Ane Sælland Christiansen

Sodium layered oxides with mixed transition metals have received significant attention as positive electrode candidates for sodium‐ion batteries because of their high reversible capacity. The phase transformations of layered compounds during electrochemical reactions are a pivotal feature for understanding the relationship between layered structures and electrochemical properties. A combination of in situ diffraction and ex situ X‐ray absorption spectroscopy reveals the phase transition mechanism for the ternary transition metal system (Fe–Mn–Co) with P2 stacking. In situ synchrotron X‐ray diffraction using a capillary‐based microbattery cell shows a structural change from P2 to O2 in P2–Na_0.7Fe_0.4Mn_0.4Co_0.2O₂ at the voltage plateau above 4.1 V on desodiation. The P2 structure is restored upon subsequent sodiation. The lattice parameter c in the O2 structure decreases significantly, resulting in a volumetric contraction of the lattice toward a fully charged state. Observations on the redox behavior of each transition metal in P2–Na_0.7Fe_0.4Mn_0.4Co_0.2O₂ using X‐ray absorption spectroscopy indicate that all transition metals are involved in the reduction/oxidation process.

In commercial Fe-based batteries the Fe²⁺/Fe³⁺ oxidation states are used, however by also utilizing the Fe⁴⁺ oxidation state, intercalation of up to two Li ions per Fe ion could be possible. In this study, we investigate whether Fe⁴⁺ can be formed and stabilized in β-Li₃Fe₂(PO₄)₃. The work includes in situ synchrotron X-ray powder diffraction studies (XRPD) during charging of β-Li₃Fe₂(PO₄)₃ up to 5.0 V vs. Li/Li⁺. A novel capillary-based micro battery cell for in situ XRPD has been designed for this. During charge, a plateau at 4.5 V was found and a small contraction in volume was observed, indicating some Li ion extraction. The volume change of the rhombohedral unit cell is anisotropic, with a decrease in the a parameter and an increase in the c parameter during the Li ion extraction. Unfortunately, no increased discharge capacity was observed and Mössbauer spectroscopy showed no evidence of Fe⁴⁺ formation. Oxidation of the organic electrolyte is inevitable at 4.5 V but this alone cannot explain the volume change. Instead, a reversible oxygen redox process (O²⁻ → O⁻) could possibly explain and charge compensate for the reversible extraction of lithium ions from β-Li₃Fe₂(PO₄)₃.

Conductivity measurements of aqueous K2CO3 were performed using the van der Pauw method and a specially designed sample holder with Pt wires as electrodes. The resistance was measured using alternating current. The conductivity of 10-50 wt% aqueous K2CO3 was measured at room temperature and ambient pressure. The conductivity was found to increase with concentration up to 30-40 wt%, and decline at higher concentrations. Furthermore, the conductivity of 5-30 wt% aqueous K2CO3 was measured up to 180-200 °C at 30 bar. The highest conductivity measured was 1.34 S/cm at 172 °C for 30 wt% K2CO3 (aq). The conductivity was found to increase with concentration and also temperature for 5-15 wt%. For 20 and 30 wt% K2CO3 (aq) the conductivity also increased with temperature up to a certain point and then a drop was seen at 150 and 180 °C, respectively. The activation energy was also obtained by fitting the data to a modified Arrhenius equation. The activation energy was found to be in the range 0.14-0.17 eV and did not appear to be concentration dependent.