Based on this we put forth two potential mechanisms through which H2 guides growth. Firstly through the continuous formation of a LiH surface layer. (Electro)chemical conversion of Li to LiH is possible in the presence of hydrogen and has been directly observed recently . While we cannot verify the presence of LiH through our experiments, a LiH surface layer has previously been found to direct growth and is consistent with our results, as we consider the presence of hydrogen to be critical.