A Bifunctional Fluorophosphate Electrolyte for Safer Sodium-Ion Batteries

Most of the currently developed sodium-ion batteries (SIBs) have potential safety hazards due to the use of highly volatile and flammable alkyl carbonate electrolytes. To overcome this challenge, we report an electrochemically compatible and nonflammable electrolyte, tris(2,2,2-triftuoroethyl) phosphate (TFEP) with low-concentration sodium bis(fluorosulfonyl)imide (0.9 M), which is designed not only to match perfectly with the hard carbon (HC) anode but also to enhance the thermal stability of SIBs. Experimental results and theoretical calculations reveal that TFEP molecules have a significantly low barrier to decompose before Na+ inserts into HC, forming a stable inorganic solid-electrolyte interface layer, thus improving the electrochemical and structural stabilities of HC anodes. An HC/NaV2(PO4)(3) full cell using TFEP electrolyte shows a high capacity retention of 89.2% after 300 cycles and a dramatically reduced exothermic heat at elevated temperature, implying its potential application for safe and low-cost larger-scale energy storage.