The development of a strategy for stabilizing interfaces in solid-state lithium-ion batteries creates opportunities.
The solid-state battery is a particularly revolutionary technology in the never-ending pursuit to load more energy into batteries while reducing their mass and size. A solid electrolyte layer replaces the liquid electrolyte layer that normally transports charges back and forth between the electrodes in these batteries. These batteries could provide significantly more energy for their size, and could also mitigate future fire hazard relating with today's lithium-ion batteries.
However, one factor has hampered the development of solid-state batteries: imbalances at the threshold between the solid electrolyte layer and the two electrodes on either side can drastically reduce the longevity of such batteries. Several research have used special coatings to enhance layer bonding, however this incorporates the cost of additional coating procedures in the fabrication process. Presently, a research group from MIT and Brookhaven National Laboratory has devised a method for meeting objectives that match or exceed the durability of coated surfaces while requiring no coatings.
The procedure entails by removing any carbon dioxide present during sintering, a critical manufacturing step in which the battery materials are heated to establish bonding between the cathode and electrolyte layers, both of which are formed of ceramic compounds. Despite the fact that the level of carbon dioxide existing in air is negligible (measured in parts per million), the effects are dramatic and harmful. According to the researchers, performing the sintering procedure in pure oxygen produces bonds that match the efficiency of the best coated surfaces without the additional cost of the coating.
The results were published in the journal Advanced Energy Materials in a paper by MIT doctoral student Younggyu Kim, Bilge Yildiz, professor of nuclear science and engineering and materials science and engineering, and Iradikanari Waluyo and Adrian Hunt of Brookhaven National Laboratory.
“Solid-state batteries have been desirable for different reasons for a long time,” Yildiz says. “The key motivating points for solid batteries are they are safer and have higher energy density,” however, she claims that factors have prevented them from commercializing on a large scale: the lower conductivity of the solid electrolyte and interface instability problems.
According to Yildiz, the conductivity issue has been adequately addressed, and reasonably high-conductivity materials have already been tested. However, resolving the instabilities that form at the interface has proven far more difficult. These instabilities can happen during both the production and electrochemical process of such batteries, but for the time being, the research team have concentrated on the manufacturing, specifically the sintering process.
Sintering is required since the connection between the ceramic layers is anything but ideal, there are still several discrepancies, and the electrical resistance across the interface is significant if they are simply pressed together. Sintering, which is typically done at temperatures of 1,000 degrees Celsius or higher for ceramic materials, induces atoms from one material to transfer into the other to form bonds. Experiments by the team revealed that at temperatures above a few hundred degrees, harmful reactions occur that raise the force at the interface — but only if carbon dioxide is present, even in trace amounts. They showed that by deterring carbon dioxide and, specifically, by retaining a pure oxygen atmosphere during sintering, they could achieve very good bonding at temperatures as high as 700 degrees Celsius while forming none of the harmful compounds.
According to Yildiz, the behavior of the cathode-electrolyte interface created using this approach was "comparable to the best interface resistances we have seen in the literature," but those were all attained with the alternative method of applying coatings. “We are finding that you can avoid that additional fabrication step, which is typically expensive.”
The group is now looking into how these bonds hold up over time during battery cycling, which is the next stage of the battery's performance.
Large corporations like Toyota are already working to commercialize early versions of solid-state lithium-ion batteries, and these research observations could help them develop the technology's economics and durability.