Lithium ion transport in self-assembled zwitterionic nanochannels containing ionic liquids

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Next-generation electrochemical energy storage technologies, such as lithium metal batteries, will require advanced electrolyte materials. These electrolytes should promote fast, selective conduction of a target ion (e.g. Li+) while also providing robust mechanical properties to inhibit dendrite growth and eliminate electrolyte leakage. Copolymer self-assembly, which can facilitate the spontaneous formation of interpenetrating structural domains and conducting domains from two chemically distinct monomers, is a powerful tool for realizing such materials. However, confined salt-in-copolymer only electrolytes have so far largely failed to reach the levels of ionic conductivity that are necessary for practical applications. Bicontinuous structures that exhibit conducting nanochannels are especially attractive, as unique interfacial effects between the two domains that may promote selective ion transport can be exploited. 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 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) in 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. While this study will focus on Li+ transport, such NSE materials may also provide a platform for enhancing the transport of other cations (Na+, H+) and/or anions in other applications such as “beyond-lithium” batteries and fuel cells.