Influence of the Polymer Binder Composition on the Charge Transfer Resistance, Morphology, and Crystallinity of LiFePO₄ Electrodes Revealed by Electrochemical Impedance Spectroscopy and Grazing Incidence Small- and Wide-Angle X-ray Scattering

Electrode materials for application in lithium-ion batteries are commonly probed by X-ray diffraction (XRD) to investigate their crystalline structure. Grazing incidence wide-angle X-ray scattering (GIWAXS) is an extension to XRD since in-plane structures are also accessible. Additionally, with grazing incidence small-angle X-ray scattering (GISAXS), morphological information on the nanoscale can be revealed. In this work, the nanostructure of battery electrodes, which consist of lithium iron phosphate (LiFePO₄) as active material, carbon black (CB) as conducting agent, and the polymeric binders polyvinylidenefluoride (PVDF) and poly((trifluoromethane) sulfonimide lithium styrene) (PSTFSILi) is studied by performing GISAXS and GIWAXS. The chemical nature of the binder is tuned by blending PVDF and PSTFSILi. Specifically, a series of LiFePO₄ electrodes with polymer blends of the common, non-conducting PVDF and the single-ion conducting PSTFSILi with different weight ratios as binders is investigated to understand the influence of the binder on the structure of the electrode in detail. Scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) complement these studies to correlate the morphology and structure with the electrochemical behavior. It is found that LiFePO₄ crystallites do not exhibit any preferred orientation with respect to the substrate, irrespective of the binder composition, but their size depends on the binder composition.