Solid-state electrolytes are a promising field of study to improve the safety and energy density of lithium-based batteries. Composite solid electrolytes seek to leverage both the desirable mechanical properties of polymers and the high conductivity of active ceramic fillers; however, Li+ transport through the composite, and especially the extent of Li+ through the ceramic phase, remains an open question.1, 2 Simulations suggest that decreasing interfacial resistance between polymer and ceramic would promote transport through ceramic particles.1 Our work seeks to improve Li+ transport in the ceramic phase by engineering both polymer and ceramic materials that minimize the interfacial resistance between the two.Traditional polymer electrolytes have a low transference number (~0.15) compared to ceramics (~1), and when combined in composites this mismatch has been suggested as a major contributor to interfacial impedance.3 We explore the effect of the transference number mismatch by using a single-ion conducting polymer poly((trifluoromethane)sulfonamide lithium methacrylate) (PMTFSILi). The Li+ transport in a composite electrolyte with high transference number PMTFSILi is compared with that of a system utilizing a lithium salt.Contaminants on ceramic surface can also contribute to interfacial resistance. We consider how lithium carbonate (Li2CO3) content on the surface of LLZTO (Li6.4La3Zr2Ta0.6O12) affects Li+ transport. Accompanying this study is a detailed description of washing procedures used to clean the LLZTO surface. Titration Mass Spectrometry (TiMS) is used to quantify Li2CO3 content in our ceramic particles and assess the effectiveness of our cleaning procedures. Inductively-coupled plasma mass spectrometry (ICP-MS) data also assists in measuring the impact (Li+ exchange and transition metal dissolution) of various washing procedures (basic, acidic) on the LLZTO particles.Solid-state NMR has proven to be a useful tool in deconvoluting Li+ transport in different phases of composite electrolytes2 and is the primary technique we use to quantify transport in our system. These findings will provide a deeper understanding into the relationship between interfacial resistance and Li+ transport in composite electrolyte systems and shed light on how material choices can better utilize the highly conductive ceramic phase for Li+ transport. Kim, HK., Barai, P., Chavan, K. et al.Transport and mechanical behavior in PEO-LLZO composite electrolytes. J Solid State Electrochem 26, 2059–2075 (2022). https://doi.org/10.1007/s10008-022-05231-wZheng J., Hu, Y. New Insights into the Compositional Dependence of Li-Ion transport in polymer-ceramic composite electrolytes. ACS Appl. Mater. Interfaces, 10, 4, 4113–4120 (2018). https://doi.org/10.1021/acsami.7b17301Mehrotra A. et al. Quantifying Polarization Losses in an Organic Liquid Electrolyte/Single Ion Conductor Interface. Electrochem. Soc. 161 A1681 (2014). https://doi.org /10.1149/2.0721410jes