Solid-state batteries could not only deliver twice the power for their size, but they could also virtually eliminate the fire hazard associated with today’s lithium-ion batteries. However, instabilities at the boundary between the layer of solid electrolyte and the two electrodes on either side can considerably reduce the lifetime of such batteries.
Some studies have used special coatings to improve the bond between layers, but this adds the cost of additional coating steps in the manufacturing process. Now, a team of researchers from MIT and Brookhaven National Laboratory has developed a way to achieve results that equal or exceed the durability of coated surfaces, but without any coating. The results are reported in an open access article in the journal Advanced Energy Materials.
The new method simply requires removing any carbon dioxide present during a critical manufacturing step, called sintering, where battery materials are heated to create a bond between the cathode and electrolyte layers, which are made of ceramic compounds.
Even though the amount of carbon dioxide present in the air is extremely small, measured in parts per million, its effects are significant and harmful. Performing the sintering step in pure oxygen creates bonds that match the performance of the best coated surfaces, without the added cost of coating, the researchers explain.
The main motivating points for solid-state batteries are that they are safer and have a higher energy density, but they have been prevented from being commercialized on a large scale by two factors, explains Professor Bilge Yildiz, corresponding author : the lower conductivity of the solid electrolyte and the interface. instability issues.
The conductivity problem has been effectively solved, and materials with reasonably high conductivity have already been demonstrated, according to Yildiz. But overcoming the instabilities that arise at the interface has been much more difficult. These instabilities can arise both during the fabrication and electrochemical operation of such batteries, but for now the researchers have focused on the fabrication, and specifically the sintering process.
Sintering is necessary because if the ceramic layers are simply pressed together, the contact between them is less than ideal, there are far too many gaps, and the electrical resistance across the interface is high. Sintering, which typically takes place at temperatures of 1,000 degrees Celsius or more for ceramic materials, causes atoms from each material to migrate into the other to form bonds.
The team’s experiments showed that at temperatures above a few hundred degrees, adverse reactions occur that increase interface resistance, but only if carbon dioxide is present, even in small amounts. They demonstrated that avoiding carbon dioxide, and in particular maintaining an atmosphere of pure oxygen during sintering, could create a very good bond at temperatures up to 700 degrees, without any of the harmful compounds being released. shape.
Reaction products (red background) at NMC622 interfaces | LLZO at different temperatures in each gaseous environment, as inferred by XAS and XRD analysis. The reaction product for the air anneal comes from previous work. Kim et al.
The performance of the cathode-electrolyte interface achieved using this method, Yildiz says, was “comparable to the best interface resistors we’ve seen in the literature”, but these were all achieved using the additional step of applying coatings. “We find that you can avoid this extra manufacturing step, which is usually expensive.”
The new findings could potentially be quickly applied to battery production, Yildiz says.
What we are proposing is a relatively simple process in making cells. It doesn’t add much energy penalty to crafting. So we think it can be adopted relatively easily into the manufacturing process.
—Professor Yildiz
The additional costs should be negligible, the researchers calculated.
Large companies such as Toyota are already working to bring early versions of solid-state lithium-ion batteries to market, and these new findings could quickly help these companies improve the economy and durability of the technology.
The research was supported by the US Army Research Office through the Institute for Soldier Nanotechnologies at MIT. The team used facilities supported by the National Science Foundation and facilities at Brookhaven National Laboratory supported by the Department of Energy.
Resources
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Younggyu Kim, Iradwikanari Waluyo, Adrian Hunt, Bilge Yildiz (2022) “Avoiding CO2 Improves thermal stability at Li interface7The3Zr2O12 Electrolyte with layered oxide cathodes » Advanced Energy Materials doi: 10.1002/aenm.202102741
