The present invention provides a method of preparing a gallium (Ga)-doped LLZO solid electrolyte represented by the chemical formula 1. According to the present invention, the method of preparing a Ga-doped solid electrolyte adjusts a Ga content and a lithium (Li) content as a starting material and performs a coprecipitation reaction through a tailor reaction method, but controls a flow rate of supplying a raw material, thereby providing an effect of enhancing ion conductivity through a high-accuracy cubic structure formation and enhanced sintering properties. In addition, the present invention has identified an effect of synthesis conditions (a doping content of Ga, a pH value, a Li excess content and a pellet sintering time) on ion conductivity, thereby providing a method of preparing a gallium-doped solid electrolyte for all-solid lithium secondary battery capable of enhancing ion conductivity by a change in synthesis conditions. The chemical formula 1 is Li_xGa_yLa_zZr_wO_12 (5 = x = 9, 0 y = 4, 2 = z = 4, 1 = w =3).
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