Anion Solvation Regulation Enables Long Cycle Stability of Graphite Cathodes

Graphite is useful as a low-cost and high-voltage cathode of dual-ion batteries (DIBs) to allow anion (de-)intercalation, yet its practical applications are hindered by low Coulombic efficiencies (CEs) and poor cyclability. Meanwhile, the solvation behavior of anions and the impact of solvation on interphase chemistry have not been well clarified. Herein, a fluorinated co-solvent-modified electrolyte is proposed for graphite cathodes in sodium-based DIBs. The introduced composition participates in the solvation of Na+ cations and PF6¯ anions. The NaF-rich interphase layer formed on the anodes effectively inhibits side reactions with the electrolyte. More importantly, the fluorinated cathode-electrolyte interphase plays significant roles in suppressing electrolyte oxidation decomposition and maintaining the structural integrity of graphite. Consequently, this modified electrolyte endows graphite cathodes with enhanced CEs and long cycle life, and it enables good cycling stability of graphite cathode-based full cells. With this full understanding of anion solvation, this work provides a guideline to design electrolytes for anion-intercalation cathodes.