Fig. 3. Evolution of ²³Na NMR signal at the start and after continued galvanostatic plating.

a, d ²³Na NMR spectra for solvated Na in the electrolyte (blue line) and for metallic Na (red line), after discharge at a specific current of 150 mA g⁻¹ (a) and after discharge at a specific current of 600 mA g⁻¹ (d). The corresponding spectra for the cell following a single charge cycle (Fig. 2a) are included for comparison as grey lines. b, e 2D ²³Na MRI of metallic (red) and dielectric (blue) sodium in the cell, after discharge at a specific current 150 mA g⁻¹ (b) and after discharge at a specific current 600 mA g⁻¹ (e). c, f 1D ²³Na MR profile of metallic and dielectric sodium in the cell, after discharge at a specific current 150 mA g⁻¹ (c) and after discharge at a specific current 600 mA g⁻¹ (f). This cell was assembled with two separators.