(637e) Uniform Electrodeposition of Lithium Metal Enabled By Thin Lithium Phosphorous Oxynitride (LiPON) Films

Lithium metal is the most promising anode for next-generation energy-dense 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. To suppress the dendritic deposition of Li metal anode, one strategy is encapsulate the Li metal with a solid electrolyte. The ideal solid electrolyte 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 electrical conductivity. Moreover, the morphology of the solid electrolyte surface is critical, and homogeneous, isotropic electrode interfaces promote the uniform electrodeposition. 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, electrical conductivity of 8×10^-13 S cm^-1, and is fabricated as a smooth isotropic, homogenous layer via physical vapor deposition. Uniform, reversible plating/stripping of Li metal under LiPON layers has been clearly achieved in thin film batteries, but fundamental questions remain regarding the physical and electrochemical processes during Li deposition in hybrid electrolyte cells, which employ LiPON-protected Li metal anodes coupled 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 into a solid solution layer, 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 reduction 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 plated Li 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 applied current density increased up to 1 mA cm^-2, the LiPON coating fractured due to localized, nonuniform lithium deposition and rough, dendritic Li morphologies were observed. In operando impedance spectroscopy during Li plating was acquired to resolve the key resistances in the plating process and assess the integrity of the LiPON layers. Efforts to further optimize these protective LiPON layers and understand the Li plating phenomena will be discussed.