Unveiling temperature adaptability of fluorinated ethylene carbonate as electrolyte additives for cross-temperature Li metal batteries

Fluorinated ethylene carbonate, e.g., the fluoroethylene carbonate (FEC) and Di-fluoroethylene carbonate (DFEC), are widely used as electrolyte additives to regulate the SEI on lithium (Li) anodes, yet the superiority or inferiority of their performance, particularly under different temperatures, remains a subject of debate. Herein, we systematically examine the effects of FEC and DFEC on Li anode performance across the temperature range from -40 °C to 60 °C. By evaluating Li plating/stripping CE, cycling stability, Li nucleation overpotential, and polarization voltage, we observe that FEC and DFEC exhibit temperature-dependent performance variations. Overall, the performance of DFEC is better under low temperatures, while FEC and DFEC each have their own advantages and disadvantages under normal temperatures (30 °C), and both will experience performance degradation at high temperatures. Mechanistic studies reveal that this temperature dependence primarily stems from differences in the additives’ reaction kinetics on the surface of the Li anode, rather than the changes in Li⁺ solvation structure or diffusion behavior. The divergent performance originates from temperature-dependent decomposition pathways and distinct reduction products of FEC and DFEC. This study provides theoretical guidance for the optimization of electrolyte additives under varied operating temperatures and the design of wide-temperature-range electrolytes for Li metal batteries.