The applications of solid polymer electrolytes in all-solid-state Li-ion batteries are restricted by their low ion conductivities at room temperature and poor mechanical and thermal stabilities. Herein, Li1.4Al0.4Ti1.6(PO4)(3) (LATP) electrolyte nanoparticles were used as active fillers to improve the properties of polyethylene oxide (PEO)-based electrolytes. The ion conductivity of the composite electrolyte with 1 wt% LATP increased to 1.2 x 10(-5) S/cm at room temperature, because of the effective inhibition of the PEO matrix crystallization by the high-conductivity LATP nanofillers. The electrochemical, mechanical and thermal stabilities were also enhanced by the LATP nanofillers. The LATP-filled composite electrolytes can effectively block Li dendrites in symmetric Li/electrolyte/Li cells during repeated Li stripping/plating with a current density of 0.10 mA/cm(2) for 600 h at 30 degrees C. Furthermore, the LATP-based all-solid-state LiFePO4/Li cells exhibited higher reversible capacity (152 mAh/g at 0.10 C), cycling stability (84% after 20 cycles at 50 degrees C) and rate capability than the LATP-free cells.