Anders Brennhagen

• Brennhagen, Anders; Skautvedt, Casper; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey & Sjåstad, Anja Olafsen [Show all 7 contributors for this article] (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), p. 12428–12436. doi: 10.1021/acsami.3c17296. Full text in Research Archive Show summary

  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), p. 23149–23155. doi: 10.1021/acs.jpcc.3c06414. Full text in Research Archive
• 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. ISSN 2296-598X. 10. doi: 10.3389/fenrg.2022.897755. Full text in Research Archive
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Show all 7 contributors for this article] (2022). Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries. Nanotechnology. ISSN 0957-4484. 33(18). doi: 10.1088/1361-6528/ac4eb5. Full text in Research Archive Show summary

  Based on the same rocking-chair principle as rechargeable Li-ion batteries, Na-ion batteries are promising solutions for energy storage benefiting from low-cost materials comprised of abundant elements. However, despite the mechanistic similarities, Na-ion batteries require a different set of active materials than Li-ion batteries. Bismuth molybdate (Bi2MoO6) is a promising NIB anode material operating through a combined conversion/alloying mechanism. We report an operando x-ray diffraction (XRD) investigation of Bi2MoO6-based anodes over 34 (de)sodiation cycles revealing both basic operating mechanisms and potential pathways for capacity degradation. Irreversible conversion of Bi2MoO6 to Bi nanoparticles occurs through the first sodiation, allowing Bi to reversibly alloy with Na forming the cubic Na3Bi phase. Preliminary electrochemical evaluation in half-cells versus Na metal demonstrated specific capacities for Bi2MoO6 to be close to 300 mAh g−1 during the initial 10 cycles, followed by a rapid capacity decay. Operando XRD characterisation revealed that the increased irreversibility of the sodiation reactions and the formation of hexagonal Na3Bi are the main causes of the capacity loss. This is initiated by an increase in crystallite sizes of the Bi particles accompanied by structural changes in the electronically insulating Na–Mo–O matrix leading to poor conductivity in the electrode. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David; Sottmann, Jonas; Koposov, Alexey & Fjellvåg, Helmer (2021). Understanding the (De)Sodiation Mechanisms in Na‐Based Batteries through Operando X‐Ray Methods. Batteries & Supercaps. ISSN 2566-6223. 4(7), p. 1039–1063. doi: 10.1002/batt.202000294. Full text in Research Archive Show summary

  Progress in the field of Na‐based batteries strongly relies on the development of new advanced materials. However, one of the main challenges of implementing new electrode materials is the understanding of their mechanisms (sodiation/desodiation) during electrochemical cycling. Operando studies provide extremely valuable insights into structural and chemical changes within different battery components during battery operation. The present review offers a critical summary of the operando X‐ray based characterization techniques used to examine the structural and chemical transformations of the active materials in Na‐ion, Na‐air and Na‐sulfur batteries during (de)sodiation. These methods provide structural and electronic information through diffraction, scattering, absorption and imaging or through a combination of these X‐ray‐based techniques. Challenges associated with cell design and data processing are also addressed herein. In addition, the present review provides a perspective on the future opportunities for these powerful techniques.

• Brennhagen, Anders; Kvamme, Kristian Breivik; Sverdlilje, Katja S. S. & Nilsen, Ola (2020). High power iron phosphate cathodes by atomic layer deposition. Solid State Ionics. ISSN 0167-2738. 353. doi: 10.1016/j.ssi.2020.115377. Full text in Research Archive Show summary

  Amorphous thin films of FePO4 and Fe4(P2O7)3 show excellent power capabilities and good stability as cathode materials in Li-ion batteries. Within our tested range of materials, 10 nm FePO4 shows the best results and can handle specific powers above 1 MW/kg. The thin films are deposited using atomic layer deposition (ALD) and we studied the growth using in situ quartz crystal microbalance (QCM) showing self-limiting growth. Their electrochemical properties were characterized as cathode materials in coin cell batteries using cyclic voltammetry (CV) and galvanostatic cycling (GC), correcting for the roughness of the substrates and addressing contributions from non-Faradaic processes.

