A team of Brown University researchers has used reduced graphene oxide (rGO) to enhance the toughness of an oxide-based solid-state lithium-ion conductor. In an open-access paper in the journal Matter, they report demonstrating materials with a greater than 2-fold enhancement in the average fracture toughness (K_IC).

To our knowledge, this is the toughest ceramic solid electrolyte yet reported. Based on these results, an analytical framework is developed to provide guidelines for the design of ultra-tough solid electrolytes using 2D materials.

—Athanasiou et al.

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. So far, research on solid electrolytes has focused on optimizing 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.

—Christos Athanasiou, a postdoc in Brown’s School of Engineering and lead author

Solid-state lithium batteries (SSLBs) can provide key improvements on conventional Li-ion battery technology that is based on liquid electrolytes, including the safe use of Li-metal anodes to increase energy density.

Solid ceramic electrolytes aren’t flammable, and there’s evidence that 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 fracture toughness (a material’s ability to withstand cracking without falling apart) while maintaining the electronic properties needed for electrolyte function.

Research shows that reduced graphene oxide (rGO) can help prevent the propagation of cracks in ceramic materials used for battery electrolytes. The findings could be a step toward making solid electrolytes that are tough enough for the mass market. Credit: Sheldon lab / Brown University

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 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.

—Christos Athanasiou

Experiments also showed that the graphene didn’t interfere with the electrical properties of the material. The key was making sure the right amount of graphene was added to the ceramic. Too little graphene wouldn’t achieve the toughening effect. Too much would cause the material to become electrically conductive, which is not desired in an electrolyte.

You want the electrolyte to conduct ions, not electricity. 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.

—Nitin Padture

Taken together, the results suggest that nanocomposites could provide a path forward to making safer solid electrolytes with mechanical properties to be used in everyday applications. The group plans to continue working to improve the material, trying nanomaterials other than graphene and different types of ceramic electrolyte.

Resources

• Athanasiou et al. (2020) “High-Toughness Inorganic Solid Electrolytes via the Use of Reduced Graphene Oxide,” Matter doi: 10.1016/j.matt.2020.05.003