Entropic Stabilization in Lithium Rich Transition Metal Oxide Cathodes

Abstract

It is well known that lithium rich transition metal oxide cathodes (LRM) experience voltage fade and capacity loss over the course of cycling, and it is generally known that these are a result of the LRM materials’ general instability 1–7. However, the mechanisms behind these losses are still unknown, due in large part to the structural ambiguity and controversy associated with the LRM material structure. In this perspective we consider the role entropy plays in LRM materials’ structural stability and electrochemical properties. With this mind set we argue that due to their metastability LRM cathodes’ structure and properties are not synthesis agnostic, and therefore establishment of consistent synthetic procedures is of paramount importance. We also propose that LRM materials could be categorized as entropically stabilized oxides through exploring their configurational entropies, phase behaviors, and thermal responses. The entropic stabilization of LRM materials is also found to be consistent with the ordering behaviors of LRM cathode transitional metal layers, with this ordering presenting both compositional, thermal, and electrochemical dependencies. The impact of these behaviors on the redox and electrochemical structure and activation of LRM materials is also explored. Finally, we seek to put these findings into the context of the wider battery field. Fell, C. R. et al. Correlation Between Oxygen Vacancy, Microstrain, and Cation Distribution in Lithium-Excess Layered Oxides During the First Electrochemical Cycle. Chem. Mater. 1621–1629 (2013) doi:10.1021/cm4000119.Lu, Z., Beaulieu, L. Y., Donaberger, R. A., Thomas, C. L. & Dahn, J. R. Synthesis, Structure, and Electrochemical Behavior of Li [Ni x Li(1/2-2x/3) Mn(2/3-x/3)]O2. J. Electrochem. Soc. 149, A778–A791 (2002).Wu, Y. & Manthiram, A. High Capacity, Surface-Modified Layered Li†Li„1−x.../3Mn„2−x.../3Nix/3Cox/3‡O2 Cathodes with Low Irreversible Capacity Loss. Electrochem. Solid-State Lett. 9, A221–A224 (2006).Dupré, N., Cuisinier, M., Legall, E., War, D. & Guyomard, D. Contribution of the oxygen extracted from overlithiated layered oxides at high potential to the formation of the interphase. J. Power Sources 299, 231–240 (2015).Jarvis, K. A. et al. Formation and effect of orientation domains in layered oxide cathodes of lithium-ion batteries. Acta Mater. 108, 264–270 (2016).Jiang, M., Key, B., Meng, Y. S. & Grey, C. P. Electrochemical and Structural Study of the Layered, ‘Li-Excess’; Lithium-Ion Battery Electrode Material Li[Li 1/9 Ni 1/3 Mn 5/9 ]O 2. Chem. Mater. 21, 2733–2745 (2009).Hy, S. et al. Performance and design considerations for lithium excess layered oxide positive electrode materials for lithium ion batteries. Energy Environ. Sci. 9, 1931–1954 (2016).