MIT researchers open new direction in search for better batteries; the potential of disordered materials
In a new paper in the journal Science, researchers at MIT and Brookhaven National Laboratory, led by MIT’s Dr. Gerbrand Ceder, report that contrary to conventional wisdom, Li-ion battery cathodes made of disordered lithium compounds can perform better than perfectly ordered ones. The group’s analysis of the performance of a lithium molybdenum chromium oxide (LMCO) material opens a new direction in the search for better battery materials, and a whole new category of materials possibilities that had previously been ignored, Dr. Ceder suggests.
In a rechargeable lithium-based battery, lithium ions move out of the battery’s cathode during the charging process, and return to the cathode as power is drained. These repeated round-trips can cause the electrode material to shrink and expand, leading to cracks and degrading performance over time. Currently, cathodes are usually made of an orderly crystalline material, sometimes in a striated structure of layers of lithium alternating with oxides of transition metals. When slight deviations from that order are introduced, the battery’s efficiency generally goes down—so disordered materials have mostly been ignored in the search for improved battery materials.
Scientists had thought the layering was necessary to provide a pathway for lithium to pass in and out of the cathodes without bumping into the transition metal oxide layer—“a channel with nothing in the way,” as Ceder says. Moreover, disorder “usually significantly reduces the lithium ion mobility,” Ceder says; high mobility is essential for an efficient rechargeable battery.
However, the MIT study found, through a combination of computer modeling and laboratory experiments, that certain kinds of disorder can provide a significant boost in cathode performance.
Nearly all high energy density cathodes for rechargeable lithium batteries are well-ordered materials where lithium and other cations occupy distinct sites. Cation-disordered materials are generally disregarded as cathodes because lithium diffusion tends to be limited by their structures. The performance of Li1.211Mo0.467Cr0.3O2 shows that lithium diffusion can be facile in disordered materials. Using ab initio computations, we demonstrate that this unexpected behavior is due to percolation of certain diffusion channels that are active in disordered structures, but is unique to Li excess materials. This leads to a unified understanding of high performance in both layered and Li excess materials, and opens up an exciting new direction for designing disordered electrode materials with high capacity and high energy density.—Lee et al.
The researchers found that a significant excess of lithium in the material changes things dramatically. In the traditional ordered structure, there is an exact balance between the number of lithium and metal atoms. “But if you get enough of a lithium excess,” Ceder says, “you get new channels, and they can take over from the channels you close off.”
While the disordered material with excess lithium produces irregular pathways, these nevertheless can still act as efficient channels for the lithium ions. Such a material offers an extra bonus: While the irregular channels let lithium pass just as easily as it does in a layered material, in the disordered material the lithium ions don’t push the layers out of shape.
The new material—in these experiments, lithium molybdenum chromium oxide (Li1.211Mo0.467Cr0.3O2)—has a very high dimensional stability. While the dimensional changes in layered materials can be as much as 5-10%, Ceder says, in the new disordered material it is only about 0.1%—“virtually zero.”
|Specific capacity vs. cycle number for LMCO/C. Source: Lee et al. SI. Click to enlarge.|
However, while lithium molybdenum chromium oxide can hold and release significantly more lithium than existing materials, it produces a lower voltage—meaning its overall performance is about the same as that of existing materials. The promise of the new work is in the opening to consideration of new types of materials that may offer much better performance.
Many new materials take decades to move from the laboratory to useful applications, but Ceder hopes to discover something better in one or two years, likely by using computational tools such as the Materials Project, which he co-founded. (Earlier post.)
Dr. Jeff Dahn, a professor of physics and atmospheric science at Dalhousie University in Nova Scotia who was not involved in this work, said that the experimental results were very surprising. While it remains to be seen “whether this finding can be translated in similar experimental results in more practical materials,” he said, this research “is a nice combination of experiment and theory.”
The research also involved postdocs Alexander Urban, Xin Li, and Geoffroy Hautier at MIT and researcher Dong Su at Brookhaven National Laboratory. It was funded by the Robert Bosch Corporation, Umicore, Samsung, and the US Department of Energy.
Jinhyuk Lee, Alexander Urban, Xin Li, Dong Su, Geoffroy Hautier, and Gerbrand Ceder (2014) “Unlocking the Potential of Cation-Disordered Oxides for Rechargeable Lithium Batteries,” Science doi: 10.1126/science.1246432