Coordination Effects in Polymer Electrolytes: Fast Li+ Transport by Weak Ion Binding

In view of the limited ionic conductivity and low lithium transference number in classical poly(ethylene oxide) (PEO)-based salt-in-polymer electrolytes, employing alternative polymer architectures, e.g., polyester homopolymers or copolymers, is a promising approach. To shed light on the influence of the coordination properties of different polymer architectures and to identify their influence on Li ion transport, different polymeric structures are compared, i.e., poly(e-caprolactone) (PCL), poly(trimethylene carbonate) (PTMC), and a PCL-co-PTMC random copolymer, combined with lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) at varying Li+/monomer ratios r. Electrophoretic NMR (H-1 and F-19 eNMR) is applied to determine the electrophoretic 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 (similar to 0.5 compared to similar to 0.2), while the total ionic conductivity is lower. However, the partial lithium conductivities are found to be 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 a maximized total conductivity or maximized Li transference number. Thus, for the promising class of polyester-based polymer electrolytes, showing excellent lithium conduction properties, a molecular level-based understanding of the electrochemical transport parameters is derived, complementing the segmental motion-based description of ion transport with the additional effects of ion coordination.