The development of renewable energy technologies and next-generation quantum and optoelectronic devices relies on the secured supply of critical elements. Traditional mining and separation methods are disruptive to the environment, consume large quantities of harsh chemicals, and are unable to access dilute resources due to low elemental selectivity. Therefore, the invention of new separation methods and the fundamental understanding of the separation processes are crucial to realizing sustainable separation while broadening minable resources. In this talk, I will first introduce a platform method, electrochemical intercalation, for selective ion separation. I will introduce several strategies to promote the separation of lithium and lanthanides based on the understanding of complex host behavior upon co-intercalation of competing ions. In the second part, I will introduce our efforts in developing methods to construct (sub)nanometer solid ionic channels and probe the water and ion transport behavior in confinement.