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• 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. 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.

• Borgan, Eldrid & Brennhagen, Anders (2024). Et nytt batteri kan lades på fem minutter. Hva skal til for at det kan brukes i elbiler? . [Internet]. forskning.no.
• 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).
• 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. 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.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Show all 7 contributors for this article] (2023). Tracing the (De)sodiation of Bi2MoO6 Through Good and Bad Times With Operando XRD. Show summary

  Na-ion batteries (NIBs) generally have slightly lower energy densities than Li-ion batteries (LIBs), but the abundancy of Na could make NIBs cheaper and more sustainable. The search for new anode materials is essential for further development of NIBs. Anode materials combining conversion and alloying mechanisms (CAMs) are promising because of their high capacity and high rate capabilities, but their complex cycling mechanisms involving amorphous phases are challenging to study [1]. Bi2MoO6 is an example of a CAM that has high initial capacity, but struggles with cycling stability and undergoes major changes during cycling. Detailed understanding of its cycling and degradation mechanism will hopefully enable us to improve the cycling stability of Bi2MoO6 and CAMs in general. Therefore, we have studied Bi2MoO6 using a combination of operando X-ray diffraction (Fig. 1), pair distribution function, X-ray absorption spectroscopy and transmission electron microscopy [3]. This revealed that during the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na, forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. During these cycles, we observe a growth in the crystallite size of the alloying particles, which is probably the leading cause of the rapid capacity decay after cycle 10 where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles.

• Brennhagen, Anders (2023). NAFUMA.
• Brennhagen, Anders (2023). New anode materials for Na-ion batteries.
• Brennhagen, Anders & Skurtveit, Amalie (2022). Anerkjennelse for UiO-studenter på batterikonferanse. [Internet]. uio.no.
• Brennhagen, Anders (2022). Batterier: Norges nye industrieventyr . Show summary

  Stadig flere kjører elbil. Da er gode batterier viktige, og nå planlegger Norge å bygge fire gigastore batterifabrikker. Bli med på batterieventyret, og hør hvordan vi på UiO forsøker å finne ut hvordan batteriene kan bli billigere, tryggere og vare lengre.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Show all 7 contributors for this article] (2022). Tracing the (De)sodiation of Bi2MoO6 Through Good and Bad Times With Operando XRD. Show summary

  Anode materials combining conversion and alloying mechanisms (CAMs) are promising for Na-ion batteries, but their complex cycling mechanisms are challenging to study. Understanding the (de)sodiation mechanism is crucial and, in several cases, requires advanced synchrotron characterization. Operando synchrotron studies are generally limited to one or two sodiation cycles, which are not fully comprehensive for CAMs. Therefore, herein, we studied the sodiation and desodiation of Bi2MoO6-based anodes with laboratory-based operando X-ray diffraction exceeding more than 30 cycles to have a complete overview of our CAM’s mechanism. This revealed important aspects of the cycling and degradation mechanisms in the material. During the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. This is followed by a rapid capacity decay where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles. To the best of our knowledge, this is due to the observed crystal growth of the Bi particles accompanied by structural changes in the insulating Na-Mo-O leading to poor conductivity in the electrodes. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Show all 7 contributors for this article] (2022). Corrigendum: Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries (Nanotechnology (2022)33 (185402) DOI: 10.1088/1361-6528/ac4eb5). Nanotechnology. ISSN 0957-4484. 33(35). doi: 10.1088/1361-6528/ac72b2.
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Show all 7 contributors for this article] (2022). Tracing the (de)sodiation of Bi2MoO6 through good and bad times with operando XRD. Show summary

