Resolving Concentration Dependent Electrochemistry of Ether-Based Electrolytes for Lithium Metal Anodes

Concentrated ether-based electrolytes are effective in stabilizing lithium metal anodes, yet the molecular origins of the distinct concentration dependences of ionic conductivity, interfacial kinetics, and lithium reversibility remain unclear. Herein, LiFSI/DME electrolytes with varied salt-to-solvent ratios serve as a model system to reveal that this performance decoupling is governed by the redistribution of functional Li+ solvation states. As the salt-to-solvent ratio decreases, Li+ coordination evolves from FSI–-participated to DME-dominated environments, accompanied by a redistribution of solvation motifs among FSI-rich, mixed, and DME-rich states. FSI-rich states, particularly bridging/multi-Li FSI– coordination with suppressed free-DME activity, favor interfacial stability and lithium reversibility. Electrochemical measurements reveal distinct composition-dependent behaviors: ionic conductivity increases with DME content, concentrated electrolytes provide higher Li||Cu Coulombic efficiency, and the exchange current density exhibits a nonmonotonic dependence and reaches its maximum for H3. Bulk and interfacial analyses further show that H2 contains the highest bulk mixed-solvation fraction, whereas H3 combines a substantial interfacial mixed-state population with the highest exchange current density. These results connect the electrochemical response to the composition-dependent redistribution of Li+ solvation environments in the bulk electrolyte and at the charged interface.