Surface-Oxygen-Tuned LLZTO in EMITFSI-Based Polymer Ionogel Electrolytes for Stable Solid-State Lithium Batteries

Composite solid electrolytes offer a practical route toward solid-state lithium batteries by combining the processability of polymer matrices with the functionality of ceramic fillers. However, ion transport in these systems is strongly affected by the chemical nature of the polymer-filler interphase. In this work, we designed an EMITFSI-based polymer ionogel electrolyte containing garnet-type LLZTO and regulated the LLZTO surface by ultraviolet ozone (UVO) treatment. The UVO-treated LLZTO exhibits increased lattice oxygen and a more negatively charged surface, which promotes Li⁺ accumulation near the ceramic–ionogel interphase and facilitates interfacial ion migration. Temperature-dependent EIS and DRT analysis reveal that the surface-controlled electrolyte (LEP/UV) shows enhanced ionic conductivity of 0.88 mS cm^-1 at 20 °C, compared with 0.69 mS cm^-1 for the untreated electrolyte (LEP), along with reduced resistance contributions. COMSOL simulation further supports that the modified interphase contributes to a more uniform ion-flux distribution across the electrolyte. This regulated transport behavior leads to the formation of a thinner and more homogeneous LiF-rich SEI on Li metal, as confirmed by XPS depth profiling and 3D LIBS mapping. As a result, LEP/UV-based full cells retain 92.47% of their initial capacity after 300 cycles at 0.2 C. These results demonstrate that surface-oxygen control of ceramic fillers is an effective strategy for optimizing ion transport and interfacial stability in polymer-based composite solid electrolytes.