Phosphating‐Silane Modification Realizes Ideal Ceramic Fillers for Durable and Conductive Composite Solid Electrolytes

Garnet‐type Li 7 La 3 Zr 2 O 12 (LLZO) solid electrolytes exhibit high bulk ionic conductivity but suffer from poor interfacial compatibility with polymer matrices and lithium metal, due to surface Li 2 CO 3 formation and high interfacial resistance. Here, we construct a dual‐functional interfacial structure via sequential in situ phosphating and silane grafting. The phosphating forms a Li 3 PO 4 layer, which chemically bonded to LLZO with P─O─La covalent bonds, serving as an ionic bridge to lower Li + transport barriers. Silane grafting creates a flexible, cross‐linked network, transforming the filler surface from rigid to viscoelastic. The dual‐coated LLZO enables uniform dispersion in PEO/PAN blends, establishing continuous ionconducting pathways with a stress‐dissipative framework. This enables the composite electrolyte to achieve a high ionic conductivity of 2.82 × 10 −4 S cm −1 while exhibiting exceptional mechanical integrity. Li symmetric cells cycle stably for over 2000 h at 0.4 mA cm −2 , and cells coupled with LiFePO 4 or LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode deliver high‐capacity retention over extended cycling. This work elucidates a synergistic interface‑engineering mechanism where the inner Li 3 PO 4 layer optimizes ion transfer kinetics and the outer silane network ensures lithium dendrite suppression, collectively overcoming the long‑standing trade‑off between ionic conductivity and interfacial stability in garnet‑based composite solid‑state electrolytes.