Dissolution of Lithium Metal in Poly(ethylene oxide)

We demonstrate that lithium metal is sparingly soluble in poly(ethylene oxide) (PEO). 7Li NMR shows that when a PEO sample is placed in contact with lithium metal at elevated temperatures, a lithium species dissolves and diffuses into the bulk polymer. A lithium/PEO/lithium electrochemical cell, containing no lithium salts, shows increasing conductivity over time when annealed at 120 °C. Chronoamperometry shows that the annealed cell obeys Ohm’s law, implying that conduction occurs without the development of concentration gradients. To explain the results, it is proposed that atomic lithium dissolves into PEO, where it exists as a lithium cation and free electron. The dissolution of lithium also affects the phase behavior of block copolymer electrolytes. These observations explain the strong adhesion between lithium metal and PEO and have important implications for lithium metal battery systems that contain PEO-based electrolytes.