Poly(ethylene oxide) (PEO) is the most popular polymer for solid-state lithium metal battery electrolytes. While many studies focus on increasing PEO ionic conductivity through doping with lithium salts, little work has addressed using PEO and Li directly to generate Li⁺-conducting species in situ. With electrochemical, spectroscopic, and computational techniques, we studied the reaction of Li metal with PEO. Electrochemical impedance spectroscopy showed the ionic conductivity of PEO thin films increases up to three orders of magnitude (from 10^-7 S cm^-1 to 10^-4 S cm^-1) when contacted with Li at elevated temperatures and analysis confirmed that the conductivity is fully ionic and not electronic. The species responsible for these changes were determined to be lithium alkoxides and organolithium species that formed through the kinetically slow cleavage of ether bonds in the PEO backbone by Li metal. The alkoxides have ionic character and likely transport Li⁺ through vehicular motion, while organolithium species propagate the ether cleavage reactions beyond the metal-electrolyte interface. A passivating Li₂O layer eventually forms, halting the reaction to yield a stable interface and steady bulk conductivity. Density functional theory analysis confirmed this mechanism as thermodynamically favorable. Preliminary work with free-standing PEO electrolytes and Li electrodes is revealing promising results for in situ doping at larger scale. Not only does this work shed new light on interfacial reactivity between Li metal and polymer electrolytes, but after 50 years of PEO electrolyte research, it also presents new pathways for increasing polymer electrolyte ionic conductivity by creating single-ion conductors in situ.

Figure 1. Reaction of Li Metal with Model PEO Molecules. (a) DFT calculation of the reaction between two Li atoms and a tetraglyme molecule as a model for the reaction mechanism of PEO with a Li surface. The resulting species display ionic Li-OR character (b) Development of conductivity in the Li-PEO system: I) Undoped PEO initially exhibits minimal conductivity; II) Li metal reduces ether bonds in PEO, generating Li-OR and Li-R at the interface; III) Continued reaction between Li and PEO, Li-OR species diffuse into the bulk, and insulating Li₂O begins to form at the interface; IV) The Li₂O layer prevents further reaction. Li-OR species are homogenized throughout the bulk and contribute to Li⁺ conductivity through vehicular motion.