| Abstract
| - The formation and evolution of the solid electrolyte interphase (SEI) film on the surface of natural graphitespheres in the electrolyte of 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (volumeratio 1:1) were investigated with use of focused ion beam (FIB) technology. Secondary electron FIB imagesclearly show the surface and cross-section morphology of the SEI film. The composition variation along thesurface and cross section of the SEI film was also explored by the elemental line scan analysis (ELSA). Theinitial SEI film with an apparent thickness range of ∼450 to ∼980 nm is rough in morphology and nonuniformin composition, and contains small splits. After certain electrochemical cycles, the thickened SEI film displaysmicroscale holes and obvious cracks on the surface, and the content of organic compounds increases. Inaddition, the concept of “internal SEI film” is first proposed based on the characterization of the cross sectionof the natural graphite spheres with the aid of FIB. Finally, the capacity fading mechanisms of the naturalgraphite spheres corresponding to different electrochemical stages are discussed.
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