The ceramic materials that make up solid-state electrolytes don’t stand up well enough for the stresses and strains of the manufacturing process or everyday use.
To address this deficiency, a team of investigators at Brown University has enlisted graphene to double its toughness.
The Problem with Liquid Electrolytes
In a lithium-ion battery (LiB), the electrolyte stands between its anode and cathode. Its purpose is to allow the flow of ions during charging or discharging. Most of today’s LiB’s employ liquid electrolytes and they do an excellent job of it.
But there is a problem and a huge one at that. The electrolytes are highly flammable. Worse still, tiny filaments of lithium can form within the liquid electrolyte causing electrical shorts. The result is that there all too many true stories of LiBs, causing fires and even exploding.
Replacing Liquid Electrolytes with Solid Electrolyte
A study at Brown University has investigated a strategy for replacing liquid electrolytes with solid electrolytes. The study results, reported in the journal Matter, could prove useful for the eventual goal of developing solid-state batteries resilient enough for the mass market.
As described by Christos Athanasiou, a postdoctoral researcher in Brown’s School of Engineering and lead author of the research, “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”
Athanasiou says, “So far, research on solid electrolytes has focused on optimizing their chemical properties. With this work, we’re focusing on the mechanical properties to make them safer and more practical for widespread use.”
Graphene bridging holds solid electrolyte together. Image credited to Brown University
Infusing Solid Ceramic with Graphene
Graphene, the well-known, super-strong allotrope of carbon, was the key to the Brown study. The work involved mixing tiny platelets of graphene oxide with powder of a ceramic called LATP. The mixture was heated, forming a ceramic-graphene composite.
Subsequent mechanical testing of the composite resulted in a higher than two-fold increase in toughness compared to the ceramic alone. According to Athanasiou, “What’s happening is that when the 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.”
The Amount of Graphene Employed is Critical
The success of the experiment depended on determining just the right amount of graphene to employ, and it proved to be a delicate balancing act. Too little graphene won’t endow the ceramic with enough toughness. Too much would cause to ceramic to behave like a conductor, rendering it useless.
According to Brown engineering professor Nitin Padture, one of the authors of the study, “You want the electrolyte to conduct ions, not electricity.”
Padture goes on to say that “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.”
The Toughest Solid Electrolyte
As described by Brian Sheldon, also a Brown engineering professor and the lead contact for the work, “To our knowledge, this is the toughest solid electrolyte that anyone has made to date.” Sheldon also comments that “I think what we’ve shown is that there’s a lot of promise in using these composites in battery applications.”
One of the significant efforts in electrical engineering today is developing components and devices that use less power. A major reason for that is the batteries are limited in how much energy they can store, and how safely they can do it.
Also, there is nothing that’s holding back the widespread adaptation of electric vehicles as much as the inadequacies of today’s LiBs. The perfection of solid-electrolyte LiBs is increasingly seen as the next necessary step on the road to an all-electric future.
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