Three-Phase Reconstruction Reveals How the Microscopic Structure of the Carbon-Binder Domain Affects Ion Transport in Lithium-Ion Batteries

Abstract

The morphology of the electrolyte-filled pore space in lithium-ion batteries is determined by the solid microstructure formed by μm-sized active material particles and the smaller-featured carbon binder domain (CBD). Tomographic reconstructions have largely neglected the CBD, resulting in inadequately defined pore space morphologies at odds with experimental ionic tortuosity values. We present a three-phase reconstruction of a LiCoO2 composite cathode by focused ion-beam scanning electron microscopy tomography. Morphological analysis proves that the reconstruction, which combines an unprecedented volume (20 μm minimum edge length) with the hitherto highest resolution (13.9×13.9×20 nm3 voxel size), represents the cathode's pore space morphology. Pore-scale diffusion simulations show consideration of the resolved CBD as indispensable to reproduce ionic tortuosity values from electrochemical impedance spectroscopy. Our results reveal the CBD as a convoluted network that dominates the pore space morphology and limits Li+ transport through tortuous and constricted diffusion pathways.