A sinter-free future for solid-state batteries?

A lithium ceramic could act as a solid electrolyte in a more powerful and cost-efficient generation of rechargeable lithium-ion batteries. Now, a team of researchers from MIT in the USA and TU Munich in Germany have developed a production method that works without sintering at high temperatures. The sinter-free method was used for the efficient, low-temperature synthesis of these ceramics in a conductive crystalline form.

A promising approach to making smaller, lighter, more powerful and safer batteries is to use solid-state cells with anodes made of metallic lithium instead of graphite. Unlike conventional lithium-ion batteries, which have liquid organic electrolytes and use a polymer film to separate the anodic and cathodic compartments, all components of a solid-state battery are solids, with a thin ceramic layer functioning as a solid electrolyte and separator. The researchers found that it is very effective against the dangerous short circuits caused by the growth of lithium dendrites and thermal runaway. In addition, they contain no easily flammable liquids.

A suitable ceramic electrolyte/separator for cells with high energy density is the garnet-type lithium oxide Li₇La₃Zr₂O_12−d (LLZO). This must be sintered together with the cathode at over 1050°C to convert the LLZO to the rapid lithium-conducting cubic crystalline phase, sufficiently densify it and bind it to the electrode. However, temperatures above 600°C destabilise sustainable low-cobalt or cobalt-free cathode materials while also increasing energy consumption. As a result, new production methods that are more sustainable are needed.

A team of researchers led by Jennifer L M Rupp at MIT have now developed such a synthetic process. This process is not based on a ceramic precursor compound, but a liquid one, which is directly densified to form LLZO in a sequential decomposition synthesis. To optimise the conditions for this synthetic route, the researchers analysed the multistep phase transformation of LLZO from an amorphous form to the required crystalline form (cLLZO) using a variety of methods and produced a time-temperature-transformation diagram.

Based on the insights they gained into the crystallisation process, they developed a route by which cLLZO is obtained as a dense, solid film after 10 hours of annealing at 500°C — with no sintering. For future battery designs, this method will allow for the integration of the solid LLZO electrolyte with sustainable cathodes that could avoid the use of critical elements such as cobalt.

Image credit: iStock.com/PhonlamaiPhoto