Visualisation of tetrahedral Li in the alkali layers of L-rich layered metal oxides

Understanding Li⁺ ion diffusion pathways in Li-rich layered transition metal (TM) oxides is crucial for understanding the sluggish kinetics in anionic O^2- redox.  Although Li diffusion within the alkali layers undergoes a low-barrier octahedral-tetrahedral-octahedral pathway, it is less clear how Li diffuses in and out of the TM layers, particularly given the complex structural arrangements that take p lace during the oxidation of O^2-.  

In the paper 'Visualisation of tetrahedral Li in the alkali layers of Li-rich layered metal oxides' published in the Journal of the American Chemical Society, the authors from this department, the Faraday Institution and the Henry Royce Institute develop simultaneous electron ptychography and annular dark field imaging methods to unlock the Li migration pathways in Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ associated with structural changes in the charge-discharge cycle.  At the end of TM oxidation and before the high-voltage O oxidation plateau, the authors demonstrate that the Li migrating out of the TM layers occupies the alkali-layer tetrahedral sites on opposite sides of the TM layers, forming Li-Li dumbbell configurations, consistent with the density functional theory calculations.

Additionally, TM migration and phase transition occurred from O3 to 07 stacking, leading to unstable tetrahedral Li and the absence of Li contrast in imaging.  Upon further Li deintercalation to 4.8V, most the tetrahedral Li were removed.  After discharging to V V, the authors did not identify the reformation of tetrahedral Li but observed permanently migrated TMs at the alkali-layer sites, disfavouring the Li occupying the tetrahedral sites for diffusion. 

The findings set out in this paper suggest a landscape of Li diffiusion pathways in Li-rich layered oxides and strategies for minimising the disruption of Li diffusion.