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Understanding the working principles and degradation mechanisms of battery active materials at the atomic scale is important for developing high-performing, long-lasting batteries. In the past decade, operando characterization has emerged as a powerful method to understand chemical changes during battery operation, providing dynamic insights into reaction mechanisms under realistic conditions. This approach helps illuminate potential degradation pathways and guides strategies for optimizing battery materials, including the recovery of materials that are approaching their end-of-life.
This Thesis investigates the operating mechanisms of negative electrode (i.e., anode) materials for Li-ion batteries (LIBs) and Na-ion batteries (NIBs) using operando X-ray-based methods, including X-ray diffraction (XRD) and X-ray Absorption Fine Structure (XAFS) spectroscopy. The first part focuses on the relaxation mechanisms in graphite anodes for LIBs, employing synchrotron operando XRD with a time resolution of 1.24 s. The relaxation process in a partially lithiated graphite was found to be driven by structural reorganization of Li-ions into Li-clusters and -islands. A complementary operando XRD relaxation study of LiFePO4 (LFP) electrodes was also conducted, and it was found that the relaxation mechanism in LFP was considerably slower compared to that in graphite but could also be attributed to a reorganization of Li-ions. These findings emphasize the importance of operando structural studies for accurately interpreting reaction mechanisms in battery materials.
The main part of this Thesis focuses on the (de)sodiation mechanisms of Sb2X3 (X = S, Se, Te) anode materials for NIBs. The electrochemical behavior of these materials is very similar, with their Na-storage mechanisms characterized by a combination of intercalation-conversion/alloying processes that form multiple phases. A detailed mechanistic study of Sb2Se3-based anodes was conducted using operando XRD and XAFS, coupled with post-cycling (ex situ) XRD, XAFS, solid-state nuclear magnetic resonance (ssNMR), and scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy. Two aspects of the (de)sodiation mechanism of Sb2Se3-based anodes were of particular interest: firstly, the reversibility of the conversion reaction, and secondly, the evolution of the Na—Se matrix compound during cycling. Upon investigating of the (de)sodiation mechanism of Sb2Se3-based anodes, the mechanism can be determined and explained through a combination of intercalation-conversion/alloying reactions forming NaxSb2Se3 (product after intercalation), Na2Se (inactive matrix after conversion) and Sb (active part after conversion), and NaxSb (product after alloying reaction). A completely new sodiated phase was discovered in operando mode – namely a ternary compound corresponding to Na5-xSbSe (sodium antimonide(3-) selenide) as the final sodiated product. Na5-xSbSe crystallizes in the tetragonal space group I4/mmm and displays an exceptional structural flexibility in accommodating/releasing Na-ions that was attributed to high inherent vacancy concentration and a disordered structurally flexible Sb—Se sublattice. This disorder creates a stable framework for fast Na diffusion that enables the fast-charging properties of Sb2Se3-based anodes. It was found that Sb2Se3-based anodes can be (de)sodiated at 5000 mA g-1 with minimal capacity losses. A complimentary operando XRD measurement of Sb2Te3-based anodes revealed the formation of a Na—Sb—Te compound as the final sodiation product, likely isostructural to Na5-xSbSe. These operando observations suggested that unique (de)sodiation pathways occur for Sb2X3 (X = Se, Te), and challenge the conventional understanding of Sb-based conversion/alloying anodes in NIBs.
A composite material, Sb2Se3/Sb, was studied via operando XRD at various cycling rates (100 mA g-1, 335 mA g-1, and 1000 mA g-1). The mechanism for this composite could be described through a combination of the respective (de)sodiation mechanisms Sb and Sb2Se3, forming Na3Sb and Na5-xSbSe at the final products of sodiation. The findings indicate that the high-rate performance of Sb2Se3-based materials is facilitated by amorphization of Na5-xSbSe after the first electrochemical cycle that further benefits rapid Na-diffusion. Despite the promising high-rate behavior observed for Sb2Se3-based anodes in NIBs (up to 5000 mAh g-1), the long-term cycling stability was limited to ~120 (de)sodiation cycles. Post-cycling SEM-EDX analysis of the cross-sections of cycled Sb2Se3/Sb-based electrodes suggested that the main pathway of degradation was caused by electrode architecture failure, such as delamination of the coating from the current collector.
Central to this work was also to investigate the promise of Sb2Se3-based materials under more realistic conditions, i.e., full-cell configurations. Sb2Se3-based anodes coupled with Prussian white (PW) cathode materials displayed promising behavior over 100 (de)sodiation cycles. Operando XRD revealed that the (de)sodiation mechanism of Sb2Se3-based anodes remained the same in full-cells as in half-cells, forming Na5-xSbSe. Presodiation strategies through direct-contact and electrochemical methods were explored in order to stabilize the capacity fade of Sb2Se3-based full-cells. The results illustrated that presodiation strategies might become for the successful implementation of high-capacity, and stable, Sb2Se3-based anodes in practical NIB applications.
