Amalie Skurtveit

• Jr, Andrew Pastusic; Skurtveit, Amalie & Koposov, Alexey Y. (2026). Comparative Study of Bi2S3 and Sb2S3 as Conversion/Alloying Anodes for Na-Ion Batteries. ACS Applied Energy Materials. doi: 10.1021/acsaem.6c02364. Full text in Research Archive Show summary

  Bi- and Sb-based materials have gained a significant interest as anode materials for Na-ion batteries. Bi2S3 and Sb2S3 are isostructural and isovalent chalcogenides that should undergo similar conversion/alloying reaction with the Na-ion when placed in a battery. However, the effect of cation (Bi vs Sb), particle morphology, and electrolyte environment on (de)sodiation mechanisms and cycling stability remains poorly understood. Herein, a systematic study of several morphologies of Bi2S3 and Sb2S3 in two electrolyte systems for Na-ion batteries is presented. Bi2S3 exhibited distinct electrochemical plateaus associated with crystalline-crystalline phase transitions, clearly resolved by operando and ex situ X-ray diffraction, following a conventional Bi-based conversion/alloying pathway via Na3Bi and Na2S during sodiation and a back-conversion reaction culminating in NaBiS2 at the end of desodiation. In contrast, Sb2S3 rapidly lost long-range order and cycled predominantly in the amorphous regime. Ex situ total X-ray scattering and X-ray absorption spectroscopy revealed that the (de)sodiation involved the formation and rearrangements of amorphous Na–Sb–S networks approximated by NaSbS0.5, Na2S4, and ternary Na5–xSbS. In both systems, ether-based electrolytes improved the cyclability relative to carbonate-based electrolytes regardless of particle morphology and size, revealing that electrolyte choice plays a decisive role in controlling the performance of conversion/alloying anodes.

• Skurtveit, Amalie; North, Eira Tiberg; Berdiell, Izar Capel; Beek, Wouter Van; Wragg, David S. & Koposov, Alexey (2026). Operando X‐Ray Diffraction and Total Scattering Characterization of Battery Materials: Not Just a Pretty Picture. Advanced Energy Materials. ISSN 1614-6832. doi: 10.1002/aenm.202506777. Full text in Research Archive Show summary

  The development of rechargeable batteries can be accelerated by the fundamental understanding of their chemistries. Multiple characterization techniques have been deployed to study the chemical processes in batteries, but structural studies at the atomic level using X‐ray diffraction (XRD) and total X‐ray scattering (TXS) show enduring popularity. These methods are relatively easy to apply in operando mode, where structural data is collected alongside electrochemical data under operating conditions. With a growing interest in this powerful methodology, it is important to review the general principles of operando XRD/TXS characterization and develop a set of guidelines for the collection, analysis, and presentation of data that are acceptable to the community and can lead to reliable, representative, and reproducible results that are easy to judge and compare. This Review aims to survey and discuss important steps in operando XRD and TXS. From cell and electrode design to data treatment, we survey the major experimental aspects of operando studies, pointing out potential pitfalls and identifying best‐practice. In addition, we review some unconventional experiments conducted in operando mode to illustrate how such methods can be used beyond studies of the early‐stage cycling mechanisms of battery active materials.

• Skurtveit, Amalie; Maddar, Faduma M.; Hasa, Ivana; Cavallo, Carmen; Wragg, David Stephen & Koposov, Alexey (2025). Origin of High-Rate Performance of Sb2Se3/Sb Composite Anodes for Na-Ion Batteries. ACS Applied Energy Materials. 8(19), p. 14058–16064. doi: 10.1021/acsaem.5c02418. Full text in Research Archive Show summary

  Improving the energy density of Na-ion batteries (NIBs) requires advances in anode materials. This study investigates Sb2Se3/Sb composite anodes, which offer better capacity and stability than the individual constituents. Operando X-ray diffraction (XRD) revealed that (de)sodiation is driven by synergies between the operating mechanisms of Sb2Se3 and Sb, enabling a reversible capacity of 360 mAh g–1 (at 1000 mA g–1). This performance is also aided by amorphization of the sodiated phases, which promotes rapid Na-diffusion. The work emphasizes the need for continuous material and structural optimizations to fully harness their benefits for enabling efficient and fast-charging NIBs.

