Study on the intergranular cracks evolution and mechanisms in PC-NCM811 particles through long-term real-time observation

Abstract

Nickel-rich polycrystalline LiNi_xCo_yMn_1-x-yO₂ (PC-NCM, 0.8 ≤ x < 1) particles suffer capacity degradation due to intergranular cracks, which catalyze side reactions at fresh interfaces, diminishing battery performance. Understanding the mechanisms behind crack evolution is essential for mitigating these issues. Real-time crack observation is crucial for this understanding, yet long-term monitoring remains unachieved. This study develops a versatile method using an optical in-situ reaction cell, modified from a coin cell structure, to enable long-term, real-time tracking of volume changes, crack evolution and lithium-ion diffusion in the particle. This method has provided new insights into the evolution of intergranular cracks and mechanisms in PC-NCM811 particles. Intergranular cracks can be categorized into main cracks, microcracks and cracks at the boundaries of inactive domains based on the stress origin. Main cracks stem from strain mismatches caused by asynchronous domains during initial activation, while their subsequent propagation is driven by alternating stresses from cycling. The initiation of microcracks is caused by stress concentration at grain boundaries due to abrupt volume contraction during charging process. Volume changes along the a-axis exacerbate the irreversible propagation of these microcracks at a high state of charge. Optical imaging shows regions with limited lithium-ion diffusion align with boundary cracks caused by uneven lithium-ion concentrations at high C-rate. These findings emphasize the value of long-term, real-time observation for understanding electrochemical-mechanical interactions. The observation and analysis method can be applied to investigate and evaluate the crack evolution of various materials under different conditions, facilitating the optimization of material design and the formulation of effective cycling protocols.