Uncovering the Electrochemically Active Surfaces in Polycrystalline NMC Battery Cathode Particles

In battery electrodes, it is widely believed that the electrochemically active surface area equals the surface area of the particles. Based on this assumption, it has been generally believed that smaller particles have faster reaction and diffusion times than larger ones. In this study, we develop a new high-throughput, single-particle electrochemistry platform to test this belief for polycrystalline NMC battery particles. Surprisingly, we observed that the smaller particles have no shorter reaction and diffusion times than larger ones. To explain this unexpected relationship, we propose that electrolyte penetrates the grain boundaries of the secondary particle into the bulk. For this reason, the electrochemically active surface is not the area of the secondary particles, as typically believed, but the area of the primary particles. This work has substantial implications in accurate modeling and control of battery materials.