Nickel, Manganese and Cobalt oxides (NMCs) are the most commonly used high-capacity cathodes in batteries. However, as the transportation sector transitions from gasoline-powered towards rechargeable battery-powered vehicles, there is a pressing need for batteries that have a higher capacity per charge without compromising on the capacity retention over long term cycling. High Ni composition NMCs, achieved by substituting the Co for Ni, are seen as one of the primary pathways to higher capacity cathodes [1]. This also augurs well for reducing the amount of Co used in the cathode, where the concerns regarding its supply chain and cost are well documented. However, this reduction in Co has negative consequences on the long-term cyclic stability of the NMC, particularly as we access higher voltages [2]. This decrease in capacity over cycling has been attributed to various factors such as the migration of Ni ions into the Li layer causing structural degradation, which causes stress heterogeneities. This in turn can lead to morphological degradation through the nucleation and propagation of cracks through secondary cathode particles over repeated cycling. Here, we present a novel, simultaneous characterization of the morphological, structural and chemical nature of degradation in NMC811 (LiNi_0.8Mn_0.1Co_0.1O₂) cathodes after long-term cycling, using synchrotron-based XRD -CT and absorption CT.

Most prior investigations into local cathode degradation focus either on morphological (imaging) or structural (scattering/diffraction) or chemical (spectroscopy) characterizations of the NMC [2]. Correlative XRD-CT combines high energy x-ray diffraction (XRD) with micro-computed tomography (XCT), to furnish structural, chemical, crystallographic and 3D morphological information simultaneously. Here, we compare micrometer resolution XRD-CT scans from pristine and cycled (1000 cycles) NMC811 cathodes. We show how the absence of Co as a stabilizing ion causes Ni ions to migrate from the metal oxide layer to the Li layer in the cathode. This creates local dead zones (on the order of 10s of micrometers in a 0.5 mm wide cathode) that cannot fully lithiate/delithiate during cell cycling, thereby affecting cyclic capacity retention. We explore the variation in density and distribution of these dead zones as a function of distance from the cathode/separator interface.

Additionally, the dead zones create strain heterogeneities within secondary cathode particles. Over repeated cycling, this cyclic strain variation results in the nucleation and propagation of intra-particle cracks in the NMC, isolating entire particles from reversibly cycling Li. This cracking thus exacerbates the loss of active cathode material, further decreasing cell capacity over hundreds of cycles. We show the spatial correlation between the presence of such dead zones and the presence of cracks after hundreds of cycles, through sub-micrometer resolution absorption contrast tomography. Due to non-uniform participation of the cathode across its depth in cycling, we also show that the sections of the NMC811 cathode closest to the separator have a higher propensity to crack, consistent with their higher participation in repeated lithiation/delithiation, and thus stress heterogeneities over cycling.

Finally, single crystal NMCs are less prone to intergranular cracking within a secondary particle compared to their polycrystalline analogues, due to the absence of high-angle grain boundaries that aid crack propagation. Thus, we compare the performance of single crystal versus polycrystal NMC811, in terms of capacity retention after 1000 cycles, as well as the presence of cracks after long-term cycling. These findings showcase the unique capability of XRD-CT and absorption CT to multimodally elucidate degradation in such cathodes over multiple length and time scales, by correlating chemical and crystallographic changes to associated cracking. This in turn provides a valuable tool in informing the design of the next generation of energy dense Ni-rich NMCs, which also show a high degree of capacity retention over long-term cycling.

[1] Ryu, Hoon-Hee, et al. "Capacity fading of Ni-rich Li [Ni x Co y Mn1–x–y] O2 (0.6≤ x≤ 0.95) cathodes for high-energy-density lithium-ion batteries: bulk or surface degradation?" Chemistry of materials 30.3 (2018): 1155-1163.

[2] Xu, Chao, et al. "Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries." Nature Materials 20.1 (2021): 84-92.

[3] Jiang, Ming, et al. "A Review of Degradation Mechanisms and Recent Achievements for Ni‐Rich Cathode‐Based Li‐Ion Batteries." Advanced Energy Materials 11.48 (2021): 2103005.