No sintering required: low-temperature synthesis of lithium ceramic for batteries
A lithium ceramic could act as a solid electrolyte in a
more powerful and cost-efficient generation of rechargeable lithium-ion
batteries. The challenge is to find a production method that works
without sintering at high temperatures. In the journal Angewandte
Chemie, a research team has now introduced a sinter-free method for
the efficient, low-temperature synthesis of these ceramics in a
conductive crystalline form.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
Two factors dominate the development of batteries for
electric vehicles: power, which determines the vehicle range; and cost,
which is critical in the competition with internal combustion engines.
The US Department of Energy aims to accelerate the transition from
gasoline-powered vehicles to electric vehicles and has set ambitious
goals for reducing production costs and increasing the energy density of
batteries by 2030. These targets cannot be achieved with conventional
lithium-ion batteries.
A highly promising approach to making smaller, lighter,
significantly more powerful, and safer batteries is to use solid-state
cells with anodes made of metallic lithium instead of graphite. In
contrast to 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. A thin ceramic layer simultaneously functions as a solid
electrolyte and separator. It is very effective against both the
dangerous short circuits caused by the growth of lithium dendrites and
thermal runaway. In addition, they contain no easily inflammable liquids.
A suitable ceramic electrolyte/separator for cells with
high energy density is the garnet-type lithium oxide Li7La3Zr2O12−d
(LLZO). This material 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 strongly bind it to the
electrode. However, temperatures above 600 °C destabilize sustainable
low-cobalt or cobalt-free cathode materials while also driving up
production costs and energy consumption. New production methods that are
more economical and sustainable are needed.
A team led by Jennifer L. M. Rupp at MIT, Cambridge, USA,
and TU Munich, Germany, has now developed such a new synthetic process.
Their new 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 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. Based on the insights they gained into the crystallization
process, they developed a route by which cLLZO is obtained as a dense,
solid film after 10 hours of annealing at the relatively low temperature
of 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 socioeconomically
critical elements such as cobalt.
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About the Author
Jennifer Rupp is Professor of Chemistry at TU Munich,
Germany and long-term guest professor at MIT, USA and one of the top
solid state ceramics and battery experts in the world. She serves on numerous industry, government and academic advisory
committees, is an activist for equality on science and tech boards and
has won multiple industry awards.
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