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NMCfatigue_main_submission_v1.pdf (8.53 MB)

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

preprint
submitted on 10.03.2020, 20:26 and posted on 11.03.2020, 11:54 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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