(450f) Quantifying Reaction and Transport Heterogeneities between Individual Particles in Li-Ion Cathodes

Li-ion battery electrodes comprise of micron-sized solid particles immersed in liquid electrolytes. Understanding diffusion rates in the solid and reaction rates at the solid-liquid interface is essential for predictive modeling and rational design of battery materials. Unfortunately, measuring these transport rates in individual micron-sized battery particles has been extremely challenging. In this work, we present two approaches of reaction and diffusion rates at the single particle length scale. Our first approach uses operando synchrotron X-ray imaging of LiFePO₄ particles in organic liquid electrolytes, which uncovered strong autocatalytic behavior upon lithium de-insertion but autoinhibitory behavior upon lithium insertion. Our second, more recent approach is to measure the electrochemical response of many individual polycrystalline Li(Ni,Mn,Co)O₂ (NMC) particles using the microelectrode array. By using potentiostatic intermittent titration and fitting it to a spherical reaction-diffusion model, we were able to measure both the reaction and diffusion time constants. To our surprise, we found that neither the diffusion time nor the reaction time depended on the size of the particle, in stark contrast to the prevailing model. Instead, due to intergranular cracking, the electrolyte appears to penetrate into the secondary particle; as a result, the diffusion length is much shorter than the diameter of the secondary particle and does not change with the secondary particle size. These results have substantial implications for battery modeling and design.