• Skurtveit, Amalie; North, Erlend Tiberg; Park, Heesoo; Chernyshov, Dmitry; Wragg, David Stephen & Koposov, Alexey (2025). Stepwise Structural Relaxation in Battery Active Materials. ACS Materials Letters. 7(1), p. 343–349. doi: 10.1021/acsmaterialslett.4c02058. Full text in Research Archive Show summary

  Whenever the cycling of Li-ion batteries is stopped, the electrode materials undergo a relaxation process, but the structural changes that occur during relaxation are not well-understood. We have used operando synchrotron X-ray diffraction with a time resolution of 1.24 s to observe the structural changes that occur when the lithiation of graphite and LiFePO4 electrodes are interrupted. Assessing the kinetics of the relaxation processes coupled with molecular dynamics simulations allows us to identify three relaxation stages in graphite. The atomistic origin for the relaxation process within the partially lithiated graphite structure is driven by the reorganization of Li ions into Li clusters. Relaxation in LiFePO4 electrodes is considerably slower than for graphite, but the observed structural changes is also attributed to reorganization of Li ions. These insights highlight the nature of the structural changes that occur during relaxation and the importance of using operando structural studies to avoid misleading conclusions about the reaction mechanisms in battery materials.

• Skurtveit, Amalie; Jr, Andrew Pastusic; Brennhagen, Anders; Maddar, Faduma M.; Mohn, Chris Erik & Karmakar, Abhoy [Show all 13 contributors for this article] (2025). Unique Na5−xSbSe Phase Enables High-Rate Performance of Sb2Se3 Anodes in Na-Ion Batteries. Advanced Energy Materials. ISSN 1614-6832. 15(31), p. 1–16. doi: 10.1002/aenm.202501433. Full text in Research Archive Show summary

  Na-ion batteries (NIBs) need new anode materials to improve energy density. Metal chalcogenides, such as Sb2Se3, represent a promising alternative to commonly used hard carbon materials, demonstrating high-rate performance up to 5 A g−1 with minimal capacity losses. However, Sb2Se3 is believed to operate under the conversion/alloying mechanism, typically linked with large structural transformations and volumetric changes—quite contrary to its performance. Herein, by combining multiple operando techniques and atomistic simulations, a new fully sodiated phase, Na5−xSbSe, is unambiguously revealed as the origin of the high-rate performance of Sb2Se3. Na5−xSbSe is stable within 0.01–0.80 V versus Na/Na+ and crystallizes in I4/mmm. The remarkable structural flexibility of Na5SbSe to changes in Na-content allows the anode to be (de)sodiated with minimal volumetric changes (≈3.4%). This unique “breathing effect” is intimately linked to high inherent vacancy concentration, disordered, and structurally flexible anion sublattice, providing a stable framework for fast Na diffusion, contributing to the fast-charging properties of Sb2Se3. The study showcases the power of operando methods for discovering new phases that are hidden in the mechanistic paths of well-studied reactions and underlines the intertwined nature of various characterization methods assisted by atomistic insights for a comprehensive understanding of complex (de)sodiation mechanisms.

• Albenga, Carla; Gott, James A.; Skurtveit, Amalie; Warnett, Jason M.; Maddar, Faduma M. & Koposov, Alexey [Show all 9 contributors for this article] (2025). Assessing the role of morphological changes as the origin of improved cycling stability of Sn-based anodes for sodium-ion batteries. Journal of Materials Chemistry A. ISSN 2050-7488. 13(37), p. 30967–30984. doi: 10.1039/D5TA03000E. Full text in Research Archive Show summary

