Revealing Solvent-Assisted Li ⁺ Transport in the Solid Electrolyte Interphase <i>operando</i>

The performance of energy-dense lithium metal batteries is critically influenced by the properties of the solid electrolyte interphase (SEI). Yet, progress in understanding this layer has been limited by the lack of accurate <i>operando</i> characterization because the SEI evolves dynamically during cycling. Here, we apply dynamic electrochemical impedance spectroscopy (dEIS) to resolve the real-time evolution of the SEI on lithium metal in ether-based electrolytes with varying degrees of fluorination. We find that faster stabilization of the compact SEI resistance correlates with improved passivation and higher Coulombic efficiency. Unexpectedly, compact SEI resistance correlates directly with Li⁺ solvation energy, revealing that weaker Li⁺ solvation increases not only bulk but also interphase resistance. These findings challenge the conventional view of the SEI as a purely solid-phase conductor and instead support a solvent-assisted Li⁺ transport mechanism within the compact SEI. This framework emphasizes the need to balance SEI ionic conductivity with the Li⁺ solvation environment to maximize lithium metal battery performance.