Hidden Subsurface Reconstruction and Its Atomic Origins in Layered Oxide Cathodes.

Structural transformations near surfaces of solid-state materials underpin functional mechanisms of a broad range of applications including catalysis, memory, and energy storage. It has been a long-standing notion that the outermost free surfaces, accompanied by broken translational symmetry and altered atomic configurations, are usually the birthplace for structural transformations. Here, in a layered oxide cathode for Li-ion batteries, we for the first time observe the incipient state of the well-documented layered-to-spinel-like structural transformation, which is surprisingly initiated from the subsurface layer, rather than the very surface. Coupling atomic level scanning transmission electron microscopy imaging with electron energy loss spectroscopy, we discover that the reconstructed subsurfaces, featuring a mix of discrete patches of layered and spinel-like structures, are associated with selective atomic species partition and consequent nanoscale nonuniform composition gradient distribution at the subsurface. Our findings provide fundamental insights on atomic-scale mechanisms of structural transformation in layered cathodes.