Ion-sieves are a class of green adsorbent for extraction Li+ from salt lakes. Here, we propose a facile synthesis of hexagonal spinel LiMn₂O₄ (LMO) precursor under mild condition which was first prepared via a modified one-pot reduction hydrothermal method using KMnO₄ and ethanol. Subsequently, the stable spinel structured λ-MnO₂ (HMO) were prepared by acidification of LMO. The as-prepared HMO shows a unique hexagonal shape and can be used for rapid adsorption-desorption process for Li+adsorption. It was found that Li⁺ adsorption capacity of HMO was 24.7 mg·g^−1 in Li+ solution and the HMO also has a stable structure with manganese dissolution loss ratio of 3.9% during desorption process. Moreover, the lithium selectivity (α Li/Mg) reaches to 1.35 × 10³ in brine and the distribution coefficients (Kd) of Li+ is much greater than that of Mg2+. The results implied that HMO can be used in extract lithium from brine or seawater containing high ratio of magnesium and lithium.