Fig. 5: In operando 23Na NMR spectra and electrochemistry during a formation charge/discharge cycle.

2D intensity plots of 23Na chemical shift, for regions around metallic signal (a) and solvated sodium (b), as a function of time, during the formation charge/discharge cycle at a specific current of 30 mA g−1. c Corresponding charge profile of the formation cycle. Spectra for time points i and ii, indicated in a and b, are shown in Fig. 6b and c, respectively.