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.

对于层状阴极，晶界处的晶间裂纹是众所周知的机械降解，这可能引发许多有害后果，从而降低循环性能。迄今为止，裂纹的原子机理，特别是动力学成核过程，还很不清楚。本文中，我们研究了LiCoO _2中相干晶界，孪晶界处的开裂机理，借助原子分辨率电子显微镜。根据裂纹的成核和演化过程，确定了两种裂纹，分别为劈裂裂纹和分解裂纹。前者是典型的变形诱发的机械故障，其特征是电化学机械疲劳退化。后者是由于热力学分解而形成的，在高压循环过程中起主要的开裂成核机理作用。我们的工作还表明，孪晶边界作为内在的平面缺陷在能量上有利于裂纹，相变和空隙的形成，这强调了机械和热力学稳定晶界对下一代锂离子电池LiCoO ₂和其他层状阴极的高电压使用至关重要。