In-situ Imaging and Analysis of Li-ion Electrode Drying and Mud Cracking with X-ray Micro-CT
Authors: Dawson Will J , Morrison Andrew RT , Boyce Adam M , Miller Thomas Samuel , Shearing Paul Robert 
Published: December 2024

The manufacture of Li-ion battery electrodes with advanced microstructures which reduce tortuosity are desirable, as they enable thicker, more energy dense electrodes, as well as improving cell capacity at high cycling rates. In recent years a variety of such manufacturing methods, such as magnetic and ice templating, have been demonstrated which allow the production of battery electrodes with engineered porous structures.1,2 Though effective, these generally require substantial changes to manufacturing equipment and method.

Mud cracking is the process whereby cracks through the electrode, from surface to current collector, form spontaneously during the final phase of the drying process of Li-ion battery electrodes, once the active particles have settled out of suspension in the solvent.3 Cracking is believed to occur due to capillary stress resulting from the evaporation of solvent from the pore network between settled active particles.4 The severity of cracking depends on coating thickness, drying temperature and the solvent used.5 Control of cracking intensity can therefore be achieved by varying of solvent composition in the electrode slurry .6 It has been suggested that mud cracks provide low-tortuosity pathways, allowing for rapid ion transport and improved performance at rates above 2C.7 Since mud cracking requires only changes to the electrode formulation, it may provide an opportunity to develop advanced, low-tortuosity electrode architectures without substantial capital expenditure.

Here we present the first detailed analysis of in-situ observation of crack nucleation and growth using synchrotron X-ray computed tomography. With this approach we characterise the development of the crack network, and the relationship between the crack volume and the changing porosity of the electrode. Complementing this in-situ study, we present an analysis of the impact of mud cracking on the rate performance of thick electrodes, utilising electrochemical models over multiple length scales. These methods, in combination, give a fuller understanding of mud cracking, from crack formation through to their impact on local active particle lithiation, as well as providing a template for the imaging and analysis of low-tortuosity thick electrodes.

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Figure 1 – Volume renderings of reconstructed X-ray tomographs of varying thicknesses, showing (a,c,e,g ) full 3D structures and (b,d,f,h) isolated crack regions; (i) corresponding gravimetric discharge capacities of electrodes at rates from C/10 to 4C; (j,k) image ortho slices from synchrotron X-ray CT imaging of cracks opening over approximately 10 minutes.

Figure 1