Researchers have doubled the toughness of a ceramic material used to make solid-state lithium-ion batteries. The strategy could be useful in bringing solid-state batteries to the mass market. So far, research on solid electrolytes has focused on optimizing their chemical properties. The new strategy focuses on the mechanical properties. There is interest in replacing the liquid electrolytes in current batteries with ceramic materials because they are safer and can provide higher energy density.
The electrolyte is the barrier between a battery’s cathode and anode through which lithium ions flow during charging or discharging. Liquid electrolytes work — they are found in most batteries in use today — but they have some problems. At high currents, tiny filaments of lithium metal can form inside the electrolytes, which cause batteries to short-circuit. And since liquid electrolytes are also highly flammable, those shorts can lead to fires.
Solid ceramic electrolytes aren’t flammable and there is evidence that they can prevent the formation of lithium filaments, which could enable batteries to operate at higher currents. Ceramics, however, are highly brittle materials that can fracture during the manufacturing process and during use.
For this new work, the researchers wanted to see if infusing a ceramic with graphene — a super-strong carbon-based nanomaterial — 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.
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 twofold increase in toughness compared to the ceramic alone. When a crack starts in a material, the graphene platelets essentially hold the broken surfaces together so that more energy is required for the crack to spread.
Experiments 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 and too much would cause the material to become electrically conductive, which is not desired in an electrolyte.
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.