Development of novel electrode materials for rechargeable batteries

Christian Lund Jakobsen

Research output: ThesisPh.D. thesis

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Abstract

In this thesis, four different materials, with promising aspect as positive electrode materials for rechargeable Li- or Na-ion batteries, are investigated with focus on their structural changes during ion-intercalation. The bearing methodology of the thesis has been operando powder X-ray diffraction and pair distribution function experiments analyzed through Rietveld and real-space refinement to obtain structural models describing the atomic scale of the electrodes and their changes during charge and discharge.
Oxide-based materials are among the best well-performing intercalation electrodes. In this thesis two oxides have been subject of investigation. One of these, rutile/pyrolusite β-MnO2, has proven to accommodate at >2 Li-ions during the first discharge. However, the ion-intercalation induces significant structural disorder. The structure of the disordered state is revolved herein to consists of nano-domains of a spinel-like LixMnO2 phase. The other oxide-material, NaCrO2, has been studied in great detail herein to elucidate both the crystalline-to-crystalline and order-disorder transitions occurring during charge and discharge. The material goes through several phase transition upon charge resulting in continuous expansion of the interlayer distance as opposed to very discrete two-phase transition during discharge. Furthermore, a previously missed charge-intermediate has been discovered in this study. As more than 60% Na is extracted from the as-synthesized material NaCrO2, long-range order is lost as Cr migrates irreversibly to the vacant Na layers forming a disordered O3-phase.
As the oxides undergoes significant structural changes, the benefits of the more stable polyanionic frameworks are quite appealing. Herein lies the very attractive phosphates of which two have been studied in this thesis. The orthorhombic polymorph β-LiVOPO4 shows core-shell formation, which block Li-migration but can be removed via high-energy ball milling. The material follows a two-phase reaction during charge and discharge but unexpected solid solution behavior in the Li-poor phase (β-VOPO4) was revealed. Finally, this thesis combines the advantages of layered XII structure with the stability from the polyanions through a layered sodium iron phosphate, Na3Fe3(PO4)4 which can accept both Li- and Na-ions reversibly. The material reacts as a solid-solution upon intercalation, where the intercalation of Na occurs in the cavities between the two iron-sites, causing cell-expansion. As the material has time to relax after discharge, the intercalated Na migrates to the Na layers, causing a contraction of the cell.
In this thesis, profound new knowledge has been presented for four materials that has a possible future as positive electrode in rechargeable batteries. The understanding of structural disorder in these materials during ion (de)intercalation has thoroughly been analyzed via state of the art characterization techniques. The discoveries in the thesis can hopefully provide knowledge to predict behaviors of more complex materials, as material compositional tuning will be imminent for the future of rechargeable batteries.
Original languageEnglish
Awarding Institution
  • University of Southern Denmark
Supervisors/Advisors
  • Ravnsbæk, Dorthe Bomholdt, Principal supervisor
Date of defence11. Nov 2022
Publisher
DOIs
Publication statusPublished - 9. Nov 2022

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