Carbonate Electrolyte Structure Engineering Enabling Ultrahigh-Rate Capability and Long-Term Cycling Stability of NFPP‖Na Batteries at −40 °C

Abstract

Sodium-ion batteries encounter challenges related to sluggish charge transfer at interfaces and high desolvation impedance at low temperatures, primarily due to the strong electronegativity of oxygen donor solvents in conventional electrolytes. This issue is particularly prominent in carbonate electrolytes. Herein, propylene carbonate (PC), known for its high chemical stability, has been utilized in the design of a localized high-concentration electrolyte by using fluorobenzene (FB) as a diluent. This approach significantly accelerates the desolvation process of Na+ ions and facilitates the formation of an anion-enhanced solvation structure. The highly reactive nature of FB contributes to the development of a NaF-rich cathode-electrolyte interphase (CEI), which interacts synergistically with anions. This interaction significantly enhances the interfacial stability of the Na4Fe3(PO4)2P2O7 (NFPP) cathode. The resulting NPF electrolyte demonstrates excellent low-temperature performance in NFPP‖Na cells, maintaining a capacity retention of 85.9 % after 100 cycles at -40 ℃, and operating steadily at -20 ℃ after 200 cycles at a high rate of 2.0 C. Additionally, the introduction of FB significantly enhances the ionic conductivity and wettability of NPF. This work provides valuable insights into the design of low-temperature electrolytes for sodium-ion batteries.