The huge challenge of improving energy storage and extending battery life while ensuring safe operation is becoming increasingly important as our reliance on this energy source increases, from portable devices to electric vehicles. A Columbia University engineering team led by Yuan Yang, assistant professor of materials science and engineering, announced on April 22, 2019, that they have developed a new method to stabilize lithium metal batteries by implanting boron nitride nanocoatings. Solid electrolytes to safely extend battery life, the findings were published in Joule. Traditional lithium-ion batteries are currently widely used in daily life, but their energy density is low, resulting in short battery life.

　　And because the battery contains highly flammable liquid electrolytes inside, it may short-circuit or even catch fire. Replacing the graphite anode used in lithium-ion batteries with lithium metal can increase energy density: the theoretical charging capacity of lithium metal is nearly 10 times higher than that of graphite. However, during the process of lithium plating, dendrites often form. If they penetrate the separator in the middle of the battery, they will cause a short circuit and raise concerns about battery safety. The research team decided to focus on solid ceramic electrolytes, which show great potential for improving safety and energy density compared with the flammable electrolytes in traditional lithium-ion batteries. Rechargeable solid-state lithium batteries are of particular interest because they are promising candidates for next-generation energy storage. Most solid electrolytes are ceramic and therefore non-flammable, eliminating safety concerns.

　　An artificial boron nitride (BN) film that is chemically and mechanically resistant to lithium electronically insulates lithium aluminum titanium phosphate (LATP) from lithium but still provides protection when penetrated by polyethylene oxide (PEO) Stable ion channels, thereby achieving stable circulation. Image: QianCheng/ColumbiaEngineering

　　In addition, solid ceramic electrolytes have high mechanical strength and can actually inhibit the growth of lithium dendrites, making lithium metal a coating of choice for battery anodes. However, most solid electrolytes are unstable to lithium ions and are easily corroded by metallic lithium and cannot be used in batteries. Qian Cheng, a postdoctoral scientist in the Department of Applied Physics and Applied Mathematics and first author of the paper, said: Lithium metal is indispensable for increasing energy density, so it is crucial that we can use it as an anode for solid electrolytes. To adapt these unstable solid electrolytes for practical applications, a chemically and mechanically stable interface needs to be developed to protect these solid electrolytes from lithium anodes.

　　In order to transport lithium ions, it is crucial that the interface is not only highly electronically insulating but also ionic conductive. Additionally, the interface must be ultra-thin to avoid reducing the battery's energy density. To address these challenges, the team collaborated with colleagues at Brookhaven National Lab and the City University of New York. A 5~10nm boron nitride (BN) nanofilm is deposited as a protective layer to isolate the electrical contact between metallic lithium and the ionic conductor (solid electrolyte), and a small amount of polymer or liquid electrolyte is added to penetrate the electrode/electrolyte interface. BN was chosen as the protective layer because it is chemically and mechanically stable with metallic lithium, providing a high degree of electronic insulation. The boron nitride layer is designed to have inherent defects that allow lithium ions to pass through it, making it an excellent separator.

　　(Illustration) The picture on the left shows that lithium aluminum titanium phosphate (LATP) particles are immediately reduced when exposed to lithium metal. Severe side reactions between lithium and the solid electrolyte can cause the battery to fail within a few cycles. Shown on the right is an artificial boron nitride film that is chemically and mechanically resistant to lithium. It electronically isolates LATP from lithium but still provides a stable ion channel when permeated by polyethylene oxide (PEO), allowing for stable cycling. Image: QianCheng/ColumbiaEngineering

　　In addition, boron nitride prepared by chemical vapor deposition method can easily form large-scale (~dm level), atomically thin scale (~nm level) and continuous films. Although early research used a polymer protective layer with a thickness of only 200 microns, the new BN protective film with a thickness of only 5 to 10 nanometers is still very thin at the limit of this protective layer without reducing the energy density of the battery. This is a perfect material that acts as a barrier to prevent metallic lithium from invading the solid electrolyte. Just like bulletproof vests, a lithium metal bulletproof vest was developed for unstable solid electrolytes, and through this innovation, lithium metal batteries with long cycle life were achieved. The researchers are now extending the new method to a broad range of unstable solid electrolytes and further optimizing the interface, hoping to create high-performance, long-cycle-life solid-state batteries.

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