  Anode materials combining conversion and alloying mechanisms (CAMs) are promising for Na-ion batteries, but their complex cycling mechanisms are challenging to study1. Understanding the (de)sodiation mechanism is crucial and, in several cases, requires advanced synchrotron characterization. Operando synchrotron studies are generally limited to one or two sodiation cycles, which are not fully comprehensive for CAMs2. Therefore, herein, we studied the sodiation and desodiation of Bi2MoO6-based anodes with laboratory-based operando X-ray diffraction exceeding more than 30 cycles to have a complete overview of our CAM’s mechanism (Figure 1)3. This revealed important aspects of the cycling and degradation mechanisms in the material. During the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. This is followed by a rapid capacity decay where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles. To the best of our knowledge, this is due to the observed crystal growth of the Bi particles accompanied by structural changes in the insulating Na-Mo-O leading to poor conductivity in the electrodes. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Brennhagen, Anders (2021). Forskning på framtidens batterier.
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Show all 7 contributors for this article] (2021). Operando XRD study on Bi2MoO6 as anode material for Na-ion batteries. Show summary

  Na-ion batteries (NIBs) could be a good alternative to Li-ion batteries (LIBs) due to the large abundance and availability of sodium resources. The search for good anode materials is one of the big challenges since graphite, which is the most common anode material for LIB, does not work well in NIBs. We are developing Bi2MoO6 as an anode material for NIBs, and will explain the main cycling mechanism. By utilizing a combination of conversion and alloying reactions, the material achieves high capacity while still maintaining a decent cycling stability. The complex cycling mechanism also makes it an interesting and challenging material to characterize during cycling. In this poster, we present high quality operando XRD data of the material during several cycles, acquired in the home lab. The data clearly shows that the reversible alloying reaction of Bi-metal to cubic Na3Bi gives the main capacity contribution, after the initial conversion reaction. This indicates formation of nanocrystalline Bi-particles, as nanocrystalline Bi-metal has shown the same reaction mechanism while microcrystalline Bi instead forms the hexagonal phase of Na3Bi. The molybdenum goes into an amorphous matrix that is only partially electrochemically active. The phases formed with molybdenum during cycling are difficult to characterize, due to their amorphous nature, and remain an unsolved mystery.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Sjåstad, Anja Olafsen; Ponniah, Vajeeston & Fjellvåg, Helmer (2021). Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries. Show summary

  Na-ion batteries (NIBs) could be a good alternative to Li-ion batteries (LIBs) due to the large abundance and availability of sodium resources. The search for good anode materials is one of the big challenges since graphite, which is the most common anode material for LIB, does not work well in NIBs. We are developing Bi2MoO6 as an anode material for NIBs, and will explain the main cycling mechanism. By utilizing a combination of conversion and alloying reactions, the material achieves high capacity while still maintaining a decent cycling stability. The complex cycling mechanism also makes it an interesting and challenging material to characterize during cycling. In this poster, we present high quality operando XRD data of the material during several cycles, acquired in the home lab. The data clearly shows that the reversible alloying reaction of Bi-metal to cubic Na3Bi gives the main capacity contribution, after the initial conversion reaction. This indicates formation of nanocrystalline Bi-particles, as nanocrystalline Bi-metal has shown the same reaction mechanism while microcrystalline Bi instead forms the hexagonal phase of Na3Bi. The molybdenum goes into an amorphous matrix that is only partially electrochemically active. The phases formed with molybdenum during cycling are difficult to characterize, due to their amorphous nature, and remain an unsolved mystery.

