Understanding Lithium-Ion Dynamics in Single-Ion and Salt-in-Polymer Perfluoropolyethers and Polyethyleneglycol Electrolytes Using Solid-State NMR

Abstract

Lithium metal batteries with high energy densities can enable a revolution in energy storage and accelerate shifts in electric transportation and electricity generation. However, several morphological and electro-chemo-mechanical challenges impede their development. Solid-state electrolytes such as those based on polymers show great promise in replacing liquid electrolytes in lithium metal batteries. Polyether-based polymer electrolytes are the most investigated but are plagued by low room-temperature ionic conductivity and poor oxidative stability. Hence, there is great need for the development and understanding of ion transport in new classes of polymer electrolytes. Perfluoropolyether (PFPE)-based electrolytes have shown improved oxidative stability, but little is understood about their lithium solvation and transport mechanism. In this work, we use multinuclear solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy to investigate the lithium cation environment and mobility in crosslinked single-ion and salt-in-polymer PFPE electrolytes and compare it directly to that of well-known polyethylene glycol (PEG) electrolytes. We show that the interaction of the lithium cation with the polymer backbone is weaker in PFPE systems compared to PEG, likely resulting in stronger ion pairing in the PFPE systems. Line shape analyses show lower lithium mobility in PFPE electrolytes despite lower activation energies being derived from spin-lattice relaxation (T1) measurements as compared to those for the PEG systems. The rapid relaxation is instead ascribed to the local fluctuations caused by polymer backbone mobility. By studying different modes of ion binding (single-ion vs salt-in-polymer), we show that differences across polymer backbones (PFPE vs PEG) have a greater effect on mobility than differences in ion binding modes within each polymer class (especially when single-ion conducting site density is not high). Our ability to use MAS NMR to study polymer electrolytes in their native state opens new opportunities to develop and understand novel polymer or hybrid solid-state electrolytes for next-generation lithium metal batteries.