Long‐Term Cycling Stability of Sodium/Sodium Ion Cells Probed by In Situ Solid‐State NMR Spectroscopy

Sodium‐ion batteries are at the forefront of new, sustainable energy systems required for the global energy transition. 23 Na in situ solid‐state nuclear magnetic resonance spectroscopy is capable of unraveling structures in working electrochemical cells during the charging and discharging processes. To evaluate its suitability for long‐term studies, local sodium environments in sodium/sodium ion cells based on silicon carbonitride and hard carbon materials are tracked for up to 49 cycles (228.5 h). The formation of dendrites as well as the decay of a secondary metallic sodium species is observed, and local structures are analyzed up to the point of capacity degradation and cell failure. Initial points of cell breakdown are reflected in the NMR data by characteristic changes in signal intensities, whereas the degradation of the cells is reflected by a cease to periodic signal intensity fluctuations. Meanwhile, ex situ 23 Na NMR spectra of the deactivated cells reveal a complex range of environments for sodium ions.