In Situ Diffuse Reflectance Infrared Fourier-Transform Spectroscopy Investigation of Fluoroethylene Carbonate and Lithium Difluorophosphate Dual Additives in SEI Formation over Cu Anode
Bikila Nagasa Olana, Leyela Hassen Adem, Shawn D. Lin, Bing‐Joe Hwang, Yi‐Chen Hsieh, Gunther Brunklaus, Martin Winter
Abstract
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.