The Impacts of Particle Size Distribution and Electrode Structuring on NMC Cathode Performance

Electrifying the transportation sector is not just a necessity but an urgent call to action in curbing carbon emissions. To facilitate the widespread adoption of electric vehicles, lithium-ion batteries with extended lifespans, increased energy densities, and rapid charging capabilities are needed. The commercial active material particles used in battery production are not uniform in shape and size distribution. This heterogeneity results in heterogeneous electrode structures, leading to uneven reactions across the battery electrodes. Moreover, mass transport limitations at high charging rates can further exacerbate the reaction heterogeneity. This non-uniform utilisation of battery electrodes not only diminishes batteries’ capacities but has also been suggested to accelerate their capacity degradation. This PhD work focuses on how particle size distribution and electrode structuring affect reaction heterogeneity and, subsequently, the battery performance, including cyclability and rate performance. To this end, polycrystalline NMC111 and single crystal NMC811, each with two different particle sizes, were synthesised. The small NMC111 particles have an average particle size of 2.3 µm (standard deviation = 0.4 µm), while the big NMC111 particles have an average particle size of 5.5 µm (standard deviation = 0.8 µm). As for NMC811, the small particles have an average particle size of 0.6 µm (standard deviation = 0.2 µm), and the big particles have an average particle size of 1.5 µm (standard deviation = 0.5 µm). The NMC111 particles were prepared by carbonate coprecipitation, followed by calcination, whereas the single crystal NMC811 particles were prepared using the molten flux method. Using the NMC111 particles, three model electrodes with different particle size distributions were fabricated: one ‘Small’, one ‘Big’, and one ‘Mix’. It was thought that the reactivity difference between the big and small particles in the ‘Mix’ electrode would lead to enhanced reaction heterogeneity, causing the polydisperse electrode to exhibit the worst cyclability. As it turned out, the ‘Big’ electrode had the worst cyclability due to the more extensive particle cracking observed in the big particles. This result showed that the electrode cyclability depends not only on particle size and size distribution, as suggested in the literature, but also on the mechanical degradation (particle cracking) of the particles in the electrode. As mentioned earlier, mass transport limitations can lead to underutilisation of battery capacity. To address this challenge, various electrode designs have been proposed to improve the transport of ions into the bulk of the electrodes. To facilitate the screening of different electrode designs, an impedance model to quantify the transport properties of electrodes was developed. This model was then used to screen two different particle size-graded electrodes; one has small particles (particle diameter = 2.5 µm) layered on top of big particles (particle diameter = 5 µm), and the other has big particles layered on top of small particles. These two electrodes were shown to have distinct transport properties. Using both big and small polycrystalline NMC111 particles mentioned earlier, similar particle size-graded electrodes were fabricated, and impedance experiments confirmed that the trend in transport properties aligned with the model’s predictions. For the cycling conditions tested, the electrodes’ cyclability showed no strong dependence on the electrode structure, highlighting the potential of graded electrodes to improve the power density of batteries without sacrificing their cyclability.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Tan, Hwee Jien

Advisors dc:contributor.advisor

• De Volder, Michael
• Grey, Clare

• Bilayer electrodes
• Finite element simulation
• Lithium-ion batteries
• Operando X-ray diffraction
• Solid state synthesis

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/bf062e04-89c5-4be3-a3e3-1f5556028748/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.117513

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/382929