Dynamics of Li ions in an intercalated polymer electrolyte: A 7Li nuclear magnetic resonance study

Restricted lithium ion dynamics in PEO-based block copolymer electrolytes measured by high-field nuclear magnetic resonance relaxation

The intrinsic ionic conductivity of polyethylene oxide (PEO)-based block copolymer electrolytes is often assumed to be identical to the conductivity of the PEO homopolymer. Here, we use high-field ⁷Li nuclear magnetic resonance (NMR) relaxation and pulsed-field-gradient (PFG) NMR diffusion measurements to probe lithium ion dynamics over nanosecond and millisecond time scales in PEO and polystyrene (PS)-b-PEO-b-PS electrolytes containing the lithium salt LiTFSI. Variable-temperature longitudinal (T₁) and transverse (T₂) ⁷Li NMR relaxation rates were acquired at three magnetic field strengths and quantitatively analyzed for the first time at such fields, enabling us to distinguish two characteristic time scales that describe fluctuations of the ⁷Li nuclear electric quadrupolar interaction. Fast lithium motions [up to O(ns)] are essentially identical between the two polymer electrolytes, including sub-nanosecond vibrations and local fluctuations of the coordination polyhedra between lithium and nearby oxygen atoms. However, lithium dynamics over longer time scales [O(10 ns) and greater] are slower in the block copolymer compared to the homopolymer, as manifested experimentally by their different transverse ⁷Li NMR relaxation rates. Restricted dynamics and altered thermodynamic behavior of PEO chains anchored near PS domains likely explain these results.

Spin relaxation studies of Li+ ion dynamics in polymer gel electrolytes

Two ternary polymer gel electrolyte systems are compared, containing either polyethylene oxide (PEO) or the poly-ionic liquid poly(diallyldimethylammonium) bis(trifluoromethyl sulfonyl)imide (PDADMA-TFSI). Both gel types are based on the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl)imide (P₁₄TFSI) and LiTFSI. We study the influence of the polymers on the local lithium ion dynamics at different polymer concentrations using ⁷Li spin-lattice relaxation data in dependence on frequency and temperature. In all cases the relaxation rates are well described by the Cole-Davidson motional model with Arrhenius dependence of the correlation time and a temperature dependent quadrupole coupling constant. For both polymers the correlation times are found to increase with polymer concentration. The activation energy of local motions slightly increases with increasing PEO concentration, and slightly decreases with increasing PDADMA-TFSI concentration. Thus the local Li⁺ motion is reduced by the presence of either polymer; however, the reduction is less effective in the PDADMA⁺ samples. We thus conclude that mechanical stabilization of a liquid electrolyte by a polymer can be achieved at a lower decrease of Li⁺ motion when a cationic polymer is used instead of PEO.