Studying the growth and morphology of metal microstructures in sodium metal batteries with ionic liquid electrolytes by operando 23Na NMR spectroscopy

Sodium metal batteries are a potentially promising energy storage solution due to the abundance and low cost of sodium and high energy density of metal anodes. However, the growth of microstructural metal during cycling remains a crucial safety and performance issue to be addressed. Herein we investigate this process in sodium metal symmetric cells using operando 23 Na nuclear magnetic resonance spectroscopy. We analyse the NMR signal arising from the microstructural sodium metal to obtain quantitative and qualitative information on its growth and morphology, observing significant differences in the evolution of the microstructures for different electrolytes, including ionic liquids with different sodium salt concentrations, water as an additive, and comparing with a more traditional organic electrolyte. The results correlate well with the previously reported cycling performance of these electrolytes, as well as ex situ SEM images of the electrode surface after cycling. • Operando 23 Na NMR provides information on microstructure growth in sodium metal batteries with ionic liquid electrolytes. • Spectra provide quantitative information on the microstructure and qualitative information on morphology. • Beneficial impact of high sodium concentration and water as an additive in ionic liquid electrolytes.