(632e) Engineering Ion Transport in Microporous Polymer Separators for Li-S Batteries

The design and control of ion transport in electrochemical systems is of fundamental and practical importance for batteries, supercapacitors, and electrocatalysis. In batteries, ion transport and reactivity can dictate battery performance and cyclability. The electrolyte phase and composition can dictate how the solid electrolyte interface (SEI) forms and can control the transport of species that have deleterious effects on battery performance. In Li-S batteries, a primary degradation mechanism is the crossover of polysulfide anions from the cathode to the anode, where it undergoes a parasitic reaction with lithium metal.

Various approaches have been implemented to combat the polysulfide crossover problem, including the trapping of polysulfide species with adsorptive functionality in the polymer and the exclusion of polysulfide anions via a size exclusion mechanism. Polysulfide trapping can extend the life of Li-S batteries, however the adsorption sites are limited and can be saturated leading to eventual crossover of polysulfides. Size exclusion has limited applicability, as it is challenging to achieve perfect selectivity for transport of the electrolyte anions over polysulfide anions.

A more promising approach to prevent crossover is to utilize a single-ion conductor as a selective phase within the electrolyte. While various polymers have been developed that display high transference numbers, these are primarily based on dense polymers with hard anions, leading to low conductivities (<10^-6 S/cm). Here, we have developed two microporous polymer systems with fixed anions that display both high conductivity (>10^-4 S/cm) and high transference numbers. These materials are based on functionalized polymers of intrinsic microporosity (PIMs) and porous aromatic frameworks (PAFs). Both of these polymer classes display high N₂ surface areas, indicating that they are permanently porous materials.

The ion transport in a series of anionic PIMs an anionic PAF have been evaluated, showing the promising charge transport characteristics of these materials. Further, utilizing these materials in Li-metal and Li-S batteries shows the applicability of high conductivity and high transference number polymers in battery systems.