Abstract
<jats:p>Fluorinated diluents have been widely adopted in lithium metal batteries in recent years and represent one of the most successful strategies for stabilizing the electrode-electrolyte interphase. Because these molecules interact only weakly with alkali metal ions and are largely excluded from the primary solvation shell, they are generally considered chemically inert. Here, however, we show that incorporating fluorinated diluents into sodium metal electrolytes unexpectedly triggers severe sodium corrosion, revealing a striking contrast between lithium and sodium systems. This behavior motivated a systematic investigation of the underlying reaction mechanism, suggesting that fluorinated diluents may not be as chemically inert as previously assumed. We propose that PF5, generated from NaPF6, catalyzes the defluorination of fluorinated diluents in the presence of sodium metal, producing NaF and carbonaceous decomposition products. Extending this study across different salts, fluorinated diluents, and alkali metals further reveals that this behavior is broadly observed across diverse electrolyte chemistries and alkali metal systems. Beyond explaining this unexpected reactivity of fluorinated diluents, we leverage these mechanistic insights to establish a new strategy for controlled in situ NaF formation that stabilizes the interphase. By tuning the diluent structure and concentration, we regulate the extent of this reaction to promote beneficial NaF formation, enabling stable anode-free sodium metal batteries cycling for over 800 cycles.</jats:p>