Operando Detection of Cracking in High Ni Battery Cathode Materials by X-Ray CT

In a drive for ever-increasing energy density and reduction in use of cobalt in lithium ion battery cathodes, high-nickel content layered cathodes (Ni&gt;80%) of the form LiNi_xMn_yCo_zO₂ (NMC) have received significant recent attention. These materials undergo considerable anisotropic lattice changes during cycling (lithiation and delithiation) which has been attributed to the creation of microcracks in polycrystalline agglomerate cathode particles. This cracking behaviour has long been suspected to cause increased surface reactions leading to impedance rise, reduction of electronic conductive pathways and eventually complete particle detachment from the conductive matrix and pulverisation; however, the pre-existence of particle cracks and defects from manufacturing and calendaring of the electrodes can make it difficult to assess the contribution of the electrochemical history alone in causing cracks[1]. In a bid to reduce this, so-called single-crystal materials have been proposed to be more resistant to degradation[2] due to their resilience to cracking[3] and lower surface exposure to electrolyte, which can cause parasitic reactions and drive oxygen loss[4].<br/><br/>Assessing the origins of cracking and its evolution with battery ageing is complex due to the small and internal nature of the cracks, often requiring either destructive imaging techniques (such as FIB SEM or TEM on lamella) or small sample sizes in non-realistic environments to facilitate non-destructive imaging by methods such as X-ray computed tomography (CT). In previous work, we have developed methods for producing electrodes with optimised tab geometries for in situ and operando X-ray CT and related characterisation methods[5, 6], which we use here to allow for a pseudo-in situ nano-CT study of electrochemically induced cracking in NMC811 particles with minimal pre-existing damage[7]. This allows for direct observation of first-cycle cracking within the particles using lab-based CT on identical particles in the discharged and charged state, showing that even on the first cycle, significant intergranular cracks can develop in the polycrystalline materials. Additional ex situ studies on non-identical particles have revealed cracking in polycrystalline materials occurs at relatively low potentials, in the early stages of charging, and is at least partially recovered on discharge[8].<br/><br/>In this presentation we will extend this methodology to assess the progression of first cycle cracking throughout the charge and discharge range on thousands of identical particles from the same electrode within a realistic pouch cell environment. The use of the tab electrode geometry within a working pouch cell and larger field of view afforded by synchrotron micro-CT studies, combined with automated image processing and analysis, allows us to follow the behaviour of around 5,000 NMC particles through multiple states of charge, revealing the diversity of cracking behaviour within working, commercially relevant, Li ion battery electrodes. Advanced grey-scale analysis of the data reveals sub-resolution limit information about the damage to the particles throughout their states of (de)lithiation and provides a statistically relevant quantification of the extent of crack closure behaviour.<br/><br/>[1] T. M. M. Heenan et al., Advanced Energy Materials, vol. 10, 2020<br/>[2] H. Li et al., Journal of The Electrochemical Society, vol. 165, 2018/04/05 2018<br/>[3] S. S. Pandurangi et al., Journal of The Electrochemical Society, vol. 170, 2023/05/23 2023<br/>[4] T. Li et al., Electrochemical Energy Reviews, vol. 3, 2020/03/01 2020<br/>[5] C. Tan et al., Journal of The Electrochemical Society, vol. 167, 2020/04/06 2020<br/>[6] C. Tan et al., Cell Reports Physical Science, vol. 2, 2021/12/22/ 2021<br/>[7] H. C. W. Parks et al., Journal of Materials Chemistry A, 10.1039/D3TA03057A vol. 11, 2023<br/>[8] A. Wade et al., Journal of The Electrochemical Society, vol. 170, 2023/07/14 2023