Ceramic−polymer solid electrolytes, combined with Li metal anodes, hold the promise for safer and more energetically dense battery technologies, as long as key interfacial challenges are fully understood and solved. Here, we investigate a garnet−PEO(LiTFSI) composite electrolyte system, the garnet filler being Li 6.55 Ga 0.15 La 3 Zr 2 O 12 (LLZO) microparticles. A "soft" mechanical milling process ensures good miscibility between the garnet and polymer phases over a wide range of volume fraction (up to 70 vol % garnet). Excellent degree of structural and chemical homogeneity is achieved without degradation nor segregation, even at the local level, as confirmed by solid-state NMR spectroscopy, electron microscopy and gel permeation chromatography. The total Li-ion conductivity of the composites is governed by the polymer matrix, as a consequence of the high interfacial resistance (∼10 4 Ω cm 2 ) between the garnet particles and the PEO(LiTFSI) matrix. However, by using 7 Li NMR 2D exchange spectroscopy (ESXY) in the solid state, it is shown that Li ions can locally exchange between the garnet surfaces to the surrounding polymer chains. This dynamic transfer phenomenon, occurring within the composite, seems to play a key role in kinetically stabilizing the interface with Li metal electrode, as observed from galvanostatic cycling and EIS experiments. Comparison of a garnet-free PEO electrolyte with a PEO−garnet (10 vol %) composite shows key performance improvements in the latter: although the Li-ion conductivity at 70 °C slightly decreases from 7.0 × 10 −4 S cm −1 , for PEO-LiTFSI, to 4.5 × 10 −4 S cm −1 for 10 vol % LLZO, the composite shows up to 1 order of magnitude lower interfacial resistance with Li metal electrode (33 vs 300 Ω cm 2 ), stable Li electrodeposition, and no dendrite formation. In contrast to previously believed, it is demonstrated that these improvements are not related to a change of the mechanical behavior but rather to a structural reorganization in the composite followed by local ion dynamics effects at the vicinity of the Li metal interface.

Oxide glasses are an integral part of the modern world, but their usefulness can be limited by their characteristic brittleness at room temperature. We show that amorphous aluminum oxide can permanently deform without fracture at room temperature and high strain rate by a viscous creep mechanism. These thin-films can reach flow stress at room temperature and can flow plastically up to a total elongation of 100%, provided that the material is dense and free of geometrical flaws. Our study demonstrates a much higher ductility for an amorphous oxide at low temperature than previous observations. This discovery may facilitate the realization of damage-tolerant glass materials that contribute in new ways, with the potential to improve the mechanical resistance and reliability of applications such as electronic devices and batteries.

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