Lina Gao,Juner Chen,Yaqin Liu,Yusuke Yamauchi,Zhenguo Huang,Xueqian Kong
Stabilizing the reactive metal anode is a critical challenge for the development of next-generation alkali metal batteries. This work demonstrates that sodium-difluoro(oxalato)borate (NaDFOB)-based carbonate-ester electrolytes have excellent compatibility with the anode in sodium metal batteries, enabling high rate performance and long cycle life. The NaDFOB-based electrolytes possess favourable electrochemical stability and effectively passivate the Na metal anode by forming a compact, robust and conductive solid-electrolyte interphase (SEI) layer. Quantitative nuclear magnetic resonance (NMR) measurements of electrolyte solvents indicated that the SEI layer forms mainly in the initial cycles, and it prevents further solvent degradation. Solid-state NMR and X-ray photoelectron spectroscopy studies revealed the chemical composition of the NaDFOB-derived SEI film, which includes a mixed phase consisting primarily of sodium diborate, tetrafluoroborate and carbonate. The formation of such a composite SEI rich in borate and tetrafluoroborate provides robust structural and chemical stability, and facilitates fast ion transport for uniform Na stripping/plating.
Sodium bis(fluorosulfonyl)imide (NaFSI) electrolytes are renowned for their superior physicochemical and electrochemical properties, making them ideal for high-performance sodium-ion batteries (SIBs). However, severe oxidative dissolution of aluminum current collectors (commonly known as Al corrosion) in NaFSI-based electrolytes occurs at high potentials. To address this challenge, aiming to understand the Al corrosion mechanism and develop strategies to inhibit corrosion, we propose dual-salt electrolytes using 0.8 mol L–1 (M) NaFSI and 0.2 M of a second fluorine-containing sodium salt dissolved in EC/PC solutions (1:1, v/v) to construct an insoluble deposits layer on the Al. Dual-salt electrolytes adopting a second sodium salt capable of passivating the Al collector have been extensively investigated through various techniques, such as cyclic voltammetry, scanning electron microscopy, chronoamperometry, X-ray photoelectron spectroscopy, and charge–discharge tests. Our findings demonstrate that introducing sodium difluoro(oxalato)borate (NaDFOB) into the NaFSI electrolytes inhibits Al corrosion, which is attributed to the formation of insoluble deposits of Al–F (AlF3) and B–F containing polymers. Moreover, the capacity retention of Na||Na3V2(PO4)3 (NVP) cells using the NaFSI-NaDFOB dual-salt electrolyte reaches 99.2% along with a Coulombic efficiency over 99.3% at a 1 C rate after 200 cycles. This research provides a practical solution for passivating Al collectors in SIBs with NaFSI electrolytes and promotes the development of sodium batteries with long calendar lifetimes.
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