posted on 2020-10-20, 20:15authored byMark P. Rosenwinkel, Rassmus Andersson, Jonas Mindemark, Monika Schönhoff
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(ε-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 (1H and 19F 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 (∼0.5
compared 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.