New insights into aging in LiNiO <sub>2</sub> cathodes from high resolution paramagnetic NMR spectroscopy

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Abstract

Heterogeneity develops in LiNiO ₂ upon long-term cycling. The structure and composition of partially electrochemically active (fatigued) phases formed in the bulk are determined by combining synchrotron XRD and high resolution ⁷Li solid-state NMR.

Abstract

Bulk degradation processes are examined in the LiNiO ₂ cathode using high resolution solid-state NMR, combined with magnetometry and X-ray diffraction. Capacity decay is correlated with bulk heterogeneity, whereby multiple structural domains coexist in the charged state, and the Li content and electrochemical activity of these domains is unraveled for the first time.

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Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries

Chao Xu, Katharina Märker, Juhan Lee … (2020)

Ni-rich layered cathode materials are among the most promising candidates for high-energy-density Li-ion batteries, yet their degradation mechanisms are still poorly understood. We report a structure-driven degradation mechanism for NMC811 (LiNi0.8Mn0.1Co0.1O2), in which a proportion of the material exhibits a lowered accessible state of charge at the end of charging after repetitive cycling and becomes fatigued. Operando synchrotron long-duration X-ray diffraction enabled by a laser-thinned coin cell shows the emergence and growth in the concentration of this fatigued phase with cycle number. This degradation is structure driven and is not solely due to kinetic limitations or intergranular cracking: no bulk phase transformations, no increase in Li/Ni antisite mixing and no notable changes in the local structure or Li-ion mobility of the bulk are seen in aged NMCs. Instead, we propose that this degradation stems from the high interfacial lattice strain between the reconstructed surface and the bulk layered structure that develops when the latter is at states of charge above a distinct threshold of approximately 75%. This mechanism is expected to be universal in Ni-rich layered cathodes. Our findings provide fundamental insights into strategies to help mitigate this degradation process.