A crystalline organic electrolyte for safe, room-temperature operable all-solid-state sodium batteries

Abstract

Owing to their mitigated safety risk and high energy density, all-solid-state sodium batteries are promising post-Li batteries. However, current solid-state electrolytes have insufficient ionic conductivity, electrochemical stability, and contacting ability with porous electrodes. To overcome these issues, this study presents sulfone-based crystalline organic electrolytes (SCOEs) composed of dimethyl sulfone (DMS) and sodium bis(fluorosulfonyl)imide (NaFSI). One particular SCOE (NaFSI/DMS, 2:8 by mol) has high ionic conductivity (7.0 × 10−4 S cm−1) at 25 °C and excellent oxidative stability (>5.5 V vs. Na+/Na). Importantly, the SCOE displays an optimum melting point (66 °C), enabling intimate contact with porous composite electrodes via the melt-casting process. Na||Na3V2(PO4)3 cells employing this SCOE show better cyclability than do cells employing a conventional carbonate electrolyte (capacity retention 91.1% vs. 60.3% after 200 cycles at 25 °C). Additionally, the SCOE has negligible flammability, unlike carbonate electrolytes, thus holding great promise as an electrolyte for safe, room-temperature operable all-solid-state sodium batteries.