Anion Screening in Ether-Based Electrolytes for Boosted Sodium Storage at Low-Temperature

Abstract

The electrolyte composition plays a crucial role in determining the electrochemical performance of sodium-ion batteries (SIBs), particularly at low temperatures (LTs), where sluggish ion transport and intensified parasitic side reactions severely compromise battery stability and efficiency. However, previous electrolyte engineering efforts have primarily focused on solvent optimization to regulate Na⁺ solvation, the fundamental role of salt anions in modulating the electrode-electrolyte interface (EEI) and solid electrolyte interphase (SEI) formation remains insufficiently explored. Herein, we systematically investigate the influence of anion chemistry in ether-based electrolytes on sodium storage performance. By evaluating eight representative anions, we elucidate their distinct effects on SEI composition, interfacial charge transfer dynamics, and Na⁺ diffusion kinetics. Our combined theoretical and experimental approach reveals that the PF₆⁻, exhibiting favorable adsorption energy on the electrode surface, facilitates the formation of an inorganic-rich SEI (comprising NaF and Na₃P). Comprehensive in-situ characterization further confirms that this tailored interface enhances interfacial stability, accelerates Na⁺ transport, improves charge transfer kinetics, and suppresses parasitic side reactions, thereby enhancing electrochemical reversibility and cycling stability. These findings provide critical insights into anion-electrode interactions, offering a fundamental framework for optimizing SEI composition, stabilizing the EEI, and improving sodium storage performance in SIBs at LTs.