A solid-state sodium-ion electrolyte based on succinonitrile (SN) and NaPF6 in a 3:1 molar, (SN)3NaPF6, 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 Tm = 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 PF6–···Na+ contacts, resulting in low ionic conductivity, σ ≈ 10–5 S cm–1, but high Na+ ion transference number, tNa+ = 0.64. Above Tm, a high concentration electrolyte (HCE) (4.16 m) is formed with σ ≈ 10–4 S cm–1 just above Tm, 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–NaPF6 system a promising solid or HCE electrolyte for next-generation high-voltage and low-temperature sodium-ion batteries.
Paul et al. (2026) studied this question.