The surface stability of layered cathode materials is a critical determinant of their chemo-mechanical reversibility during intercalation/deintercalation. This is possibly because of the migration of transition metals (TMs) into the Li layer followed by O redox, which decreases the O vacancy formation energy and thereby destabilizes the anionic framework. Hence, elucidating an optimal assembly of surface facets is a key strategy for further stabilizing the characteristic R3m structure to allow higher capacities and states of charging. In this study, three single-crystal samples of LiNi_0.33Co_0.33Mn_0.33O₂ (NCM111) with varied surface facets were synthesized and compared. After cycling, (012) facets had higher losses of Li and O atoms than (104) facets, leading to excessive Mn reduction. The formation of rocksalt structures and the loss of Li-ion migration pathways during extended cycling primarily resulted from irreversible compositional change of the (012) facets. Through electrochemical single-particle simulations we could establish a correlation between degraded surface structures and inferior electrochemical properties. The simulation indicated that the complete degradation of (012) facets is accompanied by an overpotential of approximately 0.2 V, leading to severe voltage hysteresis, which agrees well with our experimental findings. This study provides an important insight into the ideal assembly of surface facets in layered cathode materials that will help future cathode research realize more stable surface structures with even higher capacities and states of charging.

中文翻译：

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多过渡金属层状阴极的最佳小面组装以获得优异的锂离子动力学和结构稳定性


层状正极材料的表面稳定性是其嵌入/脱嵌过程中化学机械可逆性的关键决定因素。这可能是因为过渡金属（TM）迁移到Li层中，然后发生O氧化还原，这降低了O空位形成能，从而使阴离子骨架不稳定。因此，阐明表面刻面的最佳组装是进一步稳定特征 R3m 结构以实现更高容量和充电状态的关键策略。在本研究中，三个具有不同表面刻面的 LiNi _0.33 Co _0.33 Mn _0.33 O ₂ (NCM111) 单晶样品综合并比较。循环后，(012)面比(104)面损失更多的Li和O原子，导致Mn过度还原。长期循环过程中岩盐结构的形成和锂离子迁移路径的丧失主要是由于（012）晶面的不可逆成分变化造成的。通过电化学单粒子模拟，我们可以建立退化的表面结构和较差的电化学性能之间的相关性。模拟表明，(012)面的完全退化伴随着大约0.2V的过电势，导致严重的电压滞后，这与我们的实验结果非常吻合。这项研究为层状正极材料中表面晶面的理想组装提供了重要的见解，这将有助于未来的正极研究实现更稳定的表面结构以及更高的容量和充电状态。