• Capellas Espuny, Montserrat; Wragg, David Stephen & Brennhagen, Anders (2021). From Oslo to Grenoble in the search of next generation batteries. [Internet]. esrf.fr.
• Torgersen, Eivind & Brennhagen, Anders (2021). Slik bygges et batteri trinn for trinn. [Internet]. Titan.
• Torgersen, Eivind; Brennhagen, Anders; Cavallo, Carmen; Geiss, Carina & Koposov, Alexey (2021). Pursuing better, cheaper and more environmentally friendly batteries. [Internet]. Titan.
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David; Vajeeston, Ponniah; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2021). Bi4V2O11/RGO composite as conversion and alloying anode for Na-ion batteries.
• Brennhagen, Anders; Wragg, David; Fjellvåg, Helmer; Vajeeston, Ponniah; Sjåstad, Anja Olafsen & Cavallo, Carmen (2021). Beyond Insertion: Conversion alloying Bi/RGO-based materials as novel anodes for Na-ion batteries.
• Cavallo, Carmen; Brennhagen, Anders; Wragg, David & Fjellvåg, Helmer (2020). Beyond Insertion: Conversion alloying Bi/RGO-based materials as anode for Na-ion batteries .
• Brennhagen, Anders; Kvamme, Kristian Breivik; Sverdlilje, Katja S. S. & Nilsen, Ola (2019). Amorphous Iron Phosphates for Thin Film batteries.
• Brennhagen, Anders & Nilsen, Ola (2019). Forskningstorget 2019 - Batteriproduksjon på stand.
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2024). Bi metallates as conversion-alloying anodes for Na-ion batteries. Universitetet i Oslo.
• Thiagarajan, Abilash Kanish; Brennhagen, Anders; Wragg, David Stephen & Koposov, Alexey (2023). Structural changes in graphite-based anodes of Li-ion batteries elucidated through operando XRD. Kjemisk Institutt. Show summary

  Lithium-ion batteries (LIBs) with graphite-based anodes dominate the battery market around the world and have been studied extensively for the past decades, but the structural changes during cycling are still not fully understood. In this work, we used galvanostatic cycling (GC) to characterize the electrochemical performance of graphite samples in LIB. We also attempted to achieve stable capacities over hundreds of cycles to monitor the effect of long-term cycling on the mechanisms of graphite, with limited success. The fabricated coin cells experienced poor capacity retention across all graphite samples and some abnormal capacity increases that had not been observed previously. We noticed that electrolytes containing FEC made a noticeable change to the electrochemical performance as it resulted in irregular cycling, but also better capacity retention. Operando X-ray diffraction is a powerful technique to understand structural changes. We looked at multiple graphite reflections, mainly 002, 100, 101, 102 and 004, and observed that the expansion of the structure is not only 2 dimensions but all 3 dimensions as the interlayer distance and graphene layers expands during lithiation. We also monitored this expansion of graphene layers with pair distribution function (PDF) as the three C-C distances in hexagonal carbon rings, 1.41 Å, 2.41 Å and 2.85 Å, changed lengths at different points during lithiation and delithiation. We looked at diffraction peaks during lithiation and delithiation to study the mechanisms and observed that they were different. Lithiation showed solid solution like behavior indicating disordered intermediate phases, while delithiation showed two-phase transition indicating ordered structures. In this work we have used Operando X-ray diffraction to show that the structural changes graphite undergoes during cycling, transition from graphite to LiC6, is not specific to each graphite sample and the structural changes depend on the condition of the material. Pristine graphite samples transitioned fully to LiC6 during cycling with C-rate of C/6, but only LiC12 when a higher C-rate of C/2 was used. Graphite electrodes cut from commercial pouch cells that had cycled many hundreds of electrochemical cycles were able to transition to LiC6 during C/20, but only LiC12 during C/6, indicating an “ageing” mechanism.

• Skautvedt, Casper; Fjellvåg, Helmer; Koposov, Alexey; Brennhagen, Anders; Cavallo, Carmen & Sjåstad, Anja Olafsen (2022). Synthesis and characterisation of BiFeO3 as an anode material for Na-ion batteries. Universitetet i Oslo. Show summary

