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Researchers determine cause of voltage fade in NMC cathode materials, find it is reversible

Lithium-rich layered oxides (LRLO) are leading candidates for the next-generation cathode materials for energy storage, as they can deliver 50% excess capacity over commercially used compounds. However, voltage fade has prevented effective use of the excess capacity, and a major challenge has been a lack of understanding of the mechanisms underpinning the voltage fade.

Now, researchers led by a team from the University of California San Diego team have determined a mechanism for the cause of the performance-reducing voltage fade in the high-capacity LRLO material called NMC. The findings also reveal that voltage fade in LRLO is reversible and call for new paradigms for improved design of oxygen-redox active materials. A paper on their work is published in the journal Nature Energy.

The lithium-rich layered oxide (LRLO) compounds are among the most promising positive electrode materials for next-generation batteries. They exhibit high capacities of >300 mAh g-1 due to the unconventional participation of the oxygen anion redox in the charge compensation mechanism … The LRLO material is a composite of a classical layered oxide LiTMO2 (… where TM stands for Ni, Mn, Co) and Li2TMO3… In some systems, the excess capacity results from the activation of inactive monoclinic Li2TMO3 into active layered material.

… Here, we directly capture the nucleation of a dislocation network in primary nanoparticles of the high-capacity LRLO material Li1.2Ni0.133Mn0.533Co0.133O2 during electrochemical charge (lithium extraction). Based on the discovery of defect formation and first-principles calculations, we identify the origin of the voltage fade, allowing us to design and experimentally demonstrate a treatment to restore the voltage in LRLO.

—Singer et al.

The researchers identified nanoscale defects or dislocations in the NMC cathode materials as the batteries charged at a range of voltages going up to 4.7 volts.

The dislocations are extra atomic layers that don’t fit into the otherwise perfectly periodic crystal structure. Discovering these dislocations was a big surprise: if anything, we expected the extra atomic layers to occur in a completely different orientation.

—Andrej Singer, lead author, now on the faculty at Cornell University

Singer
Formation of a dislocation network during charge. Isosurface rendering of a LRLO particle measured in operando during charge. a, At a charge state of 4.0 V versus Li+ no dislocations are observed in the particle. The inset shows a schematic of a dislocation-free crystal. b, At 4.3 V two edge dislocations have formed during lithium extraction (shown by small spheres in the particle). The inset shows a schematic of an edge dislocation. c, At 4.4 V a dislocation network emerges (colors are used to represent different dislocations). The direction of the X-ray scattering vector q (perpendicular to the layers) is indicated and the size of the particle is around 300 × 300 × 500 nm3. Singer et al. Click to enlarge.

By combining experimental evidence with theory, the research team concluded that the nucleation of this specific type of dislocation results in voltage fade.

The dislocations form more readily in LRLO as compared with a classical layered oxide, suggesting a link between the defects and voltage fade. We show microscopically how the formation of partial dislocations contributes to the voltage fade. The insights allow us to design and demonstrate an effective method to recover the original high-voltage functionality.

—Singer et al.

Knowing the origin of voltage fade, the team showed that heat treating the cathode materials eliminated most of the defects and restored the original voltage. They put the heat-treated cathodes into new batteries and tested them at a range of voltages going up to 4.7 volts, demonstrating that the voltage fade had been reversed.

While the heat treating approach to reversing the defects is labor-intensive and not likely to scale, the physics and materials science-based approach to characterizing and then addressing the nano-scale defects offers promise for finding new solutions to the voltage fade problem.

Our paper is mainly about unlocking the mystery of the dislocations that cause voltage fade in Lithium-rich NMCs. We don’t have a scalable solution yet to solving the voltage fade problem in Lithium-rich NMCs, but we are making progress.

—UC San Diego nanoengineering professor Shirley Meng, co-senior author

The in situ Bragg coherent diffractive imaging technique, performed at the Argonne National Lab, allowed the researchers directly to image the interior of a nanoparticle during battery charge. The team’s analyses and reconstructions of these data offer unprecedented insights into what is actually happening while batteries are charging.

The researchers performed a number of observational studies while battery materials were charging across a range of voltages going from 4 volts up to 4.7 volts. At 4.4 volts, the researchers identified a series of defects including edge, screw and mixed dislocations.

The researchers also studied currently-commercialized non-lithium-rich NMC materials and found defects, but significantly fewer; and no new defects occurred above 4.2 volts in the non-lithium-rich NMC materials.

Resources

  • A. Singer, M. Zhang, S. Hy, D. Cela, C. Fang, T. A. Wynn, B. Qiu, Y. Xia, Z. Liu, A. Ulvestad, N. Hua, J. Wingert, H. Liu, M. Sprung, A. V. Zozulya, E. Maxey, R. Harder, Y. S. Meng & O. G. Shpyrko (2018) “Nucleation of dislocations and their dynamics in layered oxide cathode materials during battery charging” Nature Energy doi: 10.1038/s41560-018-0184-2

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