Impact of FEC Additive on SEI Structure Formed on a Carbon Negative Electrode Studied by HAXPES

 Solid electrolyte interphase (SEI) formed on a carbon negative electrode plays an important role in supporting the reversible Li ion intercalation phenomena during charge and discharge treatments.  Since the chemical and electrochemical conditions of the electrolyte/electrode interface change during electrochemical lithiation and delithiation reactions, the structure of the SEI might change even after the formation of the SEI by the initial charge and discharge treatments, although details of the reaction mechanism remain unknown. We recently successfully analyzed the behavior of the SEI of a carbon negative electrode during electrochemical lithiation and delithiation reactions by using Hard X-ray photoelectron spectroscopy (HAX-PES) technique and indicated HAX-PES was useful for analyzing SEI structure. Additives can control the structure and components of SEI, leading to the improvement of battery performance. In the present study, we applied this method to understanding of the influence of the SEI forming additives.

Lithiated and delithiated carbon anodes were prepared by using a coin-type cell with Li metal. The carbon anode was a composite graphite (active agents, carbon black, PVDF) coated on Cu foil.  The electrolyte was 1 M LiPF₆ in  90wt% ethylene carbonate / diethyl carbonate (EC / DEC) in a 1:1 volume ratio + 10 wt% fluoro ethylene carbonate (FEC).  After charge and discharge, the cell was disassembled and the anode was then retrieved, washed with dimethyl carbonate repeatedly in an argon-filled glovebox (<1 ppm H₂O, <1 ppm O₂).  The anodes were transferred from the glovebox to UHV measurement chamber without exposing to the air by using special transfer systems.  The HAXPES measurements were carried out at the BL46XU beamline at SPring-8.  The photon energy of incident x-rays was 8 keV.

Figure 1 shows C 1s HAXPES spectra for the lithiated graphite anodes with the state of charge (SOC) 0%, 50%, 100% after twice charge and discharge cycles.  The spectra were fitted with seven singlets centered at 283.5eV (Peak 1), 284.6 eV (Peak 2), 286.0eV (Peak3), 286.5 eV (Peak 4), 288.6 eV(Peak 5), 289.6 eV(Peak 6) and 290.5 eV(Peak 7).   Peak 1 can be assigned to lithiated carbon Li_xC.  Peak2 – Peak6 can be assigned to chemical components for lithium alkyl carbonate (LAC),   the organic materials of SEI. Peak 7 can be associated with a carbonate component for the Li₂CO₃, a inorganic material of SEI. Peak 2 and Peak 7 include the hydrocarbon (Peak2) and organofluorine (Peak7) components for the PVDF binder.  Li_xC was observed in the C 1s HAXPES spectra with all SOC conditions, whereas this peak wasn’t observed in the C 1s XPS spectra with Al K-α incident x-rays for the lithiated graphite anodes with SOC 50%, 100% after twice charge and discharge cycles (the C 1s XPS spectra were not shown).  This result indicates that the probing depth of HAXPES (hν = 8keV) is larger than thickness of SEI, thus HAXPES detected whole SEI and a part of the Li_xC exists underneath SEI formed on all the lithiated graphite anodes in the depth direction.  HAXPES technique allows us to analyze the variation of the chemical composition of SEI depending on electrochemical lithiation and delithiation reactions.  Figure 2 shows the concentration ratio of Li_xC, LAC, and Li₂CO₃ depending on SOC determined by the C 1s HAXPES spectra. The ratio of Li_xC decreases up to SOC 100%, while those of LAC, and Li₂CO₃ increases up to SOC 100%.  This variation shows the structural behavior of the SEI during electrochemical lithiation reactions after the formation of the SEI by the initial charge and cycles: SEI formed in the electrolyte with FEC additives continues growth and dissolution even after stable state is established.