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 1H and 23Na 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 23Na nuclear magnetic resonance spectroscopy and imaging, and their three-dimensional microstructure visualised by 1H 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.
Highlights
Sodium-ion batteries are a promising battery technology for their cost and sustainability
In situ 23Na nuclear magnetic resonance (NMR) spectroscopy has been able to quantify the deposition of microstructural Na on metallic anodes[10] and infer the chemical environment—and storage mechanisms—in non-graphitic hard carbon anodes13. 23Na NMR shows a large chemical shift difference for the metallic, compared to solvated, sodium in the electrolyte, which arises from the Knight shift of the NMR signal[10]
A schematic diagram of the sodium metal cell is shown in Fig. 1a, along with 23Na NMR spectra, 24 s. Twodimensional (2D) images and 1D profiles for a pristine cell
Summary
Sodium-ion batteries are a promising battery technology for their cost and sustainability. Operando solid-state NMR studies of hard carbons[13] have shown 23Na NMR signals attributed to both Na metal and Na+ in the electrolyte, but have shown an additional peak at −40 ppm, which shifts to +600 ppm during the low-potential (0.1 V) plateau This additional peak has been attributed to Na+ adsorbed at defect sites in the carbon, which subsequently moves downfield due to intercalation and “pooling” of sodium between graphene layers[13]. Comparison of these spectra with ex situ magic angle spinning (MAS) NMR has revealed extreme sensitivity to trace air/moisture, and the peak associated with Na in the carbon has been found to disappear over a few hours[13], which explains why it has been observed in some studies[16], but not others[17]. We observe, to our knowledge 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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