A method of preparing a lithium battery according to an embodiment of the present invention may include preparing a mixture including lithium phosphorus sulfide and metal sulfide, preparing an electrode composite by applying a physical pressure to the mixture, wherein the electrode composite includes lithium phosphorus sulfide, lithium metal sulfide, and amorphous sulfide, preparing an electrode active layer by using the electrode composite, forming an electrode current collector on one side of the electrode active layer, and forming an electrolyte layer on another side of the electrode active layer.