Nano Tomography of High Voltage Induced First Cycle Cracking in NMC811
Authors: Parks Huw Christopher William , Tan Chun , Wade Aaron , Heenan Thomas M.M. , Shearing Paul R , Brett Dan , Jervis Rhodri 
Published: July 2022

Lithium nickel-manganese-cobalt (NMC) oxide-based Li-ion batteries have emerged as the most promising successor to LiCoO2 chemistry and could potentially facilitate ubiquitous adoption of Li-ion in mobile and transport systems. High specific capacities, high-rate capability, and long-term cycling abilities are driving much research into these cathode materials, forcing a greater nickel content with each iteration. NMC811’s high specific capacity makes it highly attractive for battery suppliers and users; however, it suffers significant capacity fade via several degradation modes, one being crack formation within secondary particles. The fresh surface exposed by cracking has been linked with parasitic reactions that evolve oxygen and potentially initiate the formation of inactive crystal structures. [1]

X-ray computed tomography (CT) has been utilised extensively to image the morphology of battery particles and with the advancement of lab-based tomography systems, resolution on the nano scale is readily available.[2] Whilst in-situ studies are becoming the standard experimental operation for appraising the dynamic nature of these materials, in-situ cells are complicated for use in lab-based instruments due to their low flux, low energy and reduced working space. Thus, nano tomography rarely provides information that deconvolutes chemo-mechanical cracking from manufacturing defects and calendaring. However, in this work we have developed a unique technique to follow the progression of cracking at the nanometre scale by sequential correlated imaging of a micro-sized tab cut from the electrode.[3,4] This method allows for images of the pristine material to be acquired ex-situ before electrochemical testing, and then sequential imaging of the same region of interest in subsequent scans. Here, nano-CT imaging of the same region of the electrode while fully-lithiated and charged to 4.5 V unveils the propagation of cracks as a function of voltage, and their relaxation phenomena upon discharge.

[1] Li, Tianyu, Xiao-Zi Yuan, Lei Zhang, Datong Song, Kaiyuan Shi, and Christina Bock. "Degradation mechanisms and mitigation strategies of nickel-rich NMC-based lithium-ion batteries." Electrochemical Energy Reviews 3, no. 1 (2020): 43-80.

[2] Heenan, Thomas MM, Chun Tan, Jennifer Hack, Dan JL Brett, and Paul R. Shearing. "Developments in X-ray tomography characterization for electrochemical devices." Materials Today 31 (2019): 69-85

[3] Tan, C., S. Daemi, T. Heenan, F. Iacoviello, A. S. Leach, L. Rasha, R. Jervis, D. J. L. Brett, and P. R. Shearing. "Rapid preparation of geometrically optimal battery electrode samples for nano scale X-ray characterisation." Journal of The Electrochemical Society 167, no. 6 (2020): 060512.

[4] Tan, Chun, Andrew S. Leach, Thomas MM Heenan, Huw Parks, Rhodri Jervis, Johanna Nelson Weker, Daniel JL Brett, and Paul R. Shearing. "Nanoscale state-of-charge heterogeneities within polycrystalline nickel-rich layered oxide cathode materials." Cell Reports Physical Science (2021): 100647.

Figure 1