Stabilizing the Solid/Liquid Interface in Quasi-Solid Electrolytes via Anion-Derived Interphase Formation in Concentrated Solutions

Quasi-solid electrolytes, composed of a garnet-type solid electrolyte (lithium lanthanum zirconium oxide, LLZO) and a liquid electrolyte, are promising for next-generation batteries with high safety and high energy density. However, they have faced a critical challenge of continuously increasing Li⁺-transport resistance at the unstable LLZO/liquid electrolyte interface, which has hampered the effective utilization of both solid and liquid phases for Li⁺ conduction. Herein, we report the stabilization of the LLZO/liquid electrolyte interface by using a concentrated LiN(SO₂F)₂ (LiFSI)/sulfolane (SL) electrolyte. A stable LiF-rich interphase layer is formed on the LLZO surface through the decomposition of FSI anions in the highly concentrated LiFSI/SL, which effectively suppresses the increase in the interfacial resistance. The preferential decomposition of FSI anions results from extensive Li⁺-FSI^– ion pairing unique to high concentrations, which makes the FSI anions susceptible to nucleophilic attack. These findings provide insights into the design of highly conductive quasi-solid electrolytes through controlling the solution structure of the liquid phase.