Visualisation of Tetrahedral Li in the Alkali Layers of Li-Rich Layered Oxides

Li-rich metal oxide cathode materials for Li-ion batteries, such as Li1.2Ni0.13Mn0.54Co0.13O2 can proceed with the redox reaction of oxygen anions and transition metal (TM) cations, delivering superior capacities of over 250 mAh/g, much higher than the conventional metal oxides. However, the fading in the capacity and voltage and the sluggish kinetics are significant problems for the Li-rich oxides1,2. Understanding the Li+ ion diffusion pathway is crucial for understanding the sluggish kinetics in anionic O2- redox. Li diffusion within the alkali layers undergoes a well-known, low-barrier octahedral-tetrahedral-octahedral (o-t-o) pathway3, however, it is less clear how Li diffuses in and out of the TM layers, particularly given the complex structural rearrangements which take place during O2- oxidation.
Here, we perform electron ptychography and simultaneous annular dark field imaging to directly visualise the Li in Li1.2Ni0.13Mn0.54Co0.13O2. At the end of the TM-oxidation region and before the high voltage O oxidation plateau, we observe Li occupying alkali-layer tetrahedral sites on opposite sides of the TM layers, forming Li-Li dumbbell configurations. The presence of Li at these tetrahedral sites is demonstrated by mathematical processing of the quantitative ptychographic phase contrast. Density functional theory (DFT) calculations support the observation showing that tetrahedral Li-Li dumbbell configurations are lower in energy than corresponding tetrahedral TM configurations. We also observe the in- and out-of-plane TM migration as well as a partial phase transition from O3 to O1 stacking. In the O1 stacking phase, tetrahedral Li is absent, consistent with our DFT calculations indicating the O1 phase is not thermodynamically stable to accommodate tetrahedral Li. Upon further Li deintercalation to 4.8V, most tetrahedral Li is removed and we only observe a small amount of residual tetrahedral Li. After discharging to 2 V, we do not observe the reformation of tetrahedral Li but observe permanently migrated TMs occupying the alkali-layer sites. These migrated TMs may suppress the Li re-population of some vacant TM-layer octahedral sites by disfavouring Li occupation of the face-sharing tetrahedral sites. To maintain the maximum number of accessible tetrahedral Li sites and minimise the blocking of tetrahedral Li diffusion pathways, strategies to mitigate the irreversible TM migration and the O1 phase change should be employed.

Reference
1.House, R. A. et al. Nat Energy 5, 777, (2020).
2.Song, W. X. et al. Joule 6, 1049, (2022).
3.Kang, K., Meng, Y. S., Breger, J., Grey, C. P. & Ceder, G. Science 311, 977, (2006).
4. We acknowledge the financial support from the EPSRC (EP/K040375/1 ‘South of England Analytical Electron Microscope’), the Henry Royce Institute for Advanced Materials (EP/R00661X/1, EP/S019367/1, EP/R010145/1) and the Faraday Institution CATMAT project (FIRG016, FIRG035). We acknowledge the use of the facilities in the David Cockayne Centre for Electron Microscopy at University of Oxford.