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A team of scientists including researchers at the US Department of Energy’s (DOE) Brookhaven National Laboratory and SLAC National Accelerator Laboratory have identified the causes of degradation in nickel-rich layered cathode material (LiNi_1‐x‐yMn_xCo_yO₂) (NMC) for lithium-ion batteries, as well as possible remedies. Their findings, published in the journal Advanced Functional Materials, could lead to the development of more affordable and better performing batteries for electric vehicles.

Affordable EVs put stringent requirements on their power batteries, including high energy density, high power density, long cycle/calendar life, and low cost. The performance of batteries is closely related to the properties of the cathode materials used. Layered LiNi_1-x-yMn_xCo_yO₂ (NMC) material emerges as a practical cathode material due to its balanced good properties, including high reversible capacity (≈200 mAh g^-1) and low cost compared to commonly used cathode material LiCoO₂. Among NMC families, materials with high Ni content (Ni-rich NMC) have become the focus of the current research because of the improvement in the specific capacity.

However, Ni-rich NMC material is reported to suffer from relatively poor high-voltage stability compared to compounds with lower Ni content. Besides, significant capacity fading during cycling has been observed in Ni-rich NMC. The EV application calls for battery solutions with emphasis on power and energy density to deliver high accelerating power and long driving range, which demand the capability of high-voltage operation. Therefore, it is very important to investigate the degradation mechanism of Ni-rich NMC materials during high-voltage cycling.

—Mao et al.

Batteries are composed of an anode, a cathode, and an electrolyte, but many scientists consider the cathode to be the most pressing challenge. Researchers at Brookhaven are part of a DOE-sponsored consortium called Battery500, a group that is working to triple the energy density of the batteries that power today’s electric vehicles. (Earlier post.) One of their goals is to optimize nickel-rich layered cathode materials.

Layered materials are very attractive because they are relatively easy to synthesize, but also because they have high capacity and energy density.

—Brookhaven chemist Enyuan Hu

Lithium cobalt oxide is a layered material that has been used as the cathode for lithium-ion batteries for many years. Despite its successful application in small energy storage systems such as portable electronics, cobalt’s cost and toxicity are barriers for the material’s use in larger systems. Now, researchers are investigating how to replace cobalt with safer and more affordable elements without compromising the material’s performance.

We chose a nickel-rich layered material because nickel is less expensive and toxic than cobalt. However, the problem is that nickel-rich layered materials start to degrade after multiple charge-discharge cycles in a battery. Our goal is to pinpoint the cause of this degradation and provide possible solutions.

—Enyuan Hu

Cathode materials can degrade in several ways. For nickel-rich materials, the problem is mainly capacity fading—a reduction in the battery’s charge-discharge capacity after use. To fully understand this process in their nickel-rich layered materials, the scientists needed to use multiple research techniques to assess the material from different angles.

This is a very complex material. Its properties can change at different length scales during cycling. We needed to understand how the material’s structure changed during the charge-discharge process both physically—on the atomic scale up—and chemically, which involved multiple elements: nickel, cobalt, manganese, oxygen, and lithium.

—Enyuan Hu

To do so, Hu and his colleagues characterized the material at multiple research facilities, including two synchrotron light sources—the National Synchrotron Light Source II (NSLS-II) at Brookhaven and the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC. Both are DOE Office of Science User Facilities.

At every length scale in this material, from angstroms to nanometers and to micrometers, something is happening during the battery’s charge-discharge process. We used a technique called x-ray absorption spectroscopy (XAS) here at ISS to reveal an atomic picture of the environment around the active metal ions in the material.

—co-author Eli Stavitski, beamline scientist at NSLS-II’s Inner Shell Spectroscopy (ISS) beamline

Results from the XAS experiments at NSLS-II led the researchers to conclude that the material had a robust structure that did not release oxygen from the bulk, challenging previous beliefs. Instead, the researchers identified that the strain and local disorder was mostly associated with nickel.

To investigate further, the team conducted transmission x-ray microscopy (TXM) experiments at SSRL, mapping out all the chemical distributions in the material. This technique produces a very large set of data, so the scientists at SSRL applied machine learning to sort through the data.

These experiments produced a huge amount of data, which is where our computing contribution came in. It wouldn’t have been practical for humans to analyze all of this data, so we developed a machine learning approach that searched through the data and made judgments on which locations were problematic. This told us where to look and guided our analysis.

—co-author Yijin Liu, a SLAC staff scientist

Hu said that the major conclusion drawn from the experiment was that there were considerable inhomogeneities in the oxidation states of the nickel atoms throughout the particle. Some nickel within the particle maintained an oxidized state, and likely deactivated, while the nickel on the surface was irreversibly reduced, decreasing its efficiency.

Additional experiments revealed small cracks formed within the material’s structure.

During a battery’s charge-discharge process, the cathode material expands and shrinks, creating stress. If that stress can be released quickly then it does not cause a problem but, if it cannot be efficiently released, then cracks can occur.

—co-author Yijin Liu, SLAC

The scientists believed that they could possibly mitigate this problem by synthesizing a new material with a hollowed structure. They tested and confirmed that theory experimentally, as well as through calculations. Moving forward, the team plans to continue developing and characterizing new materials to enhance their efficiency.

Additionally, as NSLS-II continues to build up its capabilities, the scientists plan to complete more advanced TXM experiments on these kinds of materials, taking advantage of NSLS-II’s ultrabright light.

This study was supported by the National Science Foundation and DOE’s Office of Energy Efficiency and Renewable Energy. Operations at NSLS-II and SSRL are supported by DOE’s Office of Science.

Resources

• Mao, Y., Wang, X., Xia, S., Zhang, K., Wei, C., Bak, S., Shadike, Z., Liu, X., Yang, Y., Xu, R., Pianetta, P., Ermon, S., Stavitski, E., Zhao, K., Xu, Z., Lin, F., Yang, X.‐Q., Hu, E., Liu, Y. (2019) “High‐Voltage Charging‐Induced Strain, Heterogeneity, and Micro‐Cracks in Secondary Particles of a Nickel‐Rich Layered Cathode Material” Adv. Funct. Mater. doi: 10.1002/adfm.201900247