Operando visualization of kinetically induced lithium heterogeneities in single-particle layered Ni-rich cathodes
Chao Xu, Alice J. Merryweather, Shrinidhi S. Pandurangi, Zhengyan Lun, David S. Hall, V.S. Deshpande, N.A. Fleck, Christoph Schnedermann, Akshay Rao, Clare P. Grey
Abstract
Understanding how lithium-ion dynamics affect the (de)lithiation mechanisms of state-of-the-art nickel-rich layered oxide cathodes is crucial to improve electrochemical performance. Here, we directly observe two distinct kinetically induced lithium heterogeneities within single-crystal LiNixMnyCo(1−x−y)O2 (NMC) particles using recently developed operando optical microscopy, challenging the notion that uniform (de)lithiation occurs within individual particles. Upon delithiation, a rapid increase in lithium diffusivity at the beginning of charge results in particles with lithium-poor peripheries and lithium-rich cores. The slow ion diffusion at near-full lithiation states—and slow charge transfer kinetics—also leads to heterogeneity at the end of discharge, with a lithium-rich surface preventing complete lithiation. Finite-element modeling confirms that concentration-dependent diffusivity is necessary to reproduce these phenomena. Our results demonstrate how kinetic limitations cause significant first-cycle capacity losses in Ni-rich cathodes.