  Tin (Sn) is a promising anode material for sodium-ion batteries (NIBs) due to its high theoretical capacity, good conductivity and higher density compared to hard carbon, which could significantly increase energy density. However, the substantial volume changes Sn undergoes during (de-)sodiation typically result in mechanical degradation and unstable solid-electrolyte interphase (SEI). While recent research has reported unexpectedly stable cycling of micrometric Sn anodes in ether-based electrolytes, the mechanisms behind this performance remain underexplored. In this work, we investigate the influence of both Sn content and particle size on the structural and electrochemical stability of Sn-based electrodes. Contrary to expectations, increasing the Sn content does not compromise the cycling stability. Instead, higher Sn loading facilitates the formation of a robust bicontinuous porous architecture, known as coral-like structure, which enhances electrode resilience by distributing mechanical stress and accommodating volumetric expansion. This morphology is particularly prevalent in Sn-rich electrodes, where sufficient Sn enables extended structural connectivity during cycling. Interestingly, smaller Sn particles (submicron and nanoscale) result in poorer cycling performance, underscoring the importance of both particle size and network formation in achieving mechanical robustness. Moreover, we demonstrate that the development and propagation of the coral-like framework are contingent not only on the initial Sn content but also on the degree of sodiation. While Sn-rich anodes cycled in half-cells achieve a fully sodiated crystalline phase that supports dense network formation, in full-cell configurations the full sodiation is not achieved, leading to the formation of a less dense and continuous porous structures that are more susceptible to cracking. These insights highlight the need for full-cell optimization to promote complete sodiation and unlock the full potential of Sn-based anodes in practical NIB applications with higher energy density.

• Brennhagen, Anders; Skurtveit, Amalie; Wragg, David Stephen; Cavallo, Carmen; Sjåstad, Anja Olafsen & Koposov, Alexey [Show all 7 contributors for this article] (2024). (De)sodiation Mechanism of Bi2MoO6 in Na-Ion Batteries Probed by Quasi-Simultaneous Operando PDF and XAS. Chemistry of Materials. ISSN 0897-4756. 36(15), p. 7514–7524. doi: 10.1021/acs.chemmater.4c01503. Full text in Research Archive Show summary

  Operando characterization can reveal degradation processes in battery materials and are essential for the development of battery chemistries. This study reports the first use of quasi-simultaneous operando pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) of a battery cell, providing a detailed, atomic-level understanding of the cycling mechanism of Bi2MoO6 as an anode material for Na-ion batteries. This material cycles via a combined conversion-alloying reaction, where electrochemically active, nanocrystalline NaxBi particles embedded in an amorphous Na–Mo–O matrix are formed during the first sodiation. The combination of operando PDF and XAS revealed that Bi obtains a positive oxidation state at the end of desodiation, due to formation of Bi–O bonds at the interface between the Bi particles and the Na–Mo–O matrix. In addition, XAS confirmed that Mo has an average oxidation state of +6 throughout the (de)sodiation process and, thus, does not contribute to the capacity. However, the local environment of Mo6+ changes from tetrahedral coordination in the desodiated state to distorted octahedral in the sodiated state. These structural changes are linked to the poor cycling stability of Bi2MoO6, as flexibility of this matrix allows movement and coalescence of the NaxBi particles, which is detrimental to the electrochemical stability.

• Skurtveit, Amalie; Brennhagen, Anders; Park, Heesoo; Cavallo, Carmen & Koposov, Alexey (2022). Benefits and Development Challenges for Conversion-Alloying Anode Materials in Na-Ion Batteries. Frontiers in Energy Research. 10. doi: 10.3389/fenrg.2022.897755. Full text in Research Archive Show summary

  Over the past decade, Na-ion batteries (NIBs) have gained a substantial interest within the research community and relevant industry. NIBs are now emerging as a cost-effective and sustainable alternative to modern Li-ion batteries (LIBs). Similar to the parent LIB technology, NIB requires a new set of materials, which can boost battery capacity without sacrificing cycling stability, rate capabilities, and other performance targets. In NIB chemistry, anodes have received less attention compared to cathode chemistry, leaving hard carbon as a primary anode material, although its intercalation/adsorption mechanism limits the allowed number of Na-ions. Promising alternative groups of anodes are materials that undergo the combined conversion and alloying reactions (i.e., conversion-alloying anodes), due to the beneficial high theoretical capacity and good cycling stability. The conversion reaction in conversion-alloying anodes can be either reversible or irreversible, each possessing its advantages. However, the complexity of their operating mechanism(s) severely impedes their development. The present mini-review provides a survey of the recent developments of conversion-alloying-type anode materials for Na-ion batteries discussed in the context of their operation mechanism(s). Considering the chemical complexity of the conversion-alloying materials, the suggestions and guidance on characterization are provided along with theoretical considerations.

