In Situ Diffuse Reflectance Infrared Fourier-Transform Spectroscopy Investigation of Fluoroethylene Carbonate and Lithium Difluorophosphate Dual Additives in SEI Formation over Cu Anode

The synergetic effect of fluoroethylene carbonate (FEC) and lithium difluorophosphate (LiPO 2 F 2 ) dual additives on the cycling stability of lithium metal batteries has been previously reported. This study applies in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) to examine the impact of these two additives on SEI species formation over Cu anode using a base electrolyte of LiPF 6 in ethylene carbonate (EC) and diethyl carbonate (DEC). The results indicate that all electrolyte components and additives can be electrochemically reduced over the Cu anode following a potential sequence of LiPO 2 F 2 > FEC > EC > DEC. The results illustrate that LiPF 6 likely interacts with the Cu anode upon contact, resulting in Li x PF y, which can lead to a reduction peak at ∼1.44 V in CV. With the base electrolyte, reduced species from Li x PF y lead to the formation of alkyl phosphorus fluorides (RPF), which can be suppressed by the presence of FEC and/or LiPO 2 F 2 . Similar to previous reports, FEC reduction in the 1st lithiation cycle leads to the continuous formation of poly(FEC), while EC is electrochemically reduced to (CH 2 OCO 2 Li) 2 and Li 2 CO 3 and DEC is reduced to CH 3 CH 2 OCO 2 Li and Li 2 CO 3 . With only the LiPO 2 F 2 additive, the redox of LiPO 2 F 2 can be found in CV with Li x PO y as the possible reduced product. In addition, Li 2 CO 3 formation from EC and DEC reduction was relatively suppressed by the presence of LiPO 2 F 2 . The simultaneous presence of the FEC additive can suppress the redox of LiPO 2 F 2 and partly the decomposition of LiPF 6 likely via the preferential adsorption of FEC on Cu. Similar DRIFTS observations are found over the Li anode. The electrolyte with dual additives demonstrates a possible advantage from poly(FEC) and Li x PO y species formation, suppressing the reduction of Li x PF y, EC, and DEC though not completely.