Advanced Technique Development for Li-Ion Electrode Characterisation Using X-Ray Computed Tomography
Authors: Randjbar Daemi Sohrab , Tan Chun , Lu Xuekun , Cookson James , Petrucco Enrico , Palacios-Padros Anna , Brett Daniel J.L. , Shearing Paul R. 
Published: February 2020

Porous electrodes are the fundamental building block of most modern battery designs. Historically, 1-dimensional models have been used to predict battery performance at low cycling rates, but these fail to account for degradation after extended cycling due to their oversight of the 3-dimensional nature of electrode microstructures. The advancement of X-ray computed tomography (CT) has allowed for imaging of battery materials in a non-destructive way with a range of ex-situ, in-situ and operando techniques.1–3 We present the development of a suite of techniques devised to further the understanding of these complex microstructures, using Lithium Nickel Manganese Cobalt Oxide (NMC) as a model electrode and characterising it from the nano- to the micro-scale using different approaches.

In the first instance ex-situ nano- and micro-CT were applied to compare how the microstructural evolution of an electrode calendered with a pellet press differs from that of one compressed with a calendering roller. Particle shape analysis and the characterisation of the porous networks and their associated transport properties were carried out and a novel laser micro-machining technique used to manufacture geometrically optimised samples for nano-CT is also discussed. This allows creating 80 μm electrode pillars that maintain their structure and directionality, whilst providing excellent resulting image quality.4

In a second instance, to overcome the limitations of X-ray CT in imaging the carbon and binder domain (CBD), a novel correlative approach combining multi-length scale CT and focused ion beam-scanning electron microscope (FIB-SEM) is presented.5 FIB-SEM and nano-CT were used to calculate the effective diffusivity of the CBD porous network by using the aforementioned analytical tools and sample preparation techniques. The CBD diffusivity is turn incorporated into a larger NMC framework obtained via micro-CT. The specific contribution of the CBD is hence discussed in the context of overall ionic transport of a full electrode.

Finally, a technique to compress electrodes whilst imaging them in-situ with nano-CT is presented. This was developed with the intent of emulating calendering and allows the tracking of active material particles as a function of applied load. Strain hot-spots as a result of the movement of secondary particles were identified in the 3-D volume by using a digital volume correlation algorithm. A suite of microstructural characterisation and quantification techniques examining the solid volume fraction, inter-particle separation and general morphology of the electrode were also applied. Further application of this technique will allow for the development of complex multi-phase models that will be able to accurately predict the electrochemical performance of calendered electrodes as a function of load.

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