Single-Particle Electrochemical Cycling Single-Crystal and Polycrystalline NMC Particles

Abstract

Li(Ni,Mn,Co)O2 (NMC) is one of the most widely used cathode materials for lithium-ion batteries. Most commercial cathodes utilize polycrystalline particle morphologies, which have a characteristic “meatball” shape. Recently, there has been interest in replacing polycrystalline particles with single-crystal particles, which are believed to have improved cycle life due to the absence of intergranular cracking. However, the effects of single-crystal particles on rate capability show conflicting results. In this work, single-particle electrochemistry using a microelectrode array to test the kinetics of 40 polycrystalline (3–12 µm) and 13 single-crystal (2–4 µm) NMC cathode particles is conducted. The results show that single-crystal particles have much higher reaction overpotentials than polycrystalline ones. Moreover, larger single-crystal particles show higher overpotentials than smaller ones, while the polycrystalline particles display no size effects despite the greater variability. These findings are attributed to intergranular cracking and the penetration of the liquid electrolyte, which enables much faster reaction kinetics. In characterizing the electrochemistry of individual particles, the results confirm that these single-crystal particles would have much poorer rate kinetics and more challenges with fast charge and discharge compared to polycrystalline particles of the same composition.