Dissolution of Lithium Metal in Poly(ethylene oxide)*

Salt doped poly(ethylene oxide) (PEO) has been studied extensively as a model polymer electrolyte system for lithium metal battery applications. In this study, we examine the widely accepted assumption that a stable interface forms between lithium and PEO above the PEO melting temperature by studying lithium symmetric cells. Using Li⁷ NMR, we show that a lithium species dissolves from the lithium electrode and diffuses into the bulk of initially neat PEO. Impedance spectroscopy is used to demonstrate that the lithium species contributes to ionic conductivity, and small angle X-ray scattering demonstrates that the dissolution also occurs in a PEO containing block copolymer, resulting in a significant increase (>20°C) of the order-to-disorder transition temperature. The results indicate that the lithium/PEO interface is not stable at elevated temperatures and there are clear implications for lithium metal batteries using PEO-based electrolytes.