Deep eutectic solvent with film-forming fluoroethylene carbonate as a nonflammable electrolyte for lithium metal batteries

Abstract

Nonflammable deep eutectic solvents (DESs) are potential electrolytes for Li metal batteries. However, their practical application is prevented by their degradation with Li metal at low voltages. Here, we use fluoroethylene carbonate (FEC), a film-forming compound, as a cosolvent in DES-based electrolytes, demonstrating that compatibility with Li metal can be improved due to the formation of a robust and compact solid electrolyte interphase layer on the anode surface. The optimal electrolyte (50%DES) exhibits the Li+ conductivity of 1.58 mS cm−1 at room temperature. After the introduction of 50%FEC, the DES electrolyte shows a considerable reduction in viscosity from 1206.3 mPa s−1 to 20.4 mPa s−1 while retaining high flame-retardant efficiency. The Li/LiFePO4 cell with 50%DES electrolyte exhibits a reversible discharge capacity of 140.7 mAh g−1 at 50 mA g−1 and a capacity retention of 94.3 % after 200 cycles. The good cycling stability of the cell in 50%DES electrolyte can be attributed to the formation of a robust SEI layer containing inorganic-rich compounds (LiF and Li3N) with rapid Li+ transport on the Li metal anode that can improve interfacial stability and suppress Li dendrite growth.