Uniform Electrodeposition of Lithium Metal Under an Artificial Solid Electrolyte Interphase Layer

Abstract

Lithium metal is one of the most promising anode materials for high energy density electrochemical energy storage. However, non-uniform electrodeposition of Li metal and dendritic growth result in low Coulombic efficiency and potential short circuits as the dendrites intrude and span the bulk electrolyte/separator region of the cells. This is one of the key stumbling blocks to further development and commercialization of Li metal batteries. To suppress the dendritic deposition of Li metal, one strategy is to encapsulate the Li metal with an artificial solid electrolyte interface (SEI) layer. The ideal artificial SEI layer should possess high mechanical strength and stiffness to prevent the intrusion of Li filaments, high Li transference number, low interfacial resistance, and high ionic yet negligible electronic conductivity. Moreover, recent studies show that the morphology of SEI layer is critical, and it is believed that a homogeneous, isotropic surface is required. The thin film solid electrolyte, lithium phosphorous oxynitride (LiPON), satisfies all of these properties. It has a relatively high shear modulus of 7.7 GPa, unity transference number, electrochemical stability window spanning from 0 up to 5.5 V vs. Li/Li+, ionic conductivity of 2×10-6 S cm-1, electronic conductivity of 8×10-13 S cm-1, and is fabricated as a smooth isotropic, homogenous thin film via physical vapor deposition. Uniform, reversible plating/stripping of Li metal under LiPON layers has been clearly demonstrated in thin film batteries, but fundamental questions remain regarding the physical and electrochemical processes during Li deposition in hybrid electrolyte cells, where LiPON-protected Li metal anodes are employed in combination with liquid electrolytes.In this study, the electrodeposition of Li on Au current collectors coated with a thin LiPON protective layer was studied, and it was shown that compared to Cu current collectors, the Au layers promote uniform electrodeposition of Li metal. The Au layers alloy with Li, reducing the nucleation overpotential and resulting in a more spatially uniform metal deposit. The LiPON protective layer remains intact during the Li plating process, preventing reaction between the liquid electrolyte and Li metal. The effectiveness of the LiPON protective layer was assessed using x-ray photoelectron spectroscopy (XPS) to characterize the surface chemistry of both LiPON-protected and unprotected Li electrodeposits in liquid electrolytes. The effect of current density and plated capacity on the morphology of plated Li on LiPON and Au current collectors was also studied. Smooth, homogeneous deposits of Li under LiPON layers were maintained for capacities up to 3 mAh cm-2 plated at 0.1 mA cm-2. As the plating current density increased up 1 mA cm-2, the LiPON coating fractured due to the localized, nonuniform lithium deposits and rough, dendritic Li morphologies were observed. In operando impedance spectroscopy during Li plating resolved key resistances in the plating process and demonstrated the integrity of the LiPON layers. Efforts to further optimize these protective LiPON layers and deeply understand the Li plating process will be discussed. Figure 1