Designing fluorine-free electrolytes for stable sodium metal anodes and high-power seawater batteries via SEI reconstruction

Abstract

Fluorine (F) is regarded as a key element in electrolytes for sodium metal anodes (SMAs) because of the formation of NaF containing solid–electrolyte interphase (SEI) layers; however, the high-cost and HF formation issues experienced by F-based electrolytes should be addressed. Herein, F-free, cost-effective 1 M NaBH₄/ether-based electrolytes are proposed, motivated by the recent speculation that NaH is a “good SEI layer.” The time-of-flight secondary ion mass spectrometry (TOF-SIMS) results of sodium metal electrodes after galvanostatic cycling demonstrated that NaH is a major component of the SEI layer. In addition, the native oxide surface of sodium was converted into NaH and NaBO₂ after soaking in the electrolytes, implying that “SEI reconstruction” occurred by chemical reduction. Accordingly, significantly longer cyclability was obtained in the Na‖Na symmetric cell (1200 h, 1 mA cm⁻², 1 mA h cm⁻²) than in F-based electrolytes. In seawater batteries (SWBs), 1 M NaBH₄/DEGDME (diethylene glycol dimethyl ether) delivers higher power density (2.82 mW cm⁻²vs. 2.27 mW cm⁻²) and cyclability (300 h vs. 50 h) under 1 mA cm⁻² than 1 M NaOTf/TEGDME (tetraethylene glycol dimethyl ether), which is commonly used in SWBs. In conclusion, two novel contributions of this study include the demonstration that NaH can work as a “good SEI layer” apart from NaF and the proposal of a cost-effective, F-free electrolyte for practical and large-scale SWBs.