Abstract
Sodium-ion batteries are a promising battery technology for their cost and sustainability. This has led to increasing interest in the development of new sodium-ion batteries and new analytical methods to non-invasively, directly visualise battery chemistry. Here we report operando H-1 and Na-23 nuclear magnetic resonance spectroscopy and imaging experiments to observe the speciation and distribution of sodium in the electrode and electrolyte during sodiation and desodiation of hard carbon in a sodium metal cell and a sodium-ion full-cell configuration. The evolution of the hard carbon sodiation and subsequent formation and evolution of sodium dendrites, upon over-sodiation of the hard carbon, are observed and mapped by Na-23 nuclear magnetic resonance spectroscopy and imaging, and their three-dimensional microstructure visualised by H-1 magnetic resonance imaging. We also observe, for the first time, the formation of metallic sodium species on hard carbon upon first charge (formation) in a full-cell configuration. Na-ion batteries offer multiple advantages, but there is a critical need for improved materials and understanding of sodiation mechanisms. Here the authors deploy operando Na-23 magnetic resonance imaging and spectroscopy to observe sodium battery chemistry and dendrite formation, enabling new insight.