To meet the increasing energy demands of portable devices and electric vehicles, high-nickel lithium-ion cathode materials with the general formula Li(Ni_xMn_yCo_z)O₂ (NMC) have been extensively researched. Currently NMC811 is used commercially for high-energy applications. The energy density of NMC also comes with concerns over cycle life and safety^1,2. To improve the cycle life of NMC-based cells, single-crystal materials have recently gained attention to tackle the particle cracking issues found in polycrystalline cathodes³. However, for successful introduction to the lithium-ion battery market, inherent safety over a material’s lifetime also needs to be proven. Failure and degradation mechanisms both need to be fully understood to improve the stability of future cathode materials. Abusive testing, such as overheating, overcharge and nail penetration, has been used in conjunction with in­-situ and ex-situ­ X-ray computed tomography (CT) 3D imaging to perform post-mortem studies and understand the relationship between thermal failure and cathode microstructure^4,5. However, the interplay between safety characteristics, microstructural properties and material degradation remains unclear.

This work first aims to compare the safety performance of polycrystalline and single-crystal NMC811 in 200 mAh pouch cells. Accelerating rate calorimetry (ARC) with a heat-wait-search (HWS) technique is used to heat cells and determine the onset of self-heating, onset of thermal runaway and the peak thermal runaway temperature. Laboratory-based pre- and post-mortem in-situ and ex-situ X-ray CT is also used for non-destructive imaging at multiple length scales to determine how failure propagates through the cells and the impacts on the electrodes and microstructure. Pouch cells containing polycrystalline and single-crystal NMC811 cathode and graphite anode are electrochemically cycled to induce material degradation. EIS measurements and diagnostic cycles are performed to identify prevalent degradation modes in both types of cathode materials. Finally, the same ARC and X-ray CT characterisations are performed on the aged cells to determine how degradation and changes to the material structure affect the safety performance in high-nickel cathode materials.

The results of this work will improve the current understanding of capacity fade in high-nickel cathodes and the safety behaviour over the lifetime of a battery cell. This information can then be used to inform future materials development and strategies for mitigating thermal runaway in batteries.

References

1. L. Ma, M. Nie, J. Xia, and J. R. Dahn, J. Power Sources, 327, 145–150 (2016).
2. H. J. Noh, S. Youn, C. S. Yoon, and Y. K. Sun, J. Power Sources, 233, 121–130 (2013).
3. J. Langdon and A. Manthiram, Energy Storage Mater., 37, 143–160 (2021).
4. D. Patel, J. B. Robinson, S. Ball, and D. J. L. Brett, (2020).
5. D. P. Finegan et al., Phys. Chem. Chem. Phys., 18, 30912–30919 (2016).