Understanding Ion Transport in Block Copolymer Electrolytes Using X-Ray Photon Correlation Spectroscopy

To design new ion conductors and simulate battery operation and performance a fundamental understanding of ion transport is required. Quantitative knowledge of ion and mass transport in liquid and polymer electrolytes across many length scales is essential for electrochemistry and other energy related fields. Lithium-doped-salt block copolymers are attractive solid-state electrolytes for rechargeable batteries, facilitating a lithium metal anode with improved energy density. Investigating ion transport <i>in situ</i> is essential in understanding the limited electrolyte conductivity in polymers and mass transport in general.<br/>Using <i>operando</i> X-ray photon correlation spectroscopy (XPCS), small angle X-ray scattering (SAXS) and X-ray absorption mapping, we investigate ion transport in the homopolymer poly(ethylene oxide) (PEO) and compare this to transport in various phases of the block copolymer polystyrene-block-poly(ethylene oxide) (PS-<i>b</i>-PEO) doped with bis-(trifluoromethylsulfonyl)amine lithium salt (LiTFSI). We directly compare continuum level computations of ion and electrolyte velocities with those obtained from XPCS heterodyning method (1). We compare the electrolyte velocity in neat polymer PEO to that in the lamellar morphology of PS-<i>b</i>-PEO. We also reveal an unexpected strain of the block copolymer network under polarization and discuss its potential origins (2).<br/>1. Steinrück, H. G. <i>et al.</i> Concentration and velocity profiles in a polymeric lithium-ion battery electrolyte. <i>Energy Environ. Sci.</i> <b>13</b>, 4312–4321 (2020).<br/>2. Galluzzo, M. D. <i>et al.</i> Orientation-Dependent Distortion of Lamellae in a Block Copolymer Electrolyte under DC Polarization. <i>Macromolecules</i> <b>54</b>, 7808–7821 (2021).