To address these challenges, the team worked with colleagues at Brookhaven National Lab and the City University of New York. They deposited 5 to 10 nm of a boron nitride (BN) nano-film as a protective layer to isolate the electrical contact between lithium metal and the ionic conductor (the solid electrolyte), along with a trace quantity of polymer or liquid electrolyte to infiltrate the electrode/electrolyte interface. They selected BN as a protective layer because it is chemically and mechanically stable with lithium metal, providing a high degree of electronic insulation. They designed the BN layer to have intrinsic defects, through which lithium ions can pass through, allowing it to serve as an excellent separator. In addition, BN can be readily prepared by chemical vapour deposition to form large-scale, atomically thin nanometre scale, continuous films.
"While earlier studies used polymeric protection layers as thick as 200 μm, our BN protective film, at only 5 to 10 nm thick, is record-thin--at the limit of such protection layers--without lowering the energy density of batteries," said Cheng. "It's the perfect material to function as a barrier that prevents the invasion of lithium metal to solid electrolyte. Like a bullet-proof vest, we've developed a lithium-metal-proof 'vest' for unstable solid electrolytes and, with that innovation, achieved long cycling lifetime lithium metal batteries."
The researchers are now extending their method to optimising a broader range of unstable solid electrolytes to build solid state lithium metal batteries with high performance and long-cycle lifetimes.
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