Ni-rich layered cathode materials are among the most promising candidates for high-energy-density Li-ion batteries, yet their degradation mechanisms are still poorly understood. We report a structure-driven degradation mechanism for NMC811 (LiNi_0.8Mn_0.1Co_0.1O₂), in which a proportion of the material exhibits a lowered accessible state of charge at the end of charging after repetitive cycling and becomes fatigued. Operando synchrotron long-duration X-ray diffraction enabled by a laser-thinned coin cell shows the emergence and growth in the concentration of this fatigued phase with cycle number. This degradation is structure driven and is not solely due to kinetic limitations or intergranular cracking: no bulk phase transformations, no increase in Li/Ni antisite mixing and no notable changes in the local structure or Li-ion mobility of the bulk are seen in aged NMCs. Instead, we propose that this degradation stems from the high interfacial lattice strain between the reconstructed surface and the bulk layered structure that develops when the latter is at states of charge above a distinct threshold of approximately 75%. This mechanism is expected to be universal in Ni-rich layered cathodes. Our findings provide fundamental insights into strategies to help mitigate this degradation process.

富镍层状阴极材料是高能量密度锂离子电池最有希望的候选材料之一，但其降解机理仍知之甚少。我们报告了NMC811（LiNi _0.8 Mn _0.1 Co _0.1 O ₂），其中一部分材料在重复循环后的充电结束时表现出较低的可及电荷状态。通过激光稀薄的纽扣电池实现的Operando同步加速器长时间X射线衍射显示，该疲劳相的浓度随循环数的出现和增长。这种降解是结构驱动的，而不仅仅是由于动力学限制或晶间开裂：在老化过程中没有本体相变，Li / Ni反位混合没有增加，本体的局部结构或锂离子迁移率没有明显变化。 NMC。代替，我们认为，这种降解源于重建表面与块状分层结构之间的高界面晶格应变，当后者处于荷电状态时，其阈值高于大约75％的明显阈值。预期该机制在富镍层状阴极中是普遍的。我们的发现提供了对有助于缓解此降解过程的策略的基本见解。