While the process of designing more improved rechargeable batteries may appear to be hazy, an alumina lining can potentially offer a solution.
When engineers at the Brown School of Engineering of Rice University applied a thin metal oxide layer to standard cathodes, they observed a novel phenomenon that can help in developing batteries that are better geared toward more robust off-grid energy storage and electric cars.
Published in the ACS Applied Energy Materials journal of the American Chemical Society, the study elucidates a hitherto unfamiliar mechanism through which lithium gets captured in batteries and thereby restricts the number of times the batteries can be charged and discharged at full power.
However, despite that aspect, it is believed that such kinds of batteries may prove to be suitable in certain circumstances.
In the laboratory of Sibani Lisa Biswal, a chemical and biomolecular engineer at the Rice University, a sweet spot in the batteries was identified—that is, without increasing their storage capacity, the batteries can offer stable and steady cycling for applications that require such a feature.
According to Biswal, traditional lithium-ion batteries make use of graphite-based anodes whose capacity is below 400 milliamp hours per gram (mAh/g). By contrast, silicon-based anodes have possibly 10 times that capacity, but this also comes with a drawback—as silicon alloys with lithium, it expands and stresses the anode.
When the silicon was made porous and its capacity was reduced to 1000 mAh/g, the test batteries developed by the researchers offered stable cycling with a capacity that was still excellent.
Maximum capacity puts a lot of stress on the material, so this is a strategy to get capacity without the same degree of stress. 1,000 milliamp hours per gram is still a big jump.
Sibani Lisa Biswal, Professor, Department of Chemical and Biomolecular Engineering, Rice University
Headed by postdoctoral fellow Anulekha Haridas, the researchers tested the idea of combining high-voltage nickel manganese cobalt oxide (NMC) cathodes with the high-capacity and porous silicon anodes (instead of graphite). At 1000 mAh/g, the full cell lithium-ion batteries exhibited a stable cyclability over hundreds of cycles.
While certain cathodes had a 3-nm alumina layer (applied through atomic layer deposition), others lacked this alumina layer. Those that had the alumina coating prevented the cathode from being disintegrated in the presence of hydrofluoric acid. This acid forms even if trace amounts of water penetrate the liquid electrolyte.
Testing revealed that the alumina also expedited the charging rate of the battery and decreased the number of times it can be both charged and discharged.
There seems to be elaborate trapping because of the rapid transport of lithium through the alumina layer, stated Haridas. While the scientists are already aware of the potential ways through which lithium is trapped by silicon anodes and becomes unavailable to power devices, this is the first-ever study where lithium is absorbed by the alumina itself until the latter becomes saturated, added Haridas.
At that point, the alumina layer acts as a catalyst for rapid transport to and from the cathode, she further stated.
This lithium-trapping mechanism effectively protects the cathode by helping maintain a stable capacity and energy density for the full cells.
Anulekha Haridas, Postdoctoral Fellow, Rice University
Study co-authors are Quan Anh Nguyen and Botao Farren Song, both graduate students from Rice University, and Rachel Blaser, a research and development engineer at Ford Motor Co.
Biswal is a professor of chemical and biomolecular engineering and of materials science and nanoengineering. The study was supported by Ford’s University Research Program.