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• Skurtveit, Amalie; Berdiell, Izar Capel; Khairnar, Abhishek; Amrheim, Simon; Peters, Simone & Gottlinger, Mara [Show all 9 contributors for this article] (2026). Inorganic Coatings of Sulfide-Based Solid Electrolytes Through Atomic Layer Deposition. Full text in Research Archive Show summary

  Solid-state batteries (SSBs) are regarded as a promising next-generation energy storage technology, offering a safer, more sustainable, and potentially cost-effective alternative compared to conventional Li-ion batteries (LIBs). The key component for high-performing SSBs is the solid-state electrolyte (SSE) with targeted properties (such as electrochemical stability window, performance, and safety). However, introduction of SSE also brings new challenges related to ionic conductivity, processability, recyclability, and the interface between the active material and the SSE that needs to be carefully controlled. These challenges are addressed with the FUNCY-SSB (FUNctional Coatings of sulfide electrolYtes for lithium Solid-State Batteries) project.

• Skurtveit, Amalie; Jr, Andrew Pastusic; Brennhagen, Anders; Maddar, Faduma M.; Mohn, Chris Erik & Karmakar, Abhoy [Show all 13 contributors for this article] (2026). Hidden Na5SbSe Unlocks Sb2Se3 Anodes for Na-Ion Batteries. Full text in Research Archive Show summary

  Antimony selenide (Sb2Se3) is a promising alternative to commonly used NIB anodes, storing 12 Na+ per formula unit, to achieve a capacity of 670 mAh g-1. The (de)sodiation mechanism is very complex: the material progresses through multiple phases when interacting with Na+, including intercalation, con-version, and alloying reactions. These multiple chemical transformations may kinetically limit the material at high rates, however, Sb2Se3 can deliver almost 150 mAh g-1 at 5000 mA g-1. Understanding this unusual behavior is important not only to rationalize the electrochemical performance, but also for the design of other materials capable of working at high (dis)charge rates. Meanwhile, subtle phase transitions or transformations of active battery materials can be traced in operando X-ray diffraction (XRD), which enables the extraction of real-time information about potentially hidden mechanistic pathways, including the discovery of new materials. In this study, Na5SbSe was discovered as the main sodiated product of Sb2Se3 anodes. Na5SbSe is predominantly composed of Na-ions and has a highly flexible composition (breathing effect) that unlocks the exceptional high rate performance of Sb2Se3 anodes

• Skurtveit, Amalie; Jr, Andrew Pastusic; Brennhagen, Anders; Maddar, Faduma M.; Mohn, Chris Erik & Karmakar, Abhoy [Show all 13 contributors for this article] (2026). Material's Discovery via Synchrotron Operando XRD and XAS: the Case of Sb2Se3 Anodes for Na-Ion Batteries. Full text in Research Archive Show summary

  Antimony selenide (Sb2Se3) is a promising alternative to commonly used NIB anodes, storing 12 Na+ per formula unit, to achieve a capacity of 670 mAh g-1. The (de)sodiation mechanism is very complex: the material progresses through multiple phases when interacting with Na+, including intercalation, con-version, and alloying reactions. These multiple chemical transformations may kinetically limit the material at high rates, however, Sb2Se3 can deliver almost 150 mAh g-1 at 5000 mA g-1. Understanding this unusual behavior is important not only to rationalize the electrochemical performance, but also for the design of other materials capable of working at high (dis)charge rates. Meanwhile, subtle phase transitions or transformations of active battery materials can be traced in operando X-ray diffraction (XRD), which enables the extraction of real-time information about potentially hidden mechanistic pathways, including the discovery of new materials. In this study, operando XRD and X-ray absorption spectroscopy (XAS) were employed to unravel the mysteries of the (de)sodiation mechanism of Sb2Se3 anodes. Complete elucidation of the reaction pathway was further enabled by solid-state NMR, ex situ work, and ab initio simulations.

