A Columbia Engineering team led by Yuan Yang, assistant professor of materials science and engineering, has developed a new method for safely prolonging battery life by inserting a nano-coating of boron nitride (BN) to stabilize solid electrolytes in lithium metal batteries. Their findings are outlined in a new study published in Joule.
When combined with ∼1–2 μm PEO polymer electrolyte at the Li/BN interface, Li/Li symmetric cells show a cycle life of more than 500 hours at 0.3 mA·cm−2. In contrast, the same configuration with bare Lithium aluminum titanium phosphate (LATP) dies after 81 hours. The LiFePO4/LATP/BN/PEO/Li solid-state batteries show high capacity retention of 96.6% after 500 cycles.
Schematics of the protection mechanism of boron nitride (BN) and characterizations of BN nanofilm. The left visual shows that a Lithium aluminum titanium phosphate (LATP) pellet that touches lithium metal will be immediately reduced. The severe side reaction between lithium and solid electrolyte will fail the battery in several cycles. The right shows that an artificial BN film is chemically and mechanically robust against lithium. It electronically isolates LATP from lithium, but still provides stable ionic pathways when infiltrated by polyethylene oxide (PEO), and thus enables stable cycling. Credit: Qian Cheng/Columbia Engineering.
Conventional lithium ion (Li-ion) batteries have low energy density, resulting in shorter battery life, and, because of the highly flammable liquid electrolyte inside them, they can short out and even catch fire.
Energy density could be improved by using lithium metal to replace the graphite anode used in Li-ion batteries; lithium metal’s theoretical capacity for the amount of charge it can deliver is almost 10 times higher than that of graphite. But during lithium plating, dendrites often form and, if they penetrate the membrane separator in the middle of the battery, they can create short-circuits, raising concerns about battery safety.
We decided to focus on solid, ceramic electrolytes. They show great promise in improving both safety and energy density, as compared with conventional, flammable electrolytes in Li-ion batteries. We are particularly interested in rechargeable solid-state lithium batteries because they are promising candidates for next-generation energy storage.—Yuan Yang
Most solid electrolytes are ceramic, and therefore non-flammable, eliminating safety concerns. In addition, solid ceramic electrolytes have a high mechanical strength that can actually suppress lithium dendrite growth, making lithium metal a coating option for battery anodes. However, most solid electrolytes are unstable against Li—they can be easily corroded by lithium metal and cannot be used in batteries.
Lithium metal is indispensable for enhancing energy density and so it’s critical that we be able to use it as the anode for solid electrolytes. To adapt these unstable solid electrolytes for real-life applications, we needed to develop a chemically and mechanically stable interface to protect these solid electrolytes against the lithium anode. It is essential that the interface not only be highly electronically insulating, but also ionically conducting in order to transport lithium ions. Plus, this interface has to be super-thin to avoid lowering the energy density of batteries.—Qian Cheng, the paper’s lead author and a postdoctoral research scientist in the department of applied physics and applied mathematics who works in Yang’s group
To address these challenges, the team worked with colleagues at Brookhaven National Lab and the City University of New York. They deposited 5~10 nm 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 vapor deposition to form large-scale (~dm level), atomically thin scale (~nm level), and continuous films.
The researchers are now extending their method to a broad range of unstable solid electrolytes and further optimizing the interface. They expect to fabricate solid-state batteries with high performance and long-cycle lifetimes.
The study was supported by the Air Force Office of Scientific Research (FA9550-18-1-0410) and Research Corporation for Science Advancement (Award #26293), and the NSF MRSEC program through Columbia in the Center for Precision Assembly of Superstratic and Superatomic Solids (DMR-1420634).
Cheng, Qian et al. (2019) “Stabilizing Solid Electrolyte-Anode Interface in Li-Metal Batteries by Boron Nitride-Based Nanocomposite Coating” Joule doi: 10.1016/j.joule.2019.03.022