In situ synchrotron radiation investigation of charge compensation and phase evolution mechanisms in Li2FeSiO4 electrodes
The application of Synchrotron Radiation (SR) based techniques in the field of electrochemistry has been facilitated with the development of in situ or in operando experimental methods using reliable, consumable, x-ray transparent windows (e.g. Kapton®, Mylar®, graphene and silicon nitride). Specifically, the two most popular SR approaches used for in situ electrochemical studies are: (1) x-ray absorption spectroscopy (XAS), which is used to infer oxidation state, ligand coordination and bond lengths; and (2) x-ray diffraction (XRD), which is used to probe long range crystallographic structure . To this end, a combined experimental setup capable of near simultaneous XRD and XAS measurements has been constructed at 06ID-1 at the Canadian Light Source for in situ studies of secondary batteries . To affirm the utility of this experimental setup, one promising cathode material Li2FeSiO4 (LFS, desirable for its low cost and high theoretical capacity of 330mAh/g), was selected and the characterization of structure-function relationships that define the electrochemical performance of LFS were explored. In Chapter 2 the details of the experimental setup are thoroughly described, including ray-tracing and optical considerations for performed near simultaneous scattering and absorption experiments. This experimental setup is used to further understand the physicochemical behaviour of LFS. In Chapter 3, the validity of a solid-solution model is explored to describe the (de)intercalation processes in a mixed phase LFS cathode material using post-mortem samples . It is found that for the formation cycle, this process does indeed follow a solid-solution behaviour, but that crystalline phase instabilities lead to a more two-phase behaviour following repeated cycling. A fine-grained study of the one Li extraction (i.e. Li2-xFeSiO4, where x=1) process is outlined in Chapter 4, and the charging rate-dependent performance of LFS  is understood to originate from varied crystal phase stabilities among different charging kinetics. During the in situ studies outlined in Chapter 4, a subsequent phenomenon involving the spontaneous formation of a surface electrolyte interphase layer was identified. As well, the apparent restoration of Li+ was observed following a charge-discharge cycle, and is detailed in Chapter 5 .