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 1 H and 23 Na 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 23 Na nuclear magnetic resonance spectroscopy and imaging, and their threedimensional microstructure visualised by 1 H 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.

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An increase in the demand for clean energy generation has brought about a need for improved energy storage devices to distribute the power generated by renewable sources. Sodium-ion batteries have been proposed as a lower cost and more sustainable solution. In order to commercialise this emerging technology, new materials and characterisation techniques are required. Magnetic resonance imaging (MRI) is able to characterise both the electrode and electrolyte materials non-invasively, providing a more holistic view of battery chemistry. Furthermore, MRI can spatially resolve electrochemical processes, in real-time, within cells.(1-4) In this study, 23Na and 1H nuclear magnetic resonance (NMR) spectroscopy and MRI experiments have been performed, in operando, on sodium metal cells. At high cycle rates, the growth of dendrites is observed during galvanostatic plating. Operando 23Na NMR experiments on sodium full cells have also been performed, which provide unprecedented insight into the sodiation mechanisms within a hard carbon anode. The formation of metallic and quasimetallic sodium species are observed during the first charge cycle.(5) M. M. Britton, Prog. Nucl. Magn. Reson. Spectrosc., 101, 51 (2017). M. M. Britton, ChemPhysChem, 15, 1731 (2014). A. J. Davenport, M. Forsyth and M. M. Britton, Electrochem. Commun., 12, 44 (2010). J. M. Stratford, P. K. Allan, O. Pecher, P. A. Chater and C. P. Grey, Chem. Commun. (Cambridge, U.K.), 52, 12430 (2016). J. M. Bray, C. L. Doswell, G. E. Pavlovskaya, L. Chen, B. Kishore, H. Au, H. Alptekin, E. Kendrick, M. M. Titirici, T. Meersmann and M. M. Britton, Nat. Commun., 11, 2083 (2020). Figure 1

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