Kinetic Control of Anisotropic Grain Growth in Ni-Rich Layered Cathodes.

Abstract

The particle morphology governed by anisotropic grain growth of Ni-rich layered oxide cathodes plays a crucial role in electrochemical stability and can be tuned via element doping during the high-temperature lithiation process. However, the fundamental impact of dopants on the anisotropic grain growth kinetics remains unclear. Herein, this work systematically investigates how dopants with different valences (Mg²⁺, Al³⁺, Zr⁴⁺, Ta⁵⁺, and Mo⁶⁺) affect the anisotropic grain growth during the high-temperature lithiation process. The findings demonstrate that the grain growth rate along the [104] and [110] directions is initially faster, followed by a higher growth rate along the [003] direction. The coarsening exponents n for the grain growth (along the [003], [104], and [110] directions) with low-valence ions (Mg²⁺, Al³⁺) dopants are approximately equal to 2, indicating that the grain growth depends solely on short-range diffusion at the grain boundary. Interestingly, the n value along the [003] direction enlarges with an increase in valence. Notably, the higher valence of Mo⁶⁺ results in a significant elevation of the exponent n along the [003] direction to 4.5, indicating a strong pinning effect due to Mo⁶⁺ segregation at the grain boundary. This pinning impedes grain boundary migration, inhibits grain coarsening, and effectively enhances cycle stability.