We all know — or think we know — that a solid-state battery is better than a battery with a liquid or semi-liquid electrolytes. A solid-state battery has a lower risk of thermal runaway (what ordinary people call fires). It also has a higher energy density, can charge and discharge more rapidly, performs better in cold temperatures, and lasts longer. So why isn’t everyone using them to power their battery electric vehicles?
The answer is, nobody knows how to manufacture them outside of the laboratory — yet — but scientists are getting closer all the time. According to MIT, one of the main stumbling blocks to making a solid-state battery is that instabilities in the boundary between the solid electrolyte layer and the two electrodes on either side can dramatically shorten its life. Adding special coatings to improve the bonding between the layers solves some of the problems but adds to the expense of manufacturing.
A team of researchers at MIT and Brookhaven National Laboratory has come up with a way of achieving results that equal or surpass the durability of coated surfaces without the need for coatings. The key is to eliminate any trace of carbon dioxide during a critical step in the manufacturing process known as sintering.
In that process, the solid-state battery materials made of ceramic compounds are heated to create a bond between the cathode and electrolyte. Doing so in the presence of pure oxygen creates bonds that match the performance of the best coated surfaces without that extra cost of the coating. The results of the research were published recently in the journal Advanced Energy Materials.
“Solid-state batteries have been desirable for different reasons for a long time,” says researcher Bilge Yildiz. “The key motivating points for solid batteries are they are safer and have higher energy density,” but she says they have been held back from large scale commercialization by two factors — the lower conductivity of the solid electrolyte and the interface instability issues.
The conductivity dilemma has been effectively tackled and reasonably high-conductivity materials have already been demonstrated, according to Yildiz. But overcoming the instabilities that arise at the interface has been far more challenging. These instabilities can occur during both the manufacturing and the electrochemical operation of such batteries, but for now the researchers have focused on the manufacturing, and specifically the sintering process.
A Solid-State Battery & Sintering
Sintering is needed because if the ceramic layers are simply pressed onto each other, the contact between them is far from ideal. There are too many gaps and the electrical resistance across the interface is high. Sintering causes atoms from each material to migrate into the other to form bonds.
The team’s experiments showed that at temperatures anywhere above a few hundred degrees, detrimental reactions take place that increase the resistance at the interface if carbon dioxide is present, even in tiny amounts. They demonstrated that avoiding carbon dioxide, and in particular maintaining a pure oxygen atmosphere during sintering, could create very good bonding at temperatures up to 700 degrees, with none of the detrimental compounds formed.
The performance of the cathode-electrolyte interface made using this method was “comparable to the best interface resistances we have seen in the literature,” but those were all achieved using the extra step of applying coatings. “We are finding that you can avoid that additional fabrication step, which is typically expensive,” Yildiz said.
What’s Next?
The research team is now studying how these bonds hold up over the long run during battery cycling. Meanwhile, the new findings could potentially be applied rapidly to solid-state battery production, Yildiz says. “What we are proposing is a relatively simple process in the fabrication of the cells. It doesn’t add much energy penalty to the fabrication. So, we believe that it can be adopted relatively easily into the fabrication process.” The added costs are negligible, the team believes
The research was supported by the U.S. Army Research Office through the MIT Institute for Soldier Nanotechnologies [your tax dollars at work]. The team used facilities supported by the National Science Foundation and facilities at Brookhaven National Laboratory supported by the Department of Energy.
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