Abstract:

High-energy batteries are important in enabling an electricity-based transport network and achieving net carbon-zero energy usage. Layered metal oxides are the predominant cathode materials in Li-ion batteries, yet their specific capacities are constrained by the cationic redox reactions. One of the most promising high-nickel ternary oxides, Li[Ni0.8Co0.1Mn0.1]O2 (NMC811) has a theoretical capacity of 200 mAh g-1 through the redox reaction of Ni2+ and Co3+ ions1. A state-of-the-art class of cathode materials, Li-rich layered metal oxides can break through the constraint and extend the capacity beyond 250 mAh g-1, by leveraging additional redox reaction of the lattice oxygen anions, such as Li-rich Li1.2[Ni0.132+Mn0.544+Co0.133+]O2 (LRNMC)2.

Despite the enhanced capacity, oxygen redox introduces severe degradation problems in materials, including voltage hysteresis, sluggish kinetics and voltage fading upon cycling3. These issues are rooted in atomic-scale structural and chemical changes. Transmission electron microscopy (TEM) has been the main technique for determining materials’ atomic positions and local chemistry using specific scattered electrons and characteristic X-rays. However, conventional TEM struggles to fully elucidate, particularly for the evolution of the light O and Li lattice structures.

Four-dimensional scanning TEM (4D STEM) is a cutting-edge imaging technique that can deliver rich material information beyond that of conventional TEM. Ptychography is a method to process the 4D data to reconstruct the phase information of the sample with a high contrast transfer function and dose efficiency4, sensitive to the heavy metals and also the light O and Li atoms in materials. In this talk, I will present the development of 4D STEM ptychography in addressing key degradation problems associated with O and Li lattice in LRNMC5,6. I will also demonstrate how these atomic-scale understandings can inform the synthesis to design better cathode materials.

Reference

1             Xu, C. et al. Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries. Nat Mater 20, 84-92, (2021).

2             Marie, J. J. et al. Trapped O(2) and the origin of voltage fade in layered Li-rich cathodes. Nat Mater 23, 818-825, (2024).

3             Gent, W. E. et al. Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides. Nat Commun 8, 2091, (2017).

4             Yang, H., Pennycook, T. J. & Nellist, P. D. Efficient phase contrast imaging in STEM using a pixelated detector. Part II: Optimisation of imaging conditions. Ultramicroscopy 151, 232-239, (2015).

5             Song, W. X. et al. Direct imaging of oxygen shifts associated with the oxygen redox of Li-rich layered oxides. Joule 6, 1049-1065, (2022).

6             Song, W. X. et al. Visualization of Tetrahedral Li in the Alkali Layers of Li-Rich Layered Metal Oxides. J Am Chem Soc 146, 23814-23824, (2024).