Mesoscale Ionic Connectivity Reconciles Transport and Cycling Stability in Highly Concentrated Deep Eutectic...

Highly concentrated electrolytes often exhibit a counterintuitive combination of reduced bulk ionic conductivity and markedly improved cycling stability in metal batteries, yet the structural origin of this long-standing trade-off remains poorly understood. Here, using the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-succinonitrile (SN) deep-eutectic electrolyte as a model system, we establish a direct structure-property framework that reconciles transport kinetics with electrochemical durability across different concentration regimes. By integrating synchrotron small- and wide-angle X-ray scattering with vibrational spectroscopy, electrochemical characterization, and machine-learning-assisted mesoscale reconstruction, we resolve how ionic organization evolves from locally mobile, weakly correlated environments to salt-rich, structurally constrained networks. We show that the highest ionic conductivity at intermediate concentration arises from optimally connected mesoscale pathways that facilitate fast ion transport, whereas higher salt concentration induces a TFSI--dominated solvation framework with partial salt ordering that suppresses bulk transport but stabilizes long-term cycling and Coulombic efficiency. These findings suggest that the electrochemical behavior of concentrated electrolytes is closely associated with concentration-induced ionic organization, structural heterogeneity, and interphase stabilization rather than conductivity alone. The results provide a structural perspective for understanding the apparent performance paradox of salt-rich electrolytes and offer general insights for the design of next-generation battery electrolytes.