Grain Boundaries and Their Impact on Li Kinetics in Layered-Oxide Cathodes for Li-Ion Batteries

Defects are pervasive in electrochemical systems across multiple length scales. The defect chemistry largely differs from the bulk behavior and often dictates the rate performance for battery materials. However, the impact of material defects on Li kinetics remains elusive because of their complex nature and the sensitivity of the reaction kinetics on the local atomic environment. Here we focus on the grain boundaries (GBs) in layered-oxide cathodes and address their role in Li transport using the first-principles theoretical approach. We construct the coincidence site lattices of ∑2(11̅04̅), ∑3(1̅102̅), ∑5(11̅01̅), and ∑9(1̅104̅) GBs. The energy profiles for Li migration across and along the grain planes are plotted. We discuss in detail how the atomistic features associated with various grain structures such as the local structural distortion and charge redistribution determine the Li transport kinetics. Specifically, the coherent ∑2 GBs facilitate Li migration with 1–2 orders of magnitude increased diffusivity than the bulk diffusion, the asymmetric ∑3 GBs significantly impede Li diffusion, and the locally disordered ∑5 and ∑9 GBs cause slightly increased Li diffusivity at the intermediate diffusion distance (∼15 Å). We further evaluate the overall Li diffusivity and conductivity in the layered-oxide lattice by a distinction of Li transport in the bulk, across the GBs, and along the grain planes. The fundamental understanding sheds insight on a prevalent defect in the state-of-the-art cathode and its potential optimization of Li kinetics.