Inspired by the structure of wood, engineers at the University of Maryland have used modified wood as a unique architecture for the negative electrode of a lithium metal battery, seeking to prevent some of the key factors that lead to battery failure.
Lithium ion shuttling in rechargeable batteries provides energy to power phones, laptops, and even light bulbs. When the battery is charged, the lithium metal expands; and when it is discharged, the lithium metal deflates. This rapid change in size can lead to an undesirable side-effect, branch-like growths of lithium on the surface of the lithium metal. The damage builds up over time and pose safety hazards, such as overheating or fire. This novel design for a safer lithium metal battery, created by Ying Zhang, a UMD Ph.D. student in the department of materials science and engineering, can be used to boost the energy density of a battery– increasing the power available for portable electronics and electric vehicles, while reducing the risk of the battery overheating.
In this new type of battery, the engineers store lithium in the natural channels of carbonized wood, channels that were once used to carry water and nutrients, instead of storing lithium particles (ions) in a metal block.
The wood acts as a “hotel” to provide lots of rooms (channels) to accommodate many guests (lithium metal). As the lithium metal “guests” enter the wood hotel, it accommodates them all and stores them comfortably and securely in each room, while maintaining the wood’s rigid exterior structure. The number of lithium particles (guests) can increase and decrease within each room, but the overall structure will not be damaged or collapse.
A battery made this way can operate safely even with fast charge and discharge rates. Metric engineers use current density of a battery to describe how quickly the lithium metal is deposited at the surface. A high current density is equivalent to having excessive guests flow into and out of the wood “hotel,” which can cause issues when pile-ups occur at the door. These pile-ups can be avoided by simply increasing the number of doors available to the lithium ions as they enter the wood “hotel”– the approach the engineers at UMD used. If the overall number of lithium metal “guests” entering at one time remains the same, only a small number of “guests” are passing through any door at a given time. This is known as the local current density. By using the large surface area provided by the walls of each channel in the wood host, the local current density can be minimized, facilitating the controlled movement of lithium metal.
Batteries that use bulk lithium metal foil, which is the conventional alternative, can be compared to an unstable hotel with only one door for guests to enter and exit. When the battery is put to the test under high current density conditions, its single door cannot manage the large flow of guests, making it easier to crack, which can lead to hazards within the battery. On the other hand, the wood “hotel” design, with its many straight channels, provides plenty of doors for guests so the lithium metal can be corralled into individual channels, behaving in an orderly, predictable manner even under high current density (3 mA/cm2) and avoiding branch-like structures of lithium that can cause battery failure. Something that had its start as natural wood helps engineers build stronger, more stable batteries for the future.
“This is part of our ongoing research to use natural materials to improve batteries,” said senior research group leader Liangbing Hu, an Associate Professor in the Department of Materials Science and Engineering and a member of the University’s Energy Research Center (UMERC). “Using nature’s bio-structure, we can find inspiration to create new ways of storing energy, and we can use renewable materials too.”
This research was published in the Proceedings of the National Academy of Sciences on March 20, 2017, in a paper entitled, “High-capacity, low-tortuosity and channel-guided lithium metal anode.”
Credit: University of Maryland