A Multiscale View on Li⁺ Transport in Li Metal Solid-State-Batteries

The fast-expanding sector of portable electronics and electric vehicles looks for new solutions to overcome the safety and performance limitations of liquid electrolyte-based Li-ion batteries. A promising new technology is the so-called Li metal Solid State Battery (SSB), which aims to replace the flammable liquid electrolyte by solid electrolyte materials, such as ceramics or polymers. Ceramic electrolytes are the best candidates for SSB in terms of high Li+ conductivity, though they are strongly limited by their mechanical properties and the limited interfacial contact with Li metal anode. On the other hand, polymer electrolytes present good mechanical properties with a high degree of processability, whereas a low ionic conductivity limits them at RT. An alternative is to combine the desirable properties of ceramic and polymeric electrolytes into a composite concept. Generating knowledge about interactions between composite components and their influence on Li-ion mobility, electrochemical stability, and Li electrodeposition properties is thus, essential to progress towards practical applications. This work attempts to give suitable solutions to overcome the main constraints of solid-state batteries, necessary to implement metallic lithium anode into the solid-state battery concept. Primarily, the intimate and stable contact between electrolyte and electrodes, continuous and fast lithium ions mobility across the membrane and interfaces were targeted. In this contribution, we focused on the Li-metal SSB with a composite solid electrolyte system based on PEO-LiTFSI polymeric matrix enriched with Li6.55Ga0.15La3Zr2O12 garnet filler [1]. To accurately investigate the Li-ion transport properties and avoid the typical segregation issues of these immiscible mixtures. We apply a new processing method that ensures the high degree of structural and chemical homogeneity, even at the local scale, and across a broad range of ceramic filler content. Solid-state NMR and Electron microscopy were used to characterize the structure of the composites locally. The local mobility of Li-ion was investigated by 2D NMR to understand and propose possible transport mechanisms. The effect of garnet filler on salt dissociation, ions interactions, and a correlation between local and global ion mobilities was reconsidered. The impact of the garnet filler content on the macroscopic Li-ion conductivity will be discussed as well as their influence on Li metal stripping/plating and Li dendrite suppression. Finally, the implementation of these composite electrolytes in the Li metal all-solid-state full cell device will be presented. All in all, this work shows how the combination of hard-soft electrolyte materials can enhance the interfacial stability with Li metal anode upon cycling, offering new opportunities to prevent Li dendrite formation in solid-state batteries. [1] J. Zagorski, J.M. Lopez del Amo, M.J. Cordill, F. Aguesse, L. Buannic, A. Llordes. Garnet-Polymer Composite Electrolytes: New Insight on Local Li-Ion Dynamics and Electrodeposition Stability with Li Metal Anodes. ACS Applied Energy Materials 2019 2(3), 1734-1746 DOI: 10.1021/acsaem.8b01850