Nuclear Magnetic Resonance Investigation of Lithium Ion Transport in a Highly Conducting Solid Polymer Electrolyte: The Enabling Effect of High Salt Concentration

Solid polymer electrolytes based on poly(ethylene oxide) (PEO) for rechargeable lithium ion batteries significantly lack sufficiently high ionic conductivity for practical use at ambient temperature. Recent reports of high ionic conductivity and an expanded electrochemical window of aqueous electrolytes enabled by very high salt concentration (~ 20m) as well as earlier work on the “salt-in-polymer” concept have inspired this investigation of a solid PEO-salt-water system, where the dissolved salt (LiTFSI) concentration approaches its solubility limit and in combination with water association succeeds in inhibiting crystallization of the PEO matrix leading to higher conductivity. Room temperature ionic conductivity in these solid-like polymer electrolytes is about 2 mS-cm^-1, which is sufficiently high for battery application. Characterization of the ion transport process by NMR pulsed field gradient diffusion measurements (⁷Li and ¹⁹F for the cation and anion, respectively) and electrochemical impedance spectroscopy demonstrate an acceptable degree of salt ion dissociation and a lithium transport number, i.e. the Li ionic fraction of the current, exceeding 0.6, which is unusually high for polymer electrolytes. Additional electrochemical testing results will also be presented.