Effect of Water As an Electrolyte Additive for Lithium Plating

Abstract

Li metal anode is generally referred to as the “holy grail” of battery technology as it can deliver the highest possible energy density, which double the capacity of state of art graphite based cells1. At the heart of this advanced technology is the process of continuous plating and stripping of lithium during the cycling of the battery. Owing to the highly reactive nature of Li, this is a complicated process with several challenges2 during both plating (for e.g., dendrite formation and growth, dead lithium, continuous interface/interphase change, short circuit etc.) and stripping (void formation, pitting, broken interface/interphase layers etc.). The electrode architecture, choice of electrolyte, solid electrolyte interphase (SEI) formation, operating conditions etc. are key factors that determine overall stability of Li metal anodes. These issues get magnified when using liquid electrolytes, where continuous SEI formation leads to dead Lithium (electrically insulating) accumulation.In general, most solvents and salts employed for Li ion cells contain some amount of water as impurity even after maintaining strict conditions to keep them anhydrous. The presence of trace amount of water in combination with fluorine from either salt (LiPF6. LiTFSI, LiFSI) or solvent additives FEC can then lead to in-situ formation of HF, which has detrimental effects on Li plating. However, recently many groups have shown the beneficial effect of small amount of water as additives for Li metal anodes3,4. However, the exact mechanism of functioning of H2O as an additive is not very clear. This is partly because the formation of SEI is quite complex, especially when film forming type solvents/additives are present in the electrolyte addition to trace amounts of water.It is thus important to investigate the nucleation and growth of Li during the initial stages of plating and understand how the presence of H2O in the electrolyte affects the morphology andthe chemical nature of the deposited Li metal films. In this direction, we choose tetraglyme (4G), which is a relatively non-labile solvent towards Li plating conditions (reduction potential ~ 1.49 vs Li+/Li), as the solvent to avoid any contribution from the solvent itself on SEI formation. We then systematically study the effect of H2O on Li plating; both stand alone and in presence of various film forming additives. We show that water can indeed drastically affect the morphology and electrochemical properties of the deposited Li. Acknowledgement: This work is part of FUGELS (Functionally Graded Electrodes for Long-Life Lithium-Sulfur Batteries) project, which is a cconsortium of researchers from University of Hasselt, University of Gent, University of Antwerp, VITO, IMEC and IMOMEC.