• Espeland, Lovise & Skurtveit, Amalie (2025). Alver, Holsnøy | Superbatteri: Fløksand-forskar vekkjer internasjonal oppsikt. [Journal]. Avisa Nordhordland. Full text in Research Archive Show summary

  Amalie Skurtveit frå Fløksand har akkurat fullført ei doktorgrad i kjemi ved Universitetet i Oslo.

• Melteig, Elina & Skurtveit, Amalie (2025). Oppdaget et nytt batteri på grunn av magefølelsen. Det viste seg å lade 30 ganger raskere. - Titan.uio.no. [Internet]. Titan.uio.no. Full text in Research Archive Show summary

  Noen ganger kan det lønne seg å følge nysgjerrigheten. Da Amalie Skurtveit gjorde det, fant hun et materiale som kan bli et superbatteri.

• Skurtveit, Amalie (2024). Elucidating Reaction Mechanisms in Battery Materials through Advanced Characterization: from Relaxation of Lithium ions to Fast Charging Sodium-ion Batteries. Full text in Research Archive
• Brennhagen, Anders; Skurtveit, Amalie; Skautvedt, Casper; Cavallo, Carmen; Wragg, David Stephen & Vajeeston, Ponniah [Show all 9 contributors for this article] (2024). Revealing the (de)sodiation mechanisms of Bi-metallates through operando X-ray characterisation. Full text in Research Archive Show summary

  Na-ion batteries is entering the battery market as an alternative to Li-ion batteries. To improve their overall performance it is crucial to develop new types of anode materials with high capacity and long cycle life. Materials combing conversion and alloying mechanisms (CAMs) are promising anodes with their high capacity, but obtaining good cycling stability is still challenging. A comprehensive understanding of the cycling and degradation mechanism of these materials is crucial to improve their performance. Bi-metallates, with a general formula of Bi−TM−O (TM = transition metal) is a group of ternary CAMs. Their general cycling mechanism consists of an irreversible conversion reaction forming nanoparticles of Bi-metal embedded in a Na-TM-O matrix during the first sodiation, followed by a reversible two-step alloying reaction forming Na3Bi with NaBi as an intermediate phase. In this work, we have used operando X-ray diffraction (XRD), pair distribution function (PDF) analysis and X-ray absorption spectroscopy (XAS) to investigate the desodiation mechanisms of Bi2MoO6 and BiFeO3. Through this work, we discovered that Bi2MoO6 forms the cubic version of Na3Bi (c-Na3Bi) while BiFeO3 forms hexagonal Na3Bi (h-Na3Bi) in addition to c-Na3Bi during the first sodiation. In the desodiated state, the Bi-particles are partially oxidised, while still maintaining the Bi-metal structure, indicating that it is only the Bi atoms at the interface between the Bi nanoparticles and the Na−TM−O matrix that is oxidised. During cycling the NaxBi particles grow larger thus increasing the distance between them and increasing the impedance in the material. This is considered to be the main driver for the capacity degradation that was observed during the first 20 cycles. The operando XAS data also revealed that Mo6+ in Bi2MoO6 does not change oxidation state during cycling, but changes coordination between tetrahedral and distorted octahedral coordination during cycling. The cycling and degradation mechanisms of Bi2MoO6 is summarised in Figure 1.

