Polymer in low concentration electrolytes for sodium metal batteries

Abstract Polyethylene oxide (PEO)‐based polymer electrolytes have been considered as the benchmark for developing solid‐state battery systems since the 1970s. However, the limited oxidation stability (<4 V) of the ether oxygen in PEO's polymer structure renders it incompatible with high‐voltage (>4 V) cathodes, which restricts further improvements in the energy density of lithium (Li) or sodium (Na) metal batteries. In this work, we develop a novel type of polymer in a low‐concentrated electrolyte system, Na(TEGDME) 4.1 (PEO) 1.0 (mole ratio of NaFSI:TEGDME:PEO = 1:4.1:1, calculated based on NaFSI mole). For a comparison, a liquid baseline electrolyte, 1 m NaTFSI in tetraglyme (TEGDME) (NaFSI:TEGDME = 1:4.1), were selected to compare with new electrolyte in term of redox stability, thermal stability, and charge‐discharge performance. The hybrid gel electrolyte, Na(TEGDME) 4.1 (PEO) 1.0 , significantly enhanced oxidation stability by raising the onset oxidation potential from 4.0 to 4.2 V in comparing to non‐PEO electrolyte. Additionally, the thermal stability of the gel polymer electrolyte was dramatically higher than that of the liquid electrolyte, with the onset temperature increased by over 75 °C relative to the evaporation temperature of the liquid electrolyte, which begins around 140 °C. The charge–discharge curves of sodium half‐cells indicates that the gel polymer electrolyte exhibited a higher capacity compared to non‐PEO electrolyte.