Elucidating Heterogeneous Li Insertion Using Single-Crystalline\nand Freestanding Layered Oxide Thin Film

The\nkinetics of interfacial ion insertion govern the uniformity\nof electrochemical reactions, playing a crucial role in lithium-ion\nbattery performance. In two-dimensional lithium-conducting layered-oxide\nbattery particles, variation in insertion rates across insertion channels\nremains unclear due to poorly defined crystal orientation at the solid–liquid\ninterface and solid-state-lithium-diffusion length. This ambiguity\ncomplicates understanding inhomogeneous lithium-insertion channels\nactivation. A systematic study requires crystallographically predefined\ninterfaces and in situ lithium-concentration mapping. Here, we fabricated\na freestanding, (104)-oriented-LiNi<sub>1/3</sub>Mn<sub>1/3</sub>Co<sub>1/3</sub>O<sub>2</sub> single-crystal thin film using dissolution-induced\nrelease and performed in situ scanning-transmission-X-ray-microscopy\nto spatially resolve lithium-insertion at well-defined-interfaces.\nWe observed heterogeneous lithium-concentration evolution due to channel-by-channel\ninsertion rate variation, despite the potential for homogeneous lithium\ndistribution via a solid-solution-phase at equilibrium in NMC111.\nIncreasing current density exacerbates this heterogeneity, highlighting\nchannel-by-channel variation. Our findings provide critical insights\ninto battery electrode utilization and lifetime management, potentially\nguiding the design of more efficient and durable lithium-ion batteries.