  Na-ion batteries (NIBs) are currently an immature technology but have gained interest in research in the last years. A big field within NIBs is on the anode since there is none good enough yet. In this work, we have looked at BiFeO3 as an anode material for NIBs. Bi-based anodes have been investigated in the group before this work and have shown promising results. The choice of BiFeO3 was based on the interesting mechanism that occurs during charging and discharging when used as an anode. The mechanism is a conversion-alloying reaction and combines conversion and alloying reactions when BiFeO3 is sodiated. We have investigated this mechanism in detail with combined X-ray diffraction (XRD) and galvanostatic cycling (GS) with operando-XRD in the RECX-lab at the Department of Chemistry. BiFeO3 was synthesised by ceramic solid-state synthesis and sol-gel synthesis as part of the work. It has previously been synthesised with these syntheses but has proven to be difficult to be phase pure. The product of the syntheses has been characterised with XRD and refined with the Rietveld method. The ceramic solid-state synthesis gave a low yield of BiFeO3 and several impurities, but the sol-gel synthesis gave a high yield of BiFeO3 and few impurities. Various treatments of BiFeO3 were investigated, including ball milling, leaching and doping, to provide increased performance as a battery material. These tests showed that ball milling of the material gave the best results, but it was still not possible to find an optimisation that avoided a large drop in capacity during the first 30 cycles. To characterise BiFeO3 as an anode material, we test it in half-cells against Na metal. These half-cells were electrochemically analysed by GS and cyclic voltammetry (SV). We also tested different mixtures of electrode composition and electrolytes to optimise battery properties. These showed that NaPF6 was the best salt and propylene carbonate (PC) + 5 wt% fluoroethylene carbonate (FEC) gave the most stable battery cells. The data from the electrochemical characterisation and operando-XRD were used together to interpret the reaction mechanisms of BiFeO3. We saw from these that there were similarities in the alloying reaction between Na and Bi with other Bi-based anodes, with NaBi and Na3Bi as different stages in the reaction. However, there were no indications of the phase(s) of Fe after the reaction.

• Brennhagen, Anders; Nilsen, Ola; Kvamme, Kristian Breivik & Sverdlilje, Katja S. S. (2019). Synthesis and electrochemical characterization of thin film iron phosphates as cathode material for Li-ion batteries. Universitetet i Oslo. Show summary

  Solid-state batteries is one of the main contenders for domination of the future battery marked. Thin film technology is important in the development of these batteries. In this work, we have shown that amorphous thin film FePO4 with a thickness around 10 nm, deposited by atomic layer deposition (ALD), can reach a specific power above 1 MW/kg and approach theoretical capacity at lower currents. The 10 nm thin film also shows very good cycling stability at elevated currents and can retain 70 % of peak capacity after 8000 cycles at 80 µA (40C). The material also shows a significant self-enhancing mechanism leading to an increase in capacity during early cycling stages. We observed a capacity increase of 90 % for 10 nm after 100 cycles at 80 µA. In this study, we used quartz crystal microbalance (QCM) analysis to establish a stable ALD process for depositing amorphous thin films from the Fe-P-O system. By varying the pulsing ratio between the precursors, we obtained films with different compositions and chose to study Fe4(P2O7)3 and FePO4 more in detail. The films were uniform and flat with an RMS roughness below 1 nm. As the FePO4 films proved to be significantly better than the Fe4(P2O7)3, we focused mainly on FePO4. We used galvanostatic cycling (GC), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to characterize the electrochemical properties of the thin films. An important part of this study was to develop a good baseline for testing, including the use of reference batteries. In this work, we confirmed that LiClO4 is a better choice than LiPF6 as electrolyte for testing thin film cathodes, because of minimal side reactions with the steel casing. The FePO4 thin films show a combination of capacitive and redox behavior where both contribute to the capacity. In this study, we have tried to separate the two contributions and find their thickness and current dependency. In an attempt to increase the area capacity of the cathodes without increasing the film thickness, we created soot substrates with high surface 3D structures of carbon, deposited from the flames of a candle. We managed to maintain the structure and evenly coat it with FePO4. Despite the increase in mass, we obtained no higher capacity or better battery performance from the soot batteries.

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