Atomistic mechanism of cracking degradation at twin boundary of LiCoO2

Abstract Intergranular cracking at grain boundary is a well-known mechanical degradation for layered cathodes, which can trigger many detrimental consequences to degrade the cycling performance. To date, the atomistic mechanism of crack, especially the kinetic nucleation process, is still far from clear. Herein, we investigate the cracking mechanism at a coherent grain boundary, twin boundary in LiCoO2, by virtue of atomic resolution electron microscopy. Based on crack's nucleation and evolution process, two kinds of cracks are identified, the cleavage crack and the decomposition crack. The former is a typical deformation induced mechanical failure, featuring the electrochemomechanical fatigue degradation. The latter is formed due to thermodynamic decomposition, acting as the dominant cracking nucleation mechanism during high voltage cycling. Our work also demonstrates that twin boundary as an intrinsic planar defect energetically favors cracking, phase transformation and void formation, which stresses that stabilizing grain boundary mechanically and thermodynamically is vital towards high voltage usage of LiCoO2 and other layered cathodes for next generation lithium ion battery.