A team led by researchers from Argonne National Laboratory has identified a fundamental source of instability in nickel-manganese-cobalt oxide (NMC) cathodes: the domain boundaries that are present in single-crystalline cathode particles. The researchers also devised a method for producing boundary-free single crystals to improve NMC performance. A paper on the work is published in Nature Energy.

By investigating single-crystalline cathodes with different domain boundaries structures, we show that the elimination of domain boundaries enhances the reversible lattice oxygen redox while inhibiting the irreversible oxygen release. This leads to significantly suppressed structural degradation and improved mechanical integrity during battery cycling and abuse heating. The robust oxygen redox enabled through domain boundary control provides practical opportunities towards high-energy, long-cycling, safe batteries.

—Liu et al.

Current NMC cathodes pose a major barrier to operation at high voltage, explained Guiliang Xu, co-corresponding author. With charge-discharge cycling, performance rapidly declines due to cracks forming in the cathode particles. For several decades, battery researchers have been seeking ways to eliminate these cracks.

One past approach involved microscale spherical particles consisting of numerous much smaller particles. The large spherical particles are polycrystalline, with differently oriented crystalline regions. As a result, they have grain boundaries between particles—which cause cracking upon battery cycling. To prevent this, Xu and Argonne colleagues had previously developed a protective polymer coating around each particle. This coating surrounds the large spherical particles and smaller ones inside them.

A different approach to avoid this cracking involves single-crystal particles. Electron microscopy of these particles indicated they have no boundaries.

Image shows single crystals of cathode material: (A) no internal boundaries and (B) internal boundaries visible. (Image by Argonne National Laboratory.)

The problem the team faced was that cathodes made from both coated polycrystals and single crystals still formed cracks with cycling. So, they subjected these cathode materials to extensive analyses at the Advanced Photon Source (APS) and Center for Nanoscale Materials (CNM), DOE Office of Science user facilities at Argonne.

Different X-ray analyses were carried out at five APS beamlines (11-BM, 20-BM, 2-ID-D, 11-ID-C and 34-ID-E). It turned out that what scientists had believed were single crystals, as evidenced by electron and X-ray microscopy, actually had boundaries inside. Scanning and transmission electron microscopies at CNM verified the finding.

When we look at the surface morphology of these particles, they look like single crystals. But when we use a technique called synchrotron X-ray diffraction microscopy and other techniques at the APS, we find boundaries hiding inside.

—Wenjun Liu, co-author

Importantly, the team developed a method for producing boundary-free single crystals. Testing of small cells with such single-crystal cathodes at very high voltage showed a 25% increase in energy storage per unit volume, with almost no loss of performance over 100 cycles of testing. By contrast, over the same cycle life, the capacity declined by 60% to 88% in NMC cathodes composed of single crystals with many internal boundaries or with coated polycrystals.

Calculations at the atomic scale revealed the mechanism behind the capacity decline in the cathode. According to nanoscientist Maria Chan, compared to the regions away from them, boundaries are more vulnerable towards the loss of oxygen atoms when the battery is being charged. This oxygen loss leads to degradation with cell cycling.

Our calculations showed how boundaries lead to oxygen release at high voltage and, therefore, performance decline.

—Maria Chan

Eliminating the boundaries prevents oxygen release and thereby improves the cathode safety and stability with cycling. Oxygen release measurements at APS and the Advanced Light Source at DOE’s Lawrence Berkeley National Laboratory supported this finding.

We now have guidelines that battery manufacturers can use to prepare cathode material that is boundary-free and works at high voltage. And the guidelines should apply to other cathode materials besides NMC.

—Khalil Amine, an Argonne Distinguished Fellow and co-corresponding author

In addition to Xu, Amine, Liu and Chan, Argonne authors include Xiang Liu, Venkata Surya Chaitanya Kolluru, Chen Zhao, Xinwei Zhou, Yuzi Liu, Liang Yin, Amine Daali, Yang Ren, Wenqian Xu, Junjing Deng, Inhui Hwang, Chengjun Sun, Tao Zhou, Ming Du and Zonghai Chen. Also contributing to this project were scientists from Lawrence Berkeley National Laboratory (Wanli Yang, Qingtian Li and Zengqing Zhuo), Xiamen University (Jing-Jing Fan, Ling Huang and Shi-Gang Sun) and Tsinghua University (Dongsheng Ren, Xuning Feng and Minggao Ouyang).

The research was supported by the DOE Vehicle Technologies Office.

Resources

• Liu, X., Xu, GL., Kolluru, V.S.C. et al. (2022) “Origin and regulation of oxygen redox instability in high-voltage battery cathodes.” Nat Energy doi: 10.1038/s41560-022-01036-3