Solid-state electrolytes are promising to replace the traditional organic liquid electrolytes for high safety and high energy density lithium batteries. However, the poor electrode/electrolyte interfacial wettability and stability limit their practical applications. Herein, we developed a gradient poly(ethylene oxide) (PEO)-based composited electrolyte with Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) as ceramic fillers. The PEO-rich side contacting Li metal ensures dendrite-free Li deposition and a reduction in the interfacial resistance while the LLZTO-rich side contacting LiFePO₄ provides fast Li⁺ transport pathways along the grain boundaries of LLZTO with a high oxidation tolerance (5.3 V vs. Li⁺/Li) over a stable cycling. The gradient composited solid-state electrolyte achieves an ionic conductivity of up to 3.75 × 10 S cm^-1 coupled with a high Li-ion transference number of 0.66, resulting in a low-voltage hysteresis potential of 84 mV and a longer lifetime of 3700 h. Simultaneously, the all-solid-state full cell delivers a noteworthy capacity of 162.6 mA h g^-1 at 0.1C and an excellent cycling stability with 85% capacity retention after 200 cycles. This unique structure design of gradient electrolytes, featuring enhanced interfacial wettability and stability, provides a novel pathway to construct various high-energy density storage devices beyond Li batteries.