DOI: 10.25549/usctheses-c40-364207
title: Understanding the structure-property relationship in electrode materials for electrochemical energy storage

description: The development of sustainable, robust and large-scale energy storage is critical for renewable energy sources to assume a major role in our energy supply scheme. Among the pool of technologies proposed for large-scale energy storage, rechargeable Li- and Na-ion batteries offer substantial advantages in terms of mobility, efficiency, and power density. However, for such technologies to be used in a grid scale, dramatic reduction in the cost is needed for them to become viable. And although Li-ion batteries have already enjoyed certain degree of success in electric vehicles, they still face challenges such as cost, and charging rate before they can become a dominant player in the automotive market. ❧ The work presented here aims to explore a large family of mineral materials, silicates, for their potential application in electrochemical storage and other functional applications. With a wide range of characterization tools, we set out to not only understand the electrochemical performance of these materials, but also find out how their functional properties are related to their structures. We then took a similar approach in two oxide phases, Fe₂(MoO₄)₃ and ReO₃, to further understand how the structural changes in these materials correlate with their elctrochemical properties. ❧ First we found that LiFeSi₂O₆ undergoes a reversible electrochemical reaction against Li/Li⁺ centered around 2 V with capacities near 60% of the theoretical maximum. We employ high resolution synchrotron X-ray and neutron diffraction to characterize the structure and correlate the rigid connectivity with the very slow kinetics of diffusion. We also use computational tools to understand the origin of the low potential compared with other Fe-based electrodes. ❧ We then use similar low-temperature techniques to prepare high-purity NaFeSi₂O₆ and an iron-based muscovite phase with unique microstructures. Interestingly, we found that the NaFeSi₂O₆ nanowires prepared by our low-temperature technique exhibit substantial difference in temperature- and field-dependent magnetic properties. The differences can likely be attributed to the reduced particle size and increased number of spins on the surface of the nanowires. In the case of our synthetic iron muscovite, a reversible capacity equal to 0.4 mole Li per formula unit was obtained. Cyclic voltammetry analysis showed significant contribution of the electrochemical capacity come from a surface controlled process. This is probably due to the limited amount of sites available for Li⁺ intercalation. ❧ In our study of the electrochemical properties of Fe₂(MoO₄)₃, significant differences in the structural and electrochemical properties during the intercalation of Li⁺ and Na⁺ ions were observed. To understand the origin of this behavior, we use a combination of in-situ X-ray and high-resolution neutron diffraction, total scattering, electrochemical measurements, density functional theory calculations, and symmetry-mode analysis. We find that for Li⁺-intercalation, which proceeds via a two-phase monoclinic-to-orthorhombic (Pbcn) phase transition, the host lattice undergoes a concerted rotation of rigid polyhedral subunits driven by strong interactions with the Li⁺ ions, leading to an ordered lithium arrangement. Na⁺-intercalation, which proceeds via a two-stage solid solution insertion into the monoclinic structure, similarly produces rotations of the lattice polyhedral subunits. However, using a combination of total neutron scattering data and density-functional theory calculations, we find that while these rotational distortions upon Na⁺ intercalation are fundamentally the same as for Li⁺-intercalation, they result in a far less coherent final structure, with this difference attributed to the substantial difference between the ionic radii of the two alkali metals. ❧ Finally, we explored the electrochemical Li⁺- and Na⁺-intercalation into defect-perovskite type ReO₃. The polhedral rotational distortions that we found dictates the mechanism of the guest ion intercalation in Fe₂(MoO₄)₃ also seems to play an important role in ReO₃. Theoretical two Li per formula intercalation in ReO₃ is possible with multiple stages during the electrochemical cycling. However, due to the huge mismatch between the sizes of Li⁺ ions and the interstitial void, and the enormous twist of the host lattice required to accommodate the inserted Li⁺, Li⁺-intercalation shows poor reversibility. On the other hand, reversibility of Na⁺-intercalation is greatly improved if it is limited to the solid solution process on the first stage. The closer fit between Na⁺ and the interstitial space in ReO₃, which reduces the twist of host framework and the local distortion, likely explains the enhanced reversibility.