On the role of Water-as-Additive in Lithium Electrodeposition

Electroplating of flat and smooth lithium layers is key for batteries using metallic lithium as the anode, where a major failure mechanism is the self-enhancing uneven or ramified growth of lithium metal leading to capacity fading or even short-circuiting during cycling. In that regard we shed new light on an intriguing growth mode for electrodeposited lithium in this work, where lithium self-assembles in a compact columnar morphology. Such growth results in topographically smooth layers and a particular stripping behavior along the length of these nanorods, rather than from the top down. Here, we demonstrate the electroplating of smooth layers of columnar lithium using LiTFSI and LiClO4 salts, which rules out the commonly found explanation of a LiF interface layer directing growth. Rather, we find that the addition of water to these (non-hydrolising) electrolytes is key for the guided growth. We investigate its effect both during electrochemical formation (applied potentials > Li+/Li), and growth (applied potentials < Li+/Li), and observe that the presence of water is required during the whole process. We use a rotating ring disk electrode (RRDE) to study the process in-situ, and find that hydrogen is continuously generated. Notably, hydrogen is generally considered as a side-product in the LiF based mechanism, and we therefore suggest that the evolution of hydrogen is in fact the critical component for the directed growth. Such an explanation homogenizes our results with earlier reports and provides mechanistic insights for the role of water during lithium electrodeposition, which is invariably present in lithium metal batteries.