An international team of researchers has demonstrated a new way to increase the robustness and energy storage capability of a particular class of “lithium-rich” cathode materials by using a carbon dioxide-based gas mixture to create oxygen vacancies at the material’s surface. Researchers said the treatment improved the energy density of the cathode material by up to 30 to 40%.
As described in an open access paper in Nature Communications, the target material (Li[Li0.144Ni0.136Co0.136Mn0.544]O2, denoted as LR-NCM) delivers a discharge capacity as high as 301 mAh g−1 with initial Coulombic efficiency of 93.2%. After 100 cycles, a reversible capacity of 300 mAh g−1 still remains without any obvious decay in voltage. The discovery sheds light on how changing the oxygen composition of lithium-rich cathode materials could improve battery performance, particularly in high-energy applications such as electric vehicles.
The functionality of many transition metal oxides can be significantly altered by oxygen vacancies on the surface. Oxygen vacancies can behave as charge carriers for solid-oxide fuel cells, as well as important adsorption sites and as active sites for electro-photocatalysts. In Li-ion cathode materials, these vacancies play a vital role in determining the material’s electron and ion transport properties. The influence of oxygen vacancies at the surface on electrochemical performance can be completely different depending on the type of Li-ion cathode material.
… Numerous studies exemplify the contributions of bulk and surface oxygen on high charge–discharge capacity in Li-rich layered oxides, although this hypothesis has not been fully verified. … we propose a strategy based on a gas–solid interface reaction (GSIR) between Li-rich layered oxides and carbon dioxide gas to create oxygen vacancies on the particles’ surface. … As confirmed by both theoretical calculations and experimental characterization, the improved electrochemical performance is ascribed to the full utilization of oxygen activity through the creation of oxygen vacancies on the surface of the as-synthesized Li-rich layered oxides.—Qiu et al.
The researchers prepared LR-NCM using a co-precipitation method. After the GSIR process, the general morphology of the GSIR LR-NCM was the same as the pristine LR-NCM. The team assembled CR2032 coin cells with metallic Li as the counter electrode to investigate the electrochemical performance of both samples.
The GSIR LR-NCM exhibited a higher capacity than that of the pristine LR-NCM at all tested rates, as well as cycling stability and other advantages.
We’ve uncovered a new mechanism at play in this class of lithium-rich cathode materials. With this study, we want to open a new pathway to explore more battery materials in which we can control oxygen activity.—Shirley Meng, nanoengineering professor at the University of California San Diego and one of the principal investigators
Meng leads the Laboratory for Energy Storage and Conversion and is the director of the Sustainable Power and Energy Center, both at UC San Diego. A hallmark of her group’s research efforts is understanding the science behind battery materials—at the level of single atoms and molecules, and at the interfaces. Her group is one of the first to focus on the activity of oxygen atoms in battery materials. Typically, the focus has centered on lithium and transition metal atoms.
In the new study, Meng’s group collaborated with researchers from the Chinese Academy of Sciences to develop a way to introduce oxygen vacancies in a class of lithium-rich layered oxides. Although the lithium-rich layered oxides potentially house more energy than other cathode materials, they also have their drawbacks, including slow discharge rates and an issue called voltage fade, which is characterized by a drop in cell voltage with each charge-discharge cycle.
We’re presenting a new way to mitigate the issues plaguing lithium-rich cathode materials—through understanding and controlling how oxygen behaves in these materials.—Shirley Meng
The team found that treating the lithium-rich cathode particles with a carbon dioxide-based gas mixture created oxygen vacancies uniformly throughout the surface of the particles. The treatment only left oxygen vacancies within the first 10 to 20 nanometers without altering the rest of the material’s atomic structure.
This is a mild treatment that enables controlled changes in the material near the interface, explained Minghao Zhang, co-first author of the paper and a PhD student at the Jacobs School of Engineering at UC San Diego working in Meng’s group.
Through characterization studies in collaboration with groups from Brookhaven National Laboratory and Oak Ridge National Laboratory, researchers provided several reasons why oxygen vacancies improved the cathode material’s performance.
The vacancies allow lithium ions to move around more easily throughout the cathode, leading to high discharge capacity and faster discharge rates.
The vacancies also increase the material’s stability by inhibiting the formation of highly reactive oxygen radicals at the cathode material’s surface, which are typically responsible for degrading the electrolyte while the battery is operating. This could mean longer battery lifetime, researchers said.
As a next step, researchers will work on scaling up the treatment reported in this study. They will also conduct further studies on the oxygen activity in other materials and how it could be leveraged to improve battery performance.
The work performed in the United States was supported by grants from the Department of Energy.
Bao Qiu, Minghao Zhang, Lijun Wu, Jun Wang, Yonggao Xia, Danna Qian, Haodong Liu, Sunny Hy, Yan Chen, Ke An, Yimei Zhu, Zhaoping Liu & Ying Shirley Meng (2016) “Gas-solid interfacial modification of oxygen activity in layered oxide cathodes for lithium-ion batteries” Nature Communications 7, Article number: 12108 doi: 10.1038/ncomms12108