Direct lithium extraction via membrane separations has been fundamentally limited by lack of monovalent ion selectivity exhibited by conventional polymeric membranes, particularly between sodium and lithium ions. Recently, a 12-Crown-4-functionalized polynorbornene membrane was shown to have the largest lithium/sodium permeability selectivity observed in a fully aqueous system to date. Using atomistic molecular dynamics simulations, we reveal that this selectivity is due to strong interactions between sodium ions and 12-Crown-4 moieties, which reduce sodium ion diffusivity while leaving lithium ion mobility relatively unaffected. Moreover, the ion diffusivities in the membrane, when scaled by their respective solution diffusivities and free ion fractions, can be collapsed to an almost universal relationship depending on solvent volume fraction.

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12-Crown-4-官能化聚合物膜中锂/钠反渗透选择性的起源

通过膜分离直接提取锂从根本上受到传统聚合物膜缺乏单价离子选择性的限制，特别是在钠离子和锂离子之间。最近，12-Crown-4-官能化聚降冰片烯膜被证明具有迄今为止在全水系统中观察到的最大锂/钠渗透选择性。使用原子分子动力学模拟，我们揭示了这种选择性是由于钠离子和 12-Crown-4 部分之间的强相互作用，这降低了钠离子的扩散性，而锂离子的迁移率相对不受影响。此外，膜中的离子扩散率，当通过它们各自的溶液扩散率和自由离子分数来衡量时，可以根据溶剂体积分数折叠成几乎普遍的关系。