Ion transport mechanisms in salt-doped polymerized zwitterionic electrolytes

JR Keith and V Ganesan, JOURNAL OF POLYMER SCIENCE, 58, 578-588 (2020).

DOI: 10.1002/pol.20190099

Polyzwitterions (polyZIs), macromolecules with repeating ampholytic monomers, are a novel class of materials with attractive properties for battery electrolytes. In this study, we probe the ion transport characteristics and underlying mechanisms in two salt-doped (Li+-TFSI-) polyZIs of similar composition with contrasting zwitterion (ZI) ionic organization: pendant monomers organized via backbone-anion-cation (B-ZI(-)-ZI(+), Motif B) and backbone-cation-anion (B-ZI(+)-ZI(-), Motif C). Within both Motifs B and C, the counterion of the pendant-end ZI moiety shows higher mobility. Similarly, when comparing Li+ or TFSI- across motifs, it is seen that the respective pendant-end counterion possesses higher mobility than its backbone-adjacent counterpart. Furthermore, when comparing counterions to same-position ZI moieties, TFSI- is seen to possess higher mobility than Li+ in each case, a result rationalized by invoking the lower interaction strength between the TFSI- and ZI(+). Analysis of ion-transport mechanisms demonstrate that the mobility of countercharges to the pendant-end ZI moiety correlates with the ion-association relaxation timescale, similar to ideas noted in polymerized ionic liquids in past studies. However, the mobility of countercharges to the backbone-adjacent ZI moiety is shown to be correlated with a cage relaxation time, which incorporates the combined effects of frustrated motion due to the presence of the polymer backbone and pendant-end ZI moiety and the higher mobility in a population of lightly ZI-coordinated ions. (c) 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020, 58, 578-588

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