LiBr has been incorporated into Li6.25La3Zr2Al0.25O12 garnet matrix to develop a new garnet-based composite electrolyte through high-energy ball milling followed by solid-state reaction. Of importance to note in the composite materials is joint occurrence of anion doping at Li6.25La3Zr2Al0.25O12 bulks and lithiated interface at Li6.25La3Zr2Al0.25O12 grain boundaries. Lattice distortion based on Br− substitution at partial O2− sites and high densification resulted from mergence effect of LiBr have been detected in the composite electrolytes. Li-ion conductivity and transport kinetics are thereby enhanced after LiBr incorporation. Also, LiBr addition into Li6.25La3Zr2Al0.25O12 causes the appearance of finely microporous feature for the composite materials without compromising pellet densification, which can favor interfacial contact and Li-ion motion between the composite electrolytes and electrode. A full cell with a Li metal anode, the newly developed solid electrolyte, tiny amount of liquid electrolyte, and a LiCoO2 cathode has been assembled in sequence. High cycle stability of the full cell can be derived from capacity retention of 91.8% and high discharge capacity of 111.6 mAh g−1 after 70 cycles at room temperature. Consequently, favorable contributions of LiBr addition towards solid-state cell system can be further ascertained.