In this study, the compound Li3BO3 (LBO) is intended to be prepared by a polymeric complex method as a sintering aid for the densification of Li7La3Zr2O12 (LLZ) solid electrolyte. A polymeric precursor containing Li and B is heat-treated in an air atmosphere at a temperature range between 600℃ and 800℃. Instead of LBO, the compound Li2+xC1-xBxO3 (LCBO) is unexpectedly synthesized after a heat-treatment of 700℃. The effect of LCBO addition on sintering behavior and ion conductivity of LLZ is studied. It is found that the LCBO compound could lead to significant improvements in the densification and ionic conductivity of LLZ compared to pure LLZ. After sintering at 1100℃, the density of the LLZ-12wt%LBO composite is 3.72 g/cm3, with a high Li-ion conductivity of 1.18 × 10-4 Scm-1 at 28℃, while the pure LLZ specimen had a densify of 2.98 g/cm3 and Li-ion conductivity of 5.98 × 10−6 Scm-1.