Fig. 1. ²³Na NMR signal in a pristine sodium metal cell.

a Schematic diagram of the sodium metal cell with respect to the imaging coordinate system and the B₀ magnetic field direction. The working electrode (WE) and counter electrode (CE) are connected to a potentiostat (P). b ²³Na NMR spectra, acquired separately, for Na in the dielectric environment of the electrolyte (blue spectrum) and for the metallic Na (red spectrum). c 2D ²³Na MRI of metallic (red) and dielectric (blue) sodium in the cell. The dashed lines indicate the positions of the sodium metal CE and carbon WE. d 1D ²³Na MR profile of metallic (red) and dielectric (blue) sodium in the cell. This cell was assembled with five separators.