Influence of the LLZO–PEO interface on the micro- and macro-scale properties of composite polymer electrolytes for solid-state batteries

Li7La3Zr2O12 (LLZO) garnets offering high ionic conductivity and electrochemical stability are among the most promising ceramic materials for lithium metal solid-state batteries. Although their application in composite polymer electrolytes (CPEs) with poly(ethylene oxide) (PEO) has been widely studied, their surface chemistry which is influenced by their hygroscopic nature is often neglected. This work reports on how the thermal treatment and the consequent elimination of secondary phases at the LLZO–PEO interface impacts the microstructure of the garnet-rich CPEs, which in turn affects their mechanical and ion-transport properties. It is shown that LLZO heat treatment restricts local polymer chain motions, indicating reinforcement of PEO/LLZO interactions which enhances the mechanical strength and homogeneity of the CPEs. These micro-scale modifications of CPEs eventually increase their ionic conductivity and improve the solid electrolyte/lithium metal interface. Thus, the Li+ exchange at the PEO/LLZO interface was studied by using 7Li–7Li exchange spectroscopy– nuclear magnetic resonance , and it was observed that after LLZO heat treatment, the interfacial Li+ exchange has significantly decreased, in line with a lower LiOH content. This result confirms the role of LiOH as an intermediate in the Li+ exchange reaction and that surface chemistry plays a more important role in the Li+ exchange than the local Li mobilities in the individual phases.