We performed flux growth of LiCoO₂/Li₃BO₃/Li_6.75La₃Zr_1.75Nb_0.25O₁₂ composites with various composite ratio. Powder XRD profiles and cross-sectional SEM images revealed that individual LiCoO₂ and Li_6.75La₃Zr_1.75Nb_0.25O₁₂ single crystals were homogeneously dispersed in amorphous Li₃BO₃ matrix without formation of any sub-phase at the interphase even though the crystal growth process was performed under 1000 ^oC for 5h. The each crystals size and the relative density of the composite was increased as increasing time and temperature. The relative density was maximally 92%. Indicating that small amount of Li₃BO₃ plays a key rule for insulating the direct contact of LiCoO₂ with Li_6.75La₃Zr_1.75Nb_0.25O₁₂ as well as for assisting the enhancement of relative density of composites relied on liquid-state sintering principle. The Li ion conductivity of the composite with ca. 700 mm thick was 1.1×10^-4 S/cm at room temperature, as evaluated by AC-impedance spectroscopy. We also characterized their all-solid-state LIBs properties.