Succinonitrile ₃ /NaPF ₆ : A Low-Temperature Molecular Crystal and High-Concentration Electrolyte for High-Voltage Sodium Ion Batteries

A solid-state sodium-ion electrolyte based on succinonitrile (SN) and NaPF 6 in a 3:1 molar, (SN) 3 NaPF 6, was synthesized via a simple melt-mixing route. The resulting material forms a mechanically stable solid below room temperature and undergoes a reversible melt transition at approximately T m = 25 °C, enabling intimate electrode contact while maintaining solid-state integrity. In the low-temperature crystal phase (15 °C), there is an uncoordinated SN embedded in the crystal lattice. There are four C≡N···Na + and two PF 6 – ···Na + contacts, resulting in low ionic conductivity, σ ≈ 10 –5 S cm –1, but high Na + ion transference number, t Na + = 0.64. Above T m, a high concentration electrolyte (HCE) (4.16 m) is formed with σ ≈ 10 –4 S cm –1 just above T m, increasing to σ > 10 –3 S cm –1 at 50 °C, reflecting enhanced Na + mobility within the dynamically disordered SN matrix. Electrochemical measurements reveal excellent oxidative stability up to 4.5 V versus Na/Na +, making the electrolyte compatible with high-voltage sodium cathodes. The combination of low-temperature operability, wide electrochemical stability window, and favorable Na + transport properties makes the SN–NaPF 6 system a promising solid or HCE electrolyte for next-generation high-voltage and low-temperature sodium-ion batteries.