Fig. 2: 23Na NMR signal after a single charge/discharge (30 mA g−1) cycle.

a 23Na NMR spectra for solvated Na in the electrolyte (blue spectrum) and for metallic Na (red spectrum). b 2D 23Na MRI of metallic (red) and dielectric (blue) sodium in the cell. c 1D 23Na MR profile of sodium in metallic and electrolytic environments in the cell. This is cell was assembled with two separators.