Low‐Temperature and Fast‐Charging Sodium Metal Batteries Enabled by Molecular Structure Regulation of Fluorinated Solvents

Abstract

Reversible cycling of sodium metal batteries (SMBs) is limited by Na dendrite growth, unstable solid‐electrolyte interphase (SEI) formation, and poor Na+ transport/de‐solvation kinetics, especially under the conditions of low‐temperature and fast‐charging. Here, a series of electrolytes for low‐temperature and fast‐charging SMBs by regulating the molecular structure of solvents is presented, in which ethyl acetate (EA) is selected as a baseline solvent, ethyl difluoroacetate (EDFA) and 2,2‐difluoroethyl acetate (DFEA) solvents are obtained by introducing difluorination group into EA molecule at different positions. It is found that the difluorination at the space‐position of oxygen in DFEA causes the lowest binding energy with Na+, which brings faster Na+ transport/de‐solvation ability in DFEA‐based electrolytes. In addition, more ionic aggregates (AGGs) structures are also observed in DFEA‐based electrolytes, leading to a stable inorganic‐rich SEI layer formed on the Na‐metal surface. Therefore, the optimal DFEA‐based electrolyte shows prolonged cycle life (>1500 h) in Na||Na cell and evidently improves cycle performance in the Na3V2(PO4)3 (NVP)||Na cell at −20 °C, achieving high discharge capacity (93.64 mAh g−1 at 0.5C) and ultra‐stable capacity retention (99.27% after 400 cycles). Furthermore, the NVP||Na cell also reveals outstanding fast‐charging ability at room temperature, showing capacity retention of 81.84% at 10C over 1000 cycles.