(Left) crystal structure of cubic garnet-type Li7La3Zr2O12 (LLZO); (right) graph showing the temperature dependence of ionic conductivity for Ba- and Ta-substituted LLZO with different compositions. Image: Copyright Toyohashi University of Technology. All rights reserved.
(Left) crystal structure of cubic garnet-type Li7La3Zr2O12 (LLZO); (right) graph showing the temperature dependence of ionic conductivity for Ba- and Ta-substituted LLZO with different compositions. Image: Copyright Toyohashi University of Technology. All rights reserved.

Rechargeable all-solid-state lithium batteries are being developed as next-generation energy storage devices because of their high energy density, safety and excellent cycle stability. The solid electrolyte in such batteries must not only have a high lithium-ion conductivity of around 1mS/cm at room temperature, but also possess chemical stability.

Oxide-based solid electrolytes have several advantages over sulfide-based ones, including chemical stability and ease of handling. But their performance has been hampered by the difficulty of forming low resistance interfaces with the battery electrodes.

In a recent paper in Frontiers in Energy Research, Ryoji Inada and his colleagues in the Department of Electrical and Electronic Information Engineering at Toyohashi University of Technology, Japan, report developing a novel garnet-type, fast ionic conducting oxide as a solid electrolyte. Using this novel solid electrolyte, they then went on to fabricate and test a rechargeable all-solid-state battery.

To produce the solid electrolyte, the research team substituted two cations (Ba2+ and Ta5+) into a cubic garnet with the chemical formula Li7La3Zr2O12 (LLZO). They then tested the effect of this substitution on the cubic garnet’s crystal phase, microstructure and ionic conducting property. In order to stabilize the highly conductive cubic garnet phase, the researchers found they had to fix the proportion of lithium at 6.5, so that the formula of the subsequent compound became Li6.5La3-xBaxZr1.5-xTa0.5+xO12 (LLBZTO).

The researchers obtained the highest room temperature conductivity of 0.83mS/cm in an LLBZTO garnet with Ba and Ta contents of 0.1 and 1.6, respectively. They also found that the activation energy of the LLBZT garnet tended to decrease monotonically with an increasing Ba substitution level, while substituting excess Ba and Ta tended to degrade the conductivity.

In addition, they confirmed that the LLBZTO garnet has a wide electrochemical potential window, meaning that various materials for both the positive and negative electrodes can potentially be used with the garnet to construct an all-solid-state battery. To this end, they fabricated a TiNb2O7 (TNO) film electrode on LLBZTO using an aerosol deposition method to produce a TNO/LLBZTO/Li all-solid-state battery, and demonstrated its charge and discharge reaction.

These results indicate that the developed LLBZTO garnet can be used as a solid electrolyte in all-solid-state batteries for large-scale power sources, even though additional investigation will be needed in order to enhance their performance. The researchers are also carrying out further studies to increase the energy density of these solid-state batteries.

This story is adapted from material from Toyohashi University of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.