A lithium ceramic can be used as an electrolyte solid in the next generation of lithium-ion rechargeable batteries. This will make them more powerful and efficient. It is important to develop a method of production that does not require sintering. A research team published a new way in the Angewandte Chemie for the low-temperature, efficient synthesis of these ceramics.
The development of batteries for electric vehicles is influenced by two factors: power, which determines vehicle range, and cost, which is crucial in the competition against internal combustion engines. The US Department of Energy wants to accelerate the switch from gasoline to electric vehicles. It has set ambitious targets for reducing costs and increasing the energy density of batteries. These targets can’t be met with lithium-ion battery technology.
Solid-state cells, with anodes of metallic lithium in place of graphite, are a promising way to make smaller, lighter, and more powerful batteries. Solid-state batteries are all solids, unlike conventional lithium-ion cells, which use liquid organic electrolytes and a polymer to separate anodic from cathodic compartments. A thin ceramic layer functions both as an electrolyte solid and a separator. It is highly effective in preventing dangerous short circuits that are caused by lithium dendrites or thermal runaways. They also contain no easily flammable liquids.
A suitable ceramic electrolyte/separator for cells with high energy density is the garnet-type lithium oxide Li7La3Zr2O12-d (LLZO). The cathode and this material must be sintered at temperatures above 1050 degrees Celsius to transform the LLZO into the lithium-conducting cubic crystalline form, densify the material, and firmly bind it to the electrode. Temperatures above 600 degrees, however, destabilize low-cobalt and cobalt-free materials that are sustainable. They also increase production costs and energy usage. We need new production methods which are both more sustainable and economical.
This new synthetic method was developed by a team at TU Munich and MIT Cambridge led by Jennifer L. M. Rupp. The new process does not use a ceramic precursor but a liquid compound that is densified directly to form LLZO through a sequential decomposition synthesis. To optimize the conditions for this synthetic route, Rupp and her team analyzed the multistep phase transformation of LLZO from an amorphous form to the required crystalline form (cLLZO) using a variety of methods (Raman spectroscopy, dynamic differential scanning calorimetry) and produced a time-temperature-transformation diagram. They developed a technique that allows cLLZO to be obtained as a solid, dense film after 10 hours at a relatively low temperature (500 deg C) — without sintering. This method provides for future battery designs to integrate the solid LLZO electrode with sustainable cathodes, which could eliminate the use of cobalt and other socioeconomically important elements.
Before the product can be sold, researchers must solve other problems. Experts will need to reduce the thickness of the solid electrolyte to a level similar to that found in lithium-ion battery electrolytes to commercialize the all-solid-state batteries. This will increase the energy density or the amount of power the batteries can store. The team also said that the high costs of basic materials are another problem.
Solid Power, a manufacturer of advanced batteries, plans to start testing the new technology in order to evaluate its commercialization potential. Researchers said that they will continue to research ways to boost energy density.