The application of nanofiltration (NF) membrane with positive charge on the surface in the separation of lithium‑magnesium is a convenient, energy-saving and sustainable strategy. In this work, polyethyleneimine (PEI) was introduced into the composite NF membrane through two methods to investigate and make a comparation in surface morphology and properties, application in the separation of membrane to Li⁺ and Mg²⁺, respectively. Both PEI and PIP were involved in the preparation of membranes as aqueous monomers, which can reduce the cross-linking degree of active layer but reduce the electronegativity of membrane surface. The rejection of Mg²⁺ reached 98.1 % and the purity of Li⁺, which is the mass fraction of Li⁺ in cations of permeate, reached 62 %, respectively, the separation factor S_Li,Mg reached 32.6. By contrast, PEI with different molecular weight and concentration was grafted on the PA layer which improved the density of membrane but reduced the electronegativity of the membrane surface. The rejection of Mg²⁺ reached 97.2 % and the purity of Li⁺ reached 63 %, respectively, the S_Li,Mg reached 33.6 with an acceptable flux. Both of two modification methods had a preferable performance on separation of lithium‑magnesium with a higher S_Li,Mg. Therefore, it is an efficient strategy to add PEI through different methods into the preparation of NF membrane to separate lithium‑magnesium and extract the Li⁺ from salt lake brine with high Mg/Li mass ratio.

应用表面带正电荷的纳滤（NF）膜分离锂镁是一种方便、节能、可持续的策略。本工作通过两种方法将聚乙烯亚胺(PEI)引入复合NF膜中，研究并比较其表面形貌和性能，分别在膜分离Li + 和Mg 2+ 中^的应用^。PEI和PIP均作为水性单体参与膜的制备，可以降低活性层的交联度，但会降低膜表面的电负性。^{Mg 2+}的截留率达到98.1%，Li ⁺的纯度（即Li ^{+的质量分数）}渗透液中阳离子含量分别达到62%，分离因子S _Li、Mg达到32.6。相比之下，将不同分子量和浓度的PEI接枝到PA层上，提高了膜的密度，但降低了膜表面的电负性。^{Mg 2+}的截留率达到97.2%，Li ⁺的纯度达到63%，S _Li、Mg达到33.6，通量合格。两种改性方法均具有较好的锂镁分离性能，且具有较高的S _Li,Mg。因此，通过不同的方法将PEI添加到纳滤膜的制备中来分离锂镁并提取锂是一种有效的策略。⁺来自高镁/锂质量比的盐湖卤水。