Lithium-ion batteries can fall into two general types. Liquid-electrolyte systems, which potentially have safety concerns arising from flammability of the electrolyte, and solid-electrolyte systems, which both address these safety issues and possess increased energy density, but suffer from degradation of the cathode. Now, Xueli Zheng and colleagues investigate the complex and interlinked electro-chemo-mechanical degradation mechanisms of a layered oxide cathode (Sci. Adv. 11, eady7189; 2025).
Zheng and colleagues used a full-field transmission X-ray microscope to image a NMC811 particle in its discharged state after a LixIn|Li6PS5Cl|NMC811 full battery is cycled six times. By performing three-dimensional (3D) imaging to measure Ni K-edge energy, they can image both the morphology and Ni chemical state, and so the state of charge (SOC), throughout a NMC811 particle (see image, the left two panels where blue indicates maximal discharge). The distribution of lower valence Ni states within the interior of the particle is heterogeneous, while 2D slicing of the 3D tomography (right two panels; scale bar, 1 μm) shows internal cracking. In addition, some internal domains show complete isolation as they have a high SOC compared with their surroundings (see middle column of three panels, where the top panel shows X-ray absorption, the middle panel tomography and the bottom panel the overlap of both datasets; scale bar, 3 μm). Liquid electrolytes can still access these isolated domains through infiltration, but solid electrolytes cannot, and so that capacity becomes inaccessible. Zheng and colleagues conclude that capacity decay in NMC811 arises from a vicious cycle of chemical and mechanical effects. Lithium diffusion from the sulfide electrolyte into NMC811 is heterogeneous. As each grain expands and contracts upon repeated lithiation and delithiation, cracks form between grains preventing intergranular lithium diffusion. Some domains become isolated, inducing non-uniform SOC across the particle as a whole, exacerbating mechanical stress and stress gradients, causing further crack formation.
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