Improvement of Air Stability of Garnet-Type Li7La3Zr2O12 Solid-State Electrolyte through F-Doping Strategy
Authors: Chen Yi-Xiu , Slew Hoong-Zheng , Yang Tsung-Ying , Liu Chuan-Pu 
Published: December 2024

Garnet-type Li7La3Zr2O12 (LLZO) stands out among various solid-state electrolytes (SSEs) due to its exceptional potential for commercializing Li metal batteries.1 This is attributed to its high Li-ion conductivity, reaching approximately 10-4 S/cm in its cubic phase.2 Dopant stabilization strategies have been widely employed in LLZO materials to promote the formation of cubic-LLZO (c-LLZO), involving cation substitution effects on Li+, La3+, and Zr4+ lattice sites.3 However, substituting cations on the Li site can pose challenges to Li-ion conduction pathways. Consequently, one emerging trend is the use of halogen species (F-, Cl-, Br-) as dopants on the O site, aiming to enhance ion migration without sacrificing Li content.4 This novel concept shows promise in improving Li-ion conductivity in garnet-based SSEs.5However, most of the studies on halogen doping in LLZO have been conducted only through computational simulations and calculations without the experimental results verification.6 Unfortunately, the conventional sintering process at high temperatures (> 1100°C) poses challenges such as lithium loss7, formation of Li2CO3.8 To achieve excellent c-LLZO at low temperatures, PVD methods such as RF magnetron sputtering9 and PLD10 have been considered effective techniques for forming thin-film LLZO. The layer-by-layer deposition and post-annealing processes are crucial for achieving dense, highly controllable, and uniformly doped C-LLZO thin films.

In this report, we propose a rational material design approach for the layer-by-layer microstructure of LiF-LLZO using the thin-film deposition technique (see Supplementary Information for details). The comprehensive X-ray photoelectron spectroscopy (XPS) spectra trace the Li2CO3 distribution inside the depth profile evolution of LLZO after a hostile environment, shedding light on the ambient stability of SSEs. Thermal decomposed LiF serves the dual role of providing supplemental Li and F dopant after by post-annealing. Furthermore, the remained LiF protection on LLZO and F-LLZO was systematically studied about the capability of against the Li2CO3 immersing inside LLZO. Our perspective offers an innovation strategy to solve the Li2CO3 problem and Li-loss simultaneously, which can be extended to other oxide-base SSEs.

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