Virtual electrode XPS experiments on Li 3−x Sc 1−x Zr x Cl 6 (x ∼ 0.2): Oxygen-mediated interface passivation on metallic lithium anodes

Lithium metal chlorides are a promising class of solid-state electrolyte (SSE) materials that may enable the use of lithium anodes for next-generation electrochemical energy storage. Zr-substituted Li 3 ScCl 6 (Li 3-x Sc 1-x Zr x Cl 6 ), combines the high ionic conductivities of Li 3 ScCl 6 with the moisture resistance of Li 2 ZrCl 6 that facilitates processing and manufacturing. However, little is known about the solid electrolyte interphase (SEI) formation and composition at the Li 3-x Sc 1-x Zr x Cl 6 and Li metal anode interfacial region. Elucidating the relevant (electro)chemical processes is crucial for engineering Li 3-x Sc 1-x Zr x Cl 6 | Li 0 interfaces with low interfacial impedances and long-term cycling stability. Here, we utilize the virtual-electrode X-ray photoelectron spectroscopy (VE-XPS) technique study the formation and early-stage evolution of the SEI. Additionally, we investigate the long-term effects of galvanostatic cycling on the ion transport properties and how SEI evolution impacts the overall electrochemical impedance of Li 2.8 Sc 0.8 Zr 0.2 Cl 6 -based cells. Our results show the initial SEI formation diverges from thermodynamic phase decomposition predictions due to the presence of oxygen, leading to formation of a novel Li-Sc-Zr-oxide (LSZO) phase that is compositionally similar to Li 7 La 3 Zr 2 O 12 (LLZO). For the early-stage SEI, this oxygen-mediated decomposition pathway appears to mitigate reduction of the metal cations and prevent the formation of metallic phases (e.g., metallic Zr, Sc, and Sc-Zr alloys). We find that LSZO phase formation effectively passivates the Li 2.8 Sc 0.8 Zr 0.2 Cl 6 | Li 0 interface and enables plating and stripping of metallic lithium.