Lithium-Ion Transport in Poly(ionic liquid) Diblock Copolymer Electrolytes: Impact of Salt Concentration and Cation and Anion Chemistry

Highly Frustrated Poly(ionic liquid) ABC Triblock Terpolymers with Exceptionally High Morphology Factors

In this work, we report the successful synthesis of 17 unique compositions of a poly(ionic liquid) (PIL) ABC triblock terpolymer, poly(S-b-VBMIm-TFSI-b-HA), where S is styrene, VBMIm-TFSI is vinylbenzyl methylimidazolium bis(trifluoromethanesulfonyl)imide, and HA is hexyl acrylate. Nine distinct morphologies were observed, including two-phase and three-phase disordered microphase separated (D2 and D3), two-phase hexagonally packed cylinders (C2), core-shell hexagonally packed cylinders (CCS), three-phase lamellae (L3), two-phase lamellae (L2), core-shell double gyroid (Q230), spheres-in-lamellae (LSI), and a three-phase hexagonal superlattice of cylinders (CSL). The LSI morphology was unambiguously confirmed using small-angle X-ray scattering and transmission electron microscopy. Morphology type significantly impacted the ion conductivity of the PIL ABC triblock terpolymers, where remarkable changes in morphology factor (normalized ion conductivity) were observed with only small changes in the conducting volume fraction, i.e., PIL block composition. An exceptionally high morphology factor of 2.0 was observed from the PIL ABC triblock terpolymer with a hexagonal superlattice morphology due to the three-dimensional narrow, continuous PIL nanodomains that accelerate ion conduction. Overall, this work demonstrates the first systematic study of highly frustrated single-ion conducting ABC triblock terpolymers with a diverse set of morphologies and exceptionally high morphology factors, enabling the exploration of transport-morphology relationships to guide the future design of highly conductive polymer electrolytes.

Polymer Architecture-Induced Trade-off between Conductivities and Transference Numbers in Salt-Doped Polymeric Ionic Liquids

Recent experiments have demonstrated that polymeric ionic liquids that share the same cation and anion but possess different architectures can exhibit markedly different conductivity and transference number characteristics when doped with lithium salt. In this study, we used atomistic molecular simulations on polymer chemistries inspired by the experiments to probe the mechanistic origins underlying the competition between conductivity and transference numbers. Our results indicate that the architecture of the polycationic ionic liquid plays a subtle but crucial role in modulating the anion-cation interactions, especially their dynamical coordination characteristics. Chemistries leading to longer-lived anion-cation coordinations relative to lithium-anion coordinations lead to lower conductivities and higher transference numbers. Our results suggest that higher conductivities are accompanied by lower transference numbers and vice versa, revealing that alternative approaches may need to be considered to break this trade-off in salt-doped polyILs.

Self-Repairable and Flexible Polymer Network Electrolyte with Enhanced Lithium-Ion Conduction for Lithium Metal Batteries

Developing high-performance functional polymer-based electrolytes is important for realizing next generation safe lithium metal batteries. In this study, a new type of quasi-solid polymer network electrolyte (SIPH-x-y%) was prepared by combining synthesized polymer network (SIPH) containing urethane bond linked ionic liquids (ILs), polyethylene glycol (PEG), and disulfide bond moieties, lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI), and glyme type additive. It was found that SIPH-20-40% was mechanically flexible, self-healable, and showed high ionic conductivity of 2.67×10^-4  S cm^-1 . Also, SIPH-20-40% possesses a high lithium ion transference number of 0.43 and good electrochemical stability. These properties enabled the SIPH-20-40% electrolyte membrane to support Li/Li symmetrical cell to cycle stably during long term Li plating and stripping. The Li/SIPH-20-40%/LFP showed high delivered specific capacity and good stability (166.1 mAh g^-1 after 106 cycles at 0.2 C). Such glyme doped polymer network electrolyte provides new experimental findings for developing polymer-based electrolyte with excellent mechanical integrity and battery related properties.