Sodium difluorophosphate: facile synthesis, structure, and electrochemical behavior as an additive for sodium-ion batteries

Abstract

Despite the success of difluorophosphate (PO₂F₂⁻, DFP) electrolyte additives in lithium and potassium-ion batteries, their utilization in sodium-ion batteries remains unexplored due to difficulties in the synthesis of sodium difluorophosphates (NaDFP). Thus, in this study, NaDFP salt prepared via ion exchange of KDFP and NaPF₆ is characterized using single-crystal X-ray diffraction, Raman and infrared (IR) spectroscopy, energy dispersive X-ray analysis (EDX), and thermogravimetry-differential thermal analysis (TG-DTA). Electrochemical tests demonstrate enhanced cycle performance of a hard carbon electrode (capacity retention; 76.3% after 500 cycles with NaDFP vs. 59.2% after 200 cycles in the neat electrolyte), achieving a high coulombic efficiency (average of 99.9% over 500 cycles) when NaDFP is used as an electrolyte additive. Further, electrochemical impedance spectroscopy (EIS) using a HC/HC symmetric cell demonstrates significant reduction of the interfacial resistance upon addition of NaDFP. X-ray photoelectron spectroscopy (XPS) indicates presence of stable, Na⁺-conducting solid-electrolyte interphase (SEI) components formed in the presence of NaDFP. This work not only presents a feasible NaDFP synthesis method, but also demonstrates the use of NaDFP as a strategy for optimizing sodium-ion battery performance.