Lithium-ion batteries (LIBs) have garnered attention for their high energy density and portability, particularly with the growing demand of electric vehicles¹. However, the flammability of the electrolytes in conventional LIBs poses a significant safety hazard. To address this, solid-state batteries (SSBs) with solid electrolytes (SE) have emerged, offering enhanced safety¹. Among these SE, Li_6.6La₃Zr_1.6 Ta_0.4O₁₂ (LLZO-Ta) stands out for its stability against Li metal and high ionic conductivity². Yet, LLZO-Ta faces challenges, including susceptibility to surface lithium hydroxide and carbonate formation, hindering lithium-ion transfer².  In this study, we investigate the ion exchange rate in LLZO-Ta with the introduction of an indium interface layer to enhance ion transfer via tracer-based experiments using SIMS analysis with isotope couples. These kinds of experiments are limited in the literature^3,4. Previous studies have applied SIMS depth-profiling, and line scan analysis to understand grain boundary diffusion and determining bulk and grain diffusion coefficients³. However, the slow diffusion observed in LLZO is not solely due to the surface carbonate layer. It is also influenced by the relatively high activation energy needed for lithium-ion migration within the material⁴. Thus, to overcome this slow diffusion, initially, a systematic polished and annealed process is employed to remove surface carbonate layers. However, this process only yields a modest increase in the isotope exchange rate over 24 hours, which may be due to slow diffusion of lithium in this kind of systems⁴. Subsequently, LLZO-Ta surfaces are coated with indium (~10 nm thickness), confirmed through Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). The indium-coated LLZO-Ta exhibits a significantly enhanced step isotope exchange rate of 29% over 24 hours, as validated by TOF-SIMS profiling (Figure 1). The observed increase in lithium-ion exchange rate with indium deposition suggests surface property modification, possibly enhancing conductivity or altering surface chemistry to facilitate faster lithium-ion diffusion. Thus, indium interlayer plays a significant role in increasing the ion conduction in LLZO-Ta, which contributes to advancing the understanding of solid-state battery interfaces, crucial for improving battery performance and safety.