Anion-Dipole Repulsions Improve Low-Temperature Performance of μ-Sn for Advanced Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are attractive for large-scale energy storage and have to work at a wide temperature range. Compared to the performance at room temperature, the low-temperature performance of SIBs is always poor. Herein, tetrahydrofuran (THF) and 2-methyltetrahydrofuran (MTHF) are mixed as a model of weak solvating effect to improve the low-temperature performance. In the mixed electrolyte, the anion-dipole expulsion and steric hindrance in the solvation sheath increase, which weakens the cation-dipole attraction and reduces the desolvation energy. Meanwhile, the weak dipole-dipole interaction between THF and MTHF lowers the electrolyte viscosity and improves the ionic conductivity. The change in the solvation structure also results in an entropy increase, facilitates the cleavage of P-F in PF₆ ^‾, and the formation of NaF-rich solid electrolyte interphase (SEI). Using commercial Sn microparticles as a model, the mixed electrolyte enables them to deliver a capacity of 328 mAh g^-1 after 1000 cycles at -40 °C, much better than the cases only using THF or MTHF alone. The electrolyte can be also successfully adapted to Bi and hard carbon, confirming its good compatibility with different anode materials. These results provide an in-depth understanding of low-temperature electrolytes for advanced sodium-ion batteries.