Fig. 4. Microstructural and chemical evolution of the SEI and Na deposition morphology.

a Low-magnification cryo-EM image of deposited Na metal. b HRTEM images and corresponding FFT patterns of the SEI formed with COF/PNSE electrolyte. c EELS mapping images and line scans across the C, F, O, and Na K-edges. d TOF-SIMS depth profiles of NaO⁻, NaCO₃⁻, C₂HO⁻, CF₃SO₃⁻, and NaF₂⁻ fragments at the Na metal surface after cycling with COF/PNSE. e–h Depth-profile XPS analysis of cycled Na metal: e C 1 s, (f) O 1 s, (g) S 2p, (h) F 1 s spectra. i SEM image and (j) corresponding topography via AFM for Na metal cycled with PNSE. k, l SEM and AFM images for Na metal cycled with COF/PNSE electrolyte.