Battery researchers consider lithium metal electrodes to be one of the leading contenders for improving battery capacity without increasing battery weight. Unfortunately, there’s one big problem with lithium that’s so far stood in the way of its potential: as the batteries are charged, lithium deposits form on the metal surface in sprawling, finger-like patterns. This diminishes battery performance and can even lead to disabling short circuits.
But this could soon be a problem of the past, as a team of MIT researchers has now carried out the most detailed analysis yet of these lithium deposits. In fact, they’ve discovered that there are actually two distinct patterns of growth due to two different mechanisms: Clustered “mossy” deposits that grow from their roots and pointed “dendritic” projections that grow outward from their tips.
The team discovered the distinction thanks to an innovative experimental setup. Previous research made use of electrical measurements to infer what was happening inside the battery, but the research team used a glass capillary cell to visibly see the lithium deposits grow. This allowed them to observe the distinct transition from one type of growth to the other as they adjusted the level of current.
Even though both types of deposits are composed of lithium filaments, the mossy deposits are easy to control while the dendritic deposits, responsible for most of the problems, are far less tractable. Fortunately, according to the researchers, the dendritic growth should not occur in practical batteries since this type of growth results from a much higher current than normal.
The solution to the mossy deposits, which form at a lower level of current, is to add a ceramic separator made of anodic aluminum oxide (AAO). This separator, which is 60 μm thick, effectively blocks the mossy growth. “It’s a big discovery, because it answers the question of why you sometimes have better cycling [charging and discharging] performance when you use ceramic separators,” said Peng Bai, lead author of the team’s paper.
Understanding the nature of the lithium deposits and finding an effective control mechanism opens the door to more efficient batteries. Replacing conventional carbon-based anodes with lithium metal could cut lithium-ion battery weight and volume in half while maintaining capacity and output current. Thanks to the team’s findings, battery researchers will now have a much greater understanding of the limits on rate and capacity achievable with lithium metal electrodes.