Energy-loss near-edge structures and low-loss structures of polymers in a solid electrolyte interface formed from fluoroethylene carbonate on a si anode clarified by dft calculations

To shed light on the polymer bonding structure and inorganic/polymer nanocomposite structures in fluoroethylene carbonate (FEC)-based solid electrolyte interfaces (SEIs), highenergy-resolution energy-loss near-edge structures (ELNESs) and low-loss structures of inorganic/organic nanocomposites were examined using electron energy-loss spectroscopy and scanning transmission electron microscopy. The ELNES and low-loss results for the polymeric species were clarified with density functional theory (DFT) calculations. Similar features observed in the plasmon losses of Li2CO3 and polymeric species indicate that the previously reported low-loss spectrum suggested that Si was covered with a seemingly homogeneous Li2CO3 layer because they were interpreted only with lithium compounds. However, in the pre-plasmon region, these low-loss spectra showed unique features associated with an interband transition that are absent from the spectra of the inorganic species. ELNES spectra were elucidated using DFT band-structure calculations with a core hole introduced into the 1s orbitals of C atoms in FEC-based polymers, as reported in prior spectroscopy studies. Cross-linked poly(FEC) showed the most consistent peak with those of the experimental spectra. The Li k-edge spectrum revealed LiF nanoparticles distributed in the domain of cross-linked poly(FEC) rather than the inner inorganic/outer polymeric bilayer structure in the SEI, as proposed in prior spectroscopy studies.