Cracking and associated volumetric expansion of NMC811 secondary particles

• Published in: Journal of power sources Vol. 588; p. 233745
• Main Authors: Shishvan, S.S., Fleck, N.A., McMeeking, R.M., Deshpande, V.S.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.12.2023
• Subjects: Cracking; Li-ion battery; Modelling; Ni-rich NMC cathodes; Secondary particle; Cracking; Modelling; Secondary particle; Li-ion battery; Ni-rich NMC cathodes
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2023.233745

Secondary particles comprising a large number of nickel-rich single crystal primary particles are extensively used as storage particles in cathodes of lithium-ion batteries. It is well-established that crack formation in secondary particles is an important degradation mode that contributes to decline in battery performance. Recent X-ray tomographic observations suggest that, at very low C-rates, concentration gradients of lithium within an NMC811 secondary particle are negligible yet cracking still occurs. Additionally, during delithiation the primary particles shrink yet a volumetric expansion of the secondary particle occurs. These observations are explained by a numerical model of distributed cracking due to the extreme anisotropy of lithiation strain of primary particles. The incompatible deformation from grain to grain induces large self-stresses even in the absence of spatial gradients in the lithium concentration. The stress state is sufficient to drive a dynamic catastrophic fracture event, and the associated kinetic energy acquired by the primary particles moves them apart (akin to an explosive event) with the carbon and binder domain surrounding each secondary particle restricting the outward motion of the primary particles. It is predicted that a volume expansion of the secondary particles on the order of 20 % accompanies cracking, in agreement with recently reported observations. •A framework is developed to model the fracture of NMC811 secondary particles.•Fragmentation is predicted in the absence of Li concentration gradients.•Volumetric expansion of the secondary particles upon cracking is predicted.•The dynamic nature of the cracking event during delithiation is emphasised.•Predictions are in agreement with observations.