Nanoscale Miscibility in In Situ Polymerized Hybrid Electrolytes Speeds Up Ion Dynamics and Enables Stable Cycling of Li Metal Batteries

While the potential use of copolymerized electrolytes in Li metal batteries is subject to intense investigation, the fundamental understanding of the nanoscale domain formation and its effect on Li⁺ transport is still lacking. In this study, we investigated the correlation between the Li⁺ transport mechanism and the miscibility of monomers in polymer blend electrolytes derived from the in situ copolymerization of methyl methacrylate (MMA) and vinylene carbonate (VC) in the presence of polyethylene glycol dimethyl ether (PEGDME) plasticizer and bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. The addition of a polar short chain plasticizer reduced the dynamic and structural heterogeneities of the electrolyte. Small-angle X-ray scattering (SAXS) measurements and coarse-grained molecular dynamics (MD) simulations were used to investigate the nanoscale structure of the electrolytes. The distribution of relaxation times corresponding to the three distinct diffusion mechanisms of the free and interfacial Li⁺ ions at the copolymer/plasticizer and electrolyte/SEI boundaries was analyzed in a broad temperature range to elucidate the Li⁺ transport mechanism. The chemical composition of the SEI and the contribution of a ceramic lithium lanthanum zirconium oxide (LLZO, Li₇La₃Zr₂O₁₂) phase on the interfacial resistance, salt degradation, and SEI stability were studied by X-ray photoelectron spectroscopy (XPS) depth profile analysis and electrochemical testing.