The strategy, described in Matter, could be useful in bringing solid-state batteries to the mass market.
"There's huge interest in replacing the liquid electrolytes in current batteries with ceramic materials because they're safer and can provide higher energy density," said Christos Athanasiou, a postdoctoral researcher in Brown's School of Engineering and lead author of the research. "So far, research on solid electrolytes has focused on optimising their chemical properties. With this work, we're focusing on the mechanical properties, in the hope of making them safer and more practical for widespread use."
Liquid electrolytes are found in most batteries but at high currents, tiny filaments of lithium metal can form inside the electrolytes, which cause batteries to short circuit. Liquid electrolytes are highly flammable, so shorting can lead to fires.
Solid ceramic electrolytes are not flammable, and evidence suggests they can prevent the formation of lithium filaments, which could enable batteries to operate at higher currents. However, ceramics are highly brittle materials that can fracture during the manufacturing process and during use.
For this new study, the researchers wanted to see if infusing a ceramic with graphene could increase the material's ability to withstand cracking without falling apart while maintaining the electronic properties needed for electrolyte function.
Athanasiou worked with Brown engineering professors Brian Sheldon and Nitin Padture, who for years have used nanomaterials to toughen ceramics for use in the aerospace industry. For this work, the researchers made tiny platelets of graphene oxide, mixed them with powder of a ceramic called LATP, and then heated the mixture to form a ceramic-graphene composite.
Mechanical testing of the composite is said to have showed a more than two-fold increase in toughness compared to the ceramic alone.
"What's happening is that when crack starts in a material, the graphene platelets essentially hold the broken surfaces together so that more energy is required for the crack to run," Athanasiou said in a statement.
Experiments also showed that the graphene did not interfere with the electrical properties of the material. According to Brown University, the key was making sure the right amount of graphene was added to the ceramic. Too little graphene would not achieve the toughening effect, but too much would cause the material to become electrically conductive.
"You want the electrolyte to conduct ions, not electricity," Padture said. "Graphene is a good electrical conductor, so people may think we're shooting ourselves in the foot by putting a conductor in our electrolyte. But if we keep the concentration low enough, we can keep the graphene from conducting, and we still get the structural benefit."