Fig. 3: Dipole-dipole interaction mechanisms and SEI formation.

a Optical observation of the surface evolution of Na metal immersed in neat solvents, solvent mixtures, and electrolytes. b XRD patterns of surface layers formed in film-forming electrolyte NGMPE, by immersing Na foil in the fluorinated electrolyte for 2 h. c LUMO energy levels of neat solvents, solvent mixture, and electrolytes, where Na, C, H, O, P, and F is marked by purple, gray, white, red, brown, and cyan, respectively, with red and blue shades corresponding to low and high electron density regions. d NMR spectra of ¹H of neat solvents and solvent mixture. COZY spectra of (e) solvent mixture G2 + MPE showing the dipole-dipole interactions in the absence of either cations or anions, and (f) binary electrolyte NGMPE showing the enhanced dipole-dipole interactions. ToF-SIMS 3D reconstructions of depth-profiling of representative secondary ion species (individuals and overlays) in the sodium metal cycled in (g) NG2 electrolyte and (h) NGMPE electrolyte for 10 cycles at 8 mA cm⁻² and 4 mAh cm⁻². i Low-magnification cryogenic transmission electron microscopy (cryo-TEM) of the in situ formed primitive SEI after exposure to NGMPE electrolyte for 2 h. j High-resolution TEM (HRTEM) of the primitive SEI, with lattice fringes of crystalline regions marked in different colors. k Fast Fourier transform (FFT) of cryo-HRTEM of the primitive SEI.