Unusual impact behavior of molten lithium microdrops

Thin lithium films are promising candidates for anodes of next-generation lithium metal batteries due to their high energy density, safety, and efficiency. Depositing molten lithium microdrops offers a cost-effective and scalable method for producing these thin films. Here we visualize and analyze the impact, spreading, and solidification behavior of single lithium microdrops on solid substrates, a fundamental process of drop-based lithium film manufacturing. Liquid lithium exhibits unusual properties, with a surface tension approximately five times higher than water and a density about half that of water. These characteristics allow us to explore previously untested parameter regimes. For example, the Weber number of a lithium drop, with the same size and impact velocity as a water drop, is an order of magnitude lower. Our high-speed imaging experiments reveal that a lithium drop with a 0.2 mm diameter, impacting on a substrate at a velocity exceeding 10 m/s, initially spreads into a thin disc, forming a solidified layer at the bottom. However, the drop then recoils vigorously owing to its high surface tension. The recoiling process is further assisted by the slow solidification of lithium, attributable to its unusually high specific heat, which is approximately 14 times that of tin. We further discuss strategies to suppress this recoil, aiming to achieve uniform thickness in film deposition.