Knocking down the kinetic barriers towards fast-charging and low-temperature sodium metal batteries

Abstract

Current knowledge on Na metal anode has been limited on its room-temperature or high-temperature (molten Na-S system) performances. However, the properties related to its low-temperature and fast-charging performances are rarely covered. Herein, we show that, using a conventional carbonate-based electrolyte, needle-like Na deposits sprout at −20 °C with a spiking impedance of ∼2.8 × 10⁴ Ω observed in symmetric cell configuration, making an early failure of the battery within tens of hours. By knocking down the kinetic barriers of Na⁺ ion de-solvation and its subsequent diffusion through the solid electrolyte interphase (SEI), we enable flat and spherical Na deposits at −20 °C with a massively reduced interfacial impedance. This has been realized by using (i) a weakly solvated electrolyte that shows a low solvation energy towards Na⁺ ions, and (ii) a Na₁₅Sn₄/NaF biphasic artificial SEI for promoting unhindered Na⁺ ion transfer at the Na metal/electrolyte interface. Ultimately, a high-voltage Na/Na₃V₂(PO₄)₂O₂F battery is developed to stand low temperatures down to −30 °C and fast charging up to 30C. The design strategy provided herein underlines the simultaneous de-solvation and SEI control for achieving low-temperature and fast-charging sodium metal batteries and presents as a prototype of how the kinetic barriers can be overcome under extreme conditions.