Li⁺ Ion Migration in Polymer Electrolytes: How Coordination Effects Control Li⁺ Transport Mechanisms

Li salt-in-polymer systems were suggested as solid electrolytes for Li ion batteries with superior properties over common liquid electrolytes. While the classical poly(ethylene oxide) (PEO)-based electrolytes suffer from limited ionic conductivity and low lithium transference number, employing alternative polymer architectures, e.g. polyesters, is a promising approach. These differ in their Li coordination with consequences for salt dissociation and Li transference numbers.
To shed light on the influence of the coordination properties of different polymer architectures and to identify their influence on Li ion transport, we compare PEO, poly(ε-caprolactone) (PCL), poly(trimethylene carbonate) (PTMC), and a PCL-co-PTMC random co-polymer, combined with the Lithium salt LiTFSA at varying Li⁺:monomer ratio r.
Employing multinuclear Pulsed-Field-Gradient (PFG)-NMR diffusion, we obtain ion-specific transport information. Moreover, electrophoretic NMR (eNMR) allows to measure the electrophoretic mobility of ions with NMR-active nuclei. ¹H and ¹⁹F eNMR yields mobilities of both ionic species, from which partial conductivities and Li transference numbers are calculated. In comparison to PEO-based electrolytes, the ester-based systems show a much higher lithium transference number (~0.5 compared to ~0.2), while the total ionic conductivity is lower. However, the partial lithium conductivities are almost equal in PEO- and PCL-based electrolytes. The results show how via modifying the coordination strength the competition of Li⁺–polymer coordination and Li⁺ ion pair formation can be finely tuned to yield either systems with maximized total conductivity or maximized Li transference number.
[1] Rosenwinkel, M.P.; Schönhoff, M. J. Elchem. Soc. 2019, 166(10), A1977.
[2] Rosenwinkel, M.P.; Andersson, R.; Mindemark, J.; Schönhoff, M. J. Phys. Chem. C 2020, 124, 43, 23588.