Engineers at Columbia University have developed a new way to increase battery life of lithium metal batteries by nano-coating them in boron nitride.
In a study featured in the journal Joule, members of Columbia's engineering team explored the possibilities of using BN to extend the charge capacity of lithium-based batteries.
Regular lithium ion batteries are commonly used in most modern devices. However, they do not have enough energy density to help them store more power.
Li-ion cells also contain highly flammable liquid electrolyte, which is why they are prone to shorting out and even catching fire.
While energy density can be increased by replacing the graphite anodes of Li-ion batteries with lithium metal, the process can leave the power cells more susceptible to short-circuits.
The researchers were able to devise a method of improving the energy density of the batteries while still keeping the devices safe to use by consumers.
Extending The Battery Life Of Lithium Ion Batteries
Lithium metal is known to carry an energy charge that is nearly 10 times higher compared to that of graphite. However, using it to plate batteries can lead to dendrite formations. These crystals can penetrate the battery's membrane separator, causing short-circuits to occur.
To solve this problem, the researchers used solid, ceramic electrolytes for their experiment.
Yuan Yang, an assistant professor of materials science and engineering at Columbia, said they chose the material because of its potential to improve both the safety and energy density of batteries, especially when compared to the flammable electrolytes found in conventional Li-ion cells.
"We are particularly interested in rechargeable solid-state lithium batteries because they are promising candidates for next-generation energy storage," Yang said.
Solid electrolytes are often ceramic, which means they are non-flammable and safer to use compared to other forms. These materials are also less susceptible to lithium dendrite growth because of their higher mechanical strength. Both traits help make lithium metal an ideal choice for coating battery anodes.
Making Solid Electrolytes More Stable Against Lithium
Yang and his team had to address one more issue before they could develop their longer-lasting Li-ion batteries: how to make solid electrolytes more stable against lithium.
The researchers teamed up with colleagues at the City University of New York and Brookhaven National Lab to create a mechanically and chemically stable interface that would allow them to protect the solid electrolytes from corrosion caused by the lithium anode.
The interface had to be electronically insulating and ionically conducting at the same time to make sure that the device can transport lithium ions. It also needed to be super-thin to help maintain the proper energy density of the batteries.
The researchers found their solution by using boron nitride nano-film to isolate the contact points between the lithium metal and the solid electrolyte in the form of an ionic conductor. They also used a small amount of polymer (liquid electrolyte) to infiltrate the interface.
Boron nitride has a higher capacity for electronic insulation because of its mechanical and chemical stability against lithium metal. The BN layer that the Columbia engineering team used had intrinsic defects to let Li-ions to pass through.
The compound can also be easily prepared through chemical vapor deposition. This allowed the researchers to create atomically thin scale, large-scale, and continuous BN films to suit their needs.
The researchers noted that the BN film they used provided enough protection to the interface without reducing the energy density of batteries. They believe it could serve as an ideal material to prevent lithium metal from damaging the solid electrolyte.
Yang and his colleagues are now looking to test their method using a broader range of unstable solid electrolytes and to optimize their interface even further. They hope to develop high-performing solid-state batteries with longer-cycle lifetimes.