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 (NMCXYZ) have been extensively researched. Currently, NMC811 is used commercially due to its high capacity and low cobalt content. However, capacity fade is still a prominent issue, with a variety of degradation mechanisms responsible. Of particular interest is secondary particle cracking, where the stress-strain effect of c-parameter expansion in the NMC crystal lattice leads to fractures. Boundaries between the primary particle grain that make up the secondary particle structure act as nucleation points for particle cracking. The exposed surface then reacts with the liquid electrolyte, leading to oxygen evolution and capacity loss. Single-crystal materials have shown promise as a method of improving cycle life, as the lack of agglomerated primary grains appear to make the monocrystalline particles more resilient to the mechanical strain of expansion during charging¹.

In parallel with the cycle life concerns, attention over battery safety continues to increase as instances of catastrophic battery failure are publicised across the world. While the specific steps that lead to battery thermal runaway have been extensively researched, the safety and thermal stability of degraded cells remains a scarcely investigated topic². To that end, the goal of this work is to characterise the safety properties of different NMC morphologies, and study how charge-discharge cycling affects the thermal stability of these materials.

In this work, we have combined accelerating rate calorimetry (ARC) with lab-based, post-mortem macro, micro and nano X-ray CT to observe the influence of cell structure on thermal runaway of NMC811 pouch cells with single-crystal and polycrystalline cathode materials in both the pristine and aged state. The results presented show that when pristine, the larger surface area of single-crystal materials provides more reaction sites and leads to a lower self-heating onset and thermal runaway initiation temperature compared to polycrystalline, suggesting a lower thermal stability. However, as polycrystalline particles break up during thermal runaway, the fragments create fresh surface to react with the electrolyte and continue the series of exothermic reactions, leading to a higher peak temperature. Macro-CT has revealed how the wound jellyroll structure of the cell focuses the force of the explosion through the flat sides of the cells, possibly increasing the likelihood of failure propagation. It has also been observed that the polycrystalline materials show a greater material ejection out from the centre of the cell, reinforcing the findings from the ARC investigations that although polycrystalline cells are more thermally stable initially, the peak temperatures produced during catastrophic failure are higher due to particle fracture. Micro and nano-CT are used to probe this idea further, revealing that polycrystalline particles show significant damage and fracturing during failure, whereas single-crystal materials remain largely intact.

Both polycrystalline and single-crystal morphologies were cycled to 80% capacity retention, with EIS and diagnostic cycles used to identify prevalent degradation modes for both particle structures. As previously found, the single-crystal cells showed superior cycle life to the polycrystalline materials³. The same combination of ARC and X-ray CT was then applied to aged cells, where it was found that aged cells show lower thermal stability than their pristine counterparts.

Overall, this work aims to build understanding of the interplay between material structure, safety and degradation of nickel-rich cathodes, with the goal of informing future material development to produce batteries that are safer throughout their lifetimes.

1. G. Qian et al., Energy Storage Mater., 27, 140–149 (2020) https://doi.org/10.1016/j.ensm.2020.01.027.
2. X. Feng et al., Energy Storage Mater., 10, 246–267 (2018) https://doi.org/10.1016/j.ensm.2017.05.013.
3. D. Ren et al., eTransportation, 2, 100034 (2019) https://doi.org/10.1016/j.etran.2019.100034.