Enhanced Surface Interactions Enable Fast Li⁺ Conduction in Oxide/Polymer Composite Electrolyte

Li⁺ -conducting oxides are considered better ceramic fillers than Li⁺ -insulating oxides for improving Li⁺ conductivity in composite polymer electrolytes owing to their ability to conduct Li⁺ through the ceramic oxide as well as across the oxide/polymer interface. Here we use two Li⁺ -insulating oxides (fluorite Gd_0.1 Ce_0.9 O_1.95 and perovskite La_0.8 Sr_0.2 Ga_0.8 Mg_0.2 O_2.55 ) with a high concentration of oxygen vacancies to demonstrate two oxide/poly(ethylene oxide) (PEO)-based polymer composite electrolytes, each with a Li⁺ conductivity above 10^-4  S cm^-1 at 30 °C. Li solid-state NMR results show an increase in Li⁺ ions (>10 %) occupying the more mobile A2 environment in the composite electrolytes. This increase in A2-site occupancy originates from the strong interaction between the O^2- of Li-salt anion and the surface oxygen vacancies of each oxide and contributes to the more facile Li⁺ transport. All-solid-state Li-metal cells with these composite electrolytes demonstrate a small interfacial resistance with good cycling performance at 35 °C.