Dependance of linker length and composition on ionic conductivity and lithium deposition in single-ion conducting network polymers

Aubrey, M. L.; Axelson, J. C.; Englerb K. E.; Long, J. R. Macromolecules 2021, 54, 7582-7589.

Abstract

Single-ion conducting electrolytes stand as promising alternatives to state-of-the-art electrolytes in lithium batteries, although a single-ion conducting material with high Li+ conductivity, stability in contact with lithi-um, and suitable mechanical properties has been slow to emerge. Here, we describe the synthesis of a series of single-ion conducting network polymers from the reaction of tetrakis(4-(chloromethyl)-2,3,5,6-tetrafluorophenyl)borate with oligoethylene glycoxide linkers Li2O[(CH2CH2)O]n (n = 1, 2, 3, 9, and 22). Polymers with the longest linkers (n = 9 and 22; ANP-9 and ANP-10, respectively) form materials with con-ductivities of ~10–6 S cm–1 at 100 °C. With the addition of 65 wt % propylene carbonate (PC), all the net-work polymers in the series exhibit high conductivities at ambient temperatures, with the n = 1 material (ANP-6) achieving a bulk ionic conductivity of 2.5 × 10–4 S cm–1 at 25 °C. More conductive single-ion conducting gels could be prepared using the less coordinating pentanediol dilithium salt as a linker (ANP-11; σ = 3.5 × 10–4 S cm–1 at 25 °C), although this material exhibited a surprisingly high interfacial resistance in contact with a lithium electrode. In contrast, the gel formed with ANP-6 is notably stable in contact with metallic lithium electrodes, displays a lithium-ion transference number of unity, and boasts a wide electrochem-ical stability window of greater than 4.5 V. Temperature-dependent ac impedance analysis reveals that the ionic conductivity of this material—and likely the other gels in the series—matches closely to a Vogel-Tamman-Fulcher temperature model.

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