Dependence of Linker Length and Composition on Ionic Conductivity and Lithium Deposition in Single-Ion Conducting Network Polymers

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 lithium, 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 Li 2 O[(CH 2 CH 2 )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 conductivities of ∼10 –6 S cm –1 at 100 °C. With the addition of 65 wt % propylene carbonate (PC), all the network 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 by 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 electrochemical 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.