Operando visualisation of kinetically-induced lithium heterogeneities in single-particle layered Ni-rich cathodes

Abstract

Understanding how lithium-ion dynamics affect the (de)lithiation mechanisms of state-of-the-art nickel-rich layered oxide cathodes is crucial to improving 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 modelling confirms that concentration-dependent diffusivity is necessary to reproduce these phenomena. Our results show that diffusion limitations cause first-cycle capacity losses in Ni-rich cathodes.

Three-dimensional operando optical imaging of particle and electrolyte heterogeneities inside Li-ion batteries

“…High numerical aperture (N.A.) microscope objectives have recently been used with the WF technique to extend its resolution and track changes in phase 24 , state-of-charge 25 and structure 26 at the single-particle level in high energy density and fast charging battery materials.…”

“…The RI mismatch can result in severe aberrations and artifacts 29 , which will be further accentuated by the high absorption and multiple scattering 30 of electrodes. In batteries speci cally, high-resolution re ection microscopy studies have thus far been primarily limited to studying surface topography changes 19,20 or lithiation in ' at' single crystalline particles in 2D [24][25][26] . Such constraints not only fundamentally limit applications but also skews our understanding as objects and processes are inherently three-dimensional (3D) with both a surface and bulk.…”

Three-dimensional operando optical imaging of single particle and electrolyte heterogeneities inside Li-ion batteries

“…High numerical aperture (N.A.) microscope objectives have recently been used with the WF technique to extend its resolution and track changes in phase 24 , state-of-charge 25 and structure 26 at the single-particle level in high energy density and fast charging battery materials. Despite this, little framework still exists for physically understanding the reflectivity signals from such (WF) experiments, with conflicting results emerging 24,27 .…”

“…The RI mismatch can result in severe aberrations and artifacts 29 , which will be further accentuated by the high absorption and multiple scattering 30 of electrodes. In batteries specifically, high-resolution reflection microscopy studies have thus far been primarily limited to studying surface topography changes 19,20 or lithiation in 'flat' single crystalline particles in 2D [24][25][26] . Such constraints not only fundamentally limit applications but also skews our understanding as objects and processes are inherently three-dimensional (3D) with both a surface and bulk.…”