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Due to the intrinsically high ionic conductivity and good interfacial stability towards lithium, garnet-type solid electrolytes are usually introduced into polymer electrolytes as fillers to prepare polymer/garnet composite electrolytes, which can improve the ionic conductivity and enhance the mechanical strength to suppress Li dendrites. However, the surface Li₂CO₃ and/or LiOH passive layers which form when garnet is exposed to the air greatly reduce the enhancement effect of garnet on the composite electrolyte. Furthermore, compared with micro-size particles, nano-size garnet fillers exhibit a better effect on enhancing the performance of composite solid electrolytes. Nevertheless, inferior organic/inorganic interphase compatibility and high specific surface energy of nanofillers inevitably cause agglomeration, which severely hinders the effect of nanoparticles for promoting composite solid electrolytes. Herein, a cost-effective amphipathic 3-Aminopropyltriethoxysilane coupling agent is introduced to modify garnet fillers, which effectively expands the air stability of garnet and greatly improves the dispersion of garnet fillers in the polymer matrix. The well-dispersed garnet filler/polymer interface is intimate through the bridging effect of the silane coupling agent, resulting in boosted ionic conductivity (0.72 × 10⁻⁴ S/cm at room temperature) of the composite electrolyte, enhanced stability against lithium dendrites (critical current density > 0.5 mA/cm²), and prolonged cycling life of LFP/Li full cells.