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 (LiPO2F2) 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 LiPF6 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 LiPO2F2 > FEC > EC > DEC. The results illustrate that LiPF6 likely interacts with the Cu anode upon contact, resulting in LixPFy, which can lead to a reduction peak at ∼1.44 V in CV. With the base electrolyte, reduced species from LixPFy lead to the formation of alkyl phosphorus fluorides (RPF), which can be suppressed by the presence of FEC and/or LiPO2F2. 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 (CH2OCO2Li)2 and Li2CO3 and DEC is reduced to CH3CH2OCO2Li and Li2CO3. With only the LiPO2F2 additive, the redox of LiPO2F2 can be found in CV with LixPOy as the possible reduced product. In addition, Li2CO3 formation from EC and DEC reduction was relatively suppressed by the presence of LiPO2F2. The simultaneous presence of the FEC additive can suppress the redox of LiPO2F2 and partly the decomposition of LiPF6 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 LixPOy species formation, suppressing the reduction of LixPFy, EC, and DEC though not completely.