired with multiple electrode materials such as Na₃V₄(PO₄)₃, Prussian white, Na₃V₂(PO₄)₂F₃ cathodes, and hard carbon anodes. These batteries deliver exceptional cycling stability (over 2500 cycles), rate performance (up to 60C), specific capacity (163.9 mAh g^-1), and operating voltage (≥4.0 V). Moreover, the hard carbon‖Na₃V₄(PO₄)₃ full cell maintains stable cycling at 10C over 100 cycles, highlighting its promise for practical applications.