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Bulk Fatigue Induced by Surface Reconstruction in Layered Ni-Rich Oxide Cathodes for Liion Batteries

preprint
submitted on 10.03.2020 and posted on 11.03.2020 by Chao Xu, Katharina Marker, Juhan Lee, Amoghavarsha Mahadevegowda, Philip J. Reeves, Sarah Day, Matthias F. Groh, Steffen Emge, Caterina Ducati, B. Layla Mehdi, Chiu C. Tang, Clare P. Grey

Ni-rich layered cathode materials are among the most promising candidates for high energy density Li-ion batteries. However, the low cobalt containing materials suffer from rapid degradation, the underlying mechanism of which is still poorly understood. We herein report a novel structure-drive degradation mechanism for the NMC811(LiNi0.8Mn0.1Co0.1O2) cathode, in which a proportion of the material exhibits a lowered accessible state-of-charge (SoC) at the end of charge after repetitive cycling, i.e. becomes fatigued. Ex-situ and operando long- duration high-resolution X-ray diffraction enabled by a laser-thinned coin cell design clearly shows the emergence of the fatigued phase and the increase in its population as the cycling progresses. We show that the fatigue degradation is a structure-driven process rather than originating solely due to kinetic limitations or inter-granular cracking. No bulk phase transformations or increase in Li/Ni antisite mixing were observed by diffraction; no significant change in the local structure or Li-ion mobility of the bulk were observed by 7Li solid-state NMR spectroscopy. Instead, we propose that the fatigue process is a result of the high interfacial lattice strain between the reconstructed surface and the bulk layered structure when the latter is at SoCs above a distinct threshold of ~75 %. This mechanism is expected to be universal to Ni-rich layer cathodes, and our findings provide a fundamental guide for designing effective approaches to mitigate such deleterious processes.

Funding

EP/S003053/1

History

Email Address of Submitting Author

cx237@cam.ac.uk

Institution

University of Cambridge

Country

United Kingdom

ORCID For Submitting Author

0000-0001-5416-5343

Declaration of Conflict of Interest

No conflict of interest

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