The present invention relates to a lithium recovery device comprising a separator reservoir and manganese oxide, a lithium recovery method using the same, and a lithium adsorption/desorption system. The separator reservoir includes a vacant space therein and an outer wall made of a polymer or a membrane of other useful materials. The manganese oxide is contained, as an adsorbent, in the vacant space inside the separator reservoir. The lithium adsorption/desorption system enables lithium recovery through adsorption and desorption of lithium dissolved in seawater using a lithium adsorbent prepared with the manganese oxide in one system, wherein the manganese oxide can be used as a lithium adsorbent with high lithium selectivity. The invention uses a separator reservoir including an outer wall made of a porous polymer or a membrane of other useful materials. As a result, the invention can be directly applied to seawater by enabling free movement of the solution, especially seawater, without additional pressure from the outside. The invention ensures strong stability in seawater and the acidic solution using a polymer with high chemical resistance and mechanical strength, or other useful materials. Therefore, the invention can be widely used in technical fields requiring lithium recovery.