Strain-Associated Nanoscale Fluctuating Lithium Transport Within Single-Crystalline NMC Cathode Particles

The pursuit of high-performance energy storage has led to significant research on the fundamental processes in lithium-ion batteries (LIBs), where lithium transport within active materials is crucial for their rate performance, capacity, and longevity. Traditionally, lithium diffusion in solid-solution materials has been assumed to follow Fick’s law, aiming to minimize concentration gradients, similar to behavior in liquids or gases. This led to the widespread belief that lithium diffusion creates a uniform lithium distribution within solid-solution particles. However, diffusion is not solely driven by concentration gradients but by chemical potential gradients, which consider thermodynamic factors such as strain/stress, mixing free energy, and interfacial energy.

Active battery materials undergo anisotropic lattice volume changes during lithium insertion and extraction, generating strain and stress fields that affect diffusion. This suggests that lithium concentration gradients may not always align with chemical potential gradients, causing deviations from the expected diffusion direction, especially in single-crystalline solid-solution materials. If diffusion were only driven by concentration gradients, predictable patterns, such as core-shell or uniform distributions, would emerge during cycling based on the limiting regime.¹ However, direct evidence showing dynamic lithium diffusion pathways deviating from conventional patterns or concentration gradients is still lacking, highlighting the need for deeper investigation into these processes.

Using operando scanning transmission soft X-ray microscopy with high spatial resolution and chemical sensitivity to track nanoscale intraparticle lithium transport,² and post-cycling Bragg coherent diffraction X-ray imaging to directly reveal three-dimensional intraparticle strain fields, we uncovered strain-associated lithium transport dynamics within single-crystalline LiNi_1/3Mn_1/3Co_1/3O₂ (scNMC) particles during cycling. Contrary to the expected thermodynamic solid-solution behavior of scNMC, our observations reveal near-uniform but fluctuating regions of lithium-dense and lithium-dilute areas during cycling. These fluctuations suggest that nanoscale lithium diffusion can proceed counter to concentration gradients, driven by local strain fields. Additionally, we demonstrate that an increased presence of lithium-dilute regions near the surface enhances lithium surface insertion kinetics, emphasizing the importance of controlling surface lithium distribution to improve rate performance. Our study provides novel insights into nanoscale solid-state ion transport, with potential applications in batteries, solid-state fuel cells, and memristors.


References
1. C. Xu et al. Joule 6, 2535-2546 (2022).
2. J. Lim et al. Science 353, 566-572 (2016).