“Mirror-like” Electrodeposition of Lithium Metal Under a Low-Resistance Artificial Solid Electrolyte Interphase Layer

Abstract

Lithium metal is considered as the “holy grail” for next generation anode materials of energy storages. However, non-uniform electrodeposition of Li metal and dendritic growth result in low Coulombic efficiency and potential short circuits, leading to enormous capacity fading and safety issues. 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 (ASEI) 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 bulk/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. However, it remains a challenge to observe uniform, mirror-like electrodeposition of Li metal under the sole protection of LiPON layers.Herein we propose a multifunctional ASEI that combines the advantages of LiPON coating protection with the enhanced electrolyte/electrode interface wetting conferred by LixAu alloys. “Mirror-like” electrodeposition of Li up to a practical capacity of 3 mAh cm-2 was achieved, guiding a new avenue towards the limited amount of Li electrode enabled by precisely controlled electrodeposition. The lithiation of Au films (as current collectors), significantly reduces nucleation overpotential to zero, promoting uniform electrodeposition of Li metal, thereby a more spatially uniform metal deposit. The low overall resistance ASEI layer (50 Ω cm2), LiPON, separates physically deposited Li and solvents from liquid electrolyte, drastically eliminating the electrolyte reduction and improving Li deposition efficiency. Operando impedance measurements were utilized to in-situ monitor interface differences during Li deposition. Figure 1