Multi-Length Scale Analysis of Individual NMC811 Particle Degradation Using X-Ray Computed Tomography
Authors: Wade Aaron , Heenan Thomas M. M. , Leach Andrew , Kok Matthew , Shearing Paul R. 
Published: December 2020

Due to issues with toxicity and price, many manufacturers are reducing the cobalt content within lithium-ion (Li-ion) batteries by moving towards nickel-rich cathode chemistries, such as layered nickel-manganese-cobalt-oxide LiNi0.8Nm0.1Co0.1O2 (NMC811). Increasing the Ni content is accompanied by higher theoretical capacities, as high as 275 mAh g-1. However, the complex degradation of this material is not fully understood (1)(2)(3) and consequently practical capacities can be significantly lower (~200 mAh g-1) and capacity retention can be low, e.g. ~70% over 100 cycles to 4.3 V vs. Li/Li+ (4). The cracking of electrode particles (5) is a prominent degradation mechanism, but in order to gain a more comprehensive understanding of its origins and propagation, a multi-scale approach is required (6).

X-ray computed tomography (CT) has revolutionised the characterisation of energy materials as it enables the non-destructive imaging of microstructure in 3D (7) . In this work, the effect of cycle number, C-rate and upper cut-off voltage on the morphology of NMC electrodes and particles have all been investigated using lab-based X-ray CT. This was achieved through post-mortem 3D analysis of the samples using micro- and nano-CT to reveal the electrode and particle degradation across multiple length scales.

The individual particles within the 3D tomograms (Figure 1) have been quantitatively and qualitatively assessed to build a comprehensive understanding of the electrode state of health for a wide library of electrochemical histories. This has been achieved through carrying out greyscale intensity analysis in 3D on individual particles, and comparing them across cycling conditions, size ranges and categorizing each particle through machine learning.

Due to the number of particles assessed, this work presents unprecedented materials statistics with detail of the degradation of NMC811 from the electrode to sub-particle level. Here, we discuss the methodology and findings from the multi-length-scale study, investigating both the electrochemical and mechanical response due to the variations in voltage, C-rate and number of cycles, and the implications of this research on the Li-ion field.

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Figure 1