NSF-DFG CONFINE: Lithium ion transport in self-assembled zwitterionic nanochannels containing ionic liquids

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Next generation energy storage systems such as lithium metal batteries are crucial for the broader use of renewable energy resources and mobile energy. Their success relies on the development of advanced electrolyte materials that quickly and selectively conduct the target ion (e.g. Li+) while also providing robust mechanical properties. Self-assembling polymers that confine the conducting ions into nanometer-scale domains while providing a robust structure can potentially enable such materials. This type of electrolyte, however, has achieved only limited ionic conductivity levels to date. This project aims to understand how ions travel within specialty polymers that self-assemble to form zwitterionic nanometer-scale domains that can be preferentially swollen with ionic liquids. We expect that these findings will open the door to new, better-performing electrolytes for energy storage applications.The aim of this project is to study Li+ ion transport within zwitterionic (ZI) conducting nanochannels formed by the self-assembly of amphiphilic comb copolymers. To ensure sufficiently high ionic conductivity at ambient temperatures, the ZI-rich channels will be swollen with controlled amounts of a nonvolatile ionic liquid (IL) containing a dissolved Li salt. The resulting materials are referred to as nanostructured electrolytes (NSEs). The primary objective of this study is to test the hypothesis that confinement into nanochannels decorated with weakly interacting ZI interfaces will selectively enhance Li+ ion transport within NSEs. The proposed experimental plan is designed to examine the effect of ZI side-groups on Li+ ion transport as the IL-swollen nanochannels size (i.e. channel diameter of the conducting domain) inside a NSE is systematically modulated. Modulation of confinement will be achieved by carefully tuning: (i) copolymer architecture, (ii) degree of IL swelling, and (iii) rigidity of the structural domain. Overall ion transport in NSEs will be characterized by AC impedance spectroscopy and DC polarization measurements used to determine Li+ transference number values. Diffusion of individual ion species will be probed using 7Li, 19F, and 1H pulsed field gradient NMR spectroscopy. Electrophoretic NMR (eNMR) spectroscopy will be applied to NSEs for the first time to measure selective Li+ conduction directly. Physical characterization of NSEs will include DSC, TEM, SAXS/WAXS, and rheology. The proposed NSEs featuring ZI conducting nanochannels are expected to provide a valuable new strategy for electrolyte materials design to realize an enhancement of targeted ion transport within electrochemical energy storage systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.