Layered transition-metal oxides, such as Ni-rich LiNi_xMn_yCo_zO₂, are high-energy-density cathode materials for batteries, but they often suffer considerable capacity loss during cycling. Unravelling the mechanism behind this capacity fading is crucial in tackling the issue, yet it has ignited intense debate in the literature. Various explanations have been proposed, including the presence of strain-induced cracks, surface reconstructions, oxygen leaks, local structural heterogeneities, or irreversible phase transitions. There have also been suggestions that the mechanism could depend on oxide structure and experimental conditions such as the applied current rate. Now, Samuel Tardif and colleagues at the Université Grenoble Alpes propose a universal mechanism: Li loss triggers capacity degradation, irrespective of the Ni content, crystalline phase of the oxides, or applied current rate.

Using laboratory and synchrotron operando X-ray diffraction, Tardif and team study the structural evolution of a series of LiNi_xMn_yCo_zO₂ in full cells under various current rates. By analysing the correlation between the distances among neighbouring transition-metal atoms and the interlayer distance with the Li concentration during cycling, the researchers identify a critical Li concentration threshold (40%) below which the structure undergoes severe distortion. A similar lattice strain dependence on the Li concentration is also observed: once the Li concentration falls below approximately 40%, considerable strain begins to develop, regardless of the current rate and material type. Furthermore, Tardif and team suggest that the lattice distortion and strain further increase with additional Li loss. These changes induce a variety of degradation phenomena, such as the formation of irreversible cracks, transition metal dissolution, oxygen release, and structure reconstruction, thereby exacerbating the loss of electrochemical performance.

层状过渡金属氧化物，例如富镍的LiNi _x Mn _y Co _z O ₂，​​是高能量密度的电池正极材料，但它们在循环过程中通常会遭受相当大的容量损失。揭示这种能力衰退背后的机制对于解决这个问题至关重要，但它在文献中引发了激烈的争论。人们提出了各种解释，包括应变引起的裂纹、表面重建、氧气泄漏、局部结构异质性或不可逆相变的存在。还有人建议，该机制可能取决于氧化物结构和实验条件，例如施加的电流速率。现在，格勒诺布尔阿尔卑斯大学的塞缪尔·塔迪夫（Samuel Tardif）及其同事提出了一种通用机制：无论镍含量、氧化物的晶相或施加的电流速率如何，锂损失都会引发容量退化。

Tardif 和团队利用实验室和同步加速器操作 X 射线衍射研究了全电池中一系列 LiNi _x Mn _y Co _z O ₂在不同电流速率下的结构演化。通过分析循环过程中相邻过渡金属原子之间的距离和层间距离与锂浓度之间的相关性，研究人员确定了一个临界锂浓度阈值（40％），低于该阈值，结构会发生严重变形。还观察到类似的晶格应变对 Li 浓度的依赖性：一旦 Li 浓度低于约 40%，无论电流速率和材料类型如何，都会开始产生相当大的应变。此外，塔迪夫和团队认为，随着额外的锂损失，晶格畸变和应变进一步增加。这些变化引发了多种降解现象，如不可逆裂纹的形成、过渡金属溶解、氧释放和结构重建等，从而加剧了电化学性能的损失。