Unlocking the full potential of solid-state electrolytes (SSEs) is key to enabling safer and more-energy dense technologies than today's Li-ion batteries. In particular, composite materials comprising a conductive, flexible polymer matrix embedding ceramic filler particles are emerging as a good strategy to provide the combination of conductivity, mechanical and chemical stability demanded from SSEs. Yet, the electrochemical 1 activity of these materials strongly depends on their

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... interfacial Li-ion dynamics at the molecular scale, whose fundamental understanding remains elusive. While this interface has been explored for non-conductive ceramic fillers, atomistic modelling of interfaces involving a potentially more promising conductive ceramic filler is still lacking. We address this shortfall by employing Molecular Dynamics and enhanced Monte Carlo techniques to gain unprecedented insights into the interfacial Liion dynamics in a composite polymer-ceramic electrolyte, which integrates polyethylene oxide (PEO) plus LiN(CF 3 SO 2 ) 2 lithium imide salt (LiTFSI), and Li-ion conductive cubic Li 7 La 3 Zr 2 O 12 (LLZO) inclusions. Our simulations automatically produce the interfacial Li-ion distribution assumed in space-charge models and, for the first time, a long-range impact of the garnet surface on the Li-ion diffusivity is unveiled. Based on our calculations, tensile strength and ionic conductivity experimental measurements, we are able to explain a previously reported drop in conductivity at a critical filler fraction well below the theoretical percolation threshold. Our results pave the way for the computational modelling of other conductive filler/polymer combinations and the rational design of composite SSEs. All-solid-state Li-ion batteries with a thin solid electrolyte material have the potential to revolutionize the energy-storage market by allowing the safe incorporation of a metal Li anode. 1,2 Indeed, the significant increase in energy density resulting from this paradigm could power the growth of several emerging applications, including long-range all-electric vehicles and large-scale wind and solar energy generation. 3 ramics and polymers constitute the two main families of solid-state electrolyte materials. Li 7 La 3 Zr 2 O 12 (LLZO) with the cubic garnet structure attracts increasing interest among ceramics due to its high conductivity at room-temperature (RT) and chemical compatibility with metallic Li. 4,5 However, ceramic electrolytes are brittle and provide poor intimate contact with the electrodes, leading to strong interfacial resistance, mechanical failure and