• Skurtveit, Amalie; North, Erlend Tiberg; Park, Heesoo; Chernyshov, Dmitry; Wragg, David Stephen & Koposov, Alexey (2024). Elucidating relaxation mechanisms in Li-ion batteries through operando X-ray diffraction: Spontaneous Li-ion clustering as a structural origin of relaxation in battery electrodes. Full text in Research Archive
• Brennhagen, Anders; Skautvedt, Casper; Cavallo, Carmen; Skurtveit, Amalie; Wragg, David Stephen & Koposov, Alexey [Show all 9 contributors for this article] (2023). Revealing the Cycling and Degradation Mechanism of Bi-Metallates as Anode Materials fro Na-ion Batteries. Full text in Research Archive Show summary

  Na-ion batteries (NIBs) represent an emerging alternative to modern Li-ion batteries. To improve the overall performance of NIBs it is crucial to develop new types of anode materials with high capacity and long cycle life. Among the anode candidates, materials operating under combined conversion and alloying mechanisms (CAMs) are considered as promising. They are capable of delivering high capacity, however, despite theoretical predictions the cycling stability is still below expectations. Therefore, a comprehensive understanding of the cycling and degradation mechanisms of CAMs is crucial to improve their performance. However, these materials undergo multiple chemical transformations, which often involves formation of amorphous and nanocrystalline phases during cycling. Such chemical complexity substantially impedes their full characterization and understanding. Bismuth metallates, with a general formula of Bi-TM-O (TM = transition metal), is a group of ternary CAMs. Their general cycling mechanism consists of an irreversible conversion reaction, which leads to formation of Bi nanoparticles embedded in a Na-TM-O matrix during the first sodiation. This is followed by a reversible two-step alloying reaction leading to formation of Na3Bi with NaBi as an intermediate phase. In this work, we have used operando X-ray diffraction over the course of ∼30 cycles to study the cycling and degradation mechanisms of two CAMs - Bi2MoO6 and BiFeO3. With the help of ex situ X-ray absorption spectroscopy and pair distribution function (PDF) analysis, we have revealed new insights into these materials in addition to confirming the general cycling mechanism. Both materials form the cubic phase of Na3Bi (c-Na3Bi) in the sodiated state with some noticeable differences. While, the hexagonal Na3Bi (h-Na3Bi) phase starts to form after 10 cycles in Bi2MoO6, a combination of c-Na3Bi and h-Na3Bi can be observed after the first cycle in BiFeO3. The formation of c-Na3Bi is kinetically favorable in these systems, while h-Na3Bi is the more thermodynamically stable and, therefore, forms later in the cycling process. Both materials shows three distinct regions of capacity degradation, where the first is initiated by a growth in the crystallite sizes of Bi-particles. This is followed by a disappearance of the Bi-phase as the second step. The last degradation phase is related to the disappearance of the NaBi-phase and locking the system in the sodiated state. The present talk will discuss the mechanisms of these degradation steps in view of the future development of CAMs in greater details.

• Skurtveit, Amalie; Pastusic, Andrew; Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen & Koposov, Alexey (2023). High-Rate Performance of Antimony Chalcogenides (Sb2X3) Studied by Operando X-ray Diffraction (XRD). Full text in Research Archive
• Andersen, Elisabeth Kirkeng & Skurtveit, Amalie (2022). Blir nesten leit å dra herfra. [Journal]. Skolesamarbeidet. Full text in Research Archive
• Skurtveit, Amalie; Cavallo, Carmen; Wragg, David Stephen & Koposov, Alexey (2022). Revealing the (de)sodiation mechanism of antimony chalcogenides Sb2X3 with operando XRD. Full text in Research Archive
• Skurtveit, Amalie & Koposov, Alexey (2022). Revealing the (de)sodiation mechanism in antimony chalcogenides Sb2X3 through operando XRD. Full text in Research Archive
• Brennhagen, Anders & Skurtveit, Amalie (2022). Anerkjennelse for UiO-studenter på batterikonferanse. [Internet]. uio.no. Full text in Research Archive
• Skurtveit, Amalie; Koposov, Alexey; Wragg, David Stephen & Cavallo, Carmen (2025). Operando Characterization of Anode Materials for Li/Na-ion Batteries. Universitetet i Oslo. Full text in Research Archive Show summary

  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.

• Pastusic, Andrew; Koposov, Alexey & Skurtveit, Amalie (2024). Bismuth Chalcogenides as conversion/alloying anode materials in Na-ion batteries. Kjemisk Institutt, Universitetet i Oslo. Full text in Research Archive

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