Atomistic mechanism of cracking degradation at twin boundary of LiCoO2

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. Intergranular cracking triggers many detrimental consequences to degrade the cycling performance. Herein, we investigate the cracking mechanism at a coherent grain boundary, twin boundary in LiCoO2, and reveal two kinds of cracking mechanisms, which are the deformation induced cleavage crack and the material decomposition induced decomposition crack, occurring at low voltage and high voltage cycling, respectively. [Display omitted] •Twin boundary is frequently observed in commercial micrometer-sized LiCoO2 particles, revealing they are not single crystals.•Twin boundary as an intrinsic planar defect energetically favors cracking, phase transformation and void formation.•Low voltage cycling induced cleavage crack is a deformation failure due to electrochemomechanical fatigue.•High voltage cycling causes massive decomposition cracks due to LiCoO2 decomposition at high delithiation state.