Norman Fleck's KEYNOTE seminar "Mechanics aspects of solid state lithium ion batteries"

Norman_Fleck_March_09_2023.pdf (pdf 171 kB)

Abstract. Li ion batteries discharge by the transport of Li ions from an anode (such as graphite or Li metal) to a cathode comprising ceramic particles that swell upon lithiation. The next generation of batteries comprise cathode particles in the form of single crystals made from layered nickel rich materials. Recently, optical microscopy has been performed that reveal the diffusion of Li within these single crystals (“Operando visualisation of kinetically-induced lithium heterogeneities in single-particle layered Ni-rich cathodes” by Chao X, et al., Joule 6, 1-12, 2022.) This allows for a direct comparison with a fully coupled chemo-mechanical model of Li diffusion, including the role of stress. Predictions reveal that the level of induced stress in the single crystals is sufficient to induce cracking when the particles are large and the rate of discharge (lithiation) is very fast (full battery discharge in 10 minutes). In the final part of the talk, a constrained compression test is reported to simulate the mechanical state of a lithium dendrite within a solid state battery. Lithium microspheres are compressed between parallel quartz platens into a pancake shape of thickness on the order of 15 µm. Full adhesion with no slip exists between the lithium and platens, and the attendant mechanical constraint implies that the average pressure on the pancake-shaped specimens increases with increasing aspect ratio of radius to height. In addition to mechanical constraint, a thickness-dependent size effect is observed whereby the flow strength of the lithium increases from 0.72 MPa in the bulk to 4.5 MPa at a thickness of 15 µm. The lithium deforms in a power-law creeping manner at room temperature, and to simplify interpretation of the results the relative velocity of the loading platens is adjusted to ensure that the true compressive strain rate is held fixed. Additional measurements of lithium flow strength are obtained by subjecting the pancake-shaped specimens to simple shear. The size effect under shear loading is minor compared to that observed for constrained compression, and this is explained by appealing to strain gradient plasticity theory.