Visualization\nof Tetrahedral Li in the Alkali Layers\nof Li-Rich Layered Metal Oxides

Understanding Li<sup>+</sup> ion diffusion pathways in Li-rich\nlayered transition metal (TM) oxides is crucial for understanding\nthe sluggish kinetics in anionic O<sup>2–</sup> redox. Although\nLi diffusion within the alkali layers undergoes a low-barrier octahedral–tetrahedral–octahedral\npathway, it is less clear how Li diffuses in and out of the TM layers,\nparticularly given the complex structural rearrangements that take\nplace during the oxidation of O<sup>2–</sup>. Here, we develop\nsimultaneous electron ptychography and annular dark field imaging\nmethods to unlock the Li migration pathways in Li<sub>1.2</sub>Ni<sub>0.13</sub>Mn<sub>0.54</sub>Co<sub>0.13</sub>O<sub>2</sub> associated\nwith structural changes in the charge–discharge cycle. At the\nend of TM oxidation and before the high-voltage O oxidation plateau,\nwe show that the Li migrating out of the TM layers occupies the alkali-layer\ntetrahedral sites on opposite sides of the TM layers, forming Li–Li\ndumbbell configurations, consistent with the density functional theory\ncalculations. Also occurring are the TM migration and phase transition\nfrom O3 to O1 stacking, leading to unstable tetrahedral Li and the\nabsence of Li contrast in imaging. Upon further Li deintercalation\nto 4.8 V, most of the tetrahedral Li are removed. After discharging\nto 2 V, we did not identify the reformation of tetrahedral Li but\nobserved permanently migrated TMs at the alkali-layer sites, disfavoring\nthe Li occupying the tetrahedral sites for diffusion. Our findings\nsuggest a landscape of Li diffusion pathways in Li-rich layered oxides\nand strategies for minimizing the disruption of Li diffusion.