Casper Skautvedt

• Brennhagen, Anders; Skautvedt, Casper; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2024). Unraveling the (De)sodiation Mechanisms of BiFeO<inf>3</inf> at a High Rate with Operando XRD. ACS Applied Materials & Interfaces. ISSN 1944-8244. 16(10), s. 12428–12436. doi: 10.1021/acsami.3c17296. Fulltekst i vitenarkiv Vis sammendrag

  Development of new anode materials for Na-ion batteries strongly depends on a detailed understanding of their cycling mechanism. Due to instrumental limitations, the majority of mechanistic studies focus on operando materials’ characterization at low cycling rates. In this work, we evaluate and compare the (de)sodiation mechanisms of BiFeO3 in Na-ion batteries at different current densities using operando X-ray diffraction (XRD) and ex situ X-ray absorption spectroscopy (XAS). BiFeO3 is a conversion-alloying anode material with a high initial sodiation capacity of ∼600 mAh g–1, when cycled at 0.1 A g–1. It does not change its performance or cycling mechanism, except for minor losses in capacity, when the current density is increased to 1 A g–1. In addition, operando XRD characterization carried out over multiple cycles shows that the Bi ⇋ NaBi (de)alloying reaction and the oxidation of Bi at the interface with the Na–Fe–O matrix are detrimental for cycling stability. The isolated NaBi ⇋ Na3Bi reaction is less damaging to the cycling stability of the material.

• Wragg, David Stephen; Skautvedt, Casper; Brennhagen, Anders; Geiss, Carina; Checchia, Stefano & Koposov, Alexey (2023). Tracking Lithiation of Si-Based Anodes in Real Time by Total Scattering Computed Tomography. Journal of Physical Chemistry C. ISSN 1932-7447. 127(48), s. 23149–23155. doi: 10.1021/acs.jpcc.3c06414. Fulltekst i vitenarkiv

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• Brennhagen, Anders; Skurtveit, Amalie; Skautvedt, Casper; Cavallo, Carmen; Wragg, David Stephen & Vajeeston, Ponniah [Vis alle 9 forfattere av denne artikkelen] (2024). Revealing the (de)sodiation mechanisms of Bi-metallates through operando X-ray characterisation. Vis sammendrag

  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.

• Brennhagen, Anders; Skautvedt, Casper; Cavallo, Carmen; Skurtveit, Amalie; Wragg, David Stephen & Koposov, Alexey [Vis alle 9 forfattere av denne artikkelen] (2023). Revealing the Cycling and Degradation Mechanism of Bi-Metallates as Anode Materials fro Na-ion Batteries. Vis sammendrag

  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.

• Skautvedt, Casper; Fjellvåg, Helmer; Koposov, Alexey; Brennhagen, Anders; Cavallo, Carmen & Sjåstad, Anja Olafsen (2022). Syntese og karakterisering av BiFeO3 som anodemateriale i Na-ionebatterier. Universitetet i Oslo. Vis sammendrag

  Na-ionebatterier (NIB-er) er foreløpig en umoden teknologi, men har de siste åra blitt forska på i økende grad. Et stort forskningsfelt innafor NIB-er er på anoden, da det ikke finnes noen som fungerer optimalt. I dette arbeidet har vi sett på BiFeO3 som anodemateriale i NIB-er. Bi-baserte anoder har tidligere blitt undersøkt i forskningsgruppa og de har vist lovende resultater. Valget av BiFeO3 var basert på den interessante mekanismen som skjer under opp- og utladning når den brukes som anode. Mekanismen er en omsettingslegeringsreaksjon og kombinerer omsettings- og legeringsreaksjoner når BiFeO3 natrieres. Vi har undersøkt denne mekanismen i detalj med kombinert røntgendiffraksjon (XRD) og galvanostatisk sykling (GS) med operando-XRD på RECX-labben ved Kjemisk institutt. Som en del av arbeidet blei BiFeO3 syntetisert ved keramisk faststoffsyntese og våtkjemisk sol-gelsyntese. Det har tidligere blitt framstilt med disse syntesene, men har vist seg å være vanskelig å få fasereint. Produkta av syntesene har blitt karakterisert med XRD og forfina med Rietveldmetoden. Den keramiske faststoffsyntesen ga lite utbytte av BiFeO3 og flere ureinheter, men sol-gelsyntesen ga derimot et høyt utbytte av BiFeO3 og få ureinheter. Det blei undersøkt ulike behandlinger av BiFeO3, blant annet kulemølling, syrevasking og doping, for å gi økt ytelse som batterimateriale. Disse testene viste at kulemølling av materialet ga best resultater, det var likevel ikke mulig å finne en optimalisering som unngikk et stort fall i kapasiteten de første 30 syklusene. For å karakterisere BiFeO3 som anodemateriale testa vi det i halvseller mot Na-metall. Disse halvsellene blei elektrokjemisk analysert med GS og syklisk voltammetri (SV). Vi testa også ulike blandinger av elektrodesammensetning og elektrolytter for å optimalisere batteriegenskapene. Disse viste at NaPF6 var det beste saltet og propylenkarbonat (PC) + 5 vt% fluoretylenkarbonat (FEC) ga de mest stabile batterisellene. Dataene fra den elektrokjemiske karakteriseringa og operando-XRD bruktes sammen for å tolke reaksjonsmekanismene til BiFeO3. Vi så ut fra disse at det var likheter i legeringsreaksjonen mellom Na og Bi med andre Bi-baserte anoder, med NaBi og Na3Bi som ulike ledd i reaksjonen. Det var dog ingen indikasjoner på fasen/-e til Fe etter omsettingsreaksjonen.

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