(Invited) Mechanistic Study of Sodium Insertion into Bio-Waste Derived Hard Carbon Anode for Sodium-Ion Batteries
Authors: Moon Hyein , Garcia-Diez Raul , Nie Kaiqi , Wilks Regan , Baer Markus , Hasa Ivana , Passerini Stefano 
Published: December 2020

Sodium-ion-batteries (SIBs) are considered one of the most promising next-generation battery systems. [1] Owing to their potential lower cost and higher sustainability compared to the current lithium-based technology, SIBs have the potential to dominate the future stationary storage market filling the gap between energy production and utilization. [2]
Currently, hard carbon represents the anode of choice for SIBs. Bio-waste derived hard carbons, in particular, are certainly the most attractive among others in terms of sustainability and cost. [3,4]. Recently, there has been a great interest in the fundamental understanding of the sodiation process. Indeed, several mechanisms have been proposed, however, the unresolved nature of the hard carbon structure hinders a comprehensive description of the sodiation mechanism. [3] Nonetheless, the understanding of the Na uptake and release is crucial for a rational design of improved hard carbon anodes. The most commonly detected sodiation processes have been identified as:ⅰ) inter-lamellae Na-ion intercalation, ⅱ) Na-ion ad-/chemisorption at defects in the graphenic sheets, ⅲ) Nano-clustering of quasi-metallic Na on micropores, and ⅳ) Solid-electrolyte-interphase (SEI) formation, including the related irreversible Na trapping. Besides, their state of charge dependence upon cycling is still widely discussed. [3]
In this study, we attempt to elucidate the sodiation processes in a peanut shell-derived hard carbon anode. X-ray based techniques and vibrational spectroscopies are used to probe and clarify the structure/function correlation and the potential dependence of the sodiation processes. In-situ X-ray diffraction (XRD) is performed to evaluate the effect of Na ion intercalation on the interlayer distance among graphenic sheets. The carbon structure changes occurring in the electrode’s bulk and surface a upon sodiation are investigated by in-situ Raman spectroscopy. X-ray absorption (XAS), emission (XES), and photoelectron (XPS) spectroscopies in the soft and tender X-ray regimes are employed to reveal the formation and evolution of the SEI upon cycling, complemented by ex-situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy to identify the chemical species generated. Additionally, resonant inelastic X-ray scattering (RIXS) measurements at the C K-edge provide supplementary information about the Na-insertion mechanism into the anode structure due to its excellent atom-specificity and mapping of the C band structure at different (dis)charge stages.
Overall, a direct relation between structural properties of hard carbon and its sodiation mechanism is proposed, paving the way for a rational design of high-performance hard carbon anodes.

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