Insights into The Reactivity and Lithium Plating Mechanisms of Ultra-Thin Metal Oxide Coatings for Anode-Free Solid-State Lithium Metal Batteries

Solid-state batteries (SSBs) using lithium metal anodes are the best candidates for high energy density battery applications. The energy density can be further increased by eliminating the lithium metal excess in an “anode-free” cell format. However, low Coulombic efficiency from heterogeneous lithium metal plating and stripping along with charge loss due to solid electrolyte interphase (SEI) formation severely limit the cycle life of anode-free SSBs. Here, we explore the use of ultra-thin (5-20 nm) Al2O3 and ZnO coatings deposited by atomic layer deposition (ALD) on copper electrodes for anode-free cells with a solid polymer electrolyte. Voltammetry shows that lithium inventory loss due to SEI formation is reduced by over 50% with Al2O3@Cu electrodes, but these electrodes experience orders of magnitude higher interface resistances than bare Cu and ZnO@Cu electrodes due to their inherently low ionic and electronic conductivities. The electrochemical differences between Al2O3 and ZnO coatings are reflected in X-ray photoelectron spectroscopy (XPS) experiments that show Al2O3 undergoing a self-limiting lithiation reaction with Li metal, while ZnO reacts completely with Li to form a LiZn alloy and Li2O. These chemical differences at the interface result in higher and lower lithium plating nucleation overpotentials for Al2O3 (up to 220 mV) and ZnO (down to 15 mV) coatings, respectively, relative to uncoated Cu electrodes (35 mV). Time-of- flight secondary ion mass spectrometry (ToF-SIMS) reveals lithium plating underneath a reacted LiyAlOx coating and through emergent defects and pinholes with Al2O3@Cu electrodes, while it plates exclusively on top of the electronically conductive converted ZnO@Cu electrodes. Scanning electron microscopy (SEM) corroborates these mechanisms by showing sparse coverage of isolated Li clusters plated with Al2O3@Cu electrodes, while Cu and ZnO@Cu grow more dense and interconnected lithium deposits. Despite both coatings improving different aspects of anode-free battery design, unmodified Cu electrodes show higher Coulombic efficiencies (~77%) than Al2O3@Cu (up to 70%) and ZnO@Cu (up to 75%) electrodes. Increasing Al2O3 coating thickness decreases the practical current density compared to unmodified Cu (30 μA/cm2) but increasing ZnO coating thicknesses can double or triple this value. These (electro)chemical and morphological observations lead to the proposal of two mechanisms, where less-reactive metal oxides develop lithium-ion conductivity through the metal oxide structure to plate lithium underneath the coating, while more-reactive metal oxides undergo full reduction and conversion reactions that allow lithium to plate above the coating. The fundamental research here open paths for future work to leverage these mechanisms and explore other materials for high-efficiency